Physiological, Metabolic, and Mitochondrial Adaptations to a One-Week Endurance Training Camp in Recreational Athletes: An Observational Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study has practical and timely value in that it examines the physiological and psychological effects of a short-term endurance training camp conducted in a holiday-like setting in recreational endurance athletes. In particular, it is considered a strength of this study that it integrates results across multiple levels, including performance, body composition, biochemical markers, and mitochondrial-related variables. In addition, it provides meaningful baseline data by exploring short-term adaptive responses in a real-world setting. However, the current manuscript shows limitations in that the logical structure of the introduction and the theoretical background are not sufficiently aligned with the study population and study design, and therefore the main research question and novelty are not clearly presented. The specific points are as follows.

1. In the current introduction, different concepts such as structured endurance training principles, tapering strategies (reduction of training load), and long-term training camps in elite athletes are presented together within a single flow. However, this study deals with a short-term training camp in a holiday-like setting targeting recreational endurance athletes, which does not directly correspond to these concepts. In particular, tapering (load reduction) and training camps (load increase) represent different physiological stimuli, but they are described within the same context without clear distinction, which leads to logical confusion. Accordingly, the introduction would be clearer if it is organized in the following way. Presentation of the real-world background that participation in short-term training camps is increasing among recreational endurance athletes Emphasis on the lack of evidence for this population and type of intervention Clarification of the key concept of this study, namely → a situation in which increased training load and reduced psychological stress due to the holiday environment occur simultaneously
2. Most of the studies cited in the introduction are based on elite athletes (long-term training camps, altitude-based training, highly structured programs), whereas the population of this study consists of recreational endurance athletes, whose training environment and characteristics are different. Because of this, there are limitations in directly applying the mechanisms or findings from previous studies to the context of the present study. Therefore, if the authors (1) briefly distinguish between elite athletes and recreational participants and (2) explain how previous findings may be only partially applicable to this population, the logical flow of the introduction could be improved.
3. In the current manuscript, it is not entirely clear whether this study should be considered a training intervention study, a recovery (rest) study, or a combined scenario. This study actually deals with a situation in which increased training load and reduced psychological stress due to the holiday environment coexist, which differentiates it from conventional studies focusing on a single stimulus. If this characteristic is more clearly described in the latter part of the introduction and linked to the study aim, the originality of this study would be more evident.
4. Minor Comments: As “holiday-like setting” is a key element of this study, a brief definition or explanation would help readers’ understanding. Providing a schematic figure or conceptual diagram that summarizes the study design would make it easier to understand.

Comments on the Quality of English Language

This study has practical and timely value in that it examines the physiological and psychological effects of a short-term endurance training camp conducted in a holiday-like setting in recreational endurance athletes. In particular, it is considered a strength of this study that it integrates results across multiple levels, including performance, body composition, biochemical markers, and mitochondrial-related variables. In addition, it provides meaningful baseline data by exploring short-term adaptive responses in a real-world setting. However, the current manuscript shows limitations in that the logical structure of the introduction and the theoretical background are not sufficiently aligned with the study population and study design, and therefore the main research question and novelty are not clearly presented. The specific points are as follows.

1. In the current introduction, different concepts such as structured endurance training principles, tapering strategies (reduction of training load), and long-term training camps in elite athletes are presented together within a single flow. However, this study deals with a short-term training camp in a holiday-like setting targeting recreational endurance athletes, which does not directly correspond to these concepts. In particular, tapering (load reduction) and training camps (load increase) represent different physiological stimuli, but they are described within the same context without clear distinction, which leads to logical confusion. Accordingly, the introduction would be clearer if it is organized in the following way. Presentation of the real-world background that participation in short-term training camps is increasing among recreational endurance athletes Emphasis on the lack of evidence for this population and type of intervention Clarification of the key concept of this study, namely → a situation in which increased training load and reduced psychological stress due to the holiday environment occur simultaneously
2. Most of the studies cited in the introduction are based on elite athletes (long-term training camps, altitude-based training, highly structured programs), whereas the population of this study consists of recreational endurance athletes, whose training environment and characteristics are different. Because of this, there are limitations in directly applying the mechanisms or findings from previous studies to the context of the present study. Therefore, if the authors (1) briefly distinguish between elite athletes and recreational participants and (2) explain how previous findings may be only partially applicable to this population, the logical flow of the introduction could be improved.
3. In the current manuscript, it is not entirely clear whether this study should be considered a training intervention study, a recovery (rest) study, or a combined scenario. This study actually deals with a situation in which increased training load and reduced psychological stress due to the holiday environment coexist, which differentiates it from conventional studies focusing on a single stimulus. If this characteristic is more clearly described in the latter part of the introduction and linked to the study aim, the originality of this study would be more evident.
4. Minor Comments: As “holiday-like setting” is a key element of this study, a brief definition or explanation would help readers’ understanding. Providing a schematic figure or conceptual diagram that summarizes the study design would make it easier to understand.

Author Response

This study has practical and timely value in that it examines the physiological and psychological effects of a short-term endurance training camp conducted in a holiday-like setting in recreational endurance athletes. In particular, it is considered a strength of this study that it integrates results across multiple levels, including performance, body composition, biochemical markers, and mitochondrial-related variables. In addition, it provides meaningful baseline data by exploring short-term adaptive responses in a real-world setting. However, the current manuscript shows limitations in that the logical structure of the introduction and the theoretical background are not sufficiently aligned with the study population and study design, and therefore the main research question and novelty are not clearly presented. The specific points are as follows.

General Response: We thank the reviewer for his/her diligent review and the helpful comments to improve the manuscript. We provide a point-to-point response in the following paragraphs. Please note that all line numbers refer to the manuscript without track changes.

1. In the current introduction, different concepts such as structured endurance training principles, tapering strategies (reduction of training load), and long-term training camps in elite athletes are presented together within a single flow. However, this study deals with a short-term training camp in a holiday-like setting targeting recreational endurance athletes, which does not directly correspond to these concepts. In particular, tapering (load reduction) and training camps (load increase) represent different physiological stimuli, but they are described within the same context without clear distinction, which leads to logical confusion. Accordingly, the introduction would be clearer if it is organized in the following way. Presentation of the real-world background that participation in short-term training camps is increasing among recreational endurance athletes Emphasis on the lack of evidence for this population and type of intervention Clarification of the key concept of this study, namely → a situation in which increased training load and reduced psychological stress due to the holiday environment occur simultaneously

Response: Thank you very much for this suggestion. We now shortened the paragraphs focusing on elite athletes and followed your proposed organization of the introduction.

1. Most of the studies cited in the introduction are based on elite athletes (long-term training camps, altitude-based training, highly structured programs), whereas the population of this study consists of recreational endurance athletes, whose training environment and characteristics are different. Because of this, there are limitations in directly applying the mechanisms or findings from previous studies to the context of the present study. Therefore, if the authors (1) briefly distinguish between elite athletes and recreational participants and (2) explain how previous findings may be only partially applicable to this population, the logical flow of the introduction could be improved.

Response: We agree that a clearer distinction should be made and revised accordingly (lines 78 ff.)

1. In the current manuscript, it is not entirely clear whether this study should be considered a training intervention study, a recovery (rest) study, or a combined scenario. This study actually deals with a situation in which increased training load and reduced psychological stress due to the holiday environment coexist, which differentiates it from conventional studies focusing on a single stimulus. If this characteristic is more clearly described in the latter part of the introduction and linked to the study aim, the originality of this study would be more evident.

Response: We also dealt with the “problem” of a non-clearly defined study design. As stated in the title of the manuscript, we think that the most appropriate one is an observational study, as the training schedule was designed by all professional trainers and not the study team. We now included a more clearly described definition at the end of the introduction (lines 90 ff.).

 

1. Minor Comments: As “holiday-like setting” is a key element of this study, a brief definition or explanation would help readers’ understanding. Providing a schematic figure or conceptual diagram that summarizes the study design would make it easier to understand.

Response: Thank you for this suggestion. We now included as further supplemental document the proposed training schedule for all training camp participants to 1) clarify the training load and 2) show the holiday-like breaks, duration of recreational time etc. (Supplemental S2)

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In its current form, I am unable to recommend this manuscript for publication. Although the topic is potentially relevant and of practical interest, I find that the study is fundamentally limited in both its methodological design and interpretative rigor to a degree that cannot be resolved through even a major revision.

1. My primary concern lies in the lack of a coherent experimental framework. The study is presented as an investigation of the effects of a one-week training camp, yet in practice, it constitutes an uncontrolled observational report with highly heterogeneous exposure. Participants were free to select training sessions; training load was not standardized, and no objective monitoring of intensity or compliance was implemented. As a result, I do not see a clearly defined “intervention,” which makes it difficult to attribute any observed changes to a specific stimulus. In my view, this severely compromises internal validity and undermines the scientific value of the findings.
2. Closely related to this is the absence of a control group, which I consider a critical flaw rather than a minor limitation. While the authors acknowledge this issue, the manuscript nevertheless proceeds to interpret pre–post changes as if they reflect causal effects of the training camp. I find this inconsistent and methodologically problematic. Given the influence of confounding factors such as environmental change (holiday setting), altered daily routines, nutritional variability, and regression to the mean, the observed effects cannot be meaningfully isolated.
3. I am also not convinced by the selection and interpretation of outcome measures. The study includes an extensive panel of biomarkers, yet the functional outcomes—most notably endurance performance—do not show meaningful improvement. Despite this, the manuscript places considerable emphasis on molecular and biochemical changes, suggesting “adaptations” that are not supported by performance data. From my perspective, this represents a disproportionate reliance on surrogate markers, many of which (e.g., circulating mtDNA) are inherently ambiguous and may reflect stress or cellular damage rather than beneficial adaptation. I find the mechanistic narrative, therefore, speculative and, at times, overstated.
4. From a statistical standpoint, I consider the analysis insufficiently robust given the scope of the collected data. A large number of variables are tested without adjustment for multiple comparisons, increasing the likelihood of false-positive findings. At the same time, effect sizes are inconsistently reported, and the sample size appears inadequate to support reliable inference across such a broad set of endpoints. This imbalance—many variables, limited statistical control—further weakens confidence in the results.
5. In addition, I find the manuscript lacks conceptual focus. It attempts to simultaneously address performance, metabolism, mitochondrial biology, psychological stress, sleep, and nutrition, but without a clearly articulated hierarchy of outcomes or a unifying theoretical framework. The result is a descriptive aggregation of findings rather than a coherent scientific argument.
6. While the Discussion is extensive, I find it frequently extends beyond what the data can support, particularly regarding mitochondrial adaptations, systemic stress regulation, and the purported benefits of the training camp. In several instances, speculative explanations are presented in a manner that may be interpreted as evidence-based conclusions, which I consider inappropriate given the study design.

My overall assessment is that the manuscript is methodologically weak, analytically limited, and interpretatively overstretched. The combination of an uncontrolled design, poorly defined intervention, reliance on surrogate outcomes, and insufficient statistical rigor prevents the study from making a meaningful scientific contribution in its current form.

Author Response

In its current form, I am unable to recommend this manuscript for publication. Although the topic is potentially relevant and of practical interest, I find that the study is fundamentally limited in both its methodological design and interpretative rigor to a degree that cannot be resolved through even a major revision.

General Response: Although the reviewer suggests our research insufficient for publication, we nevertheless provide a point-to-point response in the following paragraphs, and thus try to address some of the concerns. Please note that all line numbers refer to the manuscript without track changes.

1. My primary concern lies in the lack of a coherent experimental framework. The study is presented as an investigation of the effects of a one-week training camp, yet in practice, it constitutes an uncontrolled observational report with highly heterogeneous exposure. Participants were free to select training sessions; training load was not standardized, and no objective monitoring of intensity or compliance was implemented. As a result, I do not see a clearly defined “intervention,” which makes it difficult to attribute any observed changes to a specific stimulus. In my view, this severely compromises internal validity and undermines the scientific value of the findings.

Response: As already pointed out in the title of the manuscript, we did not perform a clearly defined intervention but performed an observational study. As we monitored the individual training load as well as the caloric expenditure/uptake, we could estimate that training volume increased approximately three times compared to a normal training week of the participants. A clearly defined intervention would have undermined the desired real-world setting of our study and would not be sensible. Thus, we now include the training schedule as a further supplementary document (Supplemental S2).

1. Closely related to this is the absence of a control group, which I consider a critical flaw rather than a minor limitation. While the authors acknowledge this issue, the manuscript nevertheless proceeds to interpret pre–post changes as if they reflect causal effects of the training camp. I find this inconsistent and methodologically problematic. Given the influence of confounding factors such as environmental change (holiday setting), altered daily routines, nutritional variability, and regression to the mean, the observed effects cannot be meaningfully isolated.

Response: Thank you for pointing this out. Again, the whole training camp setting was used as template for the real-world training camps and their effects and effectiveness of the participants. All participants who attended the camp wanted to and performed the endurance training, thus a control group normally did not exist in a real-world setting and the analytes would not be comparable.

1. I am also not convinced by the selection and interpretation of outcome measures. The study includes an extensive panel of biomarkers, yet the functional outcomes—most notably endurance performance—do not show meaningful improvement. Despite this, the manuscript places considerable emphasis on molecular and biochemical changes, suggesting “adaptations” that are not supported by performance data. From my perspective, this represents a disproportionate reliance on surrogate markers, many of which (e.g., circulating mtDNA) are inherently ambiguous and may reflect stress or cellular damage rather than beneficial adaptation. I find the mechanistic narrative, therefore, speculative and, at times, overstated.

Response: As you said and as we pointed out in the abstract and the discussion, endurance performance did not improve, which is not surprising regarding the short training camp duration and the physiological stress by an increased training compared to the normal routine. Comparable studies in elite athletes show improvements need at least 2-4 weeks, and often in combination with altitude training. But as our study is the first one that examined the training effects in recreational athletes in a holiday setting, of course some discussion parts can be interpreted as speculative as no data is available to compare this with. Nevertheless, especially damage markers as well as objective and subjective stress markers are clearly linked one to another in our opinion. With regard to the training effects on performance, we are rather cautious about our interpretation and tried to focus the discussion on measured data and no more. Of course, we would be happy to revise the manuscript accordingly if the reviewer could give us specific examples.

1. From a statistical standpoint, I consider the analysis insufficiently robust given the scope of the collected data. A large number of variables are tested without adjustment for multiple comparisons, increasing the likelihood of false-positive findings. At the same time, effect sizes are inconsistently reported, and the sample size appears inadequate to support reliable inference across such a broad set of endpoints. This imbalance—many variables, limited statistical control—further weakens confidence in the results.

Response: Thank you for pointing this out. We now included a power analysis and added statistical information in the statistics section (lines 245 ff.). As we only had a pre-post design for the majority of the analyzed parameters, we did not adjust for multiple comparisons with this statistical approach. With regard to all variables where we had data for more than two sampling points (e.g. nutrition, the FSVS and the SRSS) and performed ANOVA, we of course adjusted for multiple comparisons.

1. In addition, I find the manuscript lacks conceptual focus. It attempts to simultaneously address performance, metabolism, mitochondrial biology, psychological stress, sleep, and nutrition, but without a clearly articulated hierarchy of outcomes or a unifying theoretical framework. The result is a descriptive aggregation of findings rather than a coherent scientific argument.

Response: As also suggested by another reviewer, we now tried to restructure our introduction as well as define more clearly the primary and secondary outcomes to show the main aim of our study. As an exploratory study approach and as no comparable studies have been available, we tried to gather as much information as possible and cover as much physiological and mental stress data as possible. Our study (as mentioned out is primary observational) should serve as a template or first data attempt for future research.

1. While the Discussion is extensive, I find it frequently extends beyond what the data can support, particularly regarding mitochondrial adaptations, systemic stress regulation, and the purported benefits of the training camp. In several instances, speculative explanations are presented in a manner that may be interpreted as evidence-based conclusions, which I consider inappropriate given the study design.

Response: As stated above we would be happy to revise the discussion section accordingly if the reviewer could provide more detailed information or specific sentences for too extensive speculations.

 

My overall assessment is that the manuscript is methodologically weak, analytically limited, and interpretatively overstretched. The combination of an uncontrolled design, poorly defined intervention, reliance on surrogate outcomes, and insufficient statistical rigor prevents the study from making a meaningful scientific contribution in its current form.

Response: We can understand the reviewer’s decision, but nevertheless would be happy if we could convince the reviewer of our contribution to a yet unknown research area with our revision.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors,

Your manuscript examines the physiological, metabolic, and mitochondrial adaptations that occur in recreational athletes after a one-week endurance training camp. This topic is timely as it addresses a genuine gap in the literature; most training camp studies have focused on elite athletes over longer periods. The authors' use of performance diagnostics, blood biomarkers, mitochondrial markers, nutritional tracking, and psychometric questionnaires reflects a comprehensive methodological approach and is commendable. However, critical issues spanning methodology, reporting, statistical transparency, and manuscript presentation must be addressed before the work can be considered for publication. Therefore, a major revision is recommended.

1. MANUSCRIPT ERROR
1.1. The opening of the Results section (lines 235–237) contains the following placeholder text from the MDPI template that was not removed by the authors: “This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, and the conclusions that can be drawn from them." This text must be deleted in the revised manuscript.

2. METHODS
2.1. No sample size calculation or power analysis is provided: Either present a formal power calculation or explicitly justify the sample size (n = 35) based on feasibility, effect sizes from comparable studies, or other transparent criteria. This is particularly important given that some key outcomes showed only trends (e.g., VO₂ max, p = 0.0551) and post-camp measurements are missing for a significant number of participants.
2.2. Unexplained participant attrition: Pre-camp body composition measurements were taken from 35 participants, but the post-camp assessment only includes 27 participants (Table 1). Similarly, post-camp lactate testing involved 34 participants (Table 2). The reasons for these discrepancies are never explained in the manuscript. According to STROBE item 13, authors must report the number of participants at each stage of the study and provide explicit reasons for nonparticipation or missing data at postcamp assessments. A participant flow diagram is strongly recommended.
2.3. VO2max calculation not described: You reported calculated VO₂ max values from an incremental field lactate test, yet you did not provide a description of the calculation method. VO₂ max derived from lactate testing requires specification of the predictive equation or extrapolation procedure used because different approaches yield substantially different values. This information is essential for reproducibility and must be added to the methods section.
2.4. Methodological error in kynurenine protocol description (lines 212–216): The kynurenine measurement protocol contains the contradictory statement: “the serum samples were subjected to a process of deproteinization using acetic acid trichloride. Following this procedure, the samples were then deproteinized.” Deproteinization is described as two separate procedures, which is scientifically nonsensical. This passage must be revised to accurately reflect the analytical protocol.
2.5. Inconsistent use of “serum” vs. “plasma”: Throughout the manuscript, the term “plasma” is consistently used for the samples (as per the blood collection and centrifugation protocol described in lines 175–179). However, in the kynurenine methods section (line 212), the term “serum” is used. This inconsistency must be resolved. If plasma was used for all analyses, as described in the sampling protocol, then “serum” should be corrected to “plasma” throughout.
2.6. Absence of study registration: No prospective registration of the study is mentioned (e.g., ClinicalTrials.gov or DRKS). The authors should clarify whether the study was registered and, if so, provide the registration number. If the study was not registered, a brief justification should be included.
2.7. Nutrition diary period not clearly defined: You state that participants recorded dietary intake "during the study period of three weeks" (line 133), but the precise start date of this period relative to the training camp is not defined. This should be clarified to allow for the accurate interpretation of pre-camp nutritional data.

3. RESULTS
3.1. Typographical error: “creatine” vs. “creatinine” (line 280): The Results section states that “creatine (p = 0.0183)” increased post-camp. However, the marker measured and displayed in Figure 2 is creatinine, a renal and muscular stress biomarker, not creatine, which is a fundamentally different molecule. This must be corrected throughout.
3.2. Inconsistency in reported fold-change values: Your text (line 307) reports an “18-fold” increase in circulating mitochondrial DNA post-camp, while the caption of Figure 4 (line 314) states “17.77-fold.” These values should be consistent. Additionally, the authors should clarify whether the fold-change values for mtDNA, B2M, and PGC1-α in Figure 4C were formally tested for statistical significance. Unlike all other figures in the manuscript, Figure 4C contains no indication of statistical testing (no p-values or significance markers), which must be addressed.
3.3. Absence of sex-stratified analyses: Your study included 18 male and 17 female participants. All results are reported as aggregated data, with no sex-stratified analysis or even a sensitivity analysis exploring sex as a potential moderator. Given the well-established sex differences in hormonal responses (e.g., cortisol), body composition, lipid metabolism, and mitochondrial adaptation to endurance exercise, this is a relevant analytical gap. The authors should either present sex-stratified results or provide a clear statistical justification for why aggregated analysis is appropriate in this context.
3.4. Insufficient reporting of SRSS data in the main text: The Short Recovery and Stress Scale (SRSS) is central to the study's claim of a beneficial “holiday effect” on psychological stress and recovery. However, the Results section devotes only five lines to this outcome (lines 274–277), referring readers almost entirely to Supplemental Figure S3. Given the conceptual importance of this finding to the study's narrative, a more detailed presentation in the main text is warranted.

4. DISCUSSION
4.1. Duplicated reference: Reference 54 (line 749) is identical to Reference 35 (line 701), both cite Bizjak et al. (2021), Frontiers in Physiology. One of these citations must be replaced with an appropriate alternative reference or removed if redundant.
4.2.  Insufficient discussion of elevated LDH in the context of foot-strike hemolysis: You attribute the post-camp increase in LDH exclusively to muscle membrane stress and metabolic load. However, LDH is also a sensitive marker of intravascular hemolysis, which is particularly relevant in running athletes due to foot-strike hemolysis. This alternative explanation should at minimum be acknowledged and discussed in relation to the observed changes in hematological markers (MCV, MCHC, hematocrit).
4.3. Lipid metabolism findings not discussed: Section 3.7.2 reports significant increases in total cholesterol, HDL, and LDL post-camp (Supplemental Figure S5), yet these findings receive no discussion whatsoever. Given that exercise, dietary changes, and hydration status can all influence circulating lipids, these findings should be contextualized in the Discussion, even briefly.
4.4. Vitamin D interpretation requires further nuance: While the authors acknowledge potential confounding from sun exposure at the Mallorca camp, the discussion does not adequately address how the exercise-induced effect on vitamin D can be distinguished from the effect of solar UV-B exposure in early March in southern Spain. A more explicit discussion of this limitation, including any available data on participants' sun exposure behaviors, would strengthen the interpretation.

5. FORMATTING
5.1. Table formatting: Both Table 1 and Table 2 are formatted as full-grid tables with vertical and internal borders. The MDPI template for *Sports* requires three-line tables (horizontal lines only: top border, header separator, and bottom border), with no vertical lines. Both tables must be reformatted accordingly.
5.2. Figure legend punctuation: Figure legends in this manuscript are inconsistent. Figures 1 and 2 correctly use a period after the figure number (e.g., "Figure 1."), while Figures 3 and 4 incorrectly use a colon (e.g., "Figure 3:"). All figure legends must follow the MDPI standard format using a period.
5.3. Abbreviation error: In the Abbreviations section (line 599), “RDE: relative distribution width" is incorrect. The correct abbreviation, consistently used in the main text (line 320) and supplementary material, is RDW (Relative Distribution Width). This must be corrected.

6. STROBE Compliance
6.1. You state that the study was reported in accordance with the STROBE guidelines (line 153–154). However, the following STROBE items are not adequately addressed in the current version:
6.1.1. Item 10 (Study size): No justification for the sample size is provided.
6.1.2. Item 13 (Participants): The flow of participants across study timepoints is not reported, and reasons for missing post-camp data are absent.
6.1.3. Item 7 (Variables): No hierarchy of primary vs. secondary outcomes is established, making it difficult to assess the risk of false positives given the large number of outcomes tested.
6.1.4. Item 17 (Other analyses): No subgroup or sensitivity analyses are reported (e.g., by sex, sport modality, or baseline fitness level).

MINOR COMMENTS
A) The introduction cites two commercial/marketing websites (References 1 and 2) as supporting evidence. While these are acceptable for illustrating the commercial landscape of training camps, the authors should ensure they are framed exclusively as examples of marketing claims and not as scientific sources.
B) The participant classification as "Tier 1 to 2" (McKay et al., 2022) should be more explicitly justified with reference to the observed VO2max values and training history data.
C) Figure S2 combines macronutrient data (panels A and B) with sleep quality data (panel C) in the same supplemental figure. These are conceptually distinct outcomes and should ideally be presented in separate figures for clarity.

Comments on the Quality of English Language

The manuscript is generally written in acceptable English. However, certain passages require revision for clarity and accuracy. Most notably, the description of the kynurenine measurement protocol in the Materials and Methods section contains a redundant and contradictory sentence in which the deproteinization step is described twice, making it seem like two separate procedures. This compromises the reader's understanding of the method. Additionally, minor inconsistencies in terminology (e.g., "serum" vs. "plasma") and awkward phrasing in the discussion section suggest that a thorough language edit (ideally by a native English-speaking scientist familiar with exercise physiology) would improve the manuscript before acceptance.

Author Response

Dear Authors,

Your manuscript examines the physiological, metabolic, and mitochondrial adaptations that occur in recreational athletes after a one-week endurance training camp. This topic is timely as it addresses a genuine gap in the literature; most training camp studies have focused on elite athletes over longer periods. The authors' use of performance diagnostics, blood biomarkers, mitochondrial markers, nutritional tracking, and psychometric questionnaires reflects a comprehensive methodological approach and is commendable. However, critical issues spanning methodology, reporting, statistical transparency, and manuscript presentation must be addressed before the work can be considered for publication. Therefore, a major revision is recommended.

General Response: We thank the reviewer for his/her diligent review and the helpful comments to improve the manuscript. We provide a point-to-point response in the following paragraphs. Please note that all line numbers refer to the manuscript without track changes.

1. MANUSCRIPT ERROR

1.1. The opening of the Results section (lines 235–237) contains the following placeholder text from the MDPI template that was not removed by the authors: “This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, and the conclusions that can be drawn from them." This text must be deleted in the revised manuscript.

Response: Thank you very much for pointing this out. We missed this part in the original manuscript and revised it accordingly.

2. METHODS

2.1. No sample size calculation or power analysis is provided: Either present a formal power calculation or explicitly justify the sample size (n = 35) based on feasibility, effect sizes from comparable studies, or other transparent criteria. This is particularly important given that some key outcomes showed only trends (e.g., VO₂ max, p = 0.0551) and post-camp measurements are missing for a significant number of participants.

Response: Thank you for this comment. As there were no comparable studies available, we tried to include as many participants as possible. As there were a total of 60 free spaces available for the whole training camp, our study population had to be recruited from this population. A power analysis revealed that at least 34 participants were needed for sound statistical testing. We now added the following information to the statistic section (lines 244 ff).

“Data were analyzed using GraphPad Prism 10.5 (San Diego, CA, USA). Normality was assessed with the Kolmogorov-Smirnov test.

For paired comparisons of pre and post camp data, two-tailed t-tests were used for normal distributed data, and Wilcoxon matched-pairs signed rank tests were applied otherwise.

One-way ANOVA with Holm-Šídák's test and Greenhouse-Geisser correction (normal data) or Kruskal-Wallis with Dunn's test (non-normal data) was used to analyse nutrition, the FSVS and the SRSS, as these questionnaires covered all weeks of the study period (pre-camp, training camp, post-camp). Effect sizes are reported as partial eta squared (η_p^2), where η_p^2=0.01 signifies small, 0.06 η_p^2 medium, and 0.14 η_p^2 large effects for parametric tests only. For non-parametric Wilcoxon matched-pairs signed rank tests, GraphPad Prism does not compute any effect sizes. Data are shown as mean ± SD unless noted; significance was set at p ≤ 0.05. For of RT-PCR analysis (MITO, B2M, PGC1-alpha), only relative fold-change during pre and post without additional testing can be reported.

An a priori power analysis was conducted using G*Power (version 3.1.9.7; Heinrich Heine University Düsseldorf, Germany) to determine the required sample size for detecting within-subject changes over the course of the one-week endurance training camp. Given the study design, a paired-samples t-test (two-tailed) was selected for primary analyses comparing pre- and post-intervention outcomes.

The significance level was set at α = 0.05, with a desired statistical power of 1−β = 0.80. As no directly comparable studies investigating short-term training camps in recreational athletes were available, effect size assumptions were based on established conventions and related exercise intervention literature [34].

For performance-related outcomes (i.e., maximal oxygen uptake [VO₂max] and lactate metabolism), only small effects were anticipated (Cohen’s d = 0.2–0.3), as substantial physiological adaptations are unlikely to occur within a one-week intervention. Detecting such small effects would require large sample sizes (n ≈ 90–200), which was not feasible within the present study framework. Therefore, analyses of performance outcomes were considered exploratory.

In contrast, moderate to large effects were expected for psychophysiological stress parameters due to the combined impact of structured training, environmental change, and group dynamics (Cohen’s d = 0.5–0.8). Based on these assumptions, the required sample size ranged between n = 15 and n = 34 participants.

To ensure adequate statistical power for the primary outcomes related to mental and physiological stress, while accounting for potential dropouts and inter-individual variability, a target sample size of at least 34 participants was defined. This sample size was deemed sufficient to detect moderate effects with adequate power in within-subject comparisons.

The power analysis followed established statistical guidelines for behavioral sciences and experimental research [34].”

2.2. Unexplained participant attrition: Pre-camp body composition measurements were taken from 35 participants, but the post-camp assessment only includes 27 participants (Table 1). Similarly, post-camp lactate testing involved 34 participants (Table 2). The reasons for these discrepancies are never explained in the manuscript. According to STROBE item 13, authors must report the number of participants at each stage of the study and provide explicit reasons for nonparticipation or missing data at postcamp assessments. A participant flow diagram is strongly recommended.

Response: You are correct in pointing this out. We added a participant flow diagram to supplemental figure S1 (study design). From initially 41 screened participants, 6 dropped out due to injury/disease or organizational reasons. One further drop-out occurred due to injury before post-testing. Seven BIAs could not be conducted due to measurement failures of the BIA scale at the end of the study (added also in lines 288 ff.).

2.3. VO2max calculation not described: You reported calculated VO₂ max values from an incremental field lactate test, yet you did not provide a description of the calculation method. VO₂ max derived from lactate testing requires specification of the predictive equation or extrapolation procedure used because different approaches yield substantially different values. This information is essential for reproducibility and must be added to the methods section.

Response: Lactate testing analysis was performed with the commercially available Ergonizer software (https://www.ergonizer.de/index-de.html). The developer of this software does not provide an exact calculation algorithm for estimation of VO2max. We cite the information provided by the developer: “Physical performance can be calculated on a treadmill and measured on an exercise bike. Assuming a constant efficiency, the subject’s energy expenditure can therefore be calculated at each exercise intensity level. Using this information, the software estimates the respective oxygen demand and maximum oxygen uptake (VO2max). Naturally, this calculated estimate is not as reliable as the measurement obtained via respiratory gas analysis. Nevertheless, the calculated value is well suited for comparative assessments.”

Thus, we added the following information in the manuscript (lines 182 ff.):

As well as lactate analysis, VO₂max was estimated using the Ergonizer software (version 5.16.2, Freiburg, Germany). The software does not rely on a single predic-tive equation but applies a multi-step computational approach based on exercise performance data. First, maximal power output (Pmax) was calculated from the final completed and partially completed stages of the incremental test using the following equation: Pmax = P_penultimate + (P_increment × t_last stage / t_stage). Based on the assumption of a constant mechanical efficiency, the corresponding energy expenditure was calculated for each workload. From this, oxygen demand was estimated and subsequently extrapolated to determine VO₂max [28].

2.4. Methodological error in kynurenine protocol description (lines 212–216): The kynurenine measurement protocol contains the contradictory statement: “the serum samples were subjected to a process of deproteinization using acetic acid trichloride. Following this procedure, the samples were then deproteinized.” Deproteinization is described as two separate procedures, which is scientifically nonsensical. This passage must be revised to accurately reflect the analytical protocol.

Response: Thank you for pointing this out. Revised.

2.5. Inconsistent use of “serum” vs. “plasma”: Throughout the manuscript, the term “plasma” is consistently used for the samples (as per the blood collection and centrifugation protocol described in lines 175–179). However, in the kynurenine methods section (line 212), the term “serum” is used. This inconsistency must be resolved. If plasma was used for all analyses, as described in the sampling protocol, then “serum” should be corrected to “plasma” throughout.

Response: We only used plasma samples. We now revised it accordingly.

2.6. Absence of study registration: No prospective registration of the study is mentioned (e.g., ClinicalTrials.gov or DRKS). The authors should clarify whether the study was registered and, if so, provide the registration number. If the study was not registered, a brief justification should be included.

Response: As this study was not designed as an intervention study and consisted more of an observational approach, we did not seem it appropriate to register it as a clinical study. We now added this information in line 162 ff.).

2.7. Nutrition diary period not clearly defined: You state that participants recorded dietary intake "during the study period of three weeks" (line 133), but the precise start date of this period relative to the training camp is not defined. This should be clarified to allow for the accurate interpretation of pre-camp nutritional data.

Response: The nutrition diary started exactly one week before the first day of the training camp, and ended exactly one week after the last day of the training camp.

We now added this information (lines 137 ff.).

3. RESULTS

3.1. Typographical error: “creatine” vs. “creatinine” (line 280): The Results section states that “creatine (p = 0.0183)” increased post-camp. However, the marker measured and displayed in Figure 2 is creatinine, a renal and muscular stress biomarker, not creatine, which is a fundamentally different molecule. This must be corrected throughout.

Response: Thank you for pointing this out. We now corrected this mistake.

3.2. Inconsistency in reported fold-change values: Your text (line 307) reports an “18-fold” increase in circulating mitochondrial DNA post-camp, while the caption of Figure 4 (line 314) states “17.77-fold.” These values should be consistent. Additionally, the authors should clarify whether the fold-change values for mtDNA, B2M, and PGC1-α in Figure 4C were formally tested for statistical significance. Unlike all other figures in the manuscript, Figure 4C contains no indication of statistical testing (no p-values or significance markers), which must be addressed.

Response: We revised the numbers accordingly. Fold-change values only show a relative change from pre to post measurements and can only be reported descriptively. A statistical analysis would only be possible with another group or another training camp etc. We added this to the statistic section.

3.3. Absence of sex-stratified analyses: Your study included 18 male and 17 female participants. All results are reported as aggregated data, with no sex-stratified analysis or even a sensitivity analysis exploring sex as a potential moderator. Given the well-established sex differences in hormonal responses (e.g., cortisol), body composition, lipid metabolism, and mitochondrial adaptation to endurance exercise, this is a relevant analytical gap. The authors should either present sex-stratified results or provide a clear statistical justification for why aggregated analysis is appropriate in this context.

Response: We deliberately did not introduce further sub-analysis due to the already minimal needed participant number of n=34 for sound statistical testing of our parameters (please see comment above regarding power analysis). As this study was mainly observational and exploratory, an approach to include all participants of this camp was selected.

3.4. Insufficient reporting of SRSS data in the main text: The Short Recovery and Stress Scale (SRSS) is central to the study's claim of a beneficial “holiday effect” on psychological stress and recovery. However, the Results section devotes only five lines to this outcome (lines 274–277), referring readers almost entirely to Supplemental Figure S3. Given the conceptual importance of this finding to the study's narrative, a more detailed presentation in the main text is warranted.

Response: We added the sentence “All items showed changed values after days 2-3 after arriving at the camp, and changed again immediately after returning from the training camp” to underline the possible holiday induced effects (lines 335-336).

4. DISCUSSION

4.1. Duplicated reference: Reference 54 (line 749) is identical to Reference 35 (line 701), both cite Bizjak et al. (2021), Frontiers in Physiology. One of these citations must be replaced with an appropriate alternative reference or removed if redundant.

Response: Your observation is correct. Thank you. The references got two different numbers assigned by our citation program and are now correctly aligned.

4.2.  Insufficient discussion of elevated LDH in the context of foot-strike hemolysis: You attribute the post-camp increase in LDH exclusively to muscle membrane stress and metabolic load. However, LDH is also a sensitive marker of intravascular hemolysis, which is particularly relevant in running athletes due to foot-strike hemolysis. This alternative explanation should at minimum be acknowledged and discussed in relation to the observed changes in hematological markers (MCV, MCHC, hematocrit).

Response: We included “In addition, LDH is also a sensitive marker of intravascular hemolysis, which is particularly relevant in running athletes due to foot-strike hemolysis [60,61]. In combination with the observed changes in hematological markers (increased MCV and RDW as well as decreased RDW), this might show first signs of increased erythrocytic syntheses after the unaccustomed exercise-induced hematologic stress. Unfortunately, we did not measure haptoglobin or reticulocytes, which would provide further insights into this possible mechanism.” (lines 547 ff.).

4.3. Lipid metabolism findings not discussed: Section 3.7.2 reports significant increases in total cholesterol, HDL, and LDL post-camp (Supplemental Figure S5), yet these findings receive no discussion whatsoever. Given that exercise, dietary changes, and hydration status can all influence circulating lipids, these findings should be contextualized in the Discussion, even briefly.

Response: Thank you for this important observation. We agree that the changes in lipid parameters warrant contextualization. We have now added the following paragraph to the Discussion addressing the observed increases in total cholesterol, HDL, and LDL following the training camp.

“Although total cholesterol, HDL, and LDL increased after the training camp, these increases may reflect acute physiological responses rather than adverse cardiometabolic changes. Previous research has shown that short-term intensive endurance exercise can transiently elevate circulating lipid concentrations, reflecting in-creased lipolysis and fatty acid mobilization during intensive endurance exercise, as well as short-term metabolic adaptations to acute exercise stimuli [62,63]. In addition, changes in dietary intake during training camps—such as increased energy or fat consumption—may further contribute to altered lipid profiles. Importantly, the concurrent rise in HDL cholesterol is generally considered a favorable adaptation associated with endurance training [62]. Therefore, the present findings likely represent short-term, exercise-induced modulation of lipid metabolism due to the training camp.” (lines 557 ff.).

4.4. Vitamin D interpretation requires further nuance: While the authors acknowledge potential confounding from sun exposure at the Mallorca camp, the discussion does not adequately address how the exercise-induced effect on vitamin D can be distinguished from the effect of solar UV-B exposure in early March in southern Spain. A more explicit discussion of this limitation, including any available data on participants' sun exposure behaviors, would strengthen the interpretation.

Response: We now added “Nevertheless, the outdoor nature of the training sessions, combined with in-creased time spent outside during the camp in Mallorca, makes it difficult to dis-entangle the relative contributions of exercise and UV-B exposure. As individual sun exposure behaviors (e.g., duration of exposure, clothing, sunscreen use) were not systematically assessed, the observed increase in vitamin D may be interpret-ed as the combined effect of environmental and behavioral factors as well as the increased exercise volume and intensity.” (lines 513 ff.).

5. FORMATTING

5.1. Table formatting: Both Table 1 and Table 2 are formatted as full-grid tables with vertical and internal borders. The MDPI template for *Sports* requires three-line tables (horizontal lines only: top border, header separator, and bottom border), with no vertical lines. Both tables must be reformatted accordingly.

Response: Done

5.2. Figure legend punctuation: Figure legends in this manuscript are inconsistent. Figures 1 and 2 correctly use a period after the figure number (e.g., "Figure 1."), while Figures 3 and 4 incorrectly use a colon (e.g., "Figure 3:"). All figure legends must follow the MDPI standard format using a period.

Response: Done

5.3. Abbreviation error: In the Abbreviations section (line 599), “RDE: relative distribution width" is incorrect. The correct abbreviation, consistently used in the main text (line 320) and supplementary material, is RDW (Relative Distribution Width). This must be corrected.

Response: Thank you. Revised.

6. STROBE Compliance

6.1. You state that the study was reported in accordance with the STROBE guidelines (line 153–154). However, the following STROBE items are not adequately addressed in the current version:

6.1.1. Item 10 (Study size): No justification for the sample size is provided.

Response: We provided an updated statistics section with the relevant information (see above).

6.1.2. Item 13 (Participants): The flow of participants across study timepoints is not reported, and reasons for missing post-camp data are absent.

Response: Included (please see above)

6.1.3. Item 7 (Variables): No hierarchy of primary vs. secondary outcomes is established, making it difficult to assess the risk of false positives given the large number of outcomes tested.

Response: We provided an updated statistics section with the relevant information (see above).

6.1.4. Item 17 (Other analyses): No subgroup or sensitivity analyses are reported (e.g., by sex, sport modality, or baseline fitness level).

Response: This is correct. We deliberately did not introduce further sub-analysis due to the already minimal needed participant number of n=34 for sound statistical testing of our parameters. As this study was mainly observational and exploratory, an approach to include all participants of this camp was selected.

MINOR COMMENTS

1. A) The introduction cites two commercial/marketing websites (References 1 and 2) as supporting evidence. While these are acceptable for illustrating the commercial landscape of training camps, the authors should ensure they are framed exclusively as examples of marketing claims and not as scientific sources.

Response: Revised. We now clearly mention that the references refer to commercial websites and serve only as examples.

1. B) The participant classification as "Tier 1 to 2" (McKay et al., 2022) should be more explicitly justified with reference to the observed VO2max values and training history data.

Response: Done

1. C) Figure S2 combines macronutrient data (panels A and B) with sleep quality data (panel C) in the same supplemental figure. These are conceptually distinct outcomes and should ideally be presented in separate figures for clarity.

Response: Done

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have sufficiently addressed the major concerns raised in the previous review, particularly regarding the revisions made to better align the structure of the introduction with the study population. These changes have improved the overall clarity of the manuscript.

The classification of the study as observational is acceptable, and the limitations are appropriately presented.

The supplemental materials were also reviewed and provide useful additional information for understanding the training structure and overall context of the study.

I believe that this study has sufficient merit to be considered for publication in this journal. However, I would recommend one final round of careful proofreading to address any minor errors and further improve the overall quality of the manuscript.

Additionally, the expression “Market(rain)ing” in the title is somewhat unclear in its intended meaning. For example, simplifying this expression to more directly reflect the content of the study (e.g., “Physiological, Metabolic, and Mitochondrial Adaptations to a One-Week Endurance Training Camp in Recreational Athletes”) could be considered as an alternative.

Comments on the Quality of English Language

The authors have sufficiently addressed the major concerns raised in the previous review, particularly regarding the revisions made to better align the structure of the introduction with the study population. These changes have improved the overall clarity of the manuscript.

The classification of the study as observational is acceptable, and the limitations are appropriately presented.

The supplemental materials were also reviewed and provide useful additional information for understanding the training structure and overall context of the study.

I believe that this study has sufficient merit to be considered for publication in this journal. However, I would recommend one final round of careful proofreading to address any minor errors and further improve the overall quality of the manuscript.

Additionally, the expression “Market(rain)ing” in the title is somewhat unclear in its intended meaning. For example, simplifying this expression to more directly reflect the content of the study (e.g., “Physiological, Metabolic, and Mitochondrial Adaptations to a One-Week Endurance Training Camp in Recreational Athletes”) could be considered as an alternative.

Author Response

The authors have sufficiently addressed the major concerns raised in the previous review, particularly regarding the revisions made to better align the structure of the introduction with the study population. These changes have improved the overall clarity of the manuscript.

The classification of the study as observational is acceptable, and the limitations are appropriately presented.

The supplemental materials were also reviewed and provide useful additional information for understanding the training structure and overall context of the study.

I believe that this study has sufficient merit to be considered for publication in this journal. However, I would recommend one final round of careful proofreading to address any minor errors and further improve the overall quality of the manuscript.

Additionally, the expression “Market(rain)ing” in the title is somewhat unclear in its intended meaning. For example, simplifying this expression to more directly reflect the content of the study (e.g., “Physiological, Metabolic, and Mitochondrial Adaptations to a One-Week Endurance Training Camp in Recreational Athletes”) could be considered as an alternative.

Response: Again, we thank the reviewer for his/her diligent review and the helpful comments to improve the manuscript and the appreciation of the revision. We did a final round of proofreading and deleted “Market(rain)ing”.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

GENERAL ASSESSMENT

 

After carefully reviewing the revised manuscript, I must conclude that the authors have only partially addressed the original concerns, and in several key areas, the revisions remain insufficient or largely declarative rather than substantive. While I acknowledge certain improvements—most notably the clearer positioning of the study as observational and the addition of a power analysis—the core methodological and interpretative limitations remain fundamentally unresolved.

In particular, I still find that the internal validity is not meaningfully strengthened, the causal language is not fully aligned with the study design, and the interpretation of biomarker data continues to exceed what the data can support. The manuscript is improved in presentation, but not to the degree required to overcome the initial major concerns.

 

DETAILED EVALUATION OF AUTHOR RESPONSES

 

1. Experimental framework/lack of intervention

I acknowledge that the authors now explicitly frame the study as observational, which is consistent with the title and methods (lines 104–107). However, this is a semantic clarification rather than a methodological improvement.

The core issue remains:

• Training exposure is still self-selected and uncontrolled (lines 128–131).
• No objective load monitoring (e.g., HR zones, external load metrics) is implemented.
• The “threefold increase” in training volume is merely descriptive, not experimentally controlled (line 311).

Conclusion:

The authors did not resolve the concern. They justified the design but did not improve its scientific robustness. Internal validity remains severely compromised.

 

2. Absence of a control group

The authors maintain that a control group is not feasible in a real-world setting. While this is understandable on pragmatic grounds, it does not address the methodological critique.

Importantly:

• The manuscript still interprets pre–post changes as meaningful adaptations (e.g., lines 405–408).
• The limitation is acknowledged (lines 597–600), but it is not adequately reflected in the interpretation.

Conclusion:

This concern is acknowledged but not resolved. The interpretative framework remains inconsistent with the design.

 

3. Outcome measures and overreliance on biomarkers

Here, I observe partial improvement, but still substantial issues.

Positive changes:

• The authors now explicitly state that performance did not improve (lines 622–624).
• They correctly caution against overinterpreting lactate shifts (lines 420–423).

However, major problems remain:

• Strong claims about “mitochondrial adaptations” persist, primarily based on indirect markers (mtDNA, PGC1-α) (lines 570–574).
• The 17.77-fold increase in mtDNA (page 12, figure 4) is interpreted as adaptive, yet this interpretation fails to adequately account for cellular damage or turnover.
• Mechanistic explanations remain speculative and unverified.

Conclusion:

Partially addressed, but the manuscript still overinterprets surrogate markers and maintains a mechanistic narrative not supported by the design.

 

4. Statistical rigor

There is a notable but incomplete improvement.

Strengths:

• A power analysis has been added (lines 259–278).
• The exploratory nature of performance outcomes is now acknowledged (lines 272–274).

However:

• Multiple comparisons are still not corrected for the large biomarker panel.
• The justification (“pre-post design”) is methodologically incorrect—multiple endpoints still require control of Type I error.
• Effect sizes are inconsistently reported and missing for non-parametric tests (lines 253–256).

!! I find this justification insufficient and somewhat compromising, as the absence of effect sizes reflects a limitation of the software used rather than a methodological constraint, and appropriate effect size measures for Wilcoxon tests should still be reported. !!

Conclusion:

Partially improved, but statistical validity remains questionable.

 

5. Conceptual focus

I see minor improvement, but the manuscript is still overly broad.

• The study still simultaneously addresses performance, metabolism, stress, sleep, nutrition, and mitochondrial biology.
• Although outcomes are better structured, the manuscript remains descriptive rather than hypothesis-driven.

 

Conclusion:

Largely unresolved. The study still lacks a clear conceptual hierarchy.

 

6. Overinterpretation in Discussion

There is some improvement in tone, but not enough.

• The authors now include more cautious language in certain sections.
• However, strong claims persist, e.g.:
• “measurable adaptations occurred at several physiological levels” (line 405)
• interpretation of stress reduction despite increased load (lines 499–503)

These statements still imply causality and adaptive benefit, despite insufficient control for confounders.

Conclusion:

Partially addressed, but interpretation still exceeds evidential support.

 

FINAL VERDICT

 

In my assessment:

• The authors improved transparency (observational framing, limitations, power analysis).
• However, they did not fundamentally resolve the key methodological concerns.

 

What remains problematic:

• No control over intervention → low internal validity
• No control group → no causal inference possible
• Overinterpretation of biomarkers → mechanistic speculation
• Incomplete statistical control → risk of false positives
• Broad scope without focus → limited scientific contribution

Author Response

GENERAL ASSESSMENT

After carefully reviewing the revised manuscript, I must conclude that the authors have only partially addressed the original concerns, and in several key areas, the revisions remain insufficient or largely declarative rather than substantive. While I acknowledge certain improvements—most notably the clearer positioning of the study as observational and the addition of a power analysis—the core methodological and interpretative limitations remain fundamentally unresolved.

In particular, I still find that the internal validity is not meaningfully strengthened, the causal language is not fully aligned with the study design, and the interpretation of biomarker data continues to exceed what the data can support. The manuscript is improved in presentation, but not to the degree required to overcome the initial major concerns.

General Response: We thank the reviewer for this AI-supported response. Although the reviewer still suggests our research is insufficient for publication, we nevertheless provide a point-to-point response in the following paragraphs, and thus again try to address some of the concerns.

 

DETAILED EVALUATION OF AUTHOR RESPONSES

1. Experimental framework/lack of intervention

I acknowledge that the authors now explicitly frame the study as observational, which is consistent with the title and methods (lines 104–107). However, this is a semantic clarification rather than a methodological improvement.

The core issue remains:

• Training exposure is still self-selected and uncontrolled (lines 128–131).
• No objective load monitoring (e.g., HR zones, external load metrics) is implemented.
• The “threefold increase” in training volume is merely descriptive, not experimentally controlled (line 311).

Conclusion:

The authors did not resolve the concern. They justified the design but did not improve its scientific robustness. Internal validity remains severely compromised.

Response: As explained before, our intent was a real-world training camp setting where participants can select their respective training load. As these training camps consist of heterogenous recreational athletes with a versatile training history, mandatory training would lead to early overloading in a part of the participants with subsequently increased injury risk. Load monitoring was covered by training and subjective load questionnaires and described in the manuscript. Although we admit that these measurements are dependent on the individual participant, we nevertheless assume that the training volume increase and load is – despite descriptive – accurate.

1. Absence of a control group

The authors maintain that a control group is not feasible in a real-world setting. While this is understandable on pragmatic grounds, it does not address the methodological critique.

Importantly:

• The manuscript still interprets pre–post changes as meaningful adaptations (e.g., lines 405–408).
• The limitation is acknowledged (lines 597–600), but it is not adequately reflected in the interpretation.

Conclusion:

This concern is acknowledged but not resolved. The interpretative framework remains inconsistent with the design.

Response: We cannot agree with that comment. A control group does not make sense in our design and thus was not considered sensible or feasible.

1. Outcome measures and overreliance on biomarkers

Here, I observe partial improvement, but still substantial issues.

Positive changes:

• The authors now explicitly state that performance did not improve (lines 622–624).
• They correctly caution against overinterpreting lactate shifts (lines 420–423).

However, major problems remain:

• Strong claims about “mitochondrial adaptations” persist, primarily based on indirect markers (mtDNA, PGC1-α) (lines 570–574).
• The 17.77-fold increase in mtDNA (page 12, figure 4) is interpreted as adaptive, yet this interpretation fails to adequately account for cellular damage or turnover.
• Mechanistic explanations remain speculative and unverified.

Conclusion:

Partially addressed, but the manuscript still overinterprets surrogate markers and maintains a mechanistic narrative not supported by the design.

Response: Thank you for admitting that we have made improvements with this issue. We still interpret the indirect mitochondrial markers as possible adaptation in this context and did not try to overinterpret this. Nevertheless, and resulting from this, we of course acknowledge that the interpretation is speculative, as this is normal in an exploratory setting without comparable studies or the possibility for muscle biopsies to directly measure mitochondrial changes. We state this in the limitation section.

1. Statistical rigor

There is a notable but incomplete improvement.

Strengths:

• A power analysis has been added (lines 259–278).
• The exploratory nature of performance outcomes is now acknowledged (lines 272–274).

However:

• Multiple comparisons are still not corrected for the large biomarker panel.
• The justification (“pre-post design”) is methodologically incorrect—multiple endpoints still require control of Type I error.
• Effect sizes are inconsistently reported and missing for non-parametric tests (lines 253–256).

!! I find this justification insufficient and somewhat compromising, as the absence of effect sizes reflects a limitation of the software used rather than a methodological constraint, and appropriate effect size measures for Wilcoxon tests should still be reported. !!

Conclusion:

Partially improved, but statistical validity remains questionable.

Response: Here, we agree with the reviewer and upgraded our statistical software. We now calculated the effect sizes independent of non-paramatic or paramatic testing and included the data in the results section. For the reviewer’s convenience, we added a table with all effect sizes on the following page.

Parameter	N	test statistic	p-value	effect size	95%-CI
Urea [mmol(L]	32	W = 137.00	.017	r = 0.42	[0.08, 0.67]
Creatinine [µmol/L]	32	W = 74.00	.010	r = 0.51	[0.15, 0.75]
GFR [ml/min]	31	W = 103.00	.022	r = -0.43	[-0.69, -0.07]
Uric Acid [µmol/L]	32	W = 200.50	.352	r = 0.17	[-0.20, 0.49]
Glucose [mg/dL]	32	W = 152.00	.060	r = 0.34	[-0.02, 0.62]
Bilirubin [µmol/L]	32	W = 124.00	.667	r = -0.09	[-0.48, 0.33]
AST [U/L]	32	W = 141.50	.160	r = 0.27	[-0.12, 0.58]
ALT [U/L]	32	W = 110.50	.007	r = 0.48	[0.16, 0.72]
GGT [U/L]	32	W = 140.50	.095	r = 0.31	[-0.06, 0.61]
CK [U/L]	32	W = 161.50	.055	r = 0.34	[-0.01, 0.61]
Ferritin [µg/L]	32	W = 50.50	< .001	r = -0.70	[-0.84, -0.45]
Vitamin D 25-OH [µg/L]	32	t(31) = 5.05	< .001	d = 0.89	[0.48, 1.30]
Cholesterol [mmol/L]	32	t(31) = 2.66	.012	d = 0.47	[0.11, 0.84]
Triglycerides [mmol/L]	32	W = 195.50	.864	r = 0.03	[-0.34, 0.40]
HDL [mmol/L]	32	W = 78.00	.013	r = 0.49	[0.12, 0.74]
LDL [mmol/L]	32	W = 79.50	.005	r = 0.53	[0.20, 0.76]
TSH [mIU/L]	32	t(31) = -0.14	.891	d = -0.02	[-0.37, 0.32]
Leukocytes [10^6/L]	32	W = 188.00	.360	r = 0.17	[-0.21, 0.50]
RBC [10^12/L]	32	W = 97.00	.337	r = -0.20	[-0.58, 0.24]
Hemoglobin [g/dL]	32	W = 150.50	.147	r = -0.27	[-0.58, 0.11]
Hematocrit [L/L]	32	t(31) = 2.06	.048	d = 0.36	[0.01, 0.72]
MCV [fl]	32	t(31) = 7.31	< .001	d = 1.29	[0.82, 1.77]
MCH [pg]	32	t(31) = -0.11	.917	d = -0.02	[-0.37, 0.33]
MCHC [g/dL]	32	W = 63.00	< .001	r = 0.64	[0.36, 0.81]
RBC distribution width [%]	32	W = 60.50	.010	r = 0.52	[0.15, 0.76]
Thrombocytes [10°6/L]	31	t(30) = 2.64	.013	d = 0.47	[0.10, 0.85]
MTV [fl]	31	W = 99.50	.148	r = 0.30	[-0.12, 0.62]
Klotho [ng/ml]	32	t(31)=9.96	< .001	d=1.76	[1.21. 2.31]
Interferon-g [pg/ml]	32	t(31)=-8.26	< .001	d=-1.46	[-1.96. -0.96]
Kynurenine [µM]	32	t(31)=-5.34	< .001	d=-0.94	[-1.36. -0.53]
LDH [pg/ml]	32	t(31)=-4.99	< .001	d=-0.88	[-1.29. -0.47]
GDF-15 [pg/ml]	32	t(31)=0.58	.567	d=0.10	[-0.25. 0.45]
Cortisol [µg/ml]	31	t(30)=-2.22	.034	d=-0.40	[-0.76. -0.03]
CRP [pg/ml]	31	t(30)=-0.05	.957	d=-0.01	[-0.36. 0.34]
Uric Acid [mg/ml]	31	t(30)=-1.90	.068	d=-0.34	[-0.70. 0.02]

 

1. Conceptual focus

I see minor improvement, but the manuscript is still overly broad.

• The study still simultaneously addresses performance, metabolism, stress, sleep, nutrition, and mitochondrial biology.
• Although outcomes are better structured, the manuscript remains descriptive rather than hypothesis-driven.

Conclusion:

Largely unresolved. The study still lacks a clear conceptual hierarchy.

Response: Again, we can only refer to our previous response that that research of course was exploratory as no comparable data is available. Thus, some aspects may seem descriptive but can lead to further, more hypothesis-driven follow-up studies of colleagues or our own research group.

 

1. Overinterpretation in Discussion

There is some improvement in tone, but not enough.

• The authors now include more cautious language in certain sections.
• However, strong claims persist, e.g.:
• “measurable adaptations occurred at several physiological levels” (line 405)
• interpretation of stress reduction despite increased load (lines 499–503)

These statements still imply causality and adaptive benefit, despite insufficient control for confounders.

Conclusion:

Partially addressed, but interpretation still exceeds evidential support.

Response:  Unfortunately, we cannot agree with this point. Concluding that we overinterpret our results based on four lines in our whole discussion seems not justified. We measured adaptations by objective and subjective biomarkers and subsequently interpreted and discussed them with the available literature.

FINAL VERDICT

 

In my assessment:

• The authors improved transparency (observational framing, limitations, power analysis).
• However, they did not fundamentally resolve the key methodological concerns.

 

What remains problematic:

• No control over intervention → low internal validity
• No control group → no causal inference possible
• Overinterpretation of biomarkers → mechanistic speculation
• Incomplete statistical control → risk of false positives
• Broad scope without focus → limited scientific contribution

Response: We can understand the reviewer’s decision, but nevertheless would be happy if we could convince the reviewer of our contribution to

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors,

Thank you for your thorough and detailed responses to the previous round of reviews. Your revised manuscript ID "sports-4245140" (v2) shows significant improvement in many areas, and the point-by-point response letter shows a real effort to address the issues that were brought up. Most of the previously identified issues have been resolved to our satisfaction, and the manuscript is considerably stronger as a result. The following comments outline the remaining concerns that must be addressed before final acceptance.

1. Paragraph formatting: A careful review of the revised manuscript reveals that the paragraph formatting does not fully comply with MDPI's template requirements. Several paragraphs throughout the manuscript, particularly in the Methods and Discussion sections, are formatted with a hanging indent (where the first line of each paragraph is not indented and subsequent lines are shifted inward), deviating from standard MDPI formatting guidelines.

2. Presentation of SRSS data in the main text: The Short Recovery and Stress Scale (SRSS) results are still not presented well enough in the main text. You added a sentence noting that changes occurred after days two to three of the camp and again immediately upon return. While this is informative, it does not adequately convey the magnitude or pattern of the observed effects across the eight SRSS subscales. Since the "holiday effect" on psychological stress and recovery is one of the central themes of this manuscript, a more detailed presentation of these results in the main text is necessary. Report the key subscale findings, particularly those related to overall stress, overall recovery, and muscular stress, with their corresponding descriptive statistics and significance values rather than directing readers almost entirely to the supplementary material.

3. Hierarchy of primary versus secondary outcomes: Although the revised statistics section distinguishes between primary (psychophysiological stress parameters) and exploratory (performance variables) outcomes, it does not explicitly state a hierarchy between the two. According to STROBE item 7, you should clearly identify which outcomes were primary and which were secondary or exploratory. This designation should be consistent with the framing of the results and discussion.

4. Sex-stratified analyses: You justified the absence of sex-stratified analyses because subdividing the data would reduce the sample size below the minimum required for adequate statistical power. This justification is understood and accepted within the constraints of the study. However, given the well-established sex differences in cortisol response, body composition, lipid metabolism, and mitochondrial adaptation to endurance exercise, we encourage you to present basic descriptive statistics stratified by sex (e.g., in a supplementary table) and include a brief discussion of how potential sex differences may have influenced the aggregated findings.

In my opinion, the submitted revisions have substantially improved the manuscript's methodological transparency, completeness of reporting, and adherence to the STROBE guidelines. The remaining concerns are minor and can be addressed without collecting additional data or restructuring the manuscript. Therefore, I recommend a minor revision.

Comments on the Quality of English Language

The manuscript has undergone noticeable improvement in clarity and structure compared to the previous version. However, several of the newly added paragraphs (particularly those addressing the power analysis, the kynurenine protocol, the lipid metabolism discussion, and the vitamin D interpretation) present linguistic inconsistencies that suggest these sections were drafted separately and not fully integrated into the manuscript's overall writing style.

Specific issues include occasional awkward sentence constructions, redundant phrasing, and minor grammatical irregularities. For example, the sentence "For of RT-PCR analysis" (lines 256–257) contains a clear grammatical error that should have been caught before resubmission. Similarly, some newly added passages in the Discussion section are noticeably less fluent than the original text.

The authors acknowledge the use of generative AI for language editing; however, the revised sections would benefit from an additional, careful review by a native English speaker or a professional language-editing service before final acceptance. This is particularly important for the newly introduced methodological descriptions and discussion paragraphs, as linguistic clarity in these sections is essential for reproducibility and accurate interpretation of the findings.

Author Response

Reviewer 3

Dear authors,

Thank you for your thorough and detailed responses to the previous round of reviews. Your revised manuscript ID "sports-4245140" (v2) shows significant improvement in many areas, and the point-by-point response letter shows a real effort to address the issues that were brought up. Most of the previously identified issues have been resolved to our satisfaction, and the manuscript is considerably stronger as a result. The following comments outline the remaining concerns that must be addressed before final acceptance.

 

1. Paragraph formatting: A careful review of the revised manuscript reveals that the paragraph formatting does not fully comply with MDPI's template requirements. Several paragraphs throughout the manuscript, particularly in the Methods and Discussion sections, are formatted with a hanging indent (where the first line of each paragraph is not indented and subsequent lines are shifted inward), deviating from standard MDPI formatting guidelines.

Response: Thank you for pointing this out. Indeed, we used a hanging indent for the whole manuscript. We now corrected this mistake throughout the manuscript.

2. Presentation of SRSS data in the main text: The Short Recovery and Stress Scale (SRSS) results are still not presented well enough in the main text. You added a sentence noting that changes occurred after days two to three of the camp and again immediately upon return. While this is informative, it does not adequately convey the magnitude or pattern of the observed effects across the eight SRSS subscales. Since the "holiday effect" on psychological stress and recovery is one of the central themes of this manuscript, a more detailed presentation of these results in the main text is necessary. Report the key subscale findings, particularly those related to overall stress, overall recovery, and muscular stress, with their corresponding descriptive statistics and significance values rather than directing readers almost entirely to the supplementary material.

Response: Due to the day-to-day assessment, we found it difficult to describe all SRSS items both detailed yet not too exhaustive. We now considered another approach and described the changes of the items with regard to the percental differences of the Pre-, Camp-, and Post-duration. Thus, we included a more detailed but still concise description of the SRSS items and added a supplementary table ST1 to the respective figure:

“In detail, mean physical and mental performance capability decreased by 8.0% and 5.0%, respectively, during the camp, but increased by 7.4% and 5.2% compared to Pre camp values. These changes were accompanied by a decreased mean overall recovery of 14.3%, and 53.8% increased muscular stress. Conversely, mean lack of activation decreased by 11.0% with concomitantly 33.8% reduction of negative emotional state during the camp and 32.2% Post camp compared to Pre values. Emotional balance increased by 6% during the camp and by further 4.5% Post training camp. Overall stress increased by 12% during the camp but decreased by 22.6% after the camp. (Supple-mental figure S4 and table ST1).” (lines 335 ff.).

3. Hierarchy of primary versus secondary outcomes: Although the revised statistics section distinguishes between primary (psychophysiological stress parameters) and exploratory (performance variables) outcomes, it does not explicitly state a hierarchy between the two. According to STROBE item 7, you should clearly identify which outcomes were primary and which were secondary or exploratory. This designation should be consistent with the framing of the results and discussion.

Response: You are absolutely right. We now revised it accordingly to clarify the primary and secondary outcomes in the statistic description:

“As the examination of the “holiday effect” on mood state and stress was regarded as primary outcomes, moderate to large effects were expected for psychophysiological stress parameters due to the combined impact of structured training, environmental change, and group dynamics (Cohen’s d = 0.5–0.8). Based on these assumptions, the required sample size ranged between n = 15 and n = 34 participants.

As for the secondary regarded performance-related outcomes (i.e., maximal oxygen uptake [VO₂max] and lactate metabolism) due to the short training camp duration, only small effects were anticipated (Cohen’s d = 0.2–0.3), as substantial physiological adaptations are unlikely to occur within a one-week training camp. Detecting such small effects would require large sample sizes (n ≈ 90–200), which was not feasible within the present study framework. Therefore, analyses of performance outcomes were considered exploratory.”

4. Sex-stratified analyses: You justified the absence of sex-stratified analyses because subdividing the data would reduce the sample size below the minimum required for adequate statistical power. This justification is understood and accepted within the constraints of the study. However, given the well-established sex differences in cortisol response, body composition, lipid metabolism, and mitochondrial adaptation to endurance exercise, we encourage you to present basic descriptive statistics stratified by sex (e.g., in a supplementary table) and include a brief discussion of how potential sex differences may have influenced the aggregated findings.

Response: We can understand that you are interested in this analysis as a sex-stratified analysis is (nearly) always suggestive. Thus, we performed the extensive statistical analysis again for all data sets and parameters. After doing descriptive statistics (supplemental S3) and testing all data on normality, we performed either One-way ANOVA (normally distributed) followed by Holm-Šídák's multiple comparisons test or Kruskal-Wallis test (non-normally distributed) followed by Dunn's multiple comparisons test. Differences between women and men were detected with HDL (higher values in females Pre and Post), LDH (only increase in females), Kynurenine (only decrease in males), Creatinine (higher values in males Pre and Post), Uric Acid in plasma (higher values in males Pre and Post), Ferritin (higher values pre in females), and higher values Pre and Post in males regarding RBC/hemoglobin/hematocrit. MCHC only changed in females Pre to Post, but were higher in males Post.

We added the sex-specific analysis at the end of the Results section as well as in Discussion (lines 604 ff.) and added a supplemental document S3 with all analyses.

Especially with regard to the supplementary document, we would like to ask the reviewer’s opinion of suitability of S3, where the statistics and figures of the subanalyses are presented. These are very exhaustive and thus normally need a more prominent place and the discussion in the manuscript, but would in our opinion make the manuscript “too large”. Thus, we would be happy to provide the data but we think that an extensive coverage and discussion may be out of the scope of this manuscript.

In my opinion, the submitted revisions have substantially improved the manuscript's methodological transparency, completeness of reporting, and adherence to the STROBE guidelines. The remaining concerns are minor and can be addressed without collecting additional data or restructuring the manuscript. Therefore, I recommend a minor revision.

General Response: We thank again the reviewer for his/her diligent review and the helpful comments to improve the manuscript.

Comments on the Quality of English Language

The manuscript has undergone noticeable improvement in clarity and structure compared to the previous version. However, several of the newly added paragraphs (particularly those addressing the power analysis, the kynurenine protocol, the lipid metabolism discussion, and the vitamin D interpretation) present linguistic inconsistencies that suggest these sections were drafted separately and not fully integrated into the manuscript's overall writing style.

Specific issues include occasional awkward sentence constructions, redundant phrasing, and minor grammatical irregularities. For example, the sentence "For of RT-PCR analysis" (lines 256–257) contains a clear grammatical error that should have been caught before resubmission. Similarly, some newly added passages in the Discussion section are noticeably less fluent than the original text.

The authors acknowledge the use of generative AI for language editing; however, the revised sections would benefit from an additional, careful review by a native English speaker or a professional language-editing service before final acceptance. This is particularly important for the newly introduced methodological descriptions and discussion paragraphs, as linguistic clarity in these sections is essential for reproducibility and accurate interpretation of the findings.

Response: Thank you for this suggestion. We revised the new sections accordingly so that they fit more into the original writing style as some parts were supported by AI language editing.

Author Response File: Author Response.pdf