Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,
Thank you for the opportunity to review this manuscript. The manuscript investigates the effect of caffeine intake (administered via chewing gum) on performance parameters during submaximal exercise in healthy, physically active young men during cycling.
The manuscript is well-written, concise, and focused, which can be considered a strength of this study.
In my assessment, despite certain limitations addressed by the authors in the study limitations section, the research protocol is well-thought-out and properly conducted, and the obtained results fully support the conclusions drawn.
However, I find the presentation of the results somewhat lacking, and the authors should make improvements in this regard:
All results presented in the text must be reflected in the tables (in the appropriate places) with the same number of decimal places.
Figure 2 must be supplemented with numerical data—specifically, the results that the authors presented only in the text. This manner of presentation is unprofessional. Figure 2 is unclear and requires supplementing not only with numerical results but also with adequate descriptions.

 

Kind regards,

reviewer

Author Response

COMMENT #1: 
All results presented in the text must be reflected in the tables (in the appropriate places) with the 
same number of decimal places. 
RESPONSE #1: 
We sincerely thank the reviewer for this constructive comment. In response, we have carefully 
reviewed and standardized the numerical results throughout the manuscript to ensure consistency in 
the reported values and decimal places between the text and tables. Specifically, we have added 
Supplementary Appendix S1 to provide a comprehensive summary of mechanical performance 
outcomes, including cadence, total work, mean power output (MPO), and peak power output (PPO) 
at both RPE 4 and RPE 7, with corresponding mean ± SD values and p-values. Additionally, 
detailed physiological responses have been organized into Supplementary Appendices S2 and S3 
to ensure completeness and consistency in data reporting. We have also incorporated the relevant 
numerical results into the revised Figure 2. We believe these revisions have improved the accuracy, 
consistency, and clarity of our results. 
COMMENT #2: 
Figure 2 must be supplemented with numerical data—specifically, the results that the authors 
presented only in the text. This manner of presentation is unprofessional. Figure 2 is unclear and 
requires supplementing not only with numerical results but also with adequate descriptions. 
RESPONSE #2: 
We sincerely thank the reviewer for this constructive comment. In response, we have revised Figure 
2 to improve its clarity and readability. Specifically, we have added numerical results (mean ± SD) 
and corresponding p-values for both the caffeine (CAF) and placebo (PLA) conditions at RPE 4 and 
RPE 7. We have also improved the figure layout, increased its resolution, and expanded the figure 
legend to provide clearer descriptions of the graphical elements. We believe these revisions have 
substantially improved the clarity, completeness, and interpretability of Figure 2.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This is an interesting and practically relevant study examining whether caffeinated gum changes self-selected cycling performance at a fixed RPE. The study is generally well designed, but several parts of the manuscript could be improved, especially the Introduction, Figure 2, and Discussion. 
Introduction

Intro
The Introduction should provide a clearer explanation of how caffeine may improve cycling performance and how this may be linked to RPE. At present, the manuscript briefly mentions adenosine receptor antagonism and reduced perceived exertion, but the connection to higher self-selected power output at the same RPE should be explained more clearly. 
The concept of RPE also needs more explanation. Since RPE is central to the study, the authors should briefly describe the main RPE scales, including the Borg 6–20 scale and the 0–10 OMNI scale used here, and explain what different values represent.
The authors should also explain why RPE 4 and RPE 7 were selected. It is currently clear that these levels were used in previous studies, but the physiological or practical reason for choosing them should be stated more clearly. 

Results 
Figure 2 is difficult to read and appears to be of insufficient resolution. Since it includes several outcomes for both RPE 4 and RPE 7, I suggest separating these two intensities into different figures, or at least improving the size and resolution of the current figure. 

Discussion
The Discussion currently repeats too many numerical results that are already clearly presented in the Results section. For example, exact percentage and watt differences in PPO and MPO do not need to be repeated in detail. Instead, the authors should focus more on the magnitude and meaning of the effects. It would be more useful to discuss whether the observed increase in mechanical output is similar to what has been reported in previous caffeine studies and whether this magnitude is practically meaningful.
References to tables and figures should also be reduced unless they are necessary for the discussion. Too many references back to the Results section interrupt the flow of the text.
Most importantly, the Discussion needs more physiological and metabolic explanation. The authors should discuss possible mechanisms that could explain why participants produced more work at the same RPE. These could include central effects of caffeine, reduced perception of effort or discomfort, increased motor drive, and possible effects on ventilation or metabolism.
The higher VO₂ with caffeine should also be discussed carefully, because it may simply reflect the higher external workload rather than a direct effect of caffeine. The same applies to the ventilatory responses. The lack of difference in blood lactate despite greater mechanical output is also interesting and deserves further discussion. 
These possible mechanisms should be presented as hypotheses, since the study was not designed to test them directly. This would also help the authors provide more specific suggestions for future research.

Practical implications
The practical implications are useful, but they could be more detailed and precise. The authors correctly point out that caffeine may allow athletes to produce more external work at the same RPE. 
They could expand this by explaining what this means for coaches using RPE to prescribe or monitor training. For example, caffeine use may need to be considered when comparing RPE-based training sessions, because the same RPE may not represent the same external workload.
At the same time, these conclusions should remain limited to the population and protocol tested: well-trained young males with low habitual caffeine intake, using 6 mg/kg of caffeine during short cycling bouts.

Author Response

Please see PDF

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

• Please report the achieved RPE values.
  RPE is the central variable underlying the research question, and the Methods state that RPE was recorded at 1-min intervals. Please report the actual RPE responses for caffeine and placebo at both RPE 4 and RPE 7 to demonstrate how closely participants maintained the prescribed targets.

• Please clarify the time-course statistical analyses.
  The Methods state that individual time-point comparisons were conducted following significant condition × time interactions. Please report the corresponding condition × time interaction statistics and clearly indicate which post-hoc comparisons were performed.

• Clarify the interpretation of the two RPE stages.
  Because RPE 4 always preceded RPE 7, comparisons between the two stages may be influenced by accumulated fatigue and stage order. This limitation is acknowledged in the Discussion, but it should be reflected consistently when interpreting differences between RPE 4 and RPE 7.

• Maintain a clear distinction between workload-related and direct physiological effects of caffeine.
  The caffeine condition produced greater external mechanical work, and therefore the higher VO₂ and ventilatory responses cannot be clearly separated from the increased workload. The Discussion appropriately recognizes this issue; the same caution should be maintained throughout the manuscript.

• Clarify the individualized caffeine dosing procedure.
  The manuscript states that participants received 6 mg/kg, that the gum was portioned using an analytical balance, and that the same number of gum sticks was provided in both conditions. Please describe more precisely how the individualized dose was obtained and matched between conditions.

• Correct the figure and manuscript inconsistencies.
  The Results refer to Figure 3 in relation to blood lactate, but Figure 3 is not present in the submitted manuscript. The inconsistent Nutrients 2025/Sports 2026 headers, placeholder DOI and editorial fields should also be corrected.

• Correct the author-contribution statement.
  “Y.G.” is mentioned in the Author Contributions section but is not included in the listed authors. Please resolve this discrepancy.

• Consider correcting the numerical percentage reported for MPO.
  The Discussion reports an approximately 10.9% increase at RPE 4 based on 88 versus 79 W; these reported means correspond to approximately 11.4%.

• Please clarify data-processing procedures for respiratory measurements.
  The manuscript states that no manual artifact removal was performed. Please clarify whether any predefined automated quality-control or artifact-detection procedure was applied before analysis.

Comments on the Quality of English Language

The manuscript is generally clear and follows an appropriate scientific structure, but it would benefit from careful language and editorial revision.

Terminology should be standardized throughout the manuscript, particularly for RPE, VO₂peak, MPO, PPO, and blood lactate concentration. Some sentences in the Discussion are relatively long and could be shortened to improve precision. The distinction between perceived exertion, self-selected external workload, and physiological response should be maintained consistently.

The manuscript also requires correction of several obvious editorial and formatting inconsistencies, including the conflicting journal headers, placeholder bibliographic information, missing Figure 3, and inconsistent author initials. These issues should be resolved before acceptance.

Author Response

Please see PDF

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The authors report here on the comparison of clamped RPE cycling with and without caffeine gum supplementation. Overall the manuscript is well written, with the introduction suitably defining the research/evidence gap and the discussion properly positions the findings acknowledging the lack of mechanistic explanation supporting data. There is some details in the methods to be improved and the presentation of the results in Table 1 & 2 should be reconsidered. 

Specific Comments

Ln 22-23 and 86; While I acknowledge that the authors have categorised the recruited participants based on McKay the consideration that they are well-trained needs to be defended as the reported peak VO2 is not that high. 

Ln 24; Change 'participants' to 'athletes'

Ln 31-33; There are no reported values for oxygen uptake in the abstract to support this statement?

Ln 77-78; The authors have used cadence as the sample size variable but the introduction focuses on greater workloads, why?

Ln 78-79; This sample size is based on a different (more liberal) statistical test than what  was actually used in the study.

Ln 104-107; While the design is marginally suitable the fact that there is no known or reported reliability and minimally important detectable change for the workload at 4 and 7 OMNI RPE limits the practical implications and should be highlighted to the reader in the limitations.

Ln 110; How was the hydration status assessed to confirm all athletes were well hydrated? If it was not assessed then it can not be assumed. 

Ln 114; More details are required for what this blood sample was, currently at this stage of the manuscript I was of the belief that this was a veinous sample. 

Ln 122; I urge the authors to more Figure 1 to here in the manuscript so that it is closer to the study design explanation for the reader. 

Ln 155-158; What is the reliability of these variables?

Ln 180-181; Indicate here the abbreviation (BLC)

Ln 188-190; The described statistical tests here are different to the sample size estimation. 

Ln 201-202; The partial eta effect size statistic has only been selectively reported and would be helpful to the reader if converted to an interpretable value for variable of interest.

Ln 242 Table 1 and 2; Use a more appropriate method to visually present this data over time. These tables are difficult to visualize the response. Are all of these variables actually necessary to report or are they reported purely because this is what the collection system will output?

Pg 10; Why has the line numbering restarted?

Discussion Ln 26-29; Yes but what is the CV% and minimal important detectable change in these variables with this protocol and an Effect size for the change? The mean values appear good but the figure clearly indicates a meaningful distribution. The lack of these details increases the uncertainty in the interpretation. 

Author Response

Please see PDF

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

I have carefully reviewed the revised manuscript and the authors' responses to my previous comments. Unfortunately, The author has not fully addressed the question I raised. Several of my major concerns have been addressed only superficially, with brief additions that fail to adequately engage with the underlying scientific issues or provide the level of critical discussion expected in a scientific publication. I would like to emphasize that responding to reviewers' comments requires substantive scientific improvements rather than merely adding a few sentences to the manuscript. Nevertheless, I am willing to give the authors one further opportunity to substantially improve their manuscript and adequately address the outstanding concerns. I strongly urge the authors to take the following comments seriously and provide thorough, scientifically rigorous, and well-supported revisions.

 

Comment 1
The authors' response to this comment is clearly insufficient and does not adequately address the fundamental concern raised in the original review. Adding a few general sentences about caffeine reducing perceived exertion and potentially allowing higher self-selected workloads does not constitute a satisfactory scientific explanation of the relationship between caffeine, RPE, and cycling performance. The authors are expected to provide a substantially more comprehensive, critical, and properly referenced review of the relevant literature, including experimental evidence, proposed physiological and perceptual mechanisms, and findings from studies examining caffeine-induced changes in exercise performance and perceived exertion. I strongly request that the authors thoroughly revise this section rather than address an important conceptual and theoretical issue through a brief and superficial addition.

Comment 3

The author's approach to this important methodological problem is inappropriate. Simply stating that RPE 4 and RPE 7 represent moderate and high perceived exertion, respectively, and that these values were used in previous studies does not provide a sufficiently rigorous physiological or methodological justification for their selection. The authors must provide a more comprehensive, literature-supported explanation of the physiological and perceptual characteristics associated with these specific intensity levels, including their relevance to exercise prescription, metabolic and cardiorespiratory demands, and the interpretation of caffeine-induced responses. I strongly encourage the authors to engage critically with the existing evidence and provide a scientifically substantiated rationale for these particular RPE targets rather than relying on general statements that add little to the methodological justification of the study.

Comment 7

The author's response to that comment was unsatisfactory, as it fails to address the central scientific issue raised in my original review. The request was not merely to list possible mechanisms of caffeine action, but to provide a detailed, physiologically grounded discussion explaining how increased mechanical output can be achieved at an equivalent level of perceived exertion, integrating central nervous system mechanisms, motor drive, neuromuscular responses, respiratory regulation, and metabolic demands. Instead, the authors have added a largely descriptive paragraph containing general statements about adenosine receptor antagonism, neuronal excitability, and motivation, followed by a separate paragraph that primarily attributes the observed cardiorespiratory changes to increased workload, without adequately integrating these mechanisms into a coherent physiological interpretation of their findings. A substantially more rigorous and critical discussion, supported by relevant experimental literature and explicitly distinguishing established physiological evidence from mechanisms that remain hypothetical in the present study, is necessary; in its current form, the revision does not satisfactorily address this major concern.

Author Response

We sincerely thank the reviewer for carefully reassessing our revised manuscript and for providing further constructive comments. We appreciate the reviewer's continued consideration of our work and acknowledge that our previous revisions did not sufficiently address the underlying scientific and methodological concerns. We apologize for the limited depth of our earlier responses.

In response to these comments, we have undertaken a more comprehensive revision of the manuscript, particularly the Introduction, Methods, and Discussion sections. Rather than making only brief textual additions, we have sought to strengthen the scientific rationale, clarify the methodological justification for the selected RPE intensities, and provide a more integrated physiological interpretation of our findings.

Specifically, we have expanded the discussion of the relationship between caffeine, perceived exertion, and exercise performance; clarified the distinction between fixed-workload and RPE-regulated exercise; strengthened the rationale for selecting RPE 4 and RPE 7; and revised the interpretation of the mechanical, cardiorespiratory, and metabolic responses observed following caffeinated gum ingestion. We have also more clearly distinguished established physiological evidence from potential mechanisms that were not directly assessed in the present study.

We are grateful for the reviewer's detailed guidance, which has helped us identify important weaknesses in the previous version and improve the scientific clarity and interpretation of the manuscript. Our point-by-point responses are provided in PDF. 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

1. Consistency between Results and tables: Please include the reported results for percentage of predicted normal VO₂ and ventilatory equivalent for CO₂ in the relevant tables or supplementary material. Please also verify the apparent inconsistencies in the 2-min VO₂/kg values. Clarify the meaning of the “RPE 7” and “Overall” p-value columns in Table 2 and ensure that the presentation is consistent with the stated statistical analyses.

2. Achieved RPE values: The addition of achieved RPE values is appreciated. However, the reported SD of 0.0 for both stages and conditions requires clarification. Please specify how these values were calculated and whether they represent participant-level averages or all individual ratings.

3. Editorial consistency: Please correct the inconsistent journal/volume headers and replace the placeholder DOI with the appropriate final metadata. These issues remain visible in the revised version.

Author Response

Thank you for your valuable comments and suggestions. We have carefully addressed all the concerns raised and revised the manuscript accordingly. Please see the attached point-by-point response for detailed explanations of the revisions.

Author Response File: Author Response.pdf