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Article

Five-Day Supplementation with an Isotonic Beetroot Juice Drink Improves Sprint Interval Exercise and Muscle Oxygenation in Physically Active Individuals: A Randomized Crossover Trial

by
Tak Hiong Wong
1,
Delicia Shu Qin Ooi
2 and
Stephen F. Burns
1,*
1
Physical Education and Sports Science, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Singapore
2
Department of Paediatrics, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 117549, Singapore
*
Author to whom correspondence should be addressed.
Beverages 2025, 11(4), 97; https://doi.org/10.3390/beverages11040097
Submission received: 23 April 2025 / Revised: 7 June 2025 / Accepted: 25 June 2025 / Published: 1 July 2025
(This article belongs to the Section Quality, Nutrition, and Chemistry of Beverages)
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Abstract

This study investigated whether the addition of nitrate (from beetroot) to an isotonic drink provided over 5 days would affect sprint interval exercise (SIE) performance and muscle oxygenation. Twenty (seven female) physically active participants (mean ± SD; age 28 ± 6 years, BMI 22.6 ± 2.0 kg/m2) completed a double-blind, randomized, crossover study where they consumed 570 mL of either isotonic drink (ISO-C) or isotonic beetroot juice drink (ISO-BR) for 5 days before performing SIE (six 10 s maximal effort, interspersed with 50 s active recovery) on a cycle ergometer. Both drinks contained equal osmolality (290 mOsm/kg) but differed in the nutrients from beetroot extract, particularly the nitrate content (ISO-C: 0 mmol, ISO-BR: 12.9 mmol). ISO-BR significantly reduced the average time to peak power (ISO-C: 2.0 ± 0.18 s vs. ISO-BR: 1.6 ± 0.37 s; main effect of drink, p = 0.003, partial η2 = 0.04) and increased muscle deoxygenation (ΔHHb) (main effect of drink, p = 0.002, partial η2 = 0.021) compared to ISO-C. Five-day supplementation with ISO-BR improved the time to peak power but not the peak or mean power output for SIE compared with ISO-C.

Graphical Abstract

1. Introduction

Beetroot juice consumption has been linked to improved skeletal muscle function and metabolism, potentially benefiting high-intensity exercise performance [1,2]. While the performance-enhancing effects of nitrate on elements of endurance performance, such as the time to exhaustion, are well documented [3,4,5,6,7], evidence of an effect on sprint interval exercise (SIE) or high-intensity interval training (HIIT) is mixed [8,9,10,11,12,13]. One systematic review of nine articles published in 2018 suggested that acute and chronic beetroot juice consumption might improve performance in intermittent, high-intensity efforts with short rest periods, potentially via faster phosphocreatine resynthesis or faster muscle shortening velocity [14]. However, the performance data within this review were not subject to any meta-analysis, and five of the nine studies included did not show significant differences in the outcome measures compared with the control condition/group [15,16,17,18,19]. We subsequently assessed the impact of dietary nitrate from beetroot juice on HIIT and SIT in 17 randomized studies between 2010 and 2019 but only 6 of 11 studies reported improved power output/performance with beetroot juice [20]. Moreover, when subjected to meta-analysis, there was no favorable outcome for beetroot juice compared with placebo for either the peak or mean power output [20]. Most recently, an umbrella review of 20 systematic reviews on nitrate supplementation and exercise performance included five reviews that examined the power output during high-intensity exercise [21]. Three of five and three of four reviews reported improvements in the peak and mean power output, respectively, whilst two of two reviews found improvements in the time to peak power. Moreover, an additional meta-analysis of primary studies intended to overcome potential overlapping issues of individual reviews also noted significant improvements in the peak power output and time to peak power output. Nevertheless, the analysis within this umbrella review included both studies that examined single sprints and those where multiple sprints within SIT/HIIT were applied. Thus, collectively, the evidence suggests that the overall effect of nitrate (including beetroot) supplementation on SIT/HIIT remains unclear.
Variations in the exercise protocol, nitrate dosage, type of beetroot product, supplementation strategy, and duration may explain some of the differing conclusions among the meta-analyses regarding the effect of beetroot on the power output during SIT/HIIT [20]. Interestingly, a subgroup analysis from our study found that a higher total nitrate dose from chronic beetroot supplementation was correlated with a greater increase in circulating plasma nitrite—an observation also seen by others [12,20,22]. This suggests potential for nitrate and nitrite storage within the body as nitric oxide (NO) reservoirs increase in proportion to the daily intake [23]. However, the evidence that chronic beetroot ingestion with higher nitrate doses may improve HIIT/SIT performance remains mixed [20].
Approximately 25% of ingested dietary nitrate is absorbed by the salivary glands and reduced to nitrite by oral bacteria, which can be converted into nitric oxide (NO) in the stomach or enter systemic circulation [23]. The use of antibacterial mouthwash has been shown to reduce the plasma nitrite concentrations post-ingestion, likely by disrupting the oral microbiota [24]. Thus, enhancing oral microbiome activity could promote NO production. Delivering nitrate through an isotonic drink may be beneficial, as these drinks improve gastric emptying and nutrient absorption [25]. Nitrate combined with ascorbic acid in an isotonic drink may also enhance NO production in the stomach [26]. Additionally, isotonic drinks can increase salivary flow rates [27,28], promoting the transfer of nitrite into the circulation.
A previous randomized study from our group found that consuming beetroot in an isotonic drink (315 mOsm/kg) enhanced the peak power output during an isolated cycling sprint by 3.9% compared to consuming a non-isotonic beetroot juice drink (420 mOsm/kg) containing equal nitrate (12.9 mmol) and carbohydrate content (6.1 g per 100 mL) [29]. Both drinks elevated the plasma nitrate/nitrite to an equal extent, but the salivary nitrite content was significantly greater with the isotonic beetroot juice beverage [29]. At present, there is no evidence for the use of isotonic beetroot juice drink in HIIT/SIE. This integration of nitrate into an isotonic beverage potentially represents a notable advancement. Isotonic beverages are acknowledged for their capacity to enhance gastric emptying and facilitate the absorption of carbohydrates, sodium, and fluids across the intestinal membrane. Furthermore, when combined with ascorbic acid in an isotonic beverage, nitrate may additionally enhance the production of nitric oxide in the acidic environment of the stomach [26]. Moreover, isotonic beverages have been demonstrated to elevate salivary flow rates [27,28], which may facilitate the transfer of nitrite from the oral cavity into the systemic circulation.
Nitrate supplementation is proposed to have an impact on muscle blood flow, oxygen utilization, mitochondrial respiration, and, indirectly, nitric oxide generation in environments with low oxygen levels. Furthermore, the muscle deoxygenation rate has been suggested as a non-invasive measure for estimating the fractional extraction of oxygen in muscle tissues during exercise [30]. Non-invasive near-infrared spectroscopy (NIRS) serves as a valuable method for obtaining real-time measurements of muscle oxygenation, deoxygenation, and blood flow within muscle tissue during exercise. These measurements may support the assessment of the efficacy of an isotonic beetroot juice beverage in terms of exercise performance.
Studies incorporating an isotonic beetroot juice drink (ISO-BR) warrant further investigation as it could ultimately increase nitrate and nitrite storage within the body [29] and improve sprint interval exercise (SIE). Thus, the primary aim of this study was to compare ISO-BR with a nitrate-free isotonic drink (ISO-C) in terms of SIE performance following five days of supplementation. The secondary aim of this study was to compare the changes in muscle oxygenation during SIE after ingestion of ISO-BR and ISO-C. The primary hypothesis was that ISO-BR would improve the power output and muscle oxygenation compared to ISO-C. Recreationally active individuals were chosen for this study as evidence suggests that trained athletes may exhibit a reduced response to the ergogenic effects of nitrate supplementation, potentially due to the inherently higher endogenous NO bioavailability [20,31].

2. Materials and Methods

2.1. Ethical Considerations

This study was conducted in accordance with the Declaration of Helsinki and received approval from the Institutional Review Board of Nanyang Technological University, Singapore (IRB-2023-972). It was prospectively registered at ClinicalTrials.gov (ID: NCT06349018). Each participant provided written informed consent prior to the initiation of any study procedures.

2.2. Participants

A priori sample size calculation was performed using G*Power (version 3.1, Universität Kiel, Kiel, Germany), based on a previous study examining the effect of a beetroot juice drink on the peak power output during high-intensity intermittent exercise consisting of 24 × 6 s sprints with 24 s of recovery between each sprint [13]. It was determined that 20 participants would be sufficient to achieve a statistical power of 0.82 using a two-tailed paired t-test design with a calculated effect size (Cohen’s d) of 0.67. This was calculated from the previous study’s mean power output data during the sprints (mean ± SD: placebo 539 ± 136 W vs. beetroot juice intervention: 568 ± 136 W) [13] with a correlation between groups of 0.95 (based on power output measures using an isotonic beetroot juice drink) [29]. The expectation of a similar effect size in the present study is justified by the comparable sprint interval exercise in the previous study with the power output as the outcome, although it is recognized that there is some variation in the number of sprints and sprint time between studies. Twenty-four physically healthy and active participants volunteered and provided written informed consent to participate. Four participants failed to complete the study because of injury (unrelated to this study, n = 2) or illness (n = 2). The remaining 13 males (mean ± SD; age 29 ± 6 years, BMI 23.2 ± 1.3 kg/m2) and seven females (age 26 ± 5 years, BMI 21.6 ± 2.7 kg/m2) completed all the study procedures. The inclusion criteria were (i) age: 21–40 years (please note that in Singapore, the minimum age to provide independent consent for research participation is 21 years, according to the Human Biomedical Research Act); (ii) Asian (group homogeneity); (iii) healthy and injury-free (i.e., fit for exercise); (iv) blood pressure < 130/90 mmHg; (v) engaging in a minimum of 150 min of moderate-intensity or 75 min of vigorous-intensity physical activity each week or the equivalent based on both; (vi) recreational cyclist (minimum 1 h cycling per week); (vii) body mass index: 18.5–25.0 kg/m2; and (viii) non-smoker. Thus, participants were considered recreationally active based on World Health Organization’s criteria and recent attempts to classify the athletic level by experts [32]. Participants were excluded if they had any diagnosed form of cardiometabolic disease, orthopedic impairments or chronic medical conditions, had any injury that acted as an impediment to exercise, were pregnant or lactating females, had any allergies to beetroot, had asthma or were within a two-week block-out period of strenuous exercise after a COVID-19 vaccination.

2.3. Experimental Design

This study was conducted over a period of five months (January 2024 to May 2024) in a controlled laboratory environment. Participants visited the laboratory three times, including a screening and familiarization visit, and two main trials were performed in a random crossover manner. Participant randomization was completed using a randomization function in Microsoft Excel, and the allocation was performed beforehand by an individual not involved in the data collection and analysis (a PhD student under SFB). This individual allocated participants to the study team via opaque envelopes. The drink manufacturer labeled both beverages and assigned a unique code to each. The order of consumption was randomized, and this information remained concealed from both the researchers and the participants to ensure unbiased results. This careful blinding procedure was meticulously implemented to minimize any potential bias in the study’s outcomes. Participants ingested 2 × 285 mL of either ISO-BR (800 mg nitrate in total) or ISO-C (0 mg nitrate) per day for four days before the SIE test. During each main trial (day 5 of the supplementation), participants ingested the last serving (2 × 285 mL) of either ISO-BR or ISO-C beverage 2 h before performing SIE. The SIE test was scheduled to take place two hours following supplementation, coinciding with the period of peak concentrations of the salivary and plasma NOx levels based on our earlier study [29]. In the subsequent trial, participants completed the same procedures with the ingested beverage reversed. The salivary total nitrate and nitrite concentrations (NOx) were measured at the start of the morning (pre-ingestion of last dose), 2 h after beverage consumption, and after the SIE (post-exercise) test. Blood samples, for the measurement of the plasma nitrate and nitrite, were taken pre-ingestion (last dose) and 2 h after consuming the drink. Immediately after consuming the drink for each trial, participants filled out a sensory evaluation form. At the end of the cycling, participants completed the final salivary nitrite test. The experimental design and exercise protocols are illustrated in Figure 1.

2.4. Screening and Familiarization

After providing consent, participants’ height and body mass were measured, and their BMI was calculated by dividing the body mass (kg) by the square of height (m). Their blood pressure was measured in duplicate while seated using standard procedures (HEM 7211, Omron Healthcare Co., Ltd., Kyoto, Japan). The physical activity level was determined through self-report using the Global Physical Activity Questionnaire (GPAQ) [33], which estimates the total volume of moderate and vigorous intensity activity completed each week, calculated by the time spent on each and the assigned metabolic equivalents (METs; 4 METs for moderate intensity and 8 METs for vigorous intensity activities). Screening for potential health issues was carried out using the Physical Activity Readiness Questionnaire (2021 PAR-Q+) [34]. Suitable participants then performed the pre-trial familiarization on a cycle ergometer (Lode Excalibur Sport Ergometer, Lode B.V., Groningen, The Netherlands), which mirrored the protocol in the main trials (see Section 2.6).

2.5. Pre-Trial Experimental Control and Protocol

Participants were instructed to avoid using antiseptic mouthwash during the entire 5-day supplementation period as it blunts any increase in salivary or plasma nitrite [35]. They reported to the laboratory in the morning (8:00 am to 10:00 am) for each main trial after an 8 to 10 h overnight fast (water only). Before their first main trial, they recorded their physical activity and the food items they consumed, verified through an exercise and food intake log, to ensure that they did not consume high-nitrate foods, health supplements, chewing gum, or sweets that could contain bactericidal substances in the 24 h preceding the test sessions. Participants were instructed to refrain from vigorous exercise on the day preceding the main trial. There was a minimum five-day washout between the following supplementation and main trials. This washout period was consistent with our previous study [29] and confirmed by salivary test strips in a five-day supplementation pilot trial, where the NOx levels remained low after a five-day washout period based on strip color change (Section 2.10 below). Female participants were asked to self-report their menstrual cycle and were tested only within days seven to fourteen of their cycle (follicular phase) after any bleeding had stopped. While this provides experimental consistency in the testing dates, there is acknowledged variability in terms of the menstrual cycle length, although the extent to which this affects exercise performance is debated [36,37]. All the female participants declared that they were not using oral contraception.
Prior to the main trial, participants underwent screening for potential health issues using the PAR-Q+ and through the measurement of their blood pressure. Subsequently, an NOx salivary test and a blood draw were performed prior to the administration of the final dose of supplementation.

2.6. Exercise Protocol

Participants were provided with their drink (ISO-C or ISO-BR) and, two hours afterwards, performed the SIE protocol on the cycle ergometer. Participants performed a standardized 5 min warm-up of cycling at 50 W. Immediately after, the exercise protocol began. Participants completed six 10 s maximal sprint cycling efforts, interspersed with 50 s active recovery (cycling at 50 W) between sprints. The peak power, time to peak power, and mean power output during sprinting were measured on the cycle ergometer over the 10 s at a torque setting of 0.7 Nm for males and 0.67 Nm for females during each maximal effort. The ergometric parameters were controlled and monitored using software (The Lode Ergometry Manager, version 10.12.0). The rate of fatigue over 10 s was calculated as the difference between the peak power and the minimum power achieved during the test, and then divided by the peak power, converted into a percentage.
Research has shown that shorter exercise work bouts (5–15 s) targeting peak power generation can promote significant increases in energy expenditure [38] and acute cardiorespiratory responses [39] compared to 30 s SIE protocols. Additionally, a study by Yamagishi and Babraj (2017) concluded that the sprint duration does not affect the reproducibility of power [40].

2.7. NIRS Measurements

A near-infrared spectroscopy device (NIRS) (Portamon, Artinis Medical System, Elst, The Netherlands) was attached to the vastus lateralis of each participant’s right thigh to measure the muscle oxygenation. Based on previous recommendations, the device was placed at 2/3 of the distance from the anterior spina iliaca superior to the lateral side of the patella [41]. The device was secured with surgical tape and covered with a dark-colored elastic bandage around the leg to minimize the influence of external light. Before placement, the area was cleaned and shaved as necessary for measurement consistency. The device’s location was marked with a surgical skin marker pen to ensure the exact device placement for the subsequent trial. The NIRS device transmits two wavelengths (±760 nm and ±850 nm) from three optodes located 30, 35 and 40 mm from the receiver. The sampling frequency was set at 5 Hz and recorded with Oxysoft software, version 3.0.97.1 (Artinis Medical Systems, The Netherlands). Two sets of information were analyzed. Firstly, the relative changes in the tissue saturation index (∆TSI) in percentages and the concentrations (µM) of oxyhemoglobin + oxymyoglobin (∆O2Hb), deoxyhemoglobin + deoxymyoglobin (∆HHb), and total hemoglobin + myoglobin (∆tHb, sum of ∆O2Hb and ∆HHb) were tabulated by subtracting the value at specified time points (last 5 s of each stage) from the baseline during the resting period before performing the exercise protocol [42,43]. Secondly, the magnitude of ∆TSI, ∆O2Hb, ∆HHb and ∆tHb were calculated based on the difference between the baseline of each stage (5 s before the start of each stage) and the endpoint (5 s before the end of each stage) (the difference between the baseline and the endpoint of each stage was interpreted as the rate of change of each stage) [44]. The magnitude analysis aimed to understand the specific changes (i.e., the magnitude or rate of change) in each stage of exercise and recovery.

2.8. Supplementation

Drinks were supplied by a third-party beverage manufacturer, Fraser and Neave Limited, Singapore, and produced in a Good Manufacturing Practice (GMP)-certified food laboratory in Singapore. ISO-BR was prepared by adding beetroot extract to a typical isotonic drink (ISO-C), which involved reconstituting beetroot juice concentrate into ISO-C to standardize the nitrate content to 400 mg/285 mL. The citric acid and salts were adjusted to both ISO-C and ISO-BR to meet the osmolality level. Both drinks were pasteurized (95 °C for 30 s) to ensure they were safe for consumption before being bottled. The supplier provided a certificate of analysis for each drink to ensure the products were safe for consumption and met the target specification, including osmolality (Table 1).
Participants ingested 2 × 285 mL of beverage (either ISO-BR with 12.9 mmol of nitrate or ISO-C with 0 mmol of nitrate) for 5 days. For the day 5 supplementation, participants ingested the beverage within 10 min and 2 h before performing the exercise tests. Previous research suggests that beetroot juice should be ingested 1.5 to 2.5 h before the exercise performance intervention [23] and our previous study demonstrated that the peak concentrations of the salivary and plasma NOx levels were achieved by 2 h post-consumption [29]. Both drinks had standardized color and flavorings for blinding purposes and contained citric acid, salts, flavorings, and vitamins (B3, B6, B12). ISO-BR contained ascorbic acids (0.31 g/285 mL), which may play specific roles in nitrate reduction and metabolism [26]. The drink manufacturer coded both drinks, and the order of consumption was blinded to both the researchers and the participants.

2.9. Sensory Evaluation

Immediately after drink ingestion, participants were given an online sensory evaluation form via a QR code. An acceptance and preference test evaluated the sensory properties of each drink. A 7-point hedonic scale [45] was used, ranging from 1 (dislike extremely) to 7 (like extremely) for scoring the overall liking. The hedonic data were categorized into three groups: the top 3 boxes’ overall liking rating (T3B), the bottom three boxes’ overall disliking rating (B3B), and the neutral overall liking (Neutral) to determine the participant’s preferences.

2.10. NOx Salivary Test

A commercially available NO salivary test strip (Berkeley Fit, San Diego, CA, USA) was used to measure the salivary NOx on day 5 of the trial before the last dose of supplementation, before and after the exercise test. The test strip measure is a composite measure of saliva-derived NO analytes (nitrate and nitrite, NOx) [46], although they are a better reflection of nitrite in the saliva as the strips are based on a modified Griess reagent reaction [47,48]. One researcher (THW) compared and ranked the color on the test pad to the NOx scale. The test strip was also scanned with a Nitric Oxide App (Berkeley Fit, USA) for a second reading. The average number of the two readings was used for the subsequent statistical analysis.

2.11. Blood Plasma Collection and Analysis

Blood samples were drawn from an antecubital vein into K2EDTA vacutainers (3.0 mL) (Becton, Dickinson and Company, Franklin Lakes, NJ, USA) and, within 2 min of collection, centrifuged at 2808× g at 4 °C for 10 min. Subsequently, the plasma was aliquoted and frozen immediately at −80 °C for later analysis by an external co-investigator. The plasma nitrite and nitrate levels were quantified using high-performance liquid chromatography (Agilent 1200 Infinity Series, Agilent Technologies, Inc., Santa Clara, CA, USA) with a diode array detection (HPLC-DAD) method as previously described by Tatarczak-Michalewska and colleagues [49]. Briefly, a phosphatidylcholine column (IAM.PC.DD2-10 µM, 300 A, 10 cm × 4.6 mm ID, Regis Technologies, Morton Grove, IL, USA) was employed on an Agilent 1200 Infinity Series Diode Array Detector to achieve separation of the analytes. The nitrate levels were detected directly at 210 nm. The nitrite concentrations were quantified indirectly; the nitrites were oxidized to nitrates using potassium permanganate in acidic conditions before detection.

2.12. Statistical Analysis

Data were analyzed using SPSS version 26.0 (IBM SPSS Statistics for Windows, Version 26.0. IBM Corp.: Armonk, NY, USA) and Microsoft Excel version 2008 (Microsoft Corporation, Redmond, WA, USA). Two-way repeated measures ANOVAs were used to compare the effect of the drinks on the peak and mean power output, time to peak power, rate of fatigue, salivary NOx, ∆TSI, ∆O2Hb, ∆HHb, and ∆tHb. Paired t-tests were performed to compare the differences in the magnitude of the changes for each stage for the muscle oxygenation measures (∆TSI, ∆O2Hb, ∆HHb, and ∆tHb). Statistical tests for the sensory evaluation were performed using XLSTAT (Version 2021.4; Addinsoft, Paris, France). Participants’ preferences were analyzed using the Wilcoxon signed-rank test. The overall liking among the samples was analyzed and compared using paired t-tests. Tukey’s test was used to determine any significant differences between the drinks. Unless otherwise stated, all the data are reported as the mean ± standard deviation (SD). Significance was set at p < 0.05.

3. Results

All participants (n = 20) tolerated both drinks, confirmed by assessing their ability to accept and finish the drink within 10 min, with no self-reported adverse effects. Participants were unaware of the expected effects of each treatment, and 50% of participants were unable to identify both drinks correctly. All participants completed the SIE test. However, due to connection interruptions and unsuccessful blood draws, muscle oxygenation data and blood plasma data were missing for two instances. Consequently, only 18 participants were included in the analysis of the muscle oxygenation and plasma nitrate/nitrite levels.

3.1. Salivary NOx Concentration

The salivary NOx remained at the baseline concentrations in participants who consumed ISO-C on day 5. In contrast, the ISO-BR trial showed significantly elevated salivary NOx levels at baseline on day 5 prior to the final dose compared to the ISO-C trial (ISO-C: 20.0 ± 0 µM; ISO-BR: 126.8 ± 104.9 µM; p < 0.001; d = 1.02). Two hours after consuming the final ISO-BR dose, the salivary NOx increased by 2.9-fold and remained elevated for the remainder of the trial period. Overall, on day 5, the salivary NOx measured two hours post-supplementation was ~18-fold higher after ISO-BR compared with ISO-C (main effect of drink, p < 0.001, partial η2 = 0.40; main effect of time, p < 0.001, partial η2 = 0.13; interaction effect, p < 0.001, partial η2 = 0.13) (Figure 2).

3.2. Plasma Nitrate and Nitrite Concentration

The plasma nitrate was significantly higher (~6-fold) with ISO-BR compared to ISO-C after 4 days of supplementation and prior to the final dose on day 5 (ISO-C: 29.2 ± 15.1 µM; ISO-BR: 174.2 ± 133.1 µM; p < 0.001; d = 1.1). Two hours after the drink on day 5, the plasma nitrate was ~24-fold greater in the ISO-BR trial than in the ISO-C trial, with negligible changes observed in the ISO-C trial (main effect of drink, p < 0.001, partial η2 = 0.82; main effect of time, p < 0.001, partial η2 = 0.70; interaction effect, p < 0.001, partial η2 = 0.69) (Figure 3A). There was no significant difference in plasma nitrite between ISO-BR and ISO-C either pre- or post-supplementation on day 5 (main effect of drink, p = 0.97, partial η2 < 0.001; main effect of time, p = 0.90, partial η2 < 0.001; interaction effect, p = 0.76, partial η2 = 0.001) (Figure 3B).

3.3. SIE Performance

In comparison to ISO-C, the average peak power over the six sprints was 2.7% higher (Figure 4A) and the mean power was 0.8% higher (Figure 4B) with ISO-BR supplementation. However, the observed increases were not statistically different: peak power (main effect of drink, p = 0.42, partial η2 = 0.003; main effect of time, p < 0.001, partial η2 = 0.16; interaction effect, p = 1.00, partial η2 < 0.001) and mean power (main effect of drink, p = 0.77, partial η2 < 0.001; main effect of time, p < 0.001, partial η2 = 0.26; interaction effect, p = 0.99, partial η2 = 0.001). Notably, the average time to peak power was significantly reduced with ISO-BR supplementation compared to ISO-C (main effect of drink, p = 0.003, partial η2 = 0.04; main effect of time, p = 0.12, partial η2 = 0.04; interaction effect, p = 0.002, partial η2 = 0.06) (Figure 4C). The rate of fatigue increased with the number of sprints, but there was no significant difference observed in the rate of fatigue between ISO-BR and ISO-C (main effect of drink, p = 0.90, partial η2 < 0.001; main effect of time, p < 0.001, partial η2 = 0.19; interaction effect, p = 0.54, partial η2 = 0.02) (Figure 4D).

3.4. Sensory Evaluation Results

Table 2 shows the sensory evaluation outcomes from both trials. There was a significant difference in the overall liking and preference for the drinks. ISO-C was significantly liked and preferred over ISO-BR.
A penalty analysis with a Just-About-Right scale was performed using XLSTAT to calculate the mean drop for each sensory attribute, identifying those that significantly impact overall liking (please refer to the Supplementary Materials, Report S1). This involves computing the mean difference between the ratings and the ideal point. Compared to ISO-C, the overall liking for ISO-BR was influenced by pronounced sensory attributes, particularly the levels of sourness and aftertaste.

3.5. Muscle Oxygenation

During the SIE test, there were two instances of missing muscle oxygenation data, which were attributed to connection interruptions. A total of 18 participants were included in the muscle oxygenation analysis. Figure 5 shows the ∆TSI (Figure 5A), ∆O2Hb (Figure 5B), ∆HHb (Figure 5C), and ∆tHb (Figure 5D) changes from the baseline. All four parameters showed significant changes over time (all main effects of time p < 0.05), and the pattern of change for each parameter was similar for both drinks (all interaction effects p > 0.05). Notably, the ∆HHb was significantly higher in the ISO-BR trial than the ISO-C trial (main effect of drink, p = 0.002, partial η2 = 0.021). Conversely, there were no main effects from the drink observed for the ∆TSI, ∆O2Hb, and ∆tHb (Figure 5).

4. Discussion

This is the first study to investigate how 5-day (chronic) supplementation with an isotonic beetroot (800 mg nitrate/day) juice drink impacts SIE performance. Following ISO-BR supplementation, no statistically significant increase was observed in the peak or mean power output during SIE compared to 5-day supplementation with a nitrate-free ISO-C drink. However, the time to peak power was significantly improved (faster) with ISO-BR supplementation. Interestingly, there were no significant differences in most muscle oxygenation measures between ISO-BR and ISO-C, but the ∆HHb was notably higher with ISO-BR than ISO-C. The salivary NOx was 18-fold higher and the plasma nitrate concentration 24-fold higher in the ISO-BR trial compared to the ISO-C trial 2 h after the final supplementation on day 5. Conversely, there was no significant difference in the plasma nitrite concentration between ISO-BR and ISO-C pre- or post-supplementation on day 5.
The ISO-BR formulation involved fortifying beetroot juice concentrate into a typical isotonic drink. Both beverages were standardized in color to ensure participant blinding. ISO-BR contained approximately 12.8 mg per 285 mL of ascorbic acid, which may play a specific role in nitrate reduction and metabolism [50,51,52]. The potential benefits of providing dietary nitrate in an isotonic drink, which could facilitate nitrate absorption and reduction and thereby promote the conversion of nitrate to nitrite via the enterosalivary circulation pathway, were investigated in our previous study [29]. Based on the observed data in the current study, the salivary NOx concentration and plasma nitrate concentration were significantly higher with ISO-BR than with ISO-C.
One surprising observation was that there was no significant elevation of the plasma nitrite concentration after 5 days of ISO-BR supplementation compared to ISO-C, in contrast to the elevations in the plasma nitrate and salivary NOx described. One possible explanation is that the NO production rate was enhanced by ascorbic acid, as suggested by Duncan and colleagues [50]. Ascorbic acid from ISO-BR may rapidly reduce nitrite to NO in the stomach [26,53], thereby increasing the available NO storage in the enterosalivary pathway rather than in the form of circulating nitrite. In addition, Kadach and colleagues observed a similar nitrate concentration in the muscle and the bloodstream following nitrate intake, but the nitrite concentration was elevated to a greater extent in the muscle than in the bloodstream [54], which could potentially explain the better NO availability and improved exercise performance [55]. Moreover, Ferguson and colleagues observed unchanged plasma nitrite in beetroot-supplemented rats [56]. Thus, it is conceivable that the observed lack of significant elevation of the plasma nitrite after 5 days of ISO-BR supplementation may be attributed to the presence of a storage pool of nitrite within the skeletal muscle [56]. Nonetheless, the underlying reasons or mechanisms for this unexpected finding remain unclear and require further investigation. While nitrite is a key intermediate in the nitrate–nitrite–NO pathway, its levels can be influenced by multiple factors, including rapid tissue uptake and conversion to nitric oxide, renal clearance, and potential methodological variability in sample handling and storage. In particular, plasma nitrite is known to be labile and sensitive to oxidative degradation, which could contribute to the variability in the measured concentrations. Additionally, nitrite may be rapidly utilized in peripheral tissues where it exerts biological effects, thereby reducing its accumulation in the circulation [57].
In the sensory evaluation, ISO-C was significantly liked and preferred over ISO-BR. Compared to ISO-C in the penalty analysis, the overall liking for ISO-BR was influenced by pronounced sensory attributes, particularly the levels of sourness and aftertaste. These sensory characteristics may have been accentuated due to the 5-day chronic supplementation, and the masking flavor used in the recipe was still unable to eliminate the typical aftertaste contributed by the beetroot concentrate. While our previous research demonstrated a significant preference for ISO-BR over beetroot juice [29], participants in this study expressed the lower palatability of ISO-BR compared to ISO-C, an isotonic drink that is well received by consumers in Singapore. Given that palatability may influence long-term adherence to ISO-BR, we recommend that the drink manufacturer consider optimizing the sensory profile of ISO-BR in future formulation and intervention studies. Nonetheless, the blinding process was considered effective as participants were unaware of the anticipated impacts of each treatment.
This investigation posited that ISO-BR would enhance the power output in SIE performance, particularly as this type of exercise is likely to cause low pH and hypoxic conditions localized within muscle tissue, which favor NO production [23,58]. Contrary to expectations, no differences were seen in the peak or mean power between the two drinks. This observation is consistent with the findings of our previous meta-analysis [20], which indicated no significant impact of beetroot juice supplementation on the mean or peak power in high-intensity interval training compared to placebo [8,9,11,19,59,60]. The variation in the exercise protocol, nitrate dosage, type of beetroot product, supplementation strategy, and duration restricts a firm conclusion based on the evidence of the effect of beetroot on the power output during repeated sprints [20]. Our data also agree with a meta-analysis conducted by Silva and colleagues, which indicated that improvements in performance are only realized with nitrate supplementation when the exercise duration exceeds 2 min [31,61,62,63]. In this study, the total duration of the sprint was limited to 60 s.
Conversely, in the realm of isolated single sprints, a consistent elevation in the peak power output has been observed with beetroot juice drinks in previous studies [1,2,64] and when ISO-BR was compared with a beetroot juice drink with equal nitrate content in our previous study [29]. In the present study, the average time required to reach peak power during SIE was significantly reduced with ISO-BR, consistent with a previous study by Jonvik and colleagues on repeated sprints [8]. The initial sprint of the SIE for ISO-BR exhibited an increased time to peak power in comparison to ISO-C (Figure 4C); however, this disparity did not attain statistical significance (p = 0.12). In contrast to isolated single sprints, which consistently demonstrate a reduced time to peak power following beetroot supplementation [1,2,64], the variation noted in the first sprint of the SIE may be attributed to the participants’ awareness of the total number of sprints. This awareness likely influences their pacing strategies during the SIE, in contrast to the approach taken during isolated single sprints [65]. It has been suggested that the differences between single and repeated sprints may stem from nitrate’s pronounced influence on the initial force generation by type II muscle fibers [23]. A study in mice reported augmented early-phase force production in fast-twitch muscles following nitrate supplementation, possibly due to enhanced calcium handling [66]. One reason for the lack of improvement in the peak power output in the present study may be inadequate recovery time between sprints, which undermines the efficacy of ISO-BR supplementation in enhancing the initial force generation by type II muscle during repeated sprint efforts [67]. In addition, Roelofs and colleagues observed that an improvement in NO production could facilitate faster recovery between sprints by enhancing oxygen extraction at the muscular level [68]. While the underlying mechanisms warrant further exploration, it is evident from the current study that a five-day regimen of ISO-BR only shortened the time to peak power in SIE and not the power output itself. Certainly, this appears to differ from the observations with isolated sprints. Future research could delve into varying the sprint duration, recovery intervals, and repetition counts within the protocols. It is possible that the effectiveness of beetroot juice or nitrate supplementation may be tailored to specific types of energy expenditure related to power outputs and sporting modalities [38].
In the present study, the ∆HHb was notably higher with ISO-BR than ISO-C supplementation during the SIE test. Nevertheless, there were no significant differences between ISO-C and ISO-BR in terms of the muscle oxygenation (∆TSI, ∆O2Hb) and total blood flow (∆tHb). It is hypothesized that ISO-BR may accelerate ATP (adenosine triphosphate)/PCr (phosphocreatine) turnover and enhance PCr recovery between repetitions. This rapid PCr breakdown suggests increased oxygen extraction, potentially coupled with improved oxygen delivery, thereby optimizing the mitochondrial respiration efficiency for ATP regeneration during SIE and promoting a shift toward anaerobic ATP resynthesis. This contradicts a previous study that examined trained cyclists undertaking high-intensity interval training (cycling at 50% above functional threshold power for 75 s, followed by a 2 min recovery at 50% of functional threshold power until exhaustion), which found that the ∆HHb concentrations were significantly lower after nitrate supplementation compared to placebo [69]. It is important to note that the present study focused on SIE (all-out) with a 50 s recovery time, which differs from the submaximal HIIT used by Broeder and colleagues with a 2 min recovery time. These different exercise modalities may explain the variations in the ∆HHb concentration. Notably, the present study aligns with previous investigations suggesting that nitrate supplementation accelerates muscle deoxygenation and pulmonary oxygen uptake ( V ˙ O2) kinetics during cycling at severe intensity [30,70].
One limitation of the current study is that we did not assess the pulmonary gas exchange variables to validate the V ˙ O2 kinetics during SIE. However, it has been suggested that the muscle deoxygenation kinetics can be a non-invasive indicator of fractional muscle oxygen extraction during exercise [30,71]. The notably higher concentration of ∆HHb with ISO-BR compared to ISO-C supplementation during the SIE test may be attributed to swifter fractional muscle oxygen extraction during the SIE exercise.
This study has several limitations. Firstly, we only compared the enhanced nitrate content of the isotonic drink from beetroot juice concentrate (ISO-BR) with ISO-C as a placebo, without including water or nitrate-depleted beetroot juice. Beetroot juice or nitrate intake is already known to enhance performance [1,2,13,18,64], and our previous study [29] showed the effectiveness of ISO-BR over a high-nitrate beetroot juice drink in boosting the isolated sprint peak power output. Having water, nitrate-depleted beetroot juice drink or nitrate-depleted isotonic beetroot juice drink as another comparator in this study could have provided a more thorough understanding of the efficiency and benefits of ISO-BR. Nevertheless, our findings make it clear that ISO-BR did not improve the power output over a low-nitrate drink (ISO-C) and the addition of these other drinks would not have changed this. Another limitation of this study is the lack of pulmonary gas exchange variables, which could have confirmed the significance of the ∆HHb and V ˙ O2 kinetics during SIE. Including additional data regarding the plasma nitrate and nitrite concentrations measured before supplementation and post-exercise would enhance this study by providing a more comprehensive understanding of the trends associated with nitrate–nitrite–NO reduction and the potential biological explanations underlying these changes. The small number of female subjects (7 out of 20) in the current study makes comparisons within this sex uncertain, while comparisons between sexes would be severely underpowered and prone to type 2 errors. However, other prior research has indicated that the salivary and plasma responses of nitrate and nitrite following the ingestion of dietary nitrate are consistent across both sexes [72]. Nevertheless, a substantial body of research comparing the impact of nitrate on different sexes has not been undertaken, and future studies should investigate this issue in greater detail. The ISO-BR drink also contained 2.7 g per 285 mL more carbohydrate content that ISO-C, primarily from sugar, starches and fiber. However, ISO-C had a greater sugar content at 1.8 g per 285 mL We cannot rule out that these differences may have been important to the experimental outcomes. Lastly, the current study relied on participants’ strict adherence to the protocol, including fasting overnight before the experiment. While participants were instructed to follow the protocol and complete the survey forms, it was difficult to confirm compliance beyond verbal affirmation, particularly regarding the use of pre-exercise mouthwash.

Practical Implications

In this study, a comparative analysis was carried out to assess the impact of a novel isotonic beetroot (nitrate-containing) drink (ISO-BR) in contrast to an isotonic (nitrate-free) drink (ISO-C) on SIE performance. The findings indicate that ISO-BR may confer an advantage in terms of the time to peak power but not the other power output measures during repeated sprints. Nonetheless, this may be a useful outcome for many sports where the rate of force development is important (e.g., sprint cycling, football). Further research is essential to comprehensively assess the efficacy of ISO-BR in meeting a broader range of athletic demands, including the enhancement of the power outputs across various sporting disciplines.

5. Conclusions

This study demonstrated that five days supplementation with dietary nitrate (beetroot juice) in an isotonic drink did not improve the peak and mean power output during SIE compared with an isotonic drink alone, although the time to peak power was significantly reduced after ISO-BR supplementation compared to ISO-C. Future research could explore variations in the sprint durations, recovery intervals, and repetition counts within the protocols, especially since it is crucial to validate the suitability and optimal supplementation strategies for different exercise modalities.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/beverages11040097/s1, Report S1: Penalty Analysis Report.

Author Contributions

Conceptualization, T.H.W. and S.F.B.; data curation, T.H.W.; formal analysis, T.H.W. and D.S.Q.O.; investigation, T.H.W. and S.F.B.; methodology, T.H.W., D.S.Q.O. and S.F.B.; project administration, T.H.W. and S.F.B.; resources, T.H.W. and S.F.B.; supervision, S.F.B.; validation, T.H.W.; visualization, T.H.W.; writing—original draft, T.H.W.; writing—review and editing, T.H.W., D.S.Q.O. and S.F.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Singapore Economic Development Board, Industrial Postgraduate Program-II: S20-10028-IPP-II-SI.

Institutional Review Board Statement

This study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board of Nanyang Technological University, Singapore (protocol code IRB-2023-972 2024-01-29 and 29 January 2024).

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study. Written informed consent has been obtained from the patients to publish this paper.

Data Availability Statement

Data related to this publication will be placed in the Data Repository of the Nanyang Technological University National Institute of Education (Singapore) and made publicly available.

Acknowledgments

The authors thank the participants for their contributions and Fraser and Neave Limited for supplying the supplementation drinks for this study.

Conflicts of Interest

S.F.B. and D.S.Q.O. declare no conflicts of interest. T.H.W. is a registered PhD candidate at Nanyang Technological University and a Fraser and Neave Limited employee. He was involved in the study design, data collection, analysis, interpretation, and manuscript writing but had no role in the participant allocation, randomization, or preparation and coding of the drinks used. He was blinded to the drink allocation. The funding source listed had no role in the study design, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
ATPAdenosine triphosphate
BMIBody mass index
∆HHbRelative changes in deoxyhemoglobin + deoxymyoglobin
∆O2HbRelative changes in oxyhemoglobin + oxymyoglobin
∆tHbRelative changes in total hemoglobin + myoglobin
∆TSIRelative changes in tissue saturation index
GMPGood Manufacturing Practice
HIITHigh-intensity interval training
ISO-BRIsotonic beetroot juice drink
ISO-CIsotonic drink
NIRSNear-infrared spectroscopy
NONitric oxide
NO2Nitrite
NO3Nitrate
NOxTotal nitrate and nitrite
PCrPhosphocreatine
SDStandard deviation
SIESprint interval exercise
SITSprint interval training
V ˙ O2Rate of oxygen uptake

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Figure 1. Schematic to illustrate the overview of the protocol. (A) Experimental design and study procedures. Participants visited the laboratory three times, including a screening and familiarization visit, and two main trials were performed in a random crossover manner. (B) Exercise protocol: S1 to S6 represent the sprint exercise bouts, while R1 to R6 indicate the active rest periods, and R7 denotes the passive rest period.
Figure 1. Schematic to illustrate the overview of the protocol. (A) Experimental design and study procedures. Participants visited the laboratory three times, including a screening and familiarization visit, and two main trials were performed in a random crossover manner. (B) Exercise protocol: S1 to S6 represent the sprint exercise bouts, while R1 to R6 indicate the active rest periods, and R7 denotes the passive rest period.
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Figure 2. Salivary (n = 20) total nitrate and nitrite (NOx) responses after ISO-C and ISO-BR supplementation. Data presented as mean ± SD. # Significant main effect of time (p < 0.001). * Significant main effect of drink (p < 0.001). ^ Significant interaction effect (p < 0.001).
Figure 2. Salivary (n = 20) total nitrate and nitrite (NOx) responses after ISO-C and ISO-BR supplementation. Data presented as mean ± SD. # Significant main effect of time (p < 0.001). * Significant main effect of drink (p < 0.001). ^ Significant interaction effect (p < 0.001).
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Figure 3. Plasma nitrate (A) and nitrite (B) responses after ISO-C and ISO-BR supplementation (n = 18). Data presented as mean ± SD. # Significant main effect of time (p < 0.001). * Significant main effect of drink (p < 0.001). ^ Significant interaction effect (p < 0.001). There was no significant difference in plasma nitrite between drinks.
Figure 3. Plasma nitrate (A) and nitrite (B) responses after ISO-C and ISO-BR supplementation (n = 18). Data presented as mean ± SD. # Significant main effect of time (p < 0.001). * Significant main effect of drink (p < 0.001). ^ Significant interaction effect (p < 0.001). There was no significant difference in plasma nitrite between drinks.
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Figure 4. Peak power (A), mean power (B), time to peak power (C), and rate of fatigue (D) during SIE (n = 20) after ISO-C and ISO-BR supplementation. There was no significant difference in peak power, mean power and rate of fatigue between drinks. The average time to peak power was significantly reduced with ISO-BR supplementation compared to ISO-C. Data presented as mean ± SD. # Significant main effect of time (p < 0.05). * Significant main effect of drink (p < 0.05). ^ Significant interaction effect (p < 0.05).
Figure 4. Peak power (A), mean power (B), time to peak power (C), and rate of fatigue (D) during SIE (n = 20) after ISO-C and ISO-BR supplementation. There was no significant difference in peak power, mean power and rate of fatigue between drinks. The average time to peak power was significantly reduced with ISO-BR supplementation compared to ISO-C. Data presented as mean ± SD. # Significant main effect of time (p < 0.05). * Significant main effect of drink (p < 0.05). ^ Significant interaction effect (p < 0.05).
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Figure 5. The ∆TSI (A), ∆O2Hb (B), ∆HHb (C) and ∆tHb (D) from baseline (n = 18) after ISO-C and ISO-BR supplementation. (C) ISO-BR had significantly higher ∆HHb than ISO-C supplementation (main effect of drink, p = 0.002, partial η2 = 0.02). Data presented as mean ± SD. # Significant main effect of time (p < 0.05). * Significant main effect of drink (p < 0.05). Table 3 shows the magnitude of the ∆TSI, ∆O2Hb, ∆HHb, and ∆tHb. The magnitudes of the ∆TSI, ∆O2Hb, ∆HHb, and ∆tHb for all the stages were not significantly different when comparing ISO-BR to ISO-C.
Figure 5. The ∆TSI (A), ∆O2Hb (B), ∆HHb (C) and ∆tHb (D) from baseline (n = 18) after ISO-C and ISO-BR supplementation. (C) ISO-BR had significantly higher ∆HHb than ISO-C supplementation (main effect of drink, p = 0.002, partial η2 = 0.02). Data presented as mean ± SD. # Significant main effect of time (p < 0.05). * Significant main effect of drink (p < 0.05). Table 3 shows the magnitude of the ∆TSI, ∆O2Hb, ∆HHb, and ∆tHb. The magnitudes of the ∆TSI, ∆O2Hb, ∆HHb, and ∆tHb for all the stages were not significantly different when comparing ISO-BR to ISO-C.
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Table 1. The specifications of ISO-C and ISO-BR. The additional carbohydrates, proteins, potassium and sodium in ISO-BR (compared to ISO-C) were contributed by beetroot juice concentrate. Each bottle of ISO-BR (285 mL) contained 400 mg of nitrate.
Table 1. The specifications of ISO-C and ISO-BR. The additional carbohydrates, proteins, potassium and sodium in ISO-BR (compared to ISO-C) were contributed by beetroot juice concentrate. Each bottle of ISO-BR (285 mL) contained 400 mg of nitrate.
SpecificationISO-CISO-BR
Nitrate content (mg/285 mL)0400 ± 30
Carbohydrate (g/285 mL)14.8 ± 0.517.5 ± 0.5
Total sugars (g/285 mL)14.0 ± 0.512.2 ± 0.5
Protein (g/285 mL)01.4 ± 0.2
Sodium (mg/285 mL)121 ± 5219 ± 5
Potassium (mg/285 mL)39 ± 5215 ± 5
Initial osmolality (mOsm/kg)290 ± 10290 ± 10
Major ingredientsSugarsBeetroot juice concentrate *
Minor ingredientsCitric acid, salts, vitamins (B3, B6, B12), permitted flavorings, colorings and preservative Citric acid, salts, natural sweeteners, vitamins (C, B3, B6, B12), permitted flavorings and preservative
Specifications were provided by the drink manufacturer (Fraser and Neave Limited, Singapore). * Reconstitution of 7% beetroot juice concentrate in ISO-BR equal to 35% beetroot juice content. Source of beetroot juice concentrate: Beta vulgaris cultivated from Karnataka, India (Sami-Sabinsa Group Limited).
Table 2. Summary of the overall liking and preference testing of ISO-C and ISO-BR (n = 20).
Table 2. Summary of the overall liking and preference testing of ISO-C and ISO-BR (n = 20).
Overall Liking and Preference
(7-Point Hedonic Scale)		ISO-CISO-BR
Top 3 box overall liking (T3B)
Neutral overall liking (Neutral)
Bottom 3 box overall liking (B3B)
Average mean score		90%
10%
0%
5.40 a		35%
10%
55%
3.85 b
Preference80% a20% b
Different letters refers to significant differences between samples, p < 0.05. There was a significant difference in overall liking (p = 0.001) and preference (p = 0.007) between drinks.
Table 3. Magnitude of the change in the ∆TSI, ∆O2Hb, ∆HHb and ∆tHb after ISO-C and ISO-BR supplementation (n = 18).
Table 3. Magnitude of the change in the ∆TSI, ∆O2Hb, ∆HHb and ∆tHb after ISO-C and ISO-BR supplementation (n = 18).
Variables (n = 18)∆TSIp Value∆O2Hbp Value∆HHbp Value∆tHbp Value
S1ISO-C−28.0 ± 19.20.714−5.8 ± 3.20.7253.8 ± 4.10.098−2.1 ± 3.50.551
ISO-BR−26.3 ± 11.9 −6.1 ± 3.8 5.0 ± 4.3 −1.4 ± 3.4
R1ISO-C25.9 ± 12.00.7867.3 ± 5.00.489−3.8 ± 3.50.6653.4 ± 2.40.783
ISO-BR25.2 ± 11.7 7.8 ± 5.1 −4.0 ± 3.7 3.6 ± 2.4
S2ISO-C−25.1 ± 12.00.886−6.9 ± 5.00.1614.6 ± 4.00.381−2.3 ± 1.90.507
ISO-BR−25.5 ± 13.6 −7.7 ± 5.3 5.0 ± 4.3 −2.6 ± 1.8
R2ISO-C25.4 ± 15.60.4507.7 ± 5.50.285−4.2 ± 4.00.4093.5 ± 2.40.782
ISO-BR20.4 ± 24.3 8.4 ± 5.9 −4.6 ± 4.1 3.7 ± 2.4
S3ISO-C−21.0 ± 7.90.970−6.8 ± 5.10.3064.6 ± 4.50.689−2.2 ± 1.50.432
ISO-BR−20.9 ± 13.0 −7.5 ± 5.8 4.8 ± 4.6 −2.5 ± 1.8
R3ISO-C21.4 ± 9.40.1737.4 ± 5.40.131−4.4 ± 4.40.4393.0 ± 1.80.262
ISO-BR27.6 ± 23.0 8.1 ± 5.7 −4.7 ± 4.6 3.4 ± 1.9
S4ISO-C−19.1 ± 8.50.971−6.8 ± 5.10.5154.5 ± 4.40.766−2.3 ± 1.30.683
ISO-BR−19.2 ± 11.3 −7.1 ± 5.5 4.6 ± 4.4 −2.4 ± 1.8
R4ISO-C18.3 ± 8.90.6607.1 ± 5.10.473−4.3 ± 4.40.9732.9 ± 1.50.331
ISO-BR16.9 ± 11.9 6.8 ± 5.4 −4.3 ± 4.4 2.5 ± 2.1
S5ISO-C−18.1 ± 7.80.858−6.7 ± 4.90.7494.3 ± 4.40.670−2.4 ± 1.20.965
ISO-BR−17.7 ± 10.5 −6.8 ± 5.8 4.5 ± 4.6 −2.3 ± 1.8
R5ISO-C18.8 ± 8.50.6076.4 ± 5.00.591−4.3 ± 4.20.9862.0 ± 1.90.426
ISO-BR17.6 ± 9.9 6.8 ± 5.8 −4.3 ± 4.6 2.4 ± 2.1
S6ISO-C−18.4 ± 10.70.563−6.3 ± 5.10.6704.3 ± 4.40.930−1.9 ± 1.50.624
ISO-BR−16.8 ± 10.3 −6.5 ± 5.9 4.3 ± 4.7 −2.1 ± 2.0
R6ISO-C20.4 ± 10.40.8369.3 ± 5.90.389−4.8 ± 5.10.5054.6 ± 3.00.657
ISO-BR19.8 ± 11.2 9.9 ± 6.3 −5.1 ± 5.4 4.9 ± 2.3
R7ISO-C17.1 ± 14.70.2472.8 ± 4.30.427−4.7 ± 2.20.697−1.4 ± 4.00.488
ISO-BR12.8 ± 8.3 3.6 ± 3.9 −4.5 ± 2.5 −0.8 ± 4.3
Values are means ± standard deviations. ISO-C = isotonic drink; ISO-BR = isotonic beetroot juice drink.
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Wong, T.H.; Ooi, D.S.Q.; Burns, S.F. Five-Day Supplementation with an Isotonic Beetroot Juice Drink Improves Sprint Interval Exercise and Muscle Oxygenation in Physically Active Individuals: A Randomized Crossover Trial. Beverages 2025, 11, 97. https://doi.org/10.3390/beverages11040097

AMA Style

Wong TH, Ooi DSQ, Burns SF. Five-Day Supplementation with an Isotonic Beetroot Juice Drink Improves Sprint Interval Exercise and Muscle Oxygenation in Physically Active Individuals: A Randomized Crossover Trial. Beverages. 2025; 11(4):97. https://doi.org/10.3390/beverages11040097

Chicago/Turabian Style

Wong, Tak Hiong, Delicia Shu Qin Ooi, and Stephen F. Burns. 2025. "Five-Day Supplementation with an Isotonic Beetroot Juice Drink Improves Sprint Interval Exercise and Muscle Oxygenation in Physically Active Individuals: A Randomized Crossover Trial" Beverages 11, no. 4: 97. https://doi.org/10.3390/beverages11040097

APA Style

Wong, T. H., Ooi, D. S. Q., & Burns, S. F. (2025). Five-Day Supplementation with an Isotonic Beetroot Juice Drink Improves Sprint Interval Exercise and Muscle Oxygenation in Physically Active Individuals: A Randomized Crossover Trial. Beverages, 11(4), 97. https://doi.org/10.3390/beverages11040097

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