Effects of Caffeine-Taurine Co-Ingestion on Endurance Cycling Performance in High Temperature and Humidity Environments

Abstract

Background:

Taurine (TAU) and caffeine (CAF), as common ergogenic aids, are known to affect exercise performance; however, the effects of their combined supplementation, particularly in high temperature and humidity environments, have not been studied.

Hypothesis:

The combination of TAU and CAF will have a greater effect on endurance cycle performance and improve changes in physiological indicators during exercise compared with TAU or CAF supplementation alone and placebo.

Study Design:

Single-blind crossover randomized controlled study.

Level of Evidence:

Level 1.

Methods:

Twelve university students majoring in physical education volunteered to receive 4 different supplement ingestions: (1) placebo (maltodextrin), (2) TAU, (3) CAF, (4) TAU + CAF. After a 7-day washout period, participants completed a time to exhaustion (TTE) test in the heat (35°C, 65% relative humidity).

Results:

All experimental groups improved TTE compared with the placebo group. Peak and mean power of countermovement jump were significantly higher in the CAF group compared with the placebo group before the exhaustion exercise (P = 0.02, d = 1.2 and P = 0.04, d = 1.1, respectively). Blood lactate was significantly lower after the exhaustion test in the TAU group compared with the CAF (P < 0.01, d = 0.8) and TAU + CAF (P < 0.01, d = 0.7) groups. Core temperature in the TAU group was significantly reduced in the placebo group later in the exhaustion test (P < 0.01, d = 1.9).

Conclusion:

In high temperature and humidity environments, acute TAU, CAF, and combined supplementation all improved TTE and did not affect recovery from lower limb neuromuscular fatigue compared with placebo, with TAU having the best effect. Combined supplementation failed to exhibit superimposed performance.

Clinical Relevance:

The results provide suggestions for the effects of TAU, CAF, and their combined intake on exercise performance in high temperature and humidity environments.

A high-temperature environment is characterized as having a living environment temperature of 35°C or higher, or an exercise environment temperature of 32°C or higher. Similarly, a high humidity environment is defined as one in which the relative humidity (RH) reaches ≥60%. Both high temperature and humidity environments inevitably exert an influence on the endurance performance of athletes. The ingestion of ergogenic aids (EA) to enhance performance has become popular in recent years and is a strategy used widely by athletes and fitness professionals. ³⁶ During training and competition, athletes also inevitably encounter high temperature and humidity environments. ⁴⁹ Previous studies have documented the positive effects of the EAs taurine (TAU) and caffeine (CAF) on endurance exercise performance under thermoneutral conditions in both animal models and human studies.^6,11,29,30 Recently, the effects of TAU and CAF on aerobic endurance and anaerobic sprint performance have also been studied in hot or humid environments^47,52,62,63; however, whether the combination of TAU and CAF is more effective in high temperature and humidity environments and whether combined use affects the physiological response and performance of athletes during endurance cycling has not been well studied.

TAU is an essential amino acid derived from cysteine metabolism, and this sulfur-containing component is abundant in skeletal muscle. ³⁸ TAU is also one of the primary ingredients in the most popular energy drinks. ²⁰ There is a lack of consensus regarding the mechanisms of the ergogenic action of TAU in humans, yet this semi-essential amino acid is responsible for improvements in endurance performance. ³⁴ Furthermore, previous studies have shown that acute oral TAU ingestion in the heat increased time to exhaustion (TTE) (∼10%), while decreasing rating of perceived exertion (RPE) and core temperature (CT) in the later stages of exercise and reducing postexercise blood lactate (BL). ⁴⁸ In addition, the profound influence of TAU on sudomotor function was also presented.^58,64 The combination of hot conditions and elevated RH has a significant influence on the physiology and aerobic performance of athletes.^7,49 Whether the effects of TAU on thermoregulation and endurance cycling performance are altered in high temperature and humidity environments has not been well investigated.

CAF is one of the most common ergogenic supplements in endurance sports.^37,41 CAF enhances performance during endurance exercise via numerous mechanisms of action, such as stimulation of the central nervous system, ¹⁸ augmented fat oxidation and muscle contraction,^9,13 and mitigated perception of pain. ¹⁴ Notably, the effect of different doses of CAF intake on exercise performance in high temperatures and humidity has been investigated in previous studies. ¹² Also, research has consistently demonstrated that CAF ingestion (3-9 mg kg^-1 body mass) produces significant improvements in endurance capacity. ⁵⁹ Despite these results, a previous study reported that a high dose of CAF has a thermogenic effect that induces a more rapid CT elevation during specific exercise conditions. ¹⁶ Ferreira et al ¹⁷ argued that heat and humidity conditions may be sufficient to mask the ergogenic benefit of CAF in cycling races of prolonged duration. However, other studies show increased rate of heat storage, and no performance benefits, with CAF ingestion before a 10-km run in hot, humid conditions. ²⁷ Meanwhile, published literature indicates that ingestion of 5 mg of CAF per kilogram of body weight improved endurance running performance in hot and humid conditions. ⁸ There are also differences in the effects of CAF intake between individual athletes. ⁶¹ A study has shown that CAF did not alter the esophageal temperature or skin blood flow responses during exercise in nonhabituated persons. ⁴⁶ Therefore, the effects of CAF supplementation in high temperature and humidity environments remain controversial. Based on previously available literature, this study further validated the effect of CAF on endurance cycling performance in high temperature and humidity environments by controlling for factors that may influence the effects of CAF.

Based on the effects of TAU and CAF on exercise performance in high-temperature or high-humidity environments described above, a reasonable hypothesis can be made. TAU would delay the rate of rise in CT by enhancing convective or evaporative cooling potential, while reducing the accumulation of lactate during exercise. ⁴⁸ CAF acts as a stimulant and stimulates the central nervous system, thereby enhancing exercise performance in hot and humid environments. ⁵¹ Does the combination of the 2 supplements have a greater effect? Little is understood about the efficacy of CAF and TAU co-ingestion on endurance cycling performance, which is surprising given the high concentration of these ingredients in popular energy drinks and their purported effects.^4,31,32 In addition, TAU combined with CAF has been shown in animal models to reduce the accumulation of BL and thus improve endurance performance. ²⁹ However, comparable evidence is lacking in humans, and very few studies have focused on endurance cycling performance in hot and humid environments.

Thus, the purpose of this study was to investigate the effects of CAF-TAU co-ingestion on endurance cycling performance in high temperature and humidity environments. It was hypothesized that the combination of TAU and CAF will have a greater effect on endurance cycle performance and improve changes in physiological indicators during exercise compared with TAU or CAF supplementation alone and placebo.

Methods

Study Design

The study adopted a single-blind randomized crossover design. All participants reported to the laboratory on 5 separate occasions. The initial visit was spent on some preliminary tests whereas the last 4 visits comprised actual experimental trials (Figure 1). All testing sessions were completed within a 72-hour time frame to minimize training effects and were finished at the same time of day (within ~1 hour) to reduce any biological variability due to circadian rhythm. Each visit was separated by no more than 7 days for each participant, which was deemed sufficient to wash out TAU and CAF. All participants gave written informed consent. Ethical approval was provided by the institutional ethics committee (Capital University of Physical Education and Sports), which was conducted in accordance with the 1964 Declaration of Helsinki.

Figure 1.

Experimental study design.

Preliminary Testing

During visit 1, participants undertook an incremental exercise test to volitional exhaustion on a mechanically braked cycle ergometer (LODE 906900) in thermoneutral conditions (25 ± 1.68°C) to determine maximum rate of oxygen consumption (VO₂ max) and the power output at the ventilatory threshold. Participants cycled for 5 minutes at 80 W to warm up and rested for 5 minutes before starting the test. The test started at a workload of 80 W and increased to 20 W min^-1 at a fixed cadence of 80 ± 5 revolutions (rev) min^-1 until volitional exhaustion or when the cadence dropped below 70 rev min^-1 for >10 seconds. Heartrate (HR) was recorded throughout the trial. VO₂ max was calculated by measuring the highest 30 second average. VO₂ peak power (PP) was measured as the highest power output recorded during the test for a full minute. After a 20-minute rest, participants undertook a familiarization session consisting of a maximum constant load exercise at a power output related to the ventilation threshold in an environmental chamber set to experimental conditions (35 ± 1°C, 65 ± 2% RH). Breath-by-breath VO₂ and ·VCO₂ data from the incremental cycling test was used to plot the ventilatory threshold, using the simplified v-slope method. ²³ This threshold was selected as it was deemed appropriate to evaluate endurance capacity at a repeatable rate.

Procedure

A 5-minute steady-state warm-up of 100 W was performed, after which another 3 minutes were provided to prepare for the test protocol. Participants were instructed to maintain a pedal cadence of 80 rev min⁻¹ at an intensity equivalent to a thermoneutral ventilatory threshold until complete exhaustion. Exhaustion was defined as voluntary withdrawal or when pedal cadence dropped below 70 rev min^-1 for >10 seconds. The coefficient of variation for this test in our laboratory is 4.1% while cycling in the heat. At 1 minute postexercise, a BL sample was taken from the right ear lobe using a lancet and analyzed by an automated analyzer. Participants received nonspecific verbal encouragement from the researcher during the exhaustion test (Figure 2).

Figure 2.

Visualization of experimental procedure flow. BL, blood lactate; CMJ, countermovement jump; CT, core temperature; HR, heartrate; RPE, rating of perceived exertion, TS, thermal sensation; TTE, time to exhaustion.

Sample Size Calculation

Given the typical effect sizes (Cohen’s d = 0.3-1.0) reported using CAF and TAU across the various dependent variables in this study,^35,47,63 G*Power (Version 3.1.9.7) was used to calculate an a priori sample size of 11, which was sufficient to identify differences between groups with a statistical power of 0.80. The chances of type I and II errors were both deemed to be 5%. However, 12 participants were selected for this study to prevent an insufficient amount of data after sample attrition.

Randomization

This experiment was conducted to avoid the influence on the experimental results due to the backward and forward order of the tests. Participants were numbered and divided randomly into 4 groups of 3 each using a random function method. The experiment was then conducted according to the contents of the cross-tab in Figure 3 by entering the different intervention protocols in turn. The order of the entire experimental intake protocol was randomized and tests spaced 7 days apart to allow for full recovery and TAU and CAF washout.

Figure 3.

Randomized grouping chart.

Participants

A total of 12 university students majoring in sports took part in this study. Researchers strictly verified that all the volunteers met the inclusion/exclusion criteria to take part in the study: (1) participants were not taking any dietary supplements within the past 3 months before the start of the study; (2) non-CAF habitués; (3) participants did not smoke; (4) participants were healthy with no diagnoses of cardiovascular, respiratory, or metabolic diseases that may impair muscle biology; (5) had no orthopaedic injury that would impact cycling performance; and (6) had to have previous experience of performing high-intensity exercise. Informed consent was obtained from all individual participants included in the study. The characteristics of the subjects are listed in Table 1.

Table 1.

Participant characteristics

Characteristics	Participants, N = 12
	Mean ± SD	95% CI
Age, y	23.75 ± 2.41	22.21, 25.29
Training experience, y	3.04 ± 1.70	1.96, 4.12
Body mass, kg	75.73 ± 7.53	70.95, 80.52
Height, m	1.78 ± 0.61	1.74, 1.82
BMI, kg/m2	23.89 ± 1.75	22.78, 24.5
ST frequency, sessions per week	1.33 ± 1.16	0.6, 2.07
ET frequency, sessions per week	1.83 ± 1.27	1.03, 2.64
Maximal oxygen uptake, VO2 max	43.68 ± 2.76	41.93, 45.43
VTP, W	187.5 ± 15.71	177.52, 197.48

BMI, body mass index; ET, endurance training; ST, strength training; VTP, ventilatory threshold power.

Supplementation Interventions

The dosages of CAF and TAU followed the recommendations of recent studies.^47,63 All supplements were prepared in powder form and measured using an analytical balance (Ji Ming) for subsequent ingestion in gelatine capsules. The capsules contained one of the following: CAF (5 mg/kg body mass [BM]), TAU (50 mg/kg BM), CAF + TAU (5 mg/kg BM + 50 mg/kg BM) or placebo (5 mg/kg BM maltodextrin). The 1-hour timing was chosen as this accounted for the peak plasma availability of both TAU and CAF after oral administration.^22,24

Measures and Data Collection

Primary Outcome

Time to Exhaustion

TTE is often used as a measure of endurance performance in a laboratory environment. Participants were instructed to maintain a pedal cadence of 70 rev min^-1 at an intensity equivalent to thermoneutral ventilatory threshold until complete exhaustion. Exhaustion was defined as voluntary withdrawal or when pedal cadence dropped below 60 rev min^-1 for >10 seconds. The TTE was recorded immediately after the exhaustion test.

Countermovement Jump

The countermovement jump (CMJ) test is a practical method for evaluating neuromuscular fatigue (NMF). ²¹ CMJ was assessed at 3 different timepoints: baseline, immediately after the exhaustion test (CMJ post), and 3 minutes after exhaustion test (CMJ post-3). CMJ is obtained by applying a vertical jump pad (Omegewave). Participants stands on a vertical jump pad with feet shoulder-width apart, place their hands on the iliac bones, squat down to 90°, rest for 2 seconds, then jump as hard as they can to the highest point before finally dropping to the mat. This movement is tested 3 times and the mean power (MP) and PP are taken. PP measurement is calculated automatically from the program according to the following formula ⁵⁷ :

$$\mathrm{PP}\left(\mathrm{W}\right)=\left[60.7\times \mathrm{jump}\mathrm{height}\left(\mathrm{cm}\right)\right]+\left[45.3\times \mathrm{BM}\left(\mathrm{kg}\right)\right]-2055$$

Secondary Outcomes

Heartrate

Participants wore HR monitors (Polar H10) upon entering the laboratory and sat still for 10 minutes to obtain basal HR. After warming up, HR was recorded at 3-minute intervals throughout the exhaustion test and at completion.

Blood Lactate

BL was measured at 3 timepoints: at rest (baseline), immediately after completion of the exhaustion test (L-post), and 3.5 minutes during recovery (L-post-3.5). The BL sample was taken from the right ear lobe using a lancet and analyzed by an automated analyzer (Lactate Scout).

Core Temperature

Infrared ear thermometers have been studied using the tympanic membrane as a CT standard. The infrared ear temperature gun (Braun, IRT6520) gave an accurate and true reflection of CT. The participants recorded basal CT 10 minutes after entering the laboratory in silence. CT was recorded at 3-minute intervals throughout the exhaustion test and at completion.

Rating of Perceived Exertion

The Borg Scale (6-20), also known as RPE, is a valuable indicator of personal effort and is used commonly in exercise science. ⁴⁴ Basal RPE was measured upon entry to the laboratory, then recorded every 3 minutes during the exhaustion test and at completion.

Thermal Sensation

A 9-point standard heat thermal sensation (TS) scale was used in this study, where participants were asked to state their true heat sensations, according to the scale. Basal TS was measured upon entry to the laboratory, then recorded every 3 minutes during the exhaustion test and at completion.

Standardization

The participants were provided with an extensive list of dietary sources containing CAF and TAU, which they were instructed to avoid throughout the test period and the week before the experiment. At 1 week before the first ingestion protocol, participants underwent a routine medical screening to ensure that they were in good health and suitable for the experiment. Participants was measured to calculate TAU and CAF doses before the start of each experiment. Participants were asked to avoid strenuous exercise for 48 hours before testing. Subject food intake was controlled for 24 hours before testing as this may affect CAF and TAU absorption rates. Environmental temperature and humidity were kept constant in all experimental trials (35 ± 1°C and 65 ± 2% RH). Standardized encouragement and feedback were given to the participants in all trials by the same researcher. The seat and handlebar positions on the cycle ergometer were obtained in familiarization trials and replicated for each participant in all trials.

Statistics and Analysis

Data are presented as mean and standard deviation. The Shapiro-Wilk test was used to determine whether all the variables obtained showed a normal distribution. A 2-way repeated measures analysis of variance (RM-ANOVA) was conducted, with supplementation (placebo, CAF, TAU, and CAF + TAU) and time as the independent variables. A Greenhouse-Geisser correction was applied when the assumption of sphericity was violated. Where interaction effects were found, post hoc analysis was performed with Bonferroni tests. TTE was analyzed using 1-way ANOVA with Bonferroni correction. Statistical significance was accepted at P < 0.05 and all analyses were performed on IBM SPSS Statistics (Version 27; IBM Corp). Effect sizes (Cohen’s d) were also calculated for all pairwise differences. Effect sizes were defined as: trivial 0.2; small 0.21 to 0.6; moderate 0.61 to 1.2; large 1.21 to 1.99; very large >2.0. The effect size was calculated with the partial eta squared coefficient (η_p²) and classified as 0.099 = small, 0.0588 = moderate, and 0.1379 = large effect. ⁵³

Results

Primary outcome

TTE values for placebo (mean = 20.48; SD = 2.79), Tau (mean = 23.89, SD = 2.86), CAF (mean = 21.80, SD = 2.45) and TAU + CAF (mean = 21.72, SD = 2.90) groups were recorded. The data showed significant differences in TTE between interventions, F(3,43.33) = 3.15, mean squared error, 7.60; P = 0.03, r = 0.42. Follow-up multiple comparisons showed that TTE was significantly higher in the TAU group than in the placebo group (t [44] = 3.03, P = 0.03, d = 1.24) (Figure 4).

Figure 4.

TTE at the power output associated with ventilatory threshold after 4 interventions (N = 12) in high-temperature and -humidity environments (35°C, 65% RH) (**P < 0.05). C, caffeine; P, placebo; RH, relative humidity; T, taurine; TTE, time to exhaustion.

For PP, there was a supplementation by time interaction (F_(3,44) = 5.35; P < 0.01; η²_partial = 0.267) as well as a time effect (F_(2,43) = 59.10; P < 0.01; η²_partial = 0.73) but no supplementation effect (F_(3,44) = 0.842; P = 0.48; η²_partial = 0.054). Post hoc analysis revealed that significant measurements (P < 0.05) were found at CMJ BL, compared with CMJ post and CMJ post-3, but no significant measurement (P > 0.05) were found at CMJ post, compared with CMJ post-3 for all supplements. In addition, the CAF group had a greater PP at CMJ BL compared with the placebo, TAU and TAU + CAF groups (CAF, 4244.81 ± 337.71; placebo, 3826.48 ± 355.54; TAU, 4036.91 ± 324.69; TAU + CAF, 3916.02 ± 325.09; P = 0.02, d = 1.2) (Figure 5a).

Figure 5.

Mean ± SD for (a) PP and (b) MP registered at CMJ BL, CMJ post, and CMJ post-3. *P < 0.05 compared with CMJ BL in placebo group; ^#P < 0.05 compared with CMJ BL in the TAU group; ^&P < 0.05 compared with CMJ BL in the CAF group; ^%P < 0.05 compared with CMJ BL in the TAU + CAF group; ^^^P < 0.05 CAF vs placebo. BL, blood lactate; C/CAF, caffeine; CMJ countermovement jump; CMJ post, immediately after the exhaustion test; CMJ post-3, 3 minutes after exhaustion test; MP, mean power; P, placebo; PP, peak power; T/TAU, taurine.

For MP, there was a supplementation by time interaction (F_(3,44) = 4.65; P = 0.01; η²_partial = 0.241) as well as a time effect (F_(2,43) = 68.16; P < 0.01; η²_partial = 0.76) but no supplementation effect (F_(3,44) = 0.697; P = 0.56; η²_partial = 0.045). Post hoc analysis revealed that significant measurements (P < 0.05) were found at CMJ BL, compared with CMJ post and CMJ post-3, but no significant measurement (P > 0.05) were found at CMJ post, compared with CMJ post-3 for all supplements. In addition, the CAF group had a greater MP at CMJ BL compared with placebo, TAU, and TAU + CAF groups (CAF, 4089.01 ± 354.72; placebo, 3688.90 ± 363.39; TAU, 3919.05 ± 326.67; TAU + CAF, 3817.88 ± 320.50; P = 0.04, d = 1.1) (Figure 5b).

Secondary Outcomes

For HR, there was a supplementation by time interaction (F_(6,41) = 2.58; P = 0.03; η²_partial = 0.274) as well as a time effect (F_(6,39) = 808.73; P < 0.01; η²_partial = 0.99) but no supplementation effect (F_(3,44) = 0.464; P = 0.71; η²_partial = 0.031). Post hoc analysis revealed that HR were higher than BL from 3 minutes to the end of the exhaustion test in all 4 supplements. There was no significant difference between the 4 supplements (Figure 6).

Figure 6.

Changes in HR during the exhaustion test in high-temperature and -humidity environments (35 °C, 65% RH). *P < 0.05 compared with CMJ BL in placebo group; ^#P < 0.05 compared with BL in the TAU group; ^&P < 0.05 compared with BL in the CAF group; ^%P < 0.05, compared with BL in the TAU + CAF group. BL, blood lactate; bpm, beats per minute; C/CAF, caffeine; CMJ countermovement jump; END, end of the exhaustion test; HR, heartrate; P, placebo; T/TAU, taurine.

This study analysis unveiled an effect of time (F_(2,43) = 728.305; P < 0.01; η²_partial = 0.971), supplementation (F_(3,44) = 6.083; P = 0.01; η²_partial = 0.293), and a supplementation by time interaction (F_(3,44) = 6.414; P < 0.01; η²_partial = 0.304). Post hoc analysis revealed that significant measurements (P < 0.05) were found at BL, compared with L-post and L-post-3, but no significant measurement (P > 0.05) were found at Lactate post, compared with L-post-3 for all supplements. In addition, the TAU group had a lower BL at Lactate post (TAU, 7.93 ± 1.34; CAF, 10.01 ± 1.49; TAU + CAF, 10.22 ± 1.71; P < 0.01) and L-post-3 (TAU, 7.71 ± 1.10; CAF, 9.63 ± 1.30; TAU + CAF, 9.73 ± 1.60; P < 0.01), compared with the CAF and TAU + CAF groups (Figure 7).

Figure 7.

Mean ± SD BL registered at BL, L-post, and L-post-3.5. *P < 0.05 compared with BL in placebo group; ^#P < 0.05 compared with BL in the TAU group; ^&P < 0.05 compared with BL in the CAF group; ^%P < 0.05 compared with BL in the TAU + CAF group; ^^P < 0.05, TAU vs CAF and TAU + CAF. BL, blood lactate; C/CAF, caffeine; L-post, BL immediately after the exhaustion test; L-post-3.5, BL 3.5 minutes after exhaustion test; P, placebo; T/TAU, taurine.

For CT, there was a supplementation by time interaction (F_(6,41) = 4.452; P = 0.01; η²_partial = 0.395) as well as a time effect (F_(6,39) = 183.546; P < 0.01; η²_partial = 0.966) but no supplementation effect (F_(3,44) = 0.748; P = 0.53; η²_partial = 0.049). Post hoc analysis revealed that CT were higher than BL from 3 minutes to the end of the exhaustion test in all 4 supplements. In addition, the TAU group had a lower CT at 15 minutes compared with the placebo and TAU + CAF groups (TAU, 37.92 ± 0.19; placebo, 38.23 ± 0.137; P < 0.01, d=1.9) (TAU, 37.92 ± 0.19; TAU + CAF, 38.22 ± 0.195, P < 0.01, d = 1.8), and at end of the exhaustion test compared with the placebo, CAF, and TAU + CAF groups (TAU, 38.2 ± 0.171; placebo, 38.42 ± 0.127; CAF, 38.5 ± 0.195; TAU + CAF, 38.43 ± 0.142; P = 0.012; P < 0.01; P < 0.01) ( Figure 8).

Figure 8.

Changes in CT during the exhaustion test in high-temperature and -humidity environments (35 °C, 65% RH). *P < 0.05 compared with CMJ BL in the placebo group; ^#P < 0.05 compared with BL in the TAU group; ^&P < 0.05 compared with BL in the CAF group; ^%P < 0.05 compared with BL in the TAU + CAF group (^^P < 0.05, 15 minutes, TAU vs placebo and TAU + CAF), (^^P < 0.05 at end of the exhaustion test, TAU vs placebo, CAF, and TAU + CAF). BL, blood lactate; C/CAF, caffeine; CMJ, countermovement jump; CT, core temperature; END, end of the exhaustion test; P, placebo; RH, relative humidity; T/TAU, taurine.

For RPE, there was a supplementation by time interaction (F_(6,41) = 2.925; P = 0.02; η²_partial = 0.30) as well as a time effect (F_(6,39) = 675.051; P < 0.01; η²_partial = 0.99) but no supplementation effect (F_{(3, 44)} = 0.179; P = 0.91; η²_partial = 0.012). Post hoc analysis revealed that RPE were higher than BL from 3 minutes to the end of the exhaustion test in all 4 supplements. There was no significant difference between the 4 supplements (Figure 9).

Figure 9.

Changes in RPE during the exhaustion test in high-temperature and -humidity environments (35 °C, 65% RH). *P < 0.05 compared with CMJ BL in the placebo group; ^#P < 0.05 compared with BL in the TAU group; ^&P < 0.05 compared with BL in the CAF group; ^%P < 0.05 compared with BL in the TAU + CAF group. BL, blood lactate; C/CAF, caffeine; CMJ countermovement jump; END, end of the exhaustion test; P, placebo; RH, relative humidity; RPE, rating of perceived exertion; T/TAU, taurine.

TS increased with time in 4 supplements (F_(6,39) = 157.448; P < 0.01; η²_partial = 0.96); however, there were no main effects for supplementation (F_(3,44) = 1.199; P = 0.32; η²_partial = 0.076), nor was there an interaction (F_(6,41) = 0.836; P = 0.55; η²_partial = 0.11). For the placebo group, TS values were higher than BL from 3 minutes to the end of the exhaustion test in all 4 supplements. There was no significant difference between the 4 supplements (Figure 10).

Figure 10.

Changes in TS during the exhaustion test in high-temperature and -humidity environments (35 °C, 65% RH). *P < 0.05 compared with CMJ BL in the placebo group; ^#P < 0.05 compared with BL in the TAU group; ^&P < 0.05 compared with BL in the CAF group; ^%P < 0.05 compared with BL in the TAU + CAF group. BL, blood lactate; C/CAF, caffeine; CMJ countermovement jump; END, end of the exhaustion test; P, placebo; RH, relative humidity; TS, thermal sensation; T/TAU, taurine.

Discussion

It is well known that high-temperature and -humid environments can have significant effects on the physiological indicators and exercise performance of athletes during exercise. ¹⁰ In addition, the effects of the ingestion of EA to improve performance have been investigated extensively.^40,42 However, the effectiveness of EA supplementation in high temperature and humidity environments is still highly controversial. Supplementation of combined EA in high temperature and humidity environments is less well studied; therefore, this study was conducted to investigate this issue.

In partial support of our hypothesis, we found that isolated TAU, CAF, and TAU + CAF supplementation in high temperature and humidity environments can improve performance during exhaustion test compared with placebo, by increasing TTE, but the effects of TAU supplementation were more significant (Figure 1). Acute supplementation with TAU, CAF, and their combination all increased CMJ, but did not alter neuromuscular recovery (Figure 2). Although there were no significant differences in HR, RPE, and TS between conditions during the exhaustion test, a lower CT was found in the TAU group during the final of exhaustion test and there was a notable reduction in BL after the exhaustion test.

Endurance exercise performance in environments with high temperature and humidity can be affected for a number of physical reasons. Studies have shown that cardiac output and stroke volume are reduced during exhaustive cycling in hot and humid conditions,^7,49 with a consequent reduction in cycling power output. ¹⁹ TAU has a well-established inotropic action on cardiac muscle fiber and has been shown to increase stroke volume. ⁵⁸ However, there were no significant variations in HR changes among the 4 groups. As a result, the increased performance and delayed fatigue in the TAU condition most likely occurred for different reasons. The effects of TAU supplementation on thermoregulation and endurance performance in hot environments have been partially investigated. Some previous studies have suggested that TAU may have the potential to regulate heat dissipation and heat production, thus assisting in maintaining thermal balance in hot environments. ⁴⁸ Cutaneous vasodilation and sweating (among other processes) support these avenues of heat loss by attenuating the rate of rise in CT during exercise. ³ In relation to the current study, smaller increases in CT have been observed in heat-stressed rabbits compared with thermoneutral controls after intrathecal infusion of TAU. ⁴⁸ In addition, TAU supplementation in hot and humid conditions is believed to provide additional energy supply and alleviate fatigue, thereby delaying the onset of fatigue. ⁶⁰ Our study also found that the TAU group significantly improved TTE. In addition, some literature indicates that TAU has been implicated in energy metabolism and may enhance aerobic energy production. ²⁶ By improving mitochondrial function and increasing the capacity for oxidative metabolism, TAU supplementation could potentially reduce the reliance on anaerobic glycolysis, which leads to lactate production. ⁶ Collectively, these thermoregulatory and BL changes might partly explain the observed improvement in performance. Literature reports showing potential mechanisms for improved endurance include postponed activation of less efficient type II muscle fibers, conversion of type IIX fibers into more fatigue resistant IIa fibers, and increased muscle mass and rate of force development. ⁴⁵ Whether exercise in high-temperature and humidity conditions has an effect on these potential mechanisms still requires further research.

Several studies have demonstrated that acute CAF ingestion before exercise can improve endurance capacity, ⁴³ delay fatigue, and enhance overall performance during prolonged aerobic activities. ⁵⁵ The ergogenic effects of CAF on endurance performance are believed to be attributed to its ability to stimulate the central nervous system, and reduce the perception of effort and fatigue ³³ ; however, there was no change in RPE and TS, which may be due to the effect of the high temperature and humidity on exercise.

HR changes were also not significantly different; the results of a study conducted by Bandyopadhyay et al ¹ support our findings. This latter study shows that ingestion of 5 mg/kg CAF improved endurance running performance but did not impose any significant effect on other individual cardiorespiratory parameters of heat-acclimated recreational runners in hot and humid conditions. Furthermore, some data indicate that heat and humidity conditions may be sufficient to mask the ergogenic benefit of CAF in cycling races of prolonged duration. ⁸ Conversely, other studies show that CAF ingestion before a 10 km run in hot and humid conditions increases the rate of heat storage but does not improve performance. ⁵⁴ Also, higher dosages of CAF lead to a greater rate of heat storage. ¹² Changes in CT may counteract the energizing effect of CAF. However, from the present data, there was no significant difference in CT, compared with the placebo group. Although the experiment was conducted on nonheat-habituated participants and a randomized crossover grouping was used to avoid heat habituation, individual differences in thermoregulation in response to exercise in high temperature and humidity environments were also observed. In addition, different people have differences in sensitivity to CAF. ⁶¹ Some studies have shown that some people may experience an increase in CT after CAF ingestion, while others may not respond significantly. ²⁸ The results of this experiment, selected for those without a CAF habit, showed that CT did not produce a significant difference compared with the placebo group. Overall, the mechanism of effect of CAF on CT is not clear, especially in high temperature and humidity environments, and the sensitivity of different people to CAF and the choice of different doses still needs to be further investigated.

Although it is known that TAU and CAF may induce changes of this type, this is the first study to report improvements on endurance cycling performance after isolated or co-ingested supplementation in high temperature and humid environments. Most of the current research on TAU combined with CAF supplementation has focused on multi-ingredient functional drinks.^15,50,52 In addition, the combination of 2 acute supplements has been studied mostly in repeated sprint performance,^35,63 and less so in endurance cycle performance. Although the results of this trial were not consistent with the expected hypothesis, there was a slight improvement in the TAU + CAF group compared with the placebo group. From the results of the experiment, it was found that there was a decrease in CT in the TAU group later in the exercise, as well as lower BL after the exhaustion test. However, the positive effects of CAF on physiological indicators and subjective sensations are not fully exploited in high temperature and humidity environments. It is important to note that the effects of combined TAU and CAF supplementation on exercise performance in high temperature and humidity environments may vary depending on individual factors, such as dosage,^6,12 timing of supplementation, ³⁹ exercise type, and training status. ⁴⁷ Therefore, further research is needed to better understand the optimal protocols and potential mechanisms underlying the combined effects of TAU and CAF on exercise performance.

The CMJ can indirectly reflect the level of neuromuscular fatigue in the lower limbs. ²¹ Our study found that CMJ was significantly higher in the CAF group than in the placebo group before the exhaustion test, and improved in both TAU and TAU + CAF groups. Consistent with the results of the present study, previous studies have shown that CAF supplementation can significantly improve CMJ performance while reducing the rate of decline in high-intensity exercise performance in the later stages of exercise. ² It is possible that this phenomenon is caused by a reduction in central fatigue or intrinsic muscle fatigue. Although some studies have shown that the isolated effects of CAF and TAU could augment force production, ²⁵ others have shown no change in force production with CAF and TAU co-ingestion compared with CAF alone. ⁶³ Similarly, the results of this experiment showed that the CMJ before the exhaustion test was not superimposed in the TAU + CAF group, but was lower than in the TAU and CAF supplementation alone in high temperature and humidity environments. Both CAF and TAU are reported to regulate intracellular Ca²⁺ handling and the sensitivity of myofibrils to Ca²⁺ in skeletal muscle fibres.^38,56 Interestingly, this study found that, although PP and MP for CMJ increased in the TAU, CAF, and TAU + CAF groups before the exhaustion test, there was no significant difference after the exhaustion test. In other words, according to our findings, acute TAU, CAF, and combined supplementation all enhance endurance cycling performance without altering neuromuscular fatigue. Currently, limited comparison data are available regarding TAU and CAF on neuromuscular fatigue after endurance cycling exercise. According to Yusuf Buzdağlı et al, ⁵ acute TAU supplementation enhances anaerobic power outputs without altering neuromuscular fatigue. Similarly, another study suggests that the main reason why CAF supplementation alters CMJ performance may be to alter the feeling of fatigue and effort during the fatigue phase of exercise. ² It is unclear which mechanisms are involved in TAU, CAF, and combined supplementation to influence CMJ performance. Therefore, whether it is the high temperature and humidity environment that has an effect on its mechanism of action is not yet explained in this experiment.

In practice, TAU and CAF, the main ingredients in common sports drinks like Red Bull, each affect exercise through different mechanisms of action. However, the current study revealed that supplementation with TAU resulted in a late-stage reduction in CT during exercise and significantly prolonged the TTE. Therefore, the contribution of TAU in sports drinks like Red Bull to athletic performance may be greater in high temperature and humidity environments due to its thermoregulatory function. The combination of specialized environments and nutritional interventions can provide insights into new sports supplementation strategies and potential roles. The effects of TAU in high temperature and humidity environments need to be further investigated.

There were significant limitations to this study. The present study is limited by its small sample size. In addition, we also did not take venous blood samples in this study to test for plasma CAF or TAU concentration, nor did we follow a double-blind research design. The effect of hot and humid environments on exercise performance focused on the peripheral causes of fatigue. The effects of combined TAU and CAF supplementation on exercise performance may vary depending on individual factors such as dosage, timing of supplementation, exercise type, and training status.

Conclusion

In high temperature and humidity environments, acute TAU, CAF, and combined supplementation all improved TTE and did not affect recovery from lower limb neuromuscular fatigue compared with the placebo group, with TAU having the best effect. However, the combined supplementation failed to exhibit superimposed performance. The effect of combined TAU and CAF supplementation on exercise performance is influenced by a number of factors in high temperature and humidity environments, such as dosage, exercise type, and subject population. Therefore, further exploration of the underlying mechanisms is necessary to find optimal supplement protocols.