Table 1.

Summary of studies included in the meta-analyses (n = 25)

Study	Design	Sample	Supplement, dose and timing	Temperature and relative humidity	Core temperature method	Exercise performance type	Outcome
Beaumont et al. [81]	Double-blind, randomised, repeated measures, cross-over	Healthy, recreationally active, non-heat acclimated males (n = 8). Age 22 ± 1 years	Caffeine 6 mg·kg−1 (60 min pre-exercise)	30 °C 50% RH	Gastrointestinal every 5 min (ECT + SCT)	60 min cycling @ 55% Wmax followed by 30 min TT	NS ~ 3% ↑ in TT performance
Cheuvront et al. [45]	Double-blind, randomised, cross-over	Healthy, physically active, moderately fit, non-heat acclimated males (n = 10). Age 23 (18–37) years	Caffeine 9 mg·kg−1 (timing not mentioned)	40 °C 20–30% RH	Rectal every 5 min (ECT + SCT)	30 min cycling @ 50% $\dot{V}{\text{O}}_{\text{2peak}}$ followed by 15 min TT	NS ~ 2.4% ↑ in TT performance
Ferreira et al. [82]a	Double-blind, randomised, cross-over	Well-trained, heat acclimated, male cyclists (n = 8). Age 23.9 ± 8.6 years	Caffeine 5 mg·kg−1 (60 min pre-exercise)	30 °C average, ranged from 28.5–32 °C 71–78% RH	Tympanic pre and post exercise (ECT)	45 km cycling TT	NS ~ 4.2% ↑ in TT performance
Ganio et al. [83]	Double-blind, randomised, cross-over	Healthy, trained, non-heat acclimated male cyclists (n = 11). Age 25 ± 6 years	Caffeine 3 mg·kg−1 (60 min pre-exercise)	33 °C 41% RH	Rectal every 15 min (ECT + SCT)	90 min cycling @ 65% thermoneutral $\dot{V}{\text{O}}_{\text{2max}}$ followed by 15 min TT	NS ~ 6.3% ↑ in TT performance
Hanson et al. [48]	Single-blind, randomised, cross-over	Trained male (n = 6) and female (n = 4) endurance runners (n = 10). Age 26 ± 9 years	Caffeine 6 mg·kg−1 (60 min pre-exercise)	30.6 °C 50% RH	Gastrointestinal every 1 km (ECT + SCT)	10 km running TT	NS ~ 0.9% ↑ in TT performance
Ping et al. [84]a	Double-blind, randomised, cross-over	Recreational, heat acclimated male runners (n = 9). Age 25.4 ± 6.9 years	Caffeine 5 mg·kg−1 (60 min pre-exercise)	31 °C 70% RH	Rectal every 10 min (ECT)	Treadmill running @ 70% $\dot{V}{\text{O}}_{\text{2max}}$	Significant ~ 27.4% ↑ in TTE
Pitchford et al. [85]	Double-blind, randomised, counterbalanced, cross-over	Highly-trained, non-heat acclimated male cyclists (n = 9). Age range 22–42 years	Caffeine 3 mg·kg−1 (90 min pre-exercise)	35 °C 25% RH	Gastrointestinal continuously (ECT + SCT)	Total work cycling TT	NS ~ 6.9% ↑ in TT performance
Roelands et al. [46]	Double-blind, randomised, cross-over	Healthy, trained, non-heat acclimated males (n = 8). Age 23 ± 5 years	Caffeine 6 mg·kg−1 (60 min pre-exercise)	30 °C 50–60% RH	Rectal every 5 min (ECT + SCT)	60 min cycling @ 55% Wmax followed by total work TT	NS ~ 3% ↓ in TT performance
Suvi et al. [86]a,b	Double-blind, randomised, cross-over	Healthy, physically active, non-heat acclimated males (n = 13) and females (n = 10; n = 23). Age 24.9 ± 4.1 vs 22.5 ± 2 years	Caffeine 6 mg·kg−1 (4 mg·kg−1 60 min and 2 mg·kg−1 0 min pre-exercise)	42 °C 20% RH	Measured but no extractable data	50 min treadmill walking @ 60% thermoneutral $\dot{V}{\text{O}}_{\text{2peak}}$ followed by TTE	NS ~ 4.3% ↓ in TTE
Kilduff et al. [87]	Double-blind, randomised, independent design	Endurance-trained, non-heat acclimated males (n = 11 vs 10; n = 21). Age 27 ± 5 vs 27 ± 4 years	Creatine 159.6 g (7 × 22.8 g·day−1)	30.3 °C 70% RH	Rectal every 5 min (ECT + SCT)	Cycling @ incremental work rate at 60–90 rpm	NS ~ 3% ↓ in TTE
Fowler et al. [49]	Double-blind, randomised, cross-over	Healthy, physically inactive, non-heat acclimated males (n = 11). Age 25 ± 5 years	Nitrate (NO3−) 46 mmol (5 × 9.2 mmol·day−1)	35 °C 28% RH	Rectal every 1 min (ECT + SCT)	Cycling @ thermoneutral gas exchange threshold at 70 rpm	NS ~ 9.7% ↑ in TTE
Kent et al. [88]	Double-blind, repeated measures, counter-balanced, cross-over	Endurance-trained male cyclists (n = 12). Age 26.6 ± 4.4 years	Nitrate (NO3−) 26 mmol (2 × 6.5 mmol·day−1 and 13 mmol 2 h pre-exercise)	35 °C 48% RH	Gastrointestinal every 20% work rate (ECT + SCT)	Total work cycling TT	NS ~ 3.1% ↑ in TT performance
McQuillan et al. [47]	Double-blind, randomised, cross-over	Healthy, well-trained endurance male cyclists (n = 8). Age 25 ± 8 years	Nitrate (NO3−) 24 mmol (2 × 8 mmol·d−1 and 8 mmol 90 min pre-exercise)	35 °C 60% RH	Rectal continuously (ECT + SCT)	20 min cycling @ 40–60% PPO followed by 4 km TT	NS ~ 0.3% ↑ in TT performance
Smith et al. [89]a	Double-blind, randomised, counterbalanced, cross-over	Recreationally-trained males (n = 12), Age 22 ± 4 years	Nitrate (NO3−) 6.2 mmol (3 h pre-exercise)	30 °C 70% RH	Tympanic post IST (ECT)	20 × 6 s sprints (114 s active recovery)	NS ~ 1.5% ↓ in mean power output
Cheuvront et al. [90]	Cross-over	Healthy, physically active, moderately fit, heat acclimated males (n = 7). Age 21 ± 2 years	BCAAs 14 g·kg−1 (0 min pre- and during exercise)	40 °C 20% RH	Rectal every 10 min (ECT + SCT)	60 min cycling @ 50% $\dot{V}{\text{O}}_{\text{2peak}}$ followed by 30 min TT	NS ~ 14.3% ↑ in TT performance
Mittleman et al. [51]	Double-blind, cross-over	Healthy, moderately-trained males (n = 7) and females (n = 6; n = 13). Age 24 ± 2.9 vs 25.6 ± 7 years	BCAAs Females (9.4 g) and males (15.8 g; 5 mL·kg−1 of 5.88 g·L−1 (Every 60 min at rest and 30 min during exercise)	34.4 °C 39% RH	Oesophageal every 5 min (ECT + SCT)	Cycling @ 40% $\dot{V}{\text{O}}_{\text{2peak}}$	Significant ~ 11.1% ↑ in TTE
Watson et al. [91]	Double-blind, randomised, cross-over	Healthy, endurance exercising, non-heat acclimated males (n = 8). Age 28.5 ± 8.2 years	BCAAs 4 × 250 ml at 12 g·L−1 (30 min intervals pre-exercise and 150 ml every 15 min during exercise)	30 °C 38% RH	Rectal every 10 min (ECT + SCT)	Cycling @ 50% $\dot{V}{\text{O}}_{\text{2peak}}$	NS ~ 6.6% ↑ in TTE
Coull et al. [92]	Double-blind, counter-balanced, cross-over	Recreationally active, non-heat acclimated males (n = 8). Age 23 ± 1 years	Tyrosine 150 mg·kg−1 (60 min pre-exercise)	40 °C 30% RH	Rectal every 5 min (ECT + SCT)	60 min treadmill walk followed by 2.4 km TT wearing a 25 kg backpack	NS ~ 5% ↑ in TT performance
Tumilty et al. [52]	Double-blind, randomised, cross-over	Healthy, endurance exercising, non-heat acclimated males (n = 8). Age 32 ± 11 years	Tyrosine 150 mg·kg−1 (60 pre-exercise)	30 °C 60% RH	Rectal every 10 min (ECT + SCT)	Cycling @ 68% $\dot{V}{\text{O}}_{\text{2peak}}$	Significant ~ 14.8% ↑ in TTE
Tumilty et al. [93]	Double-blind, randomised, cross-over	Endurance exercising, non-heat acclimated males (n = 7). Age 20 (range 26) years	Tyrosine 150 mg·kg−1 (60 pre-exercise)	30 °C 60% RH	Rectal every 5 min (ECT + SCT)	60 min cycling @ 57% $\dot{V}{\text{O}}_{\text{2peak}}$ followed by total work TT	NS ~ 1.1% ↑ in TT performance
Watson et al. [94]	Randomised, counter-balanced, cross-over	Physically active, trained, non-heat acclimated males (n = 8). Age 23 ± 3 years	Tyrosine 150 mg·kg−1 (2 h, 1 h, and during)	30 °C 50% RH	Rectal every 5 min (ECT + SCT)	Cycling @ 70% $\dot{V}{\text{O}}_{\text{2peak}}$	NS ~ 2% ↓ in TTE
Keong et al. [95]	Double-blind, randomised, cross-over	Recreational, heat acclimated male athletes (n = 18). Age 24.9 ± 1.4 years	Vitamin E No dose stated (6 week pre-exercise)	31 °C 70% RH	Rectal every 10 min (ECT + SCT)	Treadmill running @ 70% $\dot{V}{\text{O}}_{\text{2max}}$	NS ~ 5.3% ↑ in TTE
Muhamad et al. [96]a	Double-blind, randomised, cross-over	Healthy, male recreational athletes (n = 12). Age 23.3 ± 3.7 years	E. longifolia Jack 1200 mg (7 × 150 mg·d−1 and 150 mg 60 min pre-exercise)	31 °C 70% RH	Tympanic every 10 min (ECT)	60 min treadmill running @ 60% $\dot{V}{\text{O}}_{\text{2max}}$ followed by 20 min TT	NS ~ 3.6% ↑ in TT performance
Page et al. [53]	Double-blind, randomised, cross-over	Healthy, non-heat acclimated males (n = 11). Age 23 ± 2 years	Taurine 50 mg·kg−1 (2 h pre-exercise)	35 °C 40% RH	Rectal every 1 min (ECT + SCT)	Cycling @ thermoneutral ventilatory threshold at 80 rpm	Significant ~ 11.5% ↑ in TTE
Trinity et al. [97]	Double-blind, randomised, cross-over	Healthy, well-trained male cyclists (n = 12). Age 26.8 ± 5 years	Polyphenols 25,200 ppm (7 × 3600-ppm·day−1)	31.5 °C 55% RH	Rectal continuously (ECT + SCT)	10 min cycling @ 60–70% $\dot{V}{\text{O}}_{\text{2max}}$ followed by cycling @ 100% $\dot{V}{\text{O}}_{\text{2max}}$	NS ~ 3.5% ↓ in TTE

TT time-trial, TTE time-to-exhaustion, IST intermittent sprint test, NS non-significant, PPO peak power output, Wmax maximal workload, ECT end core temperature, SCT submaximal core temperature, RH relative humidity, $\dot{V}{\text{O}}_{\text{2max}}$ maximal oxygen uptake, $\dot{V}{\text{O}}_{\text{2peak}}$ peak oxygen uptake, BCAAs branched-chain amino acids

^aNot included in submaximal core temperature analysis

^bNot included in end core temperature analysis. The table is a reflection of participant characteristics, as reported by the authors of the articles