Caffeine ingestion compromises thermoregulation and does not improve cycling time to exhaustion in the heat amongst males.
Study Goal
The researchers aimed to investigate the effects of acute caffeine supplementation on cycling performance and thermoregulation in hot conditions.
Results Summary
Caffeine did not improve cycling time to exhaustion but increased heat production, sweat rate, and core temperature while decreasing thermal comfort. No significant change in perceived exertion was observed.
Population
12 healthy, caffeine-habituated, unacclimatised males.
Effective Dosage
5 mg/kg, single dose.
Duration
Acute (60 minutes post-ingestion).
Interactions
None mentioned.
| Intervention | Direction | Endpoint | Population | Dosage | Impact | Claim # |
|---|---|---|---|---|---|---|
acute caffeine supplementation | no change | cycling time to exhaustion (TTE) | 12 healthy caffeine-habituated and unacclimatised males | no significant change | There was no effect of | #1 |
caffeine | increase | pulmonary oxygen uptake | 12 healthy caffeine-habituated and unacclimatised males | by 7.4% | increased | #2 |
caffeine | increase | heat production | 12 healthy caffeine-habituated and unacclimatised males | by 7.9% | increased | #3 |
caffeine | increase | whole-body sweat rate (WBSR) | 12 healthy caffeine-habituated and unacclimatised males | by 21% | increased | #4 |
caffeine | increase | evaporative heat transfer | 12 healthy caffeine-habituated and unacclimatised males | by 16.5% | increased | #5 |
caffeine | decrease | estimated skin blood flow | 12 healthy caffeine-habituated and unacclimatised males | by 14.1% | decreased | #6 |
caffeine | increase | Core temperature | 12 healthy caffeine-habituated and unacclimatised males | by 0.6% | was higher | #7 |
caffeine | decrease | thermal comfort | 12 healthy caffeine-habituated and unacclimatised males | by - 18.3% | decreased | #8 |
caffeine | no change | rate of perceived exertion | 12 healthy caffeine-habituated and unacclimatised males | no significant change | no changes in | #9 |
5 mg/kg of caffeine | no change | performance | participants | - | did not provide a performance benefit | #10 |
5 mg/kg of caffeine | increase | the thermal strain | participants | - | increased | #11 |
PURPOSE: Caffeine is a commonly used ergogenic aid for endurance events; however, its efficacy and safety have been questioned in hot environmental conditions. The aim of this study was to investigate the effects of acute caffeine supplementation on cycling time to exhaustion and thermoregulation in the heat. METHODS: In a double-blind, randomised, cross-over trial, 12 healthy caffeine-habituated and unacclimatised males cycled to exhaustion in the heat (35 °C, 40% RH) at an intensity associated with the thermoneutral gas exchange threshold, on two separate occasions, 60 min after ingesting caffeine (5 mg/kg) or placebo (5 mg/kg). RESULTS: There was no effect of caffeine supplementation on cycling time to exhaustion (TTE) (caffeine; 28.5 ± 8.3 min vs. placebo; 29.9 ± 8.8 min, P = 0.251). Caffeine increased pulmonary oxygen uptake by 7.4% (P = 0.003), heat production by 7.9% (P = 0.004), whole-body sweat rate (WBSR) by 21% (P = 0.008), evaporative heat transfer by 16.5% (P = 0.006) and decreased estimated skin blood flow by 14.1% (P < 0.001) compared to placebo. Core temperature was higher by 0.6% (P = 0.013) but thermal comfort decreased by - 18.3% (P = 0.040), in the caffeine condition, with no changes in rate of perceived exertion (P > 0.05). CONCLUSION: The greater heat production and storage, as indicated by a sustained increase in core temperature, corroborate previous research showing a thermogenic effect of caffeine ingestion. When exercising at the pre-determined gas exchange threshold in the heat, 5 mg/kg of caffeine did not provide a performance benefit and increased the thermal strain of participants.