Abstract
Acute caffeine ingestion is considered effective in improving endurance capacity. Caffeine may also improve cognitive functions and mood. However, current knowledge about the effects of caffeine is based on the findings of studies which mainly have been conducted in temperate environmental conditions and on male subjects, whereas some physiological and mood effects of caffeine have been shown to differ between sexes. PURPOSE: The aim of this study was to compare physical and cognitive performance effects of caffeine in young women and men during constant-load treadmill walk to exhaustion in the heat. METHODS: Thirteen male (age 24.9±4.1 yr, height 1.83±0.06 m, body mass 78.8±7.9 kg, VO2peak 51.7±2.7 ml·kg−1·min−1) and ten female (age 22.5±2.0 yr, height 1.67±0.05 m, body mass 61.0±5.4 kg, VO2peak 45.6±4.0 ml·kg−1·min−1) physically active students completed two constant-load walks (60% VO2peak) on a treadmill until volitional exhaustion in a climatic chamber (air temperature 42°C and relative humidity 20%) after caffeine (CAF) and placebo (wheat flour; PLC) ingestion in a double-blind, randomly assigned, crossover manner. Both CAF and PLC were administered in gelatine capsules in 2 doses: 60 min (4 mg/kg) and immediately (2 mg/kg) prior to exercise. RESULTS: Walking time to exhaustion was 82±15 min and 76±11 min (p>0.05) in females and 83±17 min and 82±14 min (p>0.05) in males in PLC and CAF trial, respectively. CAF compared to PLC increased (p<0.05) heart rate and blood lactate concentration in both males and females, but had no impact (p>0.05) on changes in rectal and mean weighted skin temperature during walk to exhaustion. CAF had no impact (p>0.05) on changes in mood, short-term memory and working memory during exercise in either gender, however, CAF compared to PLC decreased (p<0.05) ratings of perceived exertion and fatigue in males, but not in females. In females, but not in males, stronger belief that they had administered CAF associated with shorter time to exhaustion. CONCLUSION: In both males and females, caffeine has no impact on endurance capacity, cognitive performance or mood under hot environmental conditions. Supported by the Estonian Ministry of Education and Research, institutional research funding IUT 20-58.
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