Roti, M. W., Pumereantz, A. C., Watson, G., Judelson, D. A., Larsen, M. S., Sokmen, B., Dias, J. C., Ruffin, K., Casa, D. J., & Armstrong, L. E. (2004). Influence of caffeine ingestion on fluid-electrolyte, psychological, and physiological responses during an exercise heat-tolerance test. Medicine and Science in Sports and Exercise, 36(5), Supplement abstract 142.

College males (N = 59) were stratified by age, body weight, and body composition and assigned to three groups. All Ss standardized caffeine intake to 3 mg/kg per day for days 1-6. On days 7-12 they consumed treatment doses of 0 (G0), 3 (G3), and 6 (G6) gm/kg per day. Fluid-electrolyte and physiological measures were made on day 12, one hour after caffeine intake and during the exercise heat-tolerance task (90 minutes of walking; 5.6 kph; 5% grade; dry-bulb temperature of 37.71C).

There were no differences between groups for pre-and post-exercise heat-tolerance test measures of plasma osmolality, total plasma proteins, plasma glucose, plasma lactate, acute urine osmolality, urine specific gravity, and urine color. Sweat rate, percent change in plasma volume, oxygen consumption, and respiratory exchange ratio did not differ among the groups. For variables measured every 15 minutes in the heat-tolerance test, there were no between-groups differences for heart rate, rectal and skin temperatures, ratings of perceived exertion, thermal perception, or thirst rating, although those variables increased as the exercise progressed. Post-exercise measures showed some differences. Serum sodium was higher for G6 than G3; serum potassium was lower for G6 than in the other two groups. Exercise time was greater for G3 than G0. Vigor-activity mood scores in G3 were greater for G3 than the other two groups.

Implication. Acute caffeine ingestion does not alter heat tolerance, hydration status, or subjective responses in hot conditions.

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