Caffeine during Exercise in the Heat: Thermoregulation and Fluid-Electrolyte Balance

Purpose: To investigate the effects of caffeine ingestion on thermoregulation and fluid-electrolyte losses during prolonged exercise in the heat.

Methods: Seven endurance-trained (V˙O_2max = 61 ± 8 mL.kg^-1.min^-1) heat-acclimated cyclists pedaled for 120 min at 63% V˙O_2max in a hot-dry environment (36°C; 29% humidity) on six occasions: 1) without rehydration (NF); 2) rehydrating 97% of sweat losses with water (WAT); 3) rehydrating the same volume with a 6% carbohydrate-electrolytes solution (CES); or combining these treatments with the ingestion of 6 mg caffeine[middot]kg^-1 body weight 45 min before exercise, that is, 4) C_AFF + NF; 5) C_AFF + WAT; and 6) C_AFF + CES.

Results: Without fluid replacement (NF and C_AFF + NF), final rectal temperature (T_REC) reached 39.4 ± 0.1°C, whereas it remained at 38.7 ± 0.1°C during WAT (CES and C_AFF + WAT; (P < 0.05). Caffeine did not alter heat production, forearm skin blood flow, or sweat rate. However, C_AFF + CES tended to elevate T_REC above CES alone (38.9 ± 0.1°C vs 38.6 ± 0.1°C; P = 0.07). Caffeine ingestion increased sweat losses of sodium, chloride, and potassium (∼14%; P < 0.05) and enlarged urine flow (28%; P < 0.05).

Conclusion: Caffeine ingested alone or in combination with water or a sports drink was not thermogenic or impaired heat dissipation. However, C_AFF + CES tended to have a higher T_REC than CES alone. Caffeine increased urine flow and sweat electrolyte excretion, but these effects are not enough to affect dehydration or blood electrolyte levels when exercising for 120 min in a hot environment.