Influence of caffeine ingestion on fluid-electrolyte regulating hormones, aldosterone and arginine vasopressin, and physiological responses at rest and during an exercise heat tolerance test

Date of Completion

January 2003


Biology, Animal Physiology|Health Sciences, Recreation




There is limited research on the fluid-electrolyte regulating hormonal and physiological responses to controlled chronic caffeine intake both at rest and during exercise. Because fluid-electrolyte regulation is a major component of exercise heat tolerance, serum aldosterone (ALD), plasma arginine vasopressin (AVP) and related physiological responses were examined at rest and during an exercise heat tolerance test (EHT). Fifty-nine college-aged males (mean ± se age = 21.6 ± 0.4y, height = 177.9 ± 0.8cm, body weight = 75.4 ± 1.0kg, body composition = 11.1 ± 0.7% body fat) were randomized and stratified by age, body weight and composition into 3 different dosing groups. Subjects equilibrated caffeine intake at 3 mg/kg/d for 6 days. On days 7–12 subjects consumed a specific treatment dose of 0 (G0, n = 20), 3 (G3, n = 20), or 6 (G6, n = 19) mg/kg/d in two doses, morning and afternoon. Resting hormonal and physiological measures were performed on the mornings of d6 and d12. The EHT occurred on d12, one hour post-dose and consisted of 90 min walking (5.6kph, 5% grade) in 38°C heat with 56% humidity. Mood state was assessed on d11, one hour post-dose. There were no between-group differences in resting physiological variables including: 24h urine volume, osmolality and electrolytes, acute urine osmolality, hematocrit, plasma osmolality, serum electrolytes, or fluid intake (p > 0.05). ALD (GO, 398.0 ± 37.0; G3, 374.0 ± 37.0; G6, 380.7 ± 35.9pmol/l, p > 0.05) and AVP (G0, 2.91 ± 0.51; G3, 2.70 ± 0.42; G6, 2.46 ± 0.45mg/ml, p > 0.05) also did not differ between groups on day 12. There were no between-group differences during the EHT in the following variables: ALD, AVP, percent change plasma volume, plasma osmolality, acute urine osmolality, sweat rate, heart rate, rectal temperature, skin temperature, rating of perceived exertion, thermal, or thirst (p > 0.05). Serum sodium and potassium were lower in G6 vs. G3 post-EHT (p < 0.05). EHT time was significantly greater in G3 vs. G0 (G3, 86 ± 2.0min > G0, 75 ± 3.3; p < 0.05) which corresponded to increased vigor-activity scores in G3 (p < 0.05). G6 exhibited a tendency toward a longer EHT time versus G0 (G6, 80 ± 2.8min, p > 0.05). Although many medical professionals have recommended abstaining from caffeine with exercise in the heat, the results of this study indicate that in a controlled chronic caffeine study there were no decrements in fluid-electrolyte balance, ALD and, AVP, performance or thermoregulation. ^