Thursday, May 11, 2006

Thermoregulatory responses to exercise in the heat: chronic caffeine intake has no effect.

A group at the UCONN Human Performance Lab has reported a study in the Journal, Aviat Space Environ Med. 2006 Feb;77(2):124-9 showing that chronic caffeine intake has no effect on fluid-electrolyte, exercise endurance or thermoregulatory responses during EHT when compared with no caffeine.

Divers are usually admonished against drinking beverages that might be diuretic and which might cause dehydration - a known and proven risk factor for the development of decompression illness. The abstract of the article is below.

Authorities advise individuals to refrain from caffeine intake before or during exercise, especially when performed in the heat, due to potential fluid-electrolyte imbalances that exaggerate physiological strain. Yet, military personnel are often deployed to hot environments and must perform under sleep-deprived conditions where caffeine would be an ideal intervention strategy to enhance physical and cognitive performance.

PURPOSE: To assess the effects of controlled chronic and acute caffeine ingestion on fluid-electrolyte, physiological and thermoregulatory responses during an exercise heat tolerance test (EHT).

METHODS: Subjects were 59 active, college-aged males (mean +/- SE 21.6 +/- 0.4 yr, 177.9 +/- 0.8 cm, 75.4 +/- 1.0 kg, 11.1 +/- 0.7% body fat) who were randomized and stratified by age, bodyweight, and body composition into three groups. All subjects equilibrated caffeine intake at 3 mg x kg(-1) x d(-1) for days 1-6. On days 7-12, they consumed a treatment dose of either 0 (G0), 3 (G3), or 6 (G6) mg x kg(-1) x d(-1). Fluid-electrolyte and physiological measures were made on day 12, 1 h after caffeine intake, during the EHT (90 min walking, 1.56 m x s(-1), 5% grade; dry bulb temperature, 37.7 +/- 0.1 degree C; relative humidity, 56.3 -1.5%).

RESULTS: There were no between-group differences (p > 0.05) in plasma, urinary, thermoregulatory, cardiovascular, and perceptual variables across time (pre- vs. post-EHT), although some of these variables increased significantly over time (p < 0.05). EHT time was significantly greater in G3 (86 +/- 2.0 min) vs. GO (75 +/- 3.3 min, p < 0.05).

DISCUSSION: Acute caffeine ingestion, in chronically consuming subjects (3 and 6 mg x kg(-1) x d(-1)) did not alter fluid-electrolyte, exercise endurance or thermoregulatory responses during EHT when compared with G0.

This will come as welcome news for those divers who have been holding off on their morning 'cuppa' prior to diving.