CAFFEINE METABOLISM AND EPINEPHRINE RESPONSES DURING EXERCISE IN USERS AND NONUSERS

This study compared the caffeine (CAF) metabolism and the catecholamine and metabolic responses of users and nonusers of caffeine after acute ingestion of caffeine (5 mg/kg) during 1 h of steady-state exercise (50% maximal oxygen consumption). Nonusers (n = 7) completed two exercise trials after ingesting either CAF (5 mg/kg) or placebo (PL). Users (n = 7) underwent three trials designed to control caffeine use and abstained from voluntary CAF intake for 18 days. After 4 days they had a PL trial and in the following 14 days they were given random 6 days of CAF (2 x 2.5 mg . kg-1 . day-1) or PL ingestion followed in each case on the 7th day by a CAF exercise trial identical to that of the nonusers. In nonusers CAF increased (P < 0.05) plasma epinephrine (EPI) concentration above PL values during exercise. Users did not exhibit any increased EPI with CAF, but the EPI response to exercise in all three trials was twofold greater than that of the nonusers' PL trial (P < 0.05). In all trials both groups had identical norepinephrine responses. The groups had similar plasma and urinary caffeine concentration, but plasma dimethylxanthines varied; the users had greater (P < 0.05) theophylline concentration, and the nonusers had a greater (P < 0.05) rise in paraxanthine (PX) concentration. The users and nonusers' plasma free fatty acids (FFA), glycerol and respiratory exchange ratio were similar after ingestion of CAF. Although PX may increase FFA in resting subjects, in this study PX concentrations in nonusers varied from that of the users, yet FFA data were similar. This study demonstrated that short-term and long-term caffeine habits influence both how caffeine is metabolized and are critical in determining the EPI responses to both exercise and caffeine.