CAFFEINE METABOLISM AND EPINEPHRINE RESPONSES DURING EXERCISE IN USERS AND NONUSERS

This study compared the caffeine (CAF) metabolism and the catecholamin e and metabolic responses of users and nonusers of caffeine after acut e ingestion of caffeine (5 mg/kg) during 1 h of steady-state exercise (50% maximal oxygen consumption). Nonusers (n = 7) completed two exerc ise trials after ingesting either CAF (5 mg/kg) or placebo (PL). Users (n = 7) underwent three trials designed to control caffeine use and a bstained 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 ca se on the 7th day by a CAF exercise trial identical to that of the non users. In nonusers CAF increased (P < 0.05) plasma epinephrine (EPI) c oncentration above PL values during exercise. Users did not exhibit an y increased EPI with CAF, but the EPI response to exercise in all thre e trials was twofold greater than that of the nonusers' PL trial (P < 0.05). In all trials both groups had identical norepinephrine response s. The groups had similar plasma and urinary caffeine concentration, b ut plasma dimethylxanthines varied; the users had greater (P < 0.05) t heophylline concentration, and the nonusers had a greater (P < 0.05) r ise 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 restin g subjects, in this study PX concentrations in nonusers varied from th at of the users, yet FFA data were similar. This study demonstrated th at short-term and long-term caffeine habits influence both how caffein e is metabolized and are critical in determining the EPI responses to both exercise and caffeine.