The effects of caffeine on the kinetics of O2 uptake, CO2 production and expiratory ventilation in humans during the on-transient of moderate and heavy intensity exercise

In order to test the hypothesis that glycogen sparing observed early during exercise following caffeine ingestion was a consequence of tighter metabolic control reflected in faster (V)over dot (O2) kinetics, we examined the effect of caffeine ingestion on oxygen uptake ((V)over dot (O2)), carbon dioxide production ((V)over dot (CO2)) and expiratory ventilation ((V)over dot (E)) kinetics at the onset of both moderate (MOD) and heavy (HVY) intensity exercise. Male subjects (n = 10) were assigned to either a MOD (50 % (V)over dot (O2,max), n = 5) or HVY (80 % (V)over dot (O2,max), n = 5) exercise condition. Constant-load cycle ergometer exercise was performed as a step function from loadless cycling 1 h after ingestion of either dextrose (placebo, PLAC) or caffeine (CAFF; 6 mg (kg body mass)(-1)). Alveolar gas exchange was measured breath-by-breath. A 2- or 3-component exponential model, fitted through the entire exercise transient, was used to analyse gas exchange and ventilatory data for the determination of total lag time (TLT: the time taken to attain 63 % of the total exponential increase). Caffeine had no effect on TLT for (V)over dot (O2) kinetics at either exercise intensity (MOD: 36 +/- 14 s (PLAC) and 41 +/- 10 s (CAFF); HVY: 99 +/- 30 s (PLAC) and 103 +/- 26 (CAFF) (mean +/- S.D.)). TLT for (V)over dot (E) was increased with caffeine at both exercise intensities (MOD: 50 +/- 20 s (PLAC) and 59 +/- 21 s (CAFF); HVY: 168 +/- 35 s (PLAC) and 203 +/- 48 s (CAFF)) and for (V)over dot (CO2) during MOD only (MOD: 47 +/- 14 s (PLAC) and 53 +/- 17 s (CAFF); HVY: 65 +/- 13 s (PLAC) and 69 +/- 17 s (CAFF)). Contrary to our hypothesis, the metabolic effects of caffeine did not alter the on-transient (V)over dot (O2) kinetics in moderate or heavy exercise. (V)over dot (CO2) kinetics were slowed by a reduction in CO2 stores reflected in pre-exercise and exercise end-tidal CO2 pressure (P-ET,P-CO2) and plasma P-CO2 which, we propose, contributed to slowed (V)over dot (E) kinetics.