Lipid oxidation in fit young adults during postexercise recovery

To evaluate the hypothesis that lipid oxidation predominates in postexercise recovery, we examined healthy men ( n = 6; age = 21.2 +/- 0.6 yr) and women ( n = 6; age = 22.8 +/- 2.1 yr) during and after two exercise tasks [ 89 min at 45% and 60 min at 65% of peak rate of oxygen consumption ((V) over dot O-2 peak)] as well as a time- matched resting control trial (Con). Exercise bouts were matched for energy expenditure. Respiratory exchange ratios (RER) during exercise at 65% (V) over dot O-2 peak for both men and women ( 0.95 +/- 0.01 and 0.93 +/- 0.02) were significantly higher than 45% (V) over dot O-2 peak ( 0.89 +/- 0.01 and 0.86 +/- 0.02) and Con trials ( 0.86 +/- 0.01 and 0.86 +/- 0.02, respectively). During recovery, for men RER values were 0.78 +/- 0.01 and 0.76 +/- 0.01 after 45% and 65% exercise, respectively. For women, values were 0.79 +/- 0.01 and 0.78 +/- 0.01. These were significantly lower than during both the preexercise resting period and the corresponding no-exercise Con period (0.82 +/- 0.01 and 0.83 +/- 0.01, mean RER for men and women, respectively). Hence, the contribution of lipid oxidation to energy supply increased significantly during recovery compared with preexercise levels, and it was greater after exercise than during the time-matched, no-exercise Con period. It is concluded that, although carbohydrate is the major fuel source during moderate- to high-intensity exercise, 1) there is substantial postexercise lipid oxidation; and 2) lipid oxidation is the same during postexercise recovery whether the relative power output is 45% or 65% of (V) over dot O-2 peak when energy expenditure of exercise is matched.