The Effect of Exercise Intensity on Carbohydrate Sparing Postexercise: Implications for Postexercise Hypoglycemia

The purpose of this study was to determine the effect of exercise intensity on the proportion and rate of carbohydrate oxidation and glucoregulatory hormone responses during recovery from exercise. Six physically active participants completed 1 hr of low-intensity (LI; 50% lactate threshold) or moderate-intensity (MI; 100% lactate threshold) exercise on separate days following a randomized counterbalanced design. During exercise and for 6 hr of recovery, samples of expired air were collected to determine oxygen consumption, respiratory exchange ratio, energy expenditure, and substrate oxidation rates. Blood samples were also collected to measure glucoregulatory hormones (catecholamines, GH) and metabolites (glucose, free fatty acids, lactate, pH, and bicarbonate). During exercise, respiratory exchange ratio, energy expenditure, and the proportion and rate of carbohydrate (CHO) oxidation were higher during MI compared with LI. However, during recovery from MI, respiratory exchange ratio and the proportion and rate of CHO oxidation were lower than preexercise levels and corresponding LI. During exercise and early recovery, catecholamines and growth hormone were higher in MI than LI, and there was a trend for higher levels of free fatty acids in the early recovery from MI compared with LI. In summary, CHO oxidation during exercise increases with exercise intensity but there is a preference for CHO sparing (and fat oxidation) during recovery from MI exercise compared with LI exercise. This exercise intensity-dependent shift in substrate oxidation during recovery is explained, in part, by the pattern of change of key glucoregulatory hormones including catecholamines and growth hormone and plasma fatty acid concentrations.