Effect of morning exercise on counterregulatory responses to subsequent, afternoon exercise

The aim of this study was to determine whether a bout of morning exercise (EXE1) can alter neuroendocrine and metabolic responses to subsequent afternoon exercise (EXE2) and whether these changes follow a gender-specific pattern. Sixteen healthy volunteers (8 men and 8 women, age 27 +/- 1 yr, body mass index 23 +/- 1 kg/m(2), maximal O-2 uptake 31 +/- 2 ml.kg(-1).min(-1)) were studied after an overnight fast. EXE1 and EXE2 each consisted of 90 min of cycling on a stationary bike at 48 +/- 2% of maximal O-2 uptake separated by 3 h. To avoid the confounding effects of hypoglycemia and glycogen depletion, carbohydrate (1.5 g/kg body wt po) was given after EXE1, and plasma glucose was maintained at euglycemia during both episodes of exercise by a modification of the glucose-clamp technique. Basal insulin levels (7 +/- 1 muU/ml) and exercise-induced insulin decreases (-3 muU/ml) were similar during EXE1 and EXE2. Plasma glucose was 5.2 +/- 0.1 and 5.2 +/- 0.1 mmol/l during EXE1 and EXE2, respectively. The glucose infusion rate needed to maintain euglycemia during the last 30 min of exercise was increased during EXE2 compared with EXE1 (32 +/- 4 vs. 7 +/- 2 mu mol.kg(-1).min(-1)). Although this increased need for exogenous glucose was similar in men and women, gender differences in counterregulatory responses were significant. Compared with EXE1, epinephrine, norepinephrine, growth hormone, pancreatic polypeptide, and cortisol responses were blunted during EXE2 in men, but neuroendocrine responses were preserved or increased in women. In summary, morning exercise significantly impaired the body's ability to maintain euglycemia during later exercise of similar intensity and duration. We conclude that antecedent exercise can significantly modify, in a gender-specific fashion, metabolic and neuroendocrine responses to subsequent exercise.