Metabolic adaptations in post-exercise recovery

Summary. To investigate further the hormonal and metabolic adaptations occurring when carbohydrates are ingested after prolonged exercise, we have compared the fate of a 100-g oral glucose load (using ‘naturally labelled’¹³C-glucose) in healthy volunteers after an overnight fast at rest either without previous exercise or after a 3-h exercise performed on a treadmill at about 50% of the individual V?o₂ max. In comparison to the control conditions, the oral glucose tolerance test (OGTT) performed in the post-exercise recovery period was characterized by a greater rise in peripheral blood glucose levels and delayed insulin response. Plasma glucagon values were significantly elevated at the time glucose was given (+48 ±13 pg ml^-1) and at the end of the OGTT. Plasma-free fatty acid (FFA) levels were 1675 ± 103 μEq 1^-1 when glucose was given, and subsequently reduced to values similar to those observed in the control conditions. Indirect calorimetry indicated that OGTT in post-exercise recovery was associated with decreased carbohydrate and increased lipid oxidation when compared to control conditions. Exogenous glucose oxidation was also significantly reduced: 25·1 ± 2·6 vs. 35·9 ± 1·9 g per 7 h. We suggest that the higher plasma glucagon levels and the delayed insulin response played a role in the decreased hepatic glucose retention previously described by others in post-exercise recovery. Our data also suggest that the higher lipid oxidation rate observed at the time glucose was given in the post-exercise period could explain, according to the Randle ‘glucose-fatty acid cycle’, the decreased carbohydrate oxidation and the preferential muscle glycogen repletion already well documented. The reason why the lipid oxidation rate remains increased 3–7 h after glucose ingestion in spite of the fact that FFA levels at that time are similar to those observed in control conditions is still unknown; further kinetic studies are needed to clarify this point.