Metabolic adaptations in post‐exercise recovery

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Abstract
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 V̇o2 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.