Impact of caffeine and protein on postexercise muscle glycogen synthesis.

BACKGROUND:: Both protein and caffeine co-ingestion with carbohydrate (CHO) have been suggested to represent effective dietary strategies to further accelerate post-exercise muscle glycogen synthesis in athletes. PURPOSE:: To assess the impact of protein or caffeine co-ingestion on post-exercise muscle glycogen synthesis rates when optimal amounts of carbohydrate are ingested. METHODS:: 14 male cyclists were studied on 3 different test days. Each test day started with a glycogen depleting exercise session. This was followed by a 6 h recovery period, during which subjects received 1.2 g.kg.h carbohydrate (CHO), or the same amount of carbohydrate with 0.3 g.kg.h of a protein plus leucine mixture (CHO+PRO) or 1.7 mg.kg.h caffeine (CHO+CAF). All drinks were enriched with ]U-C6[ labelled glucose to assess potential differences in the appearance rate of ingested glucose from the gut. Muscle biopsies were collected immediately after cessation of exercise and after 6 h of post-exercise recovery. RESULTS:: The plasma insulin response was higher in CHO+PRO compared with CHO and CHO+CAF (P<0.01). Plasma glucose responses and glucose appearance rates did not differ between experiments. Muscle glycogen synthesis rates averaged 31+/-4, 34+/-4 and 31+/-4 mmol.kg dw.h in CHO, CHO+PRO and CHO+CAF, respectively (P=NS). In accordance, histochemical analyses did not show any differences between net changes in type I and type II muscle fiber glycogen content between experiments. CONCLUSION:: Co-ingestion of protein or caffeine does not further accelerate post-exercise muscle glycogen synthesis when ample amounts of carbohydrate (1.2 g.kg.h) are ingested.