Carbohydrate feeding and glycogen synthesis during exercise in man

In 7 male cyclists glycogen synthesis during exercise and rest was studied. Each subject did two exercise trials (A and B), in random order. In both trials, after determining the maximal workload (Wmax), intermittent exercise was given to exhaustion. After the exhaustive exercise and taking a muscle biopsy the subjects either exercised at 40% Wmax for 3 h (trial A) or rested for 3 h (trial B), during which they consumed approximately 2 l of a 25% malto-dextrine drink in both trials. After 3 h rest (trial A) or 3 h of mild exercise (trial B) a second muscle biopsy was taken for total glycogen and histochemistry (ATPase and PAS). Blood glucose and insulin levels were elevated during the first 2 h of exercise (p less than 0.05). Glycogen depletion was most pronounced in type I and to a less extent in type IIA fibers. In trial A muscle glycogen increased from 136 +/- 66 to 199 +/- 71 mmol/kg DW, and in trial B from 145 +/- 56 to 257 +/- 79 mmol/kg DW. During exercise glycogen repletion was restricted to type IIA and IIB fibers, whereas during rest glycogen synthesis occurred both in type I and type II fibers. The present study demonstrates that oral carbohydrate administered during exercise may not only provide substrate for energy metabolism, but can also be utilized for glycogen synthesis in the non-active muscle fibers.