EFFECT OF PHYSICAL EXERCISE ON GLYCOGEN TURNOVER AND NET SUBSTRATE UTILIZATION ACCORDING TO THE NUTRITIONAL STATE

To determine the metabolic effects of a single bout of exercise perfor med after a meal or in the fasting state, nine healthy subjects were s tudied over two 8-h periods during which net substrate oxidation was m onitored by indirect calorimetry. On one occasion, exercise was perfor med 90 min after ingestion of a meal labeled with [U-C-13]glucose [pro tocol meal-exercise (M-E)]. On the second occasion, exercise was perfo rmed after an overnight fast and was followed 30 min later by ingestio n of an identical meal [protocol exercise-meal (E-M)]. Energy balances were similar in both protocols, but carbohydrate balance was positive (42.2 +/- 5.1 g), and lipid balance was negative (-11.1 +/- 2.0) duri ng E-M, whereas they were nearly even during M-E. Total glycogen synth esis was calculated as carbohydrate intake minus oxidation of exogenou s C-13-labeled carbohydrate (calculated from (CO2)-C-13 production). T otal glycogen synthesis was increased by 90% (from 47.6 +/- 3.8 to 90. 7 +/- 5.4 g, P < 0.0001) during E-M vs. M-E. Endogenous glycogen break down was calculated as net carbohydrate oxidation minus oxidation of e xogenous carbohydrate and was increased by 44% (from 35.8 +/- 5.6 to 5 1.7 +/- 6.6 g, P < 0.004) during E-M. It is concluded that exercise pe rformed in the fasting state stimulates glycogen turnover and fat oxid ation.