NONINVASIVE ASSESSMENT OF IN-VIVO GLYCOGEN KINETICS IN HUMANS - EFFECT OF INCREASED PHYSICAL-ACTIVITY ON GLYCOGEN BREAKDOWN AND SYNTHESIS

In vivo glycogen kinetics was estimated with the simultaneous use of i ndirect calorimetry and tracer technology in healthy humans during 24- h periods with low or moderate physical activity (1 or 3 exercise sess ions each day). Two C-13-carbohydrates meals were administered at 9.30 a.m. and 1.30 p.m., and one C-12- carbohydrates meal at 6.30 p.m. Net carbohydrate oxi dation (net CHO ox) was measured over a 24 h period by indirect calorimetry and oxidation of C-13-labelled carbohydrates ( C-13 CHO OX) was estimated from (CO2)-C-13 production. Glycogen breakd own, assessed for the period 8.15 a.m.-6.30 p.m. as the difference bet ween net CHO ox and C-13 CHO OX, was increased 1.6 times with three ex ercise sessions [123.3 (SEM 8.0) g] versus one session [77.9 (SEM 7.7) g, P< 0.0001]. Carbohydrate balances over 24 h were close to zero und er both conditions, indicating that glycogen breakdown was matched by an equivalent glycogen synthesis. It was concluded that simultaneous u se of indirect calorimetry and tracer technology may make possible the estimation of glycogen kinetics in humans. Moderate physical activity enhanced both glycogen breakdown and synthesis. This stimulation of g lycogen metabolism may therefore play a role in the enhanced insulin s ensitivity induced by physical exercise.