Carbohydrate metabolism during exercise in females: Effect of reduced fat availability

This study examined the effect of reduced plasma free fatty acid (FFA) avai lability on carbohydrate metabolism during exercise. Six untrained women cy cled for 60 minutes at approximately 58% of maximum oxygen uptake after ing estion of a placebo (CON) or nicotinic acid (NA), 30 minutes before exercis e (7.4 +/- 0.5 mg.kg(-1) body weight), and at 0 minutes (3.7 +/- 0.3 mg.kg( -1)) and 30 minutes (3.7 +/- 0.3 mg.kg(-1)) of exercise. Glucose kinetics w ere measured using a primed, continuous infusion of [6,6-H-2] glucose. Plas ma FFA (CON, 0.86 +/- 0.12; NA, 0.21 +/- 0.11 mmol.L-1 at 60 minutes, P < . 05) and glycerol (CON, 0.34 <plus/minus> 0.05; NA, 0.10 +/- 0.04 mmol.L-1 a t 60 minutes, P < .05) were suppressed throughout exercise. Mean respirator y exchange ratio (RER) during exercise was higher (P < .05) in NA (0.89 +/- 0.02) than CON (0.83 +/- 0.02). Plasma glucose and glucose production were similar between trials. Total glucose uptake during exercise was greater ( P < .05) in NA (1,876 <plus/minus> 161 mu mol/kg(-1)) than in CON (1,525 +/ - 107 mu mol.kg(-1)). Total fat oxidation was reduced (P < .05) by approxim ately 32% during exercise in NA. Total carbohydrate oxidized was approximat ely 42% greater (P < .05) in NA (412 +/- 40 mmol) than CON (290 +/- 37 mmol ), of which, approximately 16% (20 +/- 10 mmol) could be attributed to gluc ose. Plasma insulin and glucagon were similar between trials. Catecholamine s were higher (P < .05) during exercise in NA. In summary, during prolonged moderate exercise in untrained women, reduced FFA availability results in a compensatory increase in carbohydrate oxidation, which appears to be due predominantly to an increase in glycogen utilization, although there was a small, but significant, increase in whole body glucose uptake. Copyright <( c)> 2001 by W.B. Saunders Company.