The effects of dietary carbohydrate and fat and fatty acid availability on muscle glycogen and triglyceride and substrate utilization during and after exercise

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Zderic, Theodore William

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These studies determined the effects of altering dietary fat and carbohydrate on intramuscular substrate concentrations and subsequent substrate utilization during moderate intensity exercise, as well as on plasma triglyceride (TG) concentration. A two-day high fat / low-carbohydrate diet (HF/LC; 60% fat, 25% carbohydrate) elevated intramuscular TG (IMTG) concentration by 36% (36.8±4.8 vs. 49.9±3.4 mmol/kg dry weight, p<0.05) and decreased muscle glycogen concentration to 49% (348±36 vs. 716±68 mmol/kg dry weight, p<0.05) of normal (i.e.; with a diet of 25% fat; 65% carbohydrate). HF/LC produced a 71% increase in fat oxidation (32.4±2.2 vs. 18.8±2.4 µmol/kg/min), a 79% increase in whole body lipolysis, and a 38% decrease in muscle glycogen oxidation (all p<0.05). Elevated plasma FFA concentration was not essential for this diet-induced increase in fat oxidation because the adipose lipolysis inhibitor, Acipimox®, did not effect the marked increase in fat oxidation despite the severe suppression of plasma FFA throughout exercise. Furthermore, the increase in fat oxidation was associated with a tendency for significant IMTG breakdown during exercise (8.8±3.3 mmol/kg dw/60min, p=0.06) which fully compensated for the decrease in muscle glycogen oxidation as plasma glucose uptake (Rd glucose) was unaffected by the HF/LC diet. However, the diet induced increase in fat oxidation, presumably IMTG oxidation, was attenuated by non-selective β-adrenergic blockade (80 mg propanolol), as low muscle glycogen oxidation was also compensated by a 15% increase in Rd glucose (p<0.05). Interestingly, the addition of ~100 g fat / d to a diet containing only 2% fat (7 g/d) did not effect IMTG concentration or fat oxidation. Furthermore, HF/LC also resulted in a general lowering of plasma TG concentration at rest in the fasted and postprandial states, and during exercise (all p<0.05). In summary, low muscle glycogen concentration associated with a HF/LC diet is primarily compensated by elevated IMTG concentration and oxidation during exercise whereas increases in plasma glucose uptake occur only when the increase in fat oxidation is limited (e.g.; βblockade). It seems that a high fat diet increases IMTG concentration and oxidation which prevents excessive Rd glucose and hypoglycemia yet this requires a decrease in dietary carbohydrate and/or muscle glycogen concentration.