Fat utilization during exercise: adaptation to a fat-rich diet increases utilization of plasma fatty acids and very low density lipoprotein-triacylglycerol in humans.

1. This study was carried out to test the hypothesis that the greater fat oxidation observed during exercise after adaptation to a high-fat diet is due to an increased uptake of fat originating from the bloodstream. 2. Of 13 male untrained subjects, seven consumed a fat-rich diet (62 % fat, 21 % carbohydrate) and six consumed a carbohydrate-rich diet (20 % fat, 65 % carbohydrate). After 7 weeks of training and diet, 60 min of bicycle exercise was performed at 68 +/- 1 % of maximum oxygen uptake. During exercise [1-(13)C]palmitate was infused, arterial and venous femoral blood samples were collected, and blood flow was determined by the thermodilution technique. Muscle biopsy samples were taken from the vastus lateralis muscle before and after exercise. 3. During exercise, the respiratory exchange ratio was significantly lower in subjects consuming the fat-rich diet (0.86 +/- 0.01, mean +/- S.E.M.) than in those consuming the carbohydrate-rich diet (0.93 +/- 0.02). The leg fatty acid (FA) uptake (183 +/- 37 vs. 105 +/- 28 micromol min(-1)) and very low density lipoprotein-triacylglycerol (VLDL-TG) uptake (132 +/- 26 vs. 16 +/- 21 micromol min(-1)) were both higher (each P < 0.05) in the subjects consuming the fat-rich diet. Whole-body plasma FA oxidation (determined by comparison of (13)CO(2) production and blood palmitate labelling) was 55-65 % of total lipid oxidation, and was higher after the fat-rich diet than after the carbohydrate-rich diet (13.5 +/- 1.2 vs. 8.9 +/- 1.1 micromol min(-1) kg(-1); P < 0.05). Muscle glycogen breakdown was significantly lower in the subjects taking the fat-rich diet than those taking the carbohydrate-rich diet (2.6 +/- 0.5 vs. 4.8 +/- 0.5 mmol (kg dry weight)(-1) min(-1), respectively; P < 0.05), whereas leg glucose uptake was similar (1.07 +/- 0.13 vs. 1.15 +/- 0.13 mmol min(-1)). 4. In conclusion, plasma VLDL-TG appears to be an important substrate source during aerobic exercise, and in combination with the higher plasma FA uptake it accounts for the increased fat oxidation observed during exercise after fat diet adaptation. The decreased carbohydrate oxidation was apparently due to muscle glycogen sparing and not to diminished plasma glucose uptake.