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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.

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 [1‐¹³C]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.

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 …