We examined the effect of short-term high fat diet on blood properties and energy metabolism during prolonged exercise. Six well-trained subjects participated in two trials during which they consumed either their normal diets (~25% calories as fat) or an isoenergetic high fat diet (~60% calories as fat) for 5 days followed by a normal diet on day 6. On day 7, performance testing (60min steady-state cycling at 60% VO₂max + 7.5km time trial) was undertaken. There were no significant differences between the normal and the high fat diet on body weight, body fat and fasting plasma lipids. The different dietary treatment also did not affect the blood glucose, serum insulin and FFA levels during the steady-state exercise. Respiratory exchange ratio (R) continuously decreased during the steady-state exercise and R was lower in the high fat diet trial than in the normal diet trial. Accordingly, the rate of fat oxidation was higher during steady-state exercise in the fat diet than in the normal diet (19.1 ± 9.9% and 13.8 ± 5.7%, respectively). There was no significant difference in the time to complete 7.5km of work between the normal diet and the fat diet (705±111sec and 689±95sec, respectively). These results suggest that shortterm high fat diet may increase fat oxidation during prolonged exercise without adverse changes of plasma lipids. However there was no evidence of benefit of fat adaptation to the exercise performance.

DESCENTE SPORTS SCIENCE Vol.25/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE