Concurrent physical activity increases fat oxidation during the shift to ahigh-fat diet

Background: It takes several days to adapt to a high-fat diet. In an earlie r study, we observed a large degree of interindividual variation in the cap acity to adapt to a high-fat diet. We hypothesized that concurrent physical activity would accelerate fat oxidation during an isoenergetic high-fat di et. Objective: The objective of this study was to determine the effect of incre ased physical activity on the ability of young healthy men to increase fat oxidation during the shift to a high-fat diet. Design: Six young healthy men participated in a randomized, single-blind cr ossover study. The volunteers consumed a diet contributing 37% of energy as fat, 14% as protein, and 49% as carbohydrate for 4 d. Energy expenditure a nd macronutrient balance were then measured in a respiration chamber as the energy content of the isoenergetic diet was changed to 50% fat, 14% protei n, and 36% carbohydrate. Treadmill walking, as the physical activity, was u sed to increase total daily energy expenditure to 1.8 times the resting met abolic rate during 1 of 2 stays in the metabolic chamber. Total daily energ y expenditure was maintained at 1.4 times the resting metabolic rate for th e other stay. Results: Energy balance was not significantly different between the 2 condi tions. The 24-h respiratory quotient decreased more rapidly and to a greate r extent under conditions of increased energy expenditure. Further, there w as a decrease in the interindividual variability in the response of the res piratory quotient to a high-fat diet with increased energy expenditure (phy sical activity). Cumulative carbohydrate and protein balances were greater under conditions of increased physical activity. Conversely, cumulative fat balance was greater under sedentary conditions. Conclusion: Concurrent physical activity increases fat oxidation during the shift to a high-fat diet.