Exercise increases the proportion of fat utilization during short-term consumption of a high-fat diet.

Background: Increases in energy substrate oxidation occur at different rates after an increase in either fat or carbohydrate intake. Adaptations to increased fat intake are relatively slow and are influenced by activity level.

Objective: We tested the hypothesis that increased levels of daily activity, as influenced by added exercise, would have a graded effect on the rate of compensatory adjustment to a short-term high-fat diet.

Design: Daily total energy expenditure and macronutrient oxidation were measured at 3 physical activity levels (PALs) by using a whole-room indirect calorimeter in 10 adult women as they transitioned from a 1-d low-fat (30% of energy) control diet to a 4-d high-fat (50% of energy) diet. The 3 PALs (1.4, 1.6, and 1.8) were provided daily by increases in bicycle ergometer exercise time.

Results: An increase in physical activity led to a greater increase in the nonprotein respiratory exchange ratio (-0.047 +/- 0.02, -0.064 +/- 0.02, and -0.071 +/- 0.02; P < 0.0001) and 24-h fat oxidation (113 +/- 24, 125 +/- 19, and 147 +/- 20 g/d; P < 0.0001) for PALs of 1.4, 1.6, and 1.8, respectively, after the transition from the low-fat control diet to the high-fat diet. Random-effects analysis found a significant (P = 0.003) relation between PAL and the compensatory fat oxidation response to a high-fat diet.

Conclusions: Amounts of exercise consistent with the Institute of Medicine's recommendations reduce the time required to match fat oxidation to a change in the percentage of fat in the diet. Because short-term consumption of high-fat diets is thought to contribute to excess fat accumulation, regular exercise should be protective and should help maintain a healthy body composition.