Effects Of Exercise With And Without Energy Replacement On Substrate Utilization In The Fasting State

Objective: The present study aimed to examine the interactive effect of exercise and energy balance on energy expenditure and substrate utilization. Method: Seven men and 7 women underwent three 2-day experimental protocols in a random order. Each protocol consisted of no exercise (NE), exercise only (EO), or exercise with a matched energy replacement (ER) on day 1 followed by metabolic testing that occurred after a 12-hour overnight fasting on day 2. Both EO and ER involved treadmill running at 60% maximal oxygen uptake (VO(2)max) that induced an energy expenditure of similar to 500 kcal. The replacement meal used in ER contained similar to 500 kcal made up of 45% carbohydrate, 30% fat, and 25% protein. During metabolic testing, oxygen uptake (VO2), heart rate (HR), respiratory exchange ratio (RER), and rates of carbohydrate (COX) and fat oxidation (FOX) were determined in three successive 15-minute periods including rest and exercise at 50% and 70% VO(2)max. Results: No differences in VO2 and HR were found at rest among NE, EO, and ER. However, RER was lower in EO than NE (0.840 +/- 0.014 vs 0.889 +/- 0.012, p < 0.05), COX (g center dot min(-1)) was lower in ER than NE (0.144 +/- 0.016 vs 0.197 +/- 0.019, p < 0.05), and FOX (g center dot min(-1)) was higher in EO or ER than NE (0.054 +/- 0.010 or 0.057 +/- 0.009 vs 0.034 +/- 0.007, p < 0.05). No treatment effects were observed for all variables at either intensity. Conclusions: This study demonstrates that an exercise of moderate intensity can increase resting fat oxidation even when the exercise-induced energy expenditure is balanced by energy intake. This finding suggests that muscle action is vital in augmenting fat utilization.