The Acute Resting Metabolic Response to Energy Deficit caused by Increased Expenditure vs. Caloric Restriction.

Decreases in resting metabolic rate due to energy deficit may hinder weight loss. Caloric restriction has been shown to decrease metabolic rate, but it is not clear if energy deficit due to increased expenditure has a similar effect. PURPOSE: To determine the acute changes in resting metabolic rate (RMR) due to exercise-induced energy deficit, as compared to caloric restriction. METHODS: Nine moderately active males (Age = 20.6±1.99; BMI = 22.5±2.95) completed two 24-hr RMR trails: RMR-Diet (caloric intake = 75% of RMR) and RMR-Exercise (caloric intake = 100% of RMR + energy expenditure = 25% of RMR). Overall energy deficit of each trial was 25% of RMR. The order of the trials was randomized and at least 7 days apart. For each study, the subjects arrived fasted at 7am. RMR was measured at 8am. RMR measurement consisted of 45 min of indirect calorimetry (PARVO TrueOne 2400 with dilution hood). The first 15 min of data were excluded. The final 30 min of data were used to determine average RMR kcal/day. The standard diet (45-55% CHO, 25-35% FAT and 10-15% PRO) was separated into pre-packaged meals consumed at 8:30am, 12:30pm, 4:30pm and 7:30pm. For the RMR-Exercise trial, a moderate intensity (40-59% of HRR) exercise bout was performed at 11am. The exercise bout consisted of cycling until the required kcals were expended (Ex time = 53.8±7.49 min). Workload was adjusted as needed to maintain heart rate. Indirect calorimetry (PARVO TrueOne 2400 with mouthpiece) was used to determine kcals expended during exercise. The subjects left the facility at 5pm and were instructed to limit their activity to resting/light activity (sitting, walking short distances, bathing). Activity was monitored using an accelerometer. The subjects returned to the facility at 7am the next day for the 8am 24-hr RMR. Comparisons were made using two-way ANOVA with repeated measures. RESULTS: 24-hr RMR was significantly reduced by 6.5% due to caloric restriction (1749±220 vs 1634±178 kcal/day, p=0.003). There was no change in 24-hr RMR due to exercise-induced energy deficit (1621±229 vs 1645±206 kcal/day, p=0.417). CONCLUSION: Energy deficit due to caloric restriction results in a significant reduction in RMR at 24 hours, while energy deficit due to increased energy expenditure does not cause a similar decrease in RMR. Supported by a University Research Council grant