Twenty-four hour total and dietary fat oxidation in lean, obese and reduced-obese adults with and without a bout of exercise

Abstract

Background: It has been hypothesized that obese and reduced-obese individuals have decreased oxidative capacity, which contributes to weight gain and regain. Recent data have challenged this concept.

Objective: To determine (1) whether total and dietary fat oxidation are decreased in obese and reduced-obese adults compared to lean but increase in response to an acute exercise bout and (2) whether regular physical activity attenuates these metabolic alterations.

Design: We measured 24-hr total (whole-room calorimetry) and dietary fat (14C-oleate) oxidation in Sedentary Lean (BMI = 21.5±1.6; n = 10), Sedentary Obese (BMI = 33.6±2.5; n = 9), Sedentary Reduced-Obese (RED-SED; BMI = 26.9±3.7; n = 7) and in Physically Active Reduced-Obese (RED-EX; BMI = 27.3±2.8; n = 12) men and women with or without an acute exercise bout where energy expended during exercise was not replaced.

Results: Although Red-SED and Red-EX had a similar level of fatness, aerobic capacity and metabolic profiles were better in Red-EX only compared to Obese subjects. No significant between-group differences were seen in 24-hr respiratory quotient (RQ, Lean: 0.831±0.044, Obese: 0.852±0.023, Red-SED: 0.864±0.037, Red-EX: 0.842±0.039), total and dietary fat oxidation. A single bout of exercise increased total (+27.8%, p<0.0001) and dietary (+6.6%, p = 0.048) fat oxidation across groups. Although exercise did not impact RQ during the day, it decreased RQ during sleep (p = 0.01) in all groups. Red-EX oxidized more fat overnight than Red-SED subjects under both resting (p = 0.036) and negative energy balance (p = 0.003) conditions, even after adjustment for fat-free mass.

Conclusion: Obese and reduced-obese individuals oxidize as much fat as lean both under eucaloric and negative energy balance conditions, which does not support the hypothesis of reduced oxidative capacity in these groups. Reduced-obese individuals who exercise regularly have markers of metabolic health similar to those seen in lean adults. Both the acute and chronic effects of exercise were primarily observed at night suggesting an important role of sleep in the regulation of lipid metabolism.

Figure 1. Protocol of the tests in the room calorimeter.

Figure 2. Cumulative responses of plasma triglycerides (top) and free fatty acids (FFA; bottom) concentrations expressed as the area under the curve (AUC) over 24 h in lean (n = 10), obese (n = 9), sedentary reduced-obese (Red-SED, n = 7) and exercising reduced-obese (Red-Ex, n = 12).

Statistics are presented on the Figure.

Figure 3. Twenty four hour total fat (top), carbohydrate (middle) and protein (bottom) oxidation in lean (n = 10), obese (n = 9), sedentary reduced-obese (Red-SED, n = 7) and exercising reduced-obese (Red-Ex, n = 12) after adjustment for differences in fat-free mass (FFM).

Statistics are presented on the Figure.

Figure 4. Respiratory quotient during waking (top) and sleeping (middle) periods and total fat oxidation during sleep in lean (n = 10), obese (n = 9), sedentary reduced-obese (Red-SED, n = 7) and exercising reduced-obese (Red-Ex, n = 12).

Statistics are presented on the Figure.

Figure 5. Dietary fat oxidation, i.e. the amount of fat oxidized over 24-hr coming from the breakfast meal only before (top) and after adjustment for fat-free mass (FFM) (middle), and the ratio of total and dietary fat oxidation (bottom) in lean (n = 10), obese (n = 9), sedentary reduced-obese (Red-SED, n = 7) and exercising reduced-obese (Red-Ex, n = 12).

Statistics are presented on the Figure.