Fat oxidation, hormonal and plasma metabolite kinetics during a submaximal incremental test in lean and obese adults

Abstract

This study aimed to compare fat oxidation, hormonal and plasma metabolite kinetics during exercise in lean (L) and obese (O) men. Sixteen L and 16 O men [Body Mass Index (BMI): 22.9 ± 0.3 and 39.0 ± 1.4 kg · m(-2)] performed a submaximal incremental test (Incr) on a cycle-ergometer. Fat oxidation rates (FORs) were determined using indirect calorimetry. A sinusoidal model, including 3 independent variables (dilatation, symmetry, translation), was used to describe fat oxidation kinetics and determine the intensity (Fat(max)) eliciting maximal fat oxidation. Blood samples were drawn for the hormonal and plasma metabolite determination at each step of Incr. FORs (mg · FFM(-1) · min(-1)) were significantly higher from 20 to 30% of peak oxygen uptake (VO2peak) in O than in L and from 65 to 85% VO2peak in L than in O (p ≤ 0.05). FORs were similar in O and in L from 35 to 60% VO2peak. Fat max was 17% significantly lower in O than in L (p<0.01). Fat oxidation kinetics were characterized by similar translation, significantly lower dilatation and left-shift symmetry in O compared with L (p<0.05). During whole exercise, a blunted lipolysis was found in O [lower glycerol/fat mass (FM) in O than in L (p ≤ 0.001)], likely associated with higher insulin concentrations in O than in L (p<0.01). Non-esterified fatty acids (NEFA) were significantly higher in O compared with L (p<0.05). Despite the blunted lipolysis, O presented higher NEFA availability, likely due to larger amounts of FM. Therefore, a lower Fat(max), a left-shifted and less dilated curve and a lower reliance on fat oxidation at high exercise intensities suggest that the difference in the fat oxidation kinetics is likely linked to impaired muscular capacity to oxidize NEFA in O. These results may have important implications for the appropriate exercise intensity prescription in training programs designed to optimize fat oxidation in O.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Mean respiratory exchange ratio (RER) values represented as a function of exercise intensity [absolute power output (A) and % of peak oxygen uptake () (B)] determined during the submaximal incremental test in lean (L: blue, n = 16) and obese (O: red, n = 16) individuals.

Values are the means±SE. * p≤0.05 for differences with lean; † p≤0.05 for significant group interaction effect.

Figure 2. Mean whole-body fat oxidation kinetics in absolute [g.min−1 (A and B) and mg.FFM−1.min−1 (C)] and relative [% of maximal fat oxidation (MFO) (D)] values determined with the sinusoidal (SIN) model and during the submaximal incremental test in lean (L: blue, n = 16) and obese (O: red, n = 16) individuals.

Values are the means±SE. : peak oxygen uptake. * p≤0.05 for differences with lean; † p≤0.05 for significant group interaction effect.

Figure 3. Mean non-esterified fatty acid (NEFA) concentrations (A) and mean glycerol concentrations (B) divided by fat mass (C) during the submaximal incremental test in lean (L: blue, n = 16) and obese (O: red, n = 14) individuals.

Values are the means±SE. PPO: peak power output. * p≤0.05 for differences with lean; † p≤0.05 for significant group interaction effect.

Figure 4. Mean epinephrine (A), norepinephrine (B), atrial natriuretic peptide (C) and insulin (D) concentrations during the submaximal incremental test in lean (L: blue, n = 16) and obese (O: red, n = 14) individuals.

Values are the means±SE. PPO: peak power output. * p≤0.05 for differences with lean; † p≤0.05 for significant group interaction effect.

Figure 5. Mean glycerol concentrations (A) divided by fat mass (B), mean non-esterified fatty acid (NEFA) concentrations (C) and whole-body fat oxidation kinetics in relative (D) [% of maximal fat oxidation (MFO)] values determined with the sinusoidal (SIN) model during the submaximal incremental test in lean (L: dark and light blue) and obese (O: dark and light red) individuals.

In the sub-groups matched for aerobic fitness, O present similar MFO (O: 6.3±0.6; L: 5.5±0.4 mg.FFM−1.min−1), left-shifted (O: 0.9±0.1; L: 1.2±0.1 for symmetry; p<0.05) and less dilated (O: –0.1±0.1; L: 0.3±0.1 for dilatation; p<0.05) curve, lower Fatmax (O: 46.5±5.0; L: 55.8±2.6 %) and lower Fatmax zone (O: 25.8±2.3; L: 30.1±1.2 %) although non-significant as a consequence of the small sample size. Values are the means±SE. PPO: peak power output. * p≤0.05 for differences between sub-groups; † p≤0.05 for significant group interaction effect between sub-groups; $ for significant group effect between sub-groups [2-way repeated-measures mixed design ANOVA (exercise intensity x group) followed by contrasts].