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 (Fatmax) 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 () in O than in L and from 65 to 85% in L than in O (p≤0.05). FORs were similar in O and in L from 35 to 60% . Fatmax 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 Fatmax, 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.
Highlights
Obesity is associated with a variety of health-related risks, such as hypertension and type 2 diabetes, all of which may center around insulin resistance [1]
body mass index (BMI), fat mass (FM) and fat free mass (FFM) were significantly higher in O compared with L (Table 1)
Fasting glucose was similar in O and L, whereas fasting insulin and homeostasis assessment of insulin resistance (HOMA-IR) were significantly higher in O compared with L (Table 1)
Summary
Obesity is associated with a variety of health-related risks, such as hypertension and type 2 diabetes, all of which may center around insulin resistance [1]. Previous studies [7,10] principally focused on the muscular factor with less emphasis on the extramuscular factors (hormones and plasma metabolites) that regulate fat metabolism during exercise The latter may be altered by differences in substrate availabilities and lipolytic hormones between lean and obese individuals, especially at high exercise intensities [12,13,14,15]. This may induce a narrowing of the wholebody fat oxidation kinetics and a lower Fatmax zone (i.e., the range of exercise intensities with fat oxidation rates within 10% of MFO [16]). This implies that the ‘individualization concept of training’ must be taken into account for weight management training programs, especially in metabolic disease [17]
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