Metabolic Flexibility During Exercise In Overweight/obese Children Vs. Lean Children

Abstract

Metabolic flexibility (MF) is the ability of the body to alter its reliance on fat or carbohydrate for energy purposes in response to a stimulus. The inverse, metabolic inflexibility, has been associated with type II diabetes and obesity. Given the prevalence of these disorders, assessing and improving MF is important. However, MF during exercise, particularly with respect to fat metabolism, in children remains poorly understood. PURPOSE: This study examined MF with respect to fat metabolism during exercise in lean (n=11; 10.9±0.9 years) and overweight/obese (OW/OB; n=8; 10.4±1.2 years) children. It was hypothesized that MF with respect to fat metabolism during exercise would be impaired in the OW/OB group as indicated by reduced use of fat as an energy source. METHODS: Participants were grouped based on BMI percentiles for age and sex (Lean <85th percentile, OW/OB ≥85th percentile). On the experimental visit, participants completed two 20-minute exercise bouts separated by a 10-minute rest. Bout 1 consisted of 10 minutes at 50% VO2max and 10 minutes at 75% VO2max. Bout 2 consisted of 20 minutes at 50% VO2max. Absolute fat oxidation rate (FOR; [mg·min-1], FOR relative to body mass [mg·kg-1·min-1], FOR relative to fat-free mass [mg·kgFFM-1·min-1]), and proportional fat use (% Fat) were measured at 10 minutes of Bout 1 and at 5, 10, 15, and 20 minutes of Bout 2. RESULTS: There was a main effect for time for % Fat and for each expression of FOR, with fat oxidation values generally higher during the second bout, suggesting that exercise can be used to assess MF in children. Absolute FOR was higher in the OW/OB group (range: 121.6±57.6 to 213.7±45.7 mg·min-1) than in the Lean group (81.1±32.2 to 152.2±38.2 mg·min-1), however there were no main effects for group or interactions for % Fat (OW/OB: 29.0±14.4 to 51.1±8.5%; Lean: 23.9±10.9 to 41.8±6.5%), FOR relative to body mass (OW/OB: 2.4±1.3 to 4.1±0.7 mg·kg-1·min-1; Lean: 2.5±1.0 to 4.5±1.0 mg·kg-1·min-1), or FOR relative to FFM (OW/OB: 4.15±1.97 to 7.12±1.17 mg·kgFFM-1·min-1; Lean: 3.49±1.54 to 6.29±1.13 mg·kgFFM-1·min-1). CONCLUSION: OW/OB children in this age range do not display impaired MF with respect to fat metabolism during exercise. Future research should examine a broader range of children as well as the effects of different exercise characteristics (e.g. intensity) on MF.