Maximal Fat Oxidation

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Maximal fat oxidation (FOXMax) is the exercise intensity at which the highest caloric expenditure is attributed to fat metabolism and appears to be influenced by aerobic capacity and obesity. Determining, directly or indirectly, FOXMax holds relevance with respect to optimal intensity for exercise prescription for weight loss and maintenance. PURPOSE: To determine if FOXMax is sex-dependent and its agreement with the anaerobic threshold (AT), another intensity at which exercise can be prescribed to improve aerobic conditioning. HYPOTHESIS: We hypothesised that: 1) FOXMax in obese men and women is similar; 2) FOXMax is not concordant with AT; and 3) heart rate (HR) at FOXMax relative to real peak HR (HRpk) is equivalent to that relative to HRpk predicted from a recognized prediction formula (208-0.7*age). METHODS: Forty obese adults (BMI: 40.3±1.1 kg.m-2), women (OW, n=20) and men (OM, n=20), aged 20 to 45 years, underwent a cardiopulmonary exercise test (CPX). Oxygen uptake (VO2), carbon dioxide production (VCO2), respiratory exchange ratio (RER) and heart rate (HR) were measured at FOXMax, AT and at the peak of the test (pk). Fat oxidation rates were calculated (FOX-R=1.67*VO2-1.67*VCO2) and the highest value was set as the FOXMax. AT was defined according to the V-Slope method. T-test, MANCOVA and partial correlations were applied. α level was set at 0.05. RESULTS: VO2 at AT and FOXMax were identified at 77.4±1.4 and 58.6±1.2% of VO2pk and HR at 80.5±1.2% and 68.6±1.0% of HRpk, respectively. Although VO2 and HR at FOXMax differed from AT for both groups (p≤0.01), sex-adjusted positive correlations between them were observed (FOXMaxvs AT for VO2 [r=0.78] and for HR [r=0.61], <0.01). Sex-based comparisons showed that OW had higher FOX-R, and at the AT higher HR, VO2, percent of HRpk and VO2pk (p<0.05). However, OM had a higher percent of VO2pk at FOXMax. Similar HRpk, HR at FOXMax and percent of HRpk were observed in both groups. HR at FOXMax relative to the predicted HRpk from the formula 208-0.7*age were 66±1% in OM and 67±1% in OW. CONCLUSIONS: Sex differences exist at AT and FOXMax for VO2. Meanwhile, chronotropic responses (HR) at FOXMax are equal for both and can be inferred from a usual formula when CPX is unavailable. FUNDING: Fundação de Amparo à Pesquisa do Estado de São Paulo (2009/01842-0, 2013/15681-3), SP, Brazil.