This 12-week exercise intervention study assessed changes in cardiorespiratory fitness (CRF), energy expenditure (EE) and substrate utilization at rest and during exercise in obese, black South African (SA) women and explored associations with changes in body composition. Black SA women (BMI 30 to 40 kg/m, 20 to 35 y) were randomized into control (CTL, n = 15, maintaining usual activity) or exercise (EXE, n = 20; 12 weeks, 4 d/wk, 40 to 60 min/d at > 70%HRpeak) groups. Pre- and post-intervention testing included: VO2peak, resting and steady state (50% VO2peak) EE and respiratory exchange and body composition (dual-energy x-ray absorptiometry (DXA)). Dietary intake (4d) and daily step-count (ActivPAL) was collected at pre-testing, 4, 8 and 12 weeks. EXE increased VO2peak (24.9 +/- 2.4 to 27.6 +/- 3.4 mg/min/kg; p < 0.001) and steady state fat oxidation rates (7.5 +/- 2.5 to 9.0 +/- 2.7 mg/min/kg fat-free soft tissue mass; p = 0.003) (same relative exercise intensity). CTL remained unchanged (p > 0.05). EXE reduced proportional gynoid fat mass (%total fat mass, p = 0.002). Baseline resting carbohydrate oxidation rates (p = 0.036) and steady state fat oxidation rates (p = 0.021) explained 60.6% of the variability in gynoid FM (p < 0.001) in EXE. This 12-week exercise intervention improved CRF and steady state fat oxidation rates. Greater reliance on fat oxidation at baseline promoted proportional reductions in gynoid, not visceral, FM in response to exercise training.

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