Basal fat oxidation decreases with age. In obesity, it is not known whether this age-related process occurs independently of changes in body composition and insulin sensitivity. Therefore, body composition, resting energy expenditure, basal substrate oxidation, and maximal oxygen consumption (VO(2)max) were measured in 10 older (age, 60 +/- 4 years; mean +/- SEM) and 10 younger (age, 35 +/- 4 years) body mass index-matched, obese, normal glucose-tolerant individuals. Fasting blood samples were also collected. Older subjects had slightly elevated fat mass (32.2 +/- 7.1 vs 36.5 +/- 6.7 kg, P = .16); however, waist circumference was not different between groups (104.3 +/- 10.3 vs 102.1 +/- 12.6 cm, P = .65). Basal fat oxidation was 22% lower (1.42 +/- 0.14 vs 1.17 +/- 0.22 mg/kg fat-free mass per minute, P = .03) in older subjects. The VO(2)max was also decreased in older individuals (44.6 +/- 7.1 vs 38.3 +/- 6.0 mL/kg fat-free mass per minute, P = .03); but insulin sensitivity, lipemia, and leptinemia were not different between groups (P > .05). Fat oxidation was most related to age (r = -0.61, P = .003) and VO(2)max (r = 0.52, P = .01). These data suggest that aging per se is responsible for reduced basal fat oxidation and maximal oxidative capacity in older obese individuals, independent of changes in insulin resistance, body mass, and abdominal fat. This indicates that age, in addition to obesity, is an independent risk factor for weight gain and for the metabolic complications of elevated body fat.