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The rate of fat oxidation at rest decreases with age in women. The mechanisms for this decrease are not clear. Theoretically, a decrease in the availability of fatty acids could explain the decline in fat oxidation. In consequence, the in vivo rate of production of fatty acids as a proxy for lipolysis was measured in 21 healthy women. Eleven of the volunteers were elderly (> 65 years) and 10 were young (< 24 years), and all were characterized for body composition. The nonadjusted rate of delivery of fatty acids into the systemic circulation was similar among elderly and young individuals (609 +/- 80.3 v 597 +/- 69.9 mumol/min, respectively, P > .1). When lipolysis was adjusted for the differences in fat-free mass using analysis of covariance (ANCOVA), rates were slightly increased in the elderly group (626 +/- 80 mumol/min) and decreased in the young group (578 +/- 84 mumol/min), but remained nonstatistically significant. It is concluded that mechanisms other than lipolysis must explain the decrease of fat oxidation in aging women, i.e., a decrease in the capacity of muscle to oxidize fat and/or a decrease in its capacity for transport of long-chain fatty acids.

Aging is characterized by deleterious changes in body composition and in fat distribution. The mechanisms that determine the aging-associated changes in body composition are not well defined, but the evidence suggests that the loss of fat-free mass is at least partially attributable to physical inactivity. The increase in fat mass may be the result of alterations in fatty acid metabolism. Indeed, fat oxidation is decreased in elderly individuals in several physiological conditions: a) at rest, b) during exercise, and c) in response to meal ingestion (after weight loss). These defects are related in part to loss of fat-free mass, but may also be the consequence of estrogen loss (in women) and/or a decrease in the intrinsic capacity of muscle for fat oxidation, and are amenable to partial correction by exercise training. Special emphasis should be placed in future studies upon the role of steroid hormone in the regulation of fatty acid metabolism in elderly individuals (especially women), as well as therapeutic interventions that may increase the quantity of the fat-free mass and/or fat oxidation.

The present study tested the hypothesis that a decrease in basal fat oxidation in aging women is related to a loss of fat-free mass. Thirty-two nonsmoking women with a wide range of age (18-73 yr) were characterized for body composition (underwater weight), maximal aerobic capacity, and basal fat oxidation (indirect calorimetry). Results showed that fat oxidation was negatively correlated with age (r2 = 0.17, P = 0.017) but was positively correlated with the fat-free mass (r2 = 0.48, P < 0.0001) and with the level of aerobic fitness (maximal aerobic capacity) (r2 = 0.22, P = 0.007). Unexpectedly, fat oxidation had no relationship with fat mass (r2 = 0.07, P = 0.136). Partial correlation analysis showed that the decline in fat-free mass, and not the age or maximal O2 consumption, was the best single predictor of the decline in basal fat oxidation. These results support the theory that a decrease in fat oxidation with advancing age in healthy women is associated with a decrease in the fat-free mass and not age per se. Interventions that increase or preserve the quantity of fat-free mass (e.g., exercise training) may enhance fat oxidation and thus lessen the age-associated adiposity in women.