Aging is associated with declines in aerobic capacity, exercise tolerance, and functional endurance that lead to physical disability and loss of independence. Furthermore, the existing high prevalence of obesity in the elderly is greatly exacerbating these aging-related declines in function. To date, regular exercise is the only known therapy to consistently improve aerobic function, and perhaps delay the onset of disability. Although aerobic exercise training does benefit both aerobic capacity and endurance even in obese persons, some data show that the maximal efficacy of exercise for improving aerobic function is blunted by obesity. In addition, our preliminary data show a potential dose-response benefit of concomitant fat loss on exercise-induced improvements in aerobic function. Thus, combining an exercise intervention with caloric restriction resulting in fat loss may be more efficacious for improving aerobic function than exercise alone in obese elderly, a population at high risk for disability. The purpose of this study is to determine whether the amount of fat loss (achieved through controlled underfeeding) affects the magnitude of improvement in aerobic function (maximal aerobic capacity and endurance) in response to a standardized exercise training stimulus that follows current recommendations for older persons.