Participants in this study will be randomly assigned to the Atkins diet (low-carbohydrate, unlimited fat and protein) or a conventional USDA diet (high-carbohydrate, low-fat). The study will evaluate the effects of each dietary approach on changes in: 1) weight and body composition; 2) metabolic and organ function; and 3) exercise tolerance. Each participant will be enrolled in the study for 2 years.