There is great controversy regarding the methods in attenuating the epidemic of being overweight and obese among our population. For years, low-fat diets have been prescribed as the best approach to solving this problem, however, low-carbohydrate diets have become increasingly popular despite the widespread opinion that they result in large reductions in muscle tissue and function. The purpose of this study was to compare the effects of a Step-I and carbohydrate-restricted (ketogenic) diet on changes in body mass (BM), fat mass (FM), lean body mass (LBM), and resting metabolic rate (RMR). Eight overweight men (>25% body fat) consumed Step-I (<30% fat) and ketogenic (<10% carbohydrate) diets for 6 weeks in a randomized cross-over design. Body composition (dual energy X-ray absorptiometry) and RMR were examined before and after each diet. Energy intake was significantly reduced during the Step-I (1506 kcal/day) and ketogenic (1766 kcaL'day) diets compared to habitual dietary intake (2443 kcal/day). The following changes in body composition were observed during the Step-I and ketogenic diets, respectively: BM -4.1 and -5.9 kg, FM -3.5 and -4.4kg, LBM +0.1 and -1.7kg. The changes in RMR for the Step-I and ketogenic diets respectively were -155 and -26 kcal/day, relative RMR -0.68 and +0.81 kcal/kg BM. Although changes in BM, LBM, and FM tended to be greater during the ketogenic diet and changes in both absolute and relative RMR tended to be greater during the Step-I diet, no significant differences were noted between diets. These data indicate that hypoenergetic diets greatly differing in macronutrient distribution result in similar changes in body composition and that very low-carbohydrate diets do not result in significant losses in LBM and metabolic rate. / School of Physical Education