The rapidity of weight loss and the concommittant sense of achievement induced by protein diets, has led to their re-emergence as a popular means of weight reduction. The effect of prolonged use of these regimens on the preservation of total body nitrogen has not been established, primarily due to the restrictions imposed by currently available nitrogen balance studies. Furthermore, the effect of a hypocaloric carbohydrate diet on nitrogen equilibrium is controversial. The purpose of this study is to determine the effect of a whole protein diet on nitrogen balance, and to compare its effect with an equicaloric carbohydrate containing diet. Sixty obese subjects will consume either the protein or carbohydrate containing diets for a period of three months. This will be followed by a refeeding and weight maintenance phase for an additional three months. The integrated changes in nitrogen balance will be determined by a new, non-invasive, technique of prompt gamma neutron activation.