Nutrient partitioning can be deﬁned as the process by which the organism selects fuels for storage (including protein synthesis) or oxidation. Understanding the regulation of energy balance and nutrient partitioning can potentially facilitate the treatment of obesity. Although the factors that lead to an imbalance between energy/ fat intake and energy expenditure, and thus the development of obesity, remain incompletely understood, nutrient partitioning may be especially relevant to the development of obesity as it relates to the hypothesis of Flatt (1). The latter suggests that total food intake increases to meet carbohydrate needs. According to this theory, food intake is regulated, at least in part, to assure an adequate amount of carbohydrate. Consumption of a high-fat diet would require the intake of excess fat in order to satisfy carbohydrate needs and therefore lead to obesity under this scenario. The concept of a diet ‘‘relatively’’ deﬁcient in carbohydrate becomes important in that dysregulation of substrate partitioning could potentially aﬀect the body’s sense of what constitutes adequate carbohydrate intake. For example, if fat were preferentially shunted toward storage, more carbohydrate would be required to meet oxidative needs, thereby preventing suﬃcient repletion of glycogen stores. This process is proposed to generate signals that
stimulate appetite. Other examples where nutrient partitioning relates to obesity and body fat content include the stimulation of lean tissue synthesis at the expense of fat calories by androgens and growth hormone, and (presumably) the reverse of this process by deﬁciencies of these hormones.