GH acts directly on the liver, muscle, and adipose tissue to regulate energy metabolism (Fig. 40-18). It shifts metabolism to lipid use for energy, thereby conserving carbohydrates and proteins. GH is a protein anabolic hormone that increases cellular amino acid uptake and incorporation into protein, and it represses proteolysis. Consequently, it produces nitrogen retention (positive nitrogen balance) and decreases urea production. The muscle wasting that occurs concomitant with aging has been proposed to be caused, at least in part, by the decrease in GH secretion that occurs with aging.