The protein mass of the body is regulated by changes in the relative rates of protein synthesis and protein degradation. Whole-body protein synthesis rates represent the compilation of the rates of synthesis of protein in all tissues and organs of the body. Growth, aging, nutrition, hormones, injury, and disease can affect protein synthesis in individual tissues differently, stimulating or suppressing protein synthesis in some tissues, while having little effect in others. Thus, to understand the mechanism by which these phenomena alter protein balance, it is critical that the effects of these factors on protein synthesis be examined on an individual tissue basis rather than in the body as a whole.