A possible dietary option that would ensure maximal muscle growth while avoiding excessive nitrogen loss to the environment consists of lowering the quantity of protein in the feed and using synthetic amino acids to supplement the diet. This review was undertaken in order to improve understanding of the effects of changes in the amount of protein in the diet (variations in the total quantity of protein and in its amino acid composition) on protein synthesis and breakdown. The balance between the two latter processes regulates protein deposition. The current methods for measuring protein synthesis have been presented here, noting the assumptions and limitations for each one. The techniques for estimating protein breakdown have also been briefly described. The turnover of whole-body and tissue proteins varies according to individual animal characteristics. For example, the fractional rate of protein synthesis decreases with age, especially for the skeletal muscles and the fractional rate of protein breakdown varies with the genotype. The quantity of protein in the feed modifies the protein metabolism of the animal. These effects are due to the quantity of ingested protein as well as to fluctuations in the input level (protein starvation followed by protein re-feeding). The amino acid composition of the dietary protein also plays an important rôle. Thus a decrease in the protein intake or a deficiency in a particular amino acid may reduce the quantity of protein synthesized and, to a lesser extent, degraded. The mechanisms that regulate protein deposition in response to alterations in protein and amino acid supply, however, remain poorly understood. Further investigations of protein metabolism in different tissues and organs, including physiological and hormonal factors, need to be undertaken in order to deepen understanding of the effects of amino acid supplementation and provide a better basis for diet formulation.