<P> Protein powders--such as casein, whey, egg, rice and soy--are processed and manufactured sources of protein . These protein powders may provide an additional source of protein for bodybuilders . The type of protein is important in terms of its influence on protein metabolic response and possibly on the muscle's exercise performance . The different physical and / or chemical properties within the various types of protein may affect the rate of protein digestion . As a result, the amino acid availability and the accumulation of tissue protein is altered because of the various protein metabolic responses . </P> <P> The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition . There are multiple systems which rate proteins by their usefulness to an organism based on their relative percentage of amino acids and, in some systems, the digestibility of the protein source . They include biological value, net protein utilization, and PDCAAS (Protein Digestibility Corrected Amino Acids Score). Also see complete protein, nitrogen balance and protein combining . The PDCAAS was developed by the FDA as an improvement over the Protein efficiency ratio (PER) method . The PDCAAS rating is a fairly recent evaluation method; it was adopted by the US Food and Drug Administration (FDA) and the Food and Agricultural Organization of the United Nations / World Health Organization (FAO / WHO) in 1993 as "the preferred' best"' method to determine protein quality . These organizations have suggested that other methods for evaluating the quality of protein are inferior . </P> <P> Most proteins are decomposed to single amino acids by digestion in the gastro - intestinal tract . </P> <P> Digestion typically begins in the stomach when pepsinogen is converted to pepsin by the action of hydrochloric acid, and continued by trypsin and chymotrypsin in the small intestine . Before the absorption in the small intestine, most proteins are already reduced to single amino acid or peptides of several amino acids . Most peptides longer than four amino acids are not absorbed . Absorption into the intestinal absorptive cells is not the end . There, most of the peptides are broken into single amino acids . </P>

Most of the protein in the body is found as