Nutrition 4

VII. DIETARY PROTEIN
Humans have no dietary requirement for protein, per se, but, the protein in food does provide essential amino acids. Ten of the twenty amino acids needed for the synthesis of body proteins are essential-that is, they cannot be synthesized in humans at an adequate rate. Of these ten, eight are essential at all times, whereas two (arginine and histidine) are required only during periods of rapid tissue growth characteristic of childhood or recovery from illness.
A. Quality of proteins
The quality of a dietary protein is a measure of its ability to provide the essential amino acids required for tissue maintenance. Most government agencies have adopted the Protein Digestibility-Corrected Amino Acid Scoring (PDCAAS) as the standard by which to evaluate protein quality. PDCAAS is based on the profile of essential amino acids and the digestibility of the protein. The highest possible score under these guidelines is 1.00.This amino acid score provides a method to balance intakes of poorer quality proteins by vegetarians and others who consume limited quantities of high-quality dietary proteins.
1. Proteins from animal sources: Proteins from animal sources (meat, poultry, milk, fish) have a high quality because they contain all the essential amino acids in proportions similar to those required for synthesis of human tissue proteins (Figure ). [Note: Gelatin prepared from animal collagen is an exception; it has a low biologic value as a result of deficiencies in several essential amino acids.]