Protein is the most abundant nitrogen-containing compound in the diet and the body. Proteins are formed when L-α-amino acids polymerize via peptide bond formation. Amino acids have similar central structures with different side-chains determining the multiple metabolic and physiological roles of free amino acids. Indispensable (essential) amino acids cannot be synthesized by humans from materials ordinarily available to cells at a speed commensurate with the demands of human growth and maintenance. The requirements for indispensable amino acids can be defined as “the lowest level of intake that achieves nitrogen balance or that balances the irreversible oxidative loss of the amino acid, without requiring major changes in normal protein turnover and where there is energy balance with a modest level of physical activity.” For infants, children, and pregnant and lactating women, requirements would include protein deposited and secretion of milk proteins.