Protein.

Proteins are polymers of amino acids linked via α-peptide bonds. They can be represented as primary, secondary, tertiary, and even quaternary structures, but from a nutritional viewpoint only the primary (amino acid) sequence is of interest. Similarly, although there are many compounds in the body that can be chemically defined as amino acids, we are only concerned with the 20 canonical amino acids encoded in DNA, plus 5 others-ornithine, citrulline, γ-aminobutyrate, β-alanine, and taurine-that play quantitatively important roles in the body. We consume proteins, which are digested in the gastrointestinal tract, absorbed as small peptides (di- and tripeptides) and free amino acids, and then used for the resynthesis of proteins in our cells. Additionally, some amino acids are also used for the synthesis of specific (nonprotein) products, such as nitric oxide, polyamines, creatine, glutathione, nucleotides, glucosamine, hormones, neurotransmitters, and other factors. Again, such functions are not quantitatively important for most amino acids, and the bulk of amino acid metabolism is directly related to protein turnover (synthesis and degradation). For an individual in nitrogen balance, an amount of protein equal to that of the daily protein (nitrogen) intake is degraded each day with the nitrogen being excreted as urea and ammonia (with limited amounts of creatinine and uric acid). The carbon skeletons of the amino acids degraded to urea and ammonia are recovered through gluconeogenesis or ketone synthesis, or oxidized to carbon dioxide. Of the 20 amino acids present in proteins, 9 are considered nutritionally indispensable (essential) in adult humans because the body is not able to synthesize their carbon skeletons. These 9 amino acids are leucine, valine, isoleucine, histidine, lysine, methionine, threonine, tryptophan, and phenylalanine. In addition, 2 others are made from their indispensable precursors: cysteine from methionine, and tyrosine from phenylalanine. Although arginine is needed in neonates, it appears that adults, with the possible exceptions of pregnancy in females and spermatogenesis in males, can synthesize sufficient arginine to maintain a nitrogen balance. The others, glutamate, glutamine, aspartate, asparagine, serine, glycine, proline, and alanine, can all be synthesized from glucose and a suitable nitrogen source. Under some conditions, glutamine, glutamate, glycine, proline, and arginine may be considered as conditionally indispensable, meaning that the body is not capable of synthesizing them in sufficient quantities for a specific physiologic or pathologic condition (1). Thus, any discussion of dietary protein must consider not only quantity but also quality (ratio of indispensable amino acids).