Amino adds dietary requirements

Ah 20 types of amino adds are required for protein synthesis. These amino adds can be derived from digesting dietary protein and absorbing their constituent amino acids or, alternatively, by synthesizing them de novo. 

The 10 amino adds listed in Table 1-8-1 cannot be synthesized in humans and therefore must be provided from dietary sources. These are called the essential amino adds. Arginine is required only during periods of growth, or positive nitrogen balance. 

Pyndoxal phosphate is also a cofactor for transamination reactions, In these reactions, an amino group is transferred from an amino acid to an or-keto acid, thus founing a new amino acid and a new or-keto acid, Transamination reactions are important for the synthesis of amino acids from non-protein metabolites and for the degradation of amino acids for energy production. Since pyridoxal phosphate is intimately involved ill amino add metabolism, the dietary requirement for vitamin B6 increases as the protein content of the diet increases. 

The influence of cysteine on the methionine requirement is demonstrated by the following study. The subject was a student who was awarded a Ph.U. after conducting the study on himself. The subject consumed diets that were complete except that the concentration of methionine was varied (Table 8.12), The dietary amino acids were supplied in the form of pure amino acids, rather than as protein, to allow full control over the levels of amino adds supplied by the diet. Cysteine was supplied as cy.stine. Cystine is a dimer of cysteine, in which the two cysteine residues are connected via a disulfide bond (R——R). Cystine is readily converted to cysteine in the body. The methionine requirement was assessed by determining the conditions that supported a zero or slightly positive nitrogen balance. 

While dietary carbohydrate in excess of immediate requirements may be stored in the form of glycogen, and lipid may be stored as triglyceride, there is no analogous short-term storage of amino adds. Amino acids in excess of requirements are immediately degraded. In fact, it has been shown that after eating a balanced meal, degradation of excess amino adds precedes the metabolism of carbohydrates and fats. 

Overall, dietary protein requirements represent the amount of protein that is required to support net protein synthesis (growth, repair of damaged tissues, lactation, pregnancy, muscle hypertrophy, enzyme synthesis), amino acid oxidation, and the inefficiency inherent in the amino add recycling process. The two methods of determining protein requirements are the NBAL method and those using isotope tracers. 

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