Histidine, dietary requirement

Humans have no dietary requirement for protein, per se, but, the protein in food does provide essential amino acids (see Figure 20.2, p. 260). 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. 

Pyrrolysine trait is restricted to several microbes, and only one organism has both Pyl and Sec. Of the 22 standard amino acids, 8 are called essential amino acids because the human body cannot synthesize them from other compounds at the level needed for normal growth, so they must be obtained from food. In addition, cysteine, taurine, tyrosine, histidine and arginine are semiessential amino-acids in children, because the metabolic pathways that synthesize these amino acids are not fully developed. The amounts required also depend on the age and health of the individual, so it is hard to make general statements about the dietary requirement for some amino acids. 

Our results have been recapitulated with other proteins of varying nutritional value to S. exigua and H. zea they include soy protein, tomato foliar protein, corn gluten and zein. In all cases, more than 2.5% dietary protein was required to alleviate antinutritional effects, because these proteins are less nutritious than casein (Table III). The ability of a protein to alleviate the toxicity of o-quinones is proportional to its nutritional value to the insect (Table III). The proteins ability to function as an alkylatable sink (alleviation of antinutritive effects) is correlated with the relative amounts of alkylatable amino acids (e.g., lysine, cysteine, histidine, methionine Felton and Duffey, unpublished data). 

Subjects who are maintained for prolonged periods by total parenteral nutrition are obviously wholly dependent on what is provided in the nutrient mixture, normally with no contribution from intestinal bacteria. A great deal has been learned from such patients, including the essentiality of the amino acid histidine, and evidence that endogenous synthesis of taurine (Section 14.5.3) and carnitine (Section 14.1.2) may not be adequate to meet requirements without some dietary provision. However, for obvious ethical reasons, such patients have not been subjected to trials of graded intakes of vitamins, but are generally provided with amounts calculated to be adequate and in excess of minimum requirements. 

Figure 8,23 reveals that the histidine requirement for the rat is about 0,33%, The data in Figure 8,26 show that dietary histidine is most effectively used at levels of up to 0-25% and that higher levels have an increasing tendency to be broken down, lire amount of breakdown depends on the exact level of amino acid in the diet. 

Protein is an essential nutrient for human growth, development, and homeostasis. The nutritive value of dietary proteins depends on its amino acid composition and digestibility. Dietary proteins supply essential amino acids, which are not synthesized in the body. Nonessential amino acids can be synthesized from appropriate precursor substances (Chapter 17). In human adults, essential amino acids are valine, leucine, isoleucine, lysine, methionine, phenylalanine, tryptophan, and threonine. Histidine (and possibly arginine) appears to also be required for support of normal growth in children. In the absence from the diet of an essential amino acid, cellular protein synthesis does not occur. The diet must contain these amino acids in the proper proportions. Thus, quality and quantity of dietary protein consumption and adequate intake of energy (carbohydrates and lipids) are essential. Protein constitutes about 10-15% of the average total energy intake. 

What are essential amino acids Some species, including humans, cannot synthesize all the amino acids required for protein synthesis and must therefore obtain these essential amino acids from dietary sources. About half of the standard 20 amino acids are essential in humans including arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. 

The amino acid composition of a given food protein will be relatively constant (see Appendix 2, Table A2.1.3), but that of the protein to be synthesised will vary considerably with the type of animal and the various functions it has to perform. For the normal growth of rats, pigs and chicks, for example, lysine, tryptophan, histidine, methionine, phenylalanine, leucine, isoleucine, threonine, valine and arginine are dietary essentials. Humans do not require histidine, whereas chicks need glycine, in addition to those acids required by the rat, to ensure optimum growth. On the other hand, arginine is not a dietary essential for maintenance of the rat or pig. 

Restricting dietary histidine will bring the blood histidine level back to normal and eliminates the urinary imidazole metabolites in patients with histidinemia and urocanase deficiency. However, no urgent treatment is required because of the benign nature of this condition. 

These shortcomings of the nitrogen balance method led us to attempt a supplementation of these data by measurements of the urinary amino acids and other metabolites. It is interesting to note that the urinary bound amino-N-output has been found to characterize the infant metabolism of various milk protein preparations more significantly than the N-retention values (36). This approach as will be discussed later has also afforded us a supplementary means of ascertaining the human requirements of tryptophan, lysine, and methionine and a measure of the dietary lack of amino acids, histidine and cystine, which fail to induce negative N-balance. 

It was early discovered that ergothioneine could not replace, even partially, the dietary histidine requirement of the growing rat (Eagles and Cox, 1928). Similar negative results have been obtained with thiolhistidine... 

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