Dietary protein biological value

Tt is well recognized that the nutritional value of dietary proteins de-pends primarily on the content of their constituent amino acids, especially of their essential amino acids. Because of deficiencies of lysine and methionine, and to a lesser extent of a few other amino acids, proteins from plants and other alternative sources have low biological quality. Moreover, incomplete digestion of the protein also may result in a lack of complete availability (I) of the essential amino acids and may further reduce its value. In many raw plant foodstuffs such as soybeans, common beans, or unprocessed protein foods, undenatured... 

The biological value of dietary proteins is related to the extent to which they provide all the necessary amino acids. Proteins of animal origin generally have a high biological value, whereas plant proteins may be deficient in lysine, methionine and tryptophan, and are generally less digestible than animal proteins. 

The quality of different proteins can be expressed according to their chemical score, protein efficiency ratio (PER), biological value (BV), and net protein utilization (NPU). These parameters refer to the different tests used to define the protein s quality. The chemical score refers only to the property of the protein in question. The PER, BV, and NPU refer to relationship.s between the dietary protein and the consumer. The values of the PER, BV, and NPU depend on properties of both the protein in question and the needs of the animal. The chemical score, PER, BV, and NPU of proteins in various foods, including eggs, fish, rice, and maize, are listed in Table 8,1 3. 

The effect of proteins on pollutant toxicity includes both quantitative and qualitative aspects. Experiments show that animals fed proteins of low biological value exhibited a lowered microsomal oxidase activity when dietary proteins were supplemented with tryptophan, the enzyme activity was enhanced. Alteration of xenobiotic metabolism by protein deprivation may lead to enhanced or decreased toxicity, depending on whether metabolites are more or less toxic than the parent compound. For example, rats fed a protein-deficient diet show decreased metabolism but increased mortality with respect to pentobarbital, parathion, malathion, DDT, and toxaphene (Table 6.4). On the other hand, rats treated under the same conditions may show a decreased mortality with respect to heptachlor, CC14, and aflatoxin. It is known that, in the liver, heptachlor is metabolized to epoxide, which is more toxic than heptachlor itself, while CC14 is metabolized to CC13, a highly reactive free radical. As for aflatoxin, the decreased mortality is due to reduced binding of its metabolites to DNA. 

Proteins are not required for their caloric value, but for their content of amino acids. This presents two nutritional problems (1) Does a food protein contain the correct numbers of types of amino acids (2) How accessible are the amino acids, i.e., how digestible is the food These qualities of a dietary protein are expressed as its biological value. 

About 20,000 tons of Lys per year are produced in Japan by a specific fermentation prcx ess ( Kyowa fermentation industry). This Lys is used as a supplement to foodstuffs and to increase the biological value of low value plant dietary proteins The growth rate of poltry and pigs is appredably increased by the addition of 0.1-0.3% Lys to their feed. Also of technical importance is the enzymatic synthesis of Lys from DL-2-aminocaprolactam. using microbial L-aminoca-prolactam hydrolase. The remaining D-2-aminocapro-lactam is racemized enzymatically, then converted entirely to t-Lys Lys was first isolated by Drechsel from casein in 1889. 

A system for the quantitative nutrition of ruminant animals should embody the processes described, which requires that factors such as degradabUity, efficiency of nitrogen captme, microbial protein yield, digestibihty of microbial protein, digestibihty of dietary imdegraded protein and the trae biological value of the absorbed nitrogen or its essential amino acid content be quantified. 

Persons using dialysis require restriction of sodium, potassium and water intake. Protein intake is also closely controlled and will vary from 30 to 60 g daily. The aim is to provide about 3/4 of the protein allowance as protein of high biological value. Furthermore, to prevent high blood urea and potassium levels, it is especially important that adequate energy be supplied to prevent the catabolism of body protein. Dialysis patients also require vitamin supplementation in addition to the dietary controls. 

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