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  • W.B.C.S. Optional Notes – Biological Value Of Protein – Animal Husbandry And Veterinary Science.
    Posted on June 20th, 2019 in Animal Husbandry and Veterinary Science
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    W.B.C.S. Optional Notes – Biological Value Of Protein – Animal Husbandry And Veterinary Science.

    WBCS পরীক্ষার ঐচ্ছিক নোট – প্রোটিনের জৈবিক মূল্য – পশুপালন এবং পশু চিকিৎসা বিজ্ঞান।

    The biological value of a protein extends beyond its amino-acid composition and digestibility, and can be influenced by additional factors in a tissue-specific manner. In healthy individuals, the slow appearance of dietary amino acids in the portal vein and subsequently in the systemic circulation in response to bolus protein ingestion improves nitrogen retention and decreases urea production. This is promoted by slow absorption when only protein is ingested (e.g. casein). When a full meal is ingested, whey achieves slightly better nitrogen retention than soy or casein, which is very likely achieved by its high content of essential amino acids (especially leucine). Elderly people exhibit ‘anabolic resistance’ implying that more protein is required to reach maximal rates of muscle protein synthesis compared to young individuals.Continue Reading W.B.C.S. Optional Notes – Biological Value Of Protein – Animal Husbandry And Veterinary Science.

    Protein utilization in inflammatory or traumatic conditions increases substantially in the splanchnic tissues containing most of the immune system, and in wounds and growing tissues. This happens especially in the elderly, which often suffer from chronic inflammatory activity due to disease, physical inactivity and/or the aging process itself. Consequently, the proportion of protein absorbed in the gut and utilized for muscle protein synthesis decreases in these situations. This compromises dietary-protein-induced stimulation of muscle protein synthesis and ultimately results in increased requirements of protein (∼1.2 g/kg body weight/day) to limit gradual muscle loss with age. To optimally preserve muscle mass, physical exercise is required. Exercise has both direct effects on muscle mass and health, and indirect effects by increasing the utilization of dietary protein (especially whey) to enhance rates of muscle protein synthesis.The biological value of a protein, as defined and measured by the method developed in the preceding paper, can hardly beexpected to remain constant under all conditions. It may be expected to vary with the use to which the protein is put in thebody. The maintenance of the nitrogenousintegrity of the tissues, the synthesis of new tissue in growth, and the synthesis of milk proteins in lactation may be expected to require different proportions of amino-acids, so that the nitrogen of a given proteinmay be expected to be more completely utilized in serving some one of these functions than in serving another. These differences in biological value are concerned with the amino-acid make-up of the protein as compared with the several distinct requirements for amino-acids. It is generally believed, in fact, that the chemical constitution of a protein determines entirely its biological value. This belief tacitly assumes that the economy with which the cells utilize the amino-acids supplied to them is unaffected by the abundance of the supply, particularly by the rate at which the amino-acids are transported to the cells from the intestinal tract. It also assumes that the undigested fraction of a dietary protein is representative in its amino-acid make-up of the protein as consumed, so that factors influencing the digestibility of the protein would have no effect on the proportions of the different amino-acids absorbed into the blood. It must also assume that no great differences in the rate at which different amino-acids are liberated from a protein during digestion occur, or that if such differences do occur, the storage capacity of the tissues for absorbed amino-acids is suffciently great to insure the presence there throughout the digestive period of optimum proportions for synthetic purposes of the several amino-acids. It also assumes that until the demands of the body
    for protein are covered by the protein intake, amino-acids are not oxidized in the body except in so far as they are unavailable for synthetic purposes, and hence that factors affecting the intensity of the oxidation processes can have no effect on the utilization of
    amino-acids for structural purposes. These assumptions are either extremely improbable or highly debatable.

    Human nutrition is vastly more complicated than the the simplified information provided to Americans through dietary guidelines, nutrition facts panels, and ingredient lists on products. Scientific understanding is evolving as well. If once all fats were grouped as one, today we understand that we need to look at the building blocks of fats – fatty acids – to really understand the effects of fats on our metabolism.

    The same is true of protein, the darling macro-nutrient everyone is clamoring to consume more of, even though the average American diet already overloads us with more protein than the body actually needs.

    Proteins are composed of of 21 biological amino acids. 9 of these are “essential amino acids”, which means our bodies cannot produce them, and they must be derived from food sources. The essential amino acids are phenylalanine (25 milligrams per kg of body weight), leucine (39), lysine (30), valine (26), threonine (15), methionine (15), isoleucine (20), histidine (10), and tryptophan (4). When we digest a food with protein, it breaks down into its amino acids, and each is used by the body for slightly different purposes.

    A complete protein is one that includes all 9 essential amino acids. Most animal sources are complete proteins, and some plant proteins are as well. By combining several types of plant proteins (beans and rice for example), even non-meat eaters get complete protein.

    The biological value of a protein:

    Let’s dive a bit deeper now. When a protein contains the essential amino acids in a proportion similar to that required by the body, it has a high Biological Value. When one or more of the essential amino acids are missing or present in low numbers, the protein is has a low biological value.

    The biological value of a protein is a number from 100 down to 0, that describes how well it is absorbed by the body. More precisely, it is a measure of the percentage of the protein that is actually incorporated into the proteins of the human body.

    The table of biological values shows that the proteins in eggs (white AND yolk) have the highest biological value. Rice (brown) and quinoa have higher biological value than beef or chicken! Of course this is per gram of protein consumed. Beef and chicken are much denser in protein per serving than rice is.

    Why is biological value so important?

    Unlike carbs or fats that can be stored in the body for future use, unused amino acids (protein) are excreted. Consuming a lot of food with a protein that has low biological value will not be very effective because most of the protein will not be utilized.

    The biological value is determined by the amino acid composition of the protein and can only be as high as the amount of limiting amino acid present – the amino acid present in the smallest amount in the protein. In many cases, the limiting amino acid is lysine, which is not as common as the other amino acids.

    The limiting amino acid tends to be different in different proteins, so when two plant based protein sources are eaten at a meal, say a grain (rice) and a pulse (lentils), the amino acids of one protein may compensate for the limitations of the other, resulting in a combination of higher biological value.

    Other factors influencing Biological Value:

    Methods of food preparation also have an impact on the biological value of protein in a food source; they may damage some of the essential amino acids.

    High protein diets automatically reduce the biological value of all proteins consumed, because the body is getting more amino acids than it can actually metabolize at any given time.

    The table of biological values provides information for an individual food consumed in a sterile lab situation. Everyday consumption is very different – with a combination of protein sources being eaten at the same time. The result is normally a higher biological value.

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