Proteins dietary requirements

Excessive heat can cause destruction of amino acid residues. The amino acid most susceptible to direct heat destruction is cystine. Although not an essential amino acid, cystine does have a sparing effect on the dietary requirement for methionine. As a result, cystine destruction can be nutritionally important. In addition, many vegetable proteins are limiting in the sulfur amino acids. Cystine destruction would be particularly harmful for these proteins. 

The classical niacin deficiency disease is pellagra, which is characterized by symptoms including diarrhoea, dermatitis, dementia and eventually death. High-protein diets are rarely deficient in niacin since, in addition to the preformed vitamin, such diets supply sufficient tryptophan to meet dietary requirements. Large doses of niacin can cause the dilation of capillaries, resulting in a painful tingling sensation. 

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. 

Plants, of course, also require protein to build and maintain their life processes and, consequently, are protein sources for the animal diet, In the case of herbivores, plants are essentially the exclusive source of proteins, energy, and all other dietary elements. 

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. 

In addition to their importance as essential amino acids for humans, the quantitative determination of cysteine and methionine seems to be growing in importance in the animal feed industry. The dietary requirements for the sulfur amino acids tend to be very high in many animals. This is presumably due to the magnitude of hair/feather growth and the fact that the structural proteins that comprise hair/feathers often have high cyst(e)ine content. 

Although a protein requirement per se is no longer appropriate in requirement tables, stating a dietary requirement for both protein and EAA is a convenient way to ensure that all AAs needed physiologically are provided correctly in the diet (NRC, 1994). 

Dietary requirements for AAs and protein usually are stated as proportions of the diet. However, the level of feed consumption has to be taken into account to ensure that the total intake of protein and AAs is appropriate. The protein and AA requirements derived by the NRC (1994) relate to poultry kept in moderate temperatures (18-24°C). Ambient temperatures outside of this range cause an inverse response in feed consumption i.e. the lower the temperature, the greater is the feed intake and vice versa (NRC, 1994). Consequently, the dietary levels of protein and AAs to meet the requirements should be increased in warmer environments and decreased in cooler environments, in accordance with expected differences in feed intake. These adjustments are designed to help ensure the required daily intake of AAs. 

Based on a lengthy metabolic study with men, Sandstead et al. (8 ) have reported that the dietary requirement for zinc increased as the level of protein Intake increased. All of these investigations suggest that a higher amount of protein in the diet results in poorer utilization and/or increased metabolic need for zinc. 

Protein in herbivore diets is likely to be adequate in amount to satisfy the dietary requirements for net growth of an herbivore (approx 1% of bulk diet), and perhaps also the requirement for tissue turnover The distribution of individual amino acids, however, may not even be close to those required for proper tissue growth or the turnover of existing tissues As a result, it would be expected that a substantial amount of amino acid synthesis would occur, and that the collagen of herbivores would closely reflect the isotopic composition of the herbivore s diet ... 

A certain amount of dietary protein is required to synthesize endogenous proteins such as albumin (plasma protein), myosin(muscle filament), actin and hemoglobin. The basis for the dietary requirement of protein is the bodies inability to synthesize certain amino acids which is call essential amino cells (Table 12.5). 

Dietary lipid consists mostly of triacylglycerol from plant and animal sources. It supplies about 45% of the energy in a typical Western diet. Except for the essential polyene fatty acids (Chapter 18), the dietary requirement for lipid can be met by carbohydrate or protein. However, the Eskimo has a satisfactory diet consisting of 80-90% lipid. Digestion of triacylglycerol is discussed in Chapter 12. A... 

Of the 20 amino acids in proteins, the body can readily synthesize eight if an appropriate nitrogen source is available. Two others can be synthesized from other amino acids of the diet tyrosine from phenylalanine and cysteine from methionine. The rest must be provided in the diet (Chapter 17), since the body can synthesize none or an insufficient amount. The dietary requirement depends on several factors. Beside essential amino acids, the diet should provide the nitrogen required for synthesis of the nonessential amino acids. 

Iron, whether in the form Fe(II) or Fe(III), is usually found in the body in association with proteins. Little or no iron can be found free in the blood. Because iron-containing proteins are ubiquitous, there is a dietary requirement for this mineral. Severe deficits can lead to iron-deficiency anemia. 

Birds appear unable to synthesise any arginine via the urea cycle (see Section 5.4), which may be because of lack of carbamoyl phosphate synthetase I in mitochondria (Baker, 1991), and, as a result, the dietary requirement for arginine is higher than in growing mammals. However, they do appear to have a carbamoyl phosphate synthetase II in the cytosol (Maresh, Kwan Kalman, 1969). This may be part of the multienzyme protein G D (carbamoyl phosphate synthase-aspartate carbamoyl transferase-dihydroorotase), responsible for the biosynthesis of 3ihydroorotate, a pyrimidine precursor, but this is bound on the multienzyme protein, and it seems unlikely that it would be available for arginine biosynthesis (see Price Stevens, 1989). 

The dietary requirements for carbohydrates and fats are less exacting than those for proteins. Fats and carbohydrates form the main energy sources in a typical avian diet, and although protein can also be an energy source, this only occurs in situations in which it is present in excess or where there is a shortage of fat and/or carbohydrate. The energy yields of the major nutrients, as determined by bomb calorimetry, are carbohydrate (17.15 kj/g), protein (22.59 kj/g) and fat (38.91 kJ/g). The actual amount of energy obtainable from these nutrients by birds is somewhat less, as this would require each of the nutrients to be absorbed by the gut with 100% efficiency. Both fats and carbohydrate can be fully... 

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