Phosphate dietary requirements

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. 

Dietary phosphate deficiency is relatively rare because the phosphate content in plant and animal foods is well above the requirement and because of the efficient absorption of phosphate (50-90%). Phosphate deficiency can occur in a number of situations. It can occur with the chronic intake of aluminum-based antacids, particularly with a low-phosphate diet. These antacids form a complex with dietary phosphate, preventing its absorption and resulting in the deficiency. Deficiency can occur with increased urinary excretion of phosphate that occurs with starvation and in diabetics experiencing ketoacidosis. Chronic alcoholics may be phosphate deficient because of decreased dietary intake, impaired absorption, and increased urinary excretion (Berner and Shike, 1988). Phosphate deficiency has been observed in the small, premature infant. The premature infant has a higher requirement for phosphate than the term infant, as it grows at a relatively greater rate. Phosphate is required for the S5mthesis of soft tissues and bone. The small, premature infant s requirement for phosphate cannot be fully supplied by human mUk. 

The water-soluble vitamins with hsted DVs are vitamin G, which is necessary for the prevention of scurvy (Section 4.3), and the B vitamins—niacin, pantothenic acid, vitamin Bg, riboflavin, thiamine, fohc acid, biotin, and vitamin Bj2. The B vitamins are the precursors of the metabohcally important coenzymes listed in Table 7.1, where references to the reactions in which the coenzymes play a role are given. We have seen many pathways in which NADH, NADPH, FAD, TPP, biotin, pyridoxal phosphate, and coenzyme A were found, all of which came from vitamins. A summary of vitamins and their metabolic roles is given in Table 24.2. Frequently, the actual biochemical role is played by a metabolite of the vitamin rather than by the vitamin itself, but this point does not affect the dietary requirement. 

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 requirement for nicotinamide is also related to the requirement for tryptophan. Dietary tryptophan can be converted with varying efficiencies into nicotinamide, thus dietary tryptophan spares the requirement for nicotinamide. In the domestic fowl and grain-eating birds, there is rarely an excess of tryptophan. Pyridoxal phosphate (vitamin B ) is required for the interconversion of tryptophan to nicotinamide, and so the requirements for nicotinamide are moderated by the amounts of pyridoxal and of tryptophan residues present in the diet. The ability to convert tryptophan to nicotinamide also appears to depend on the level of picolinic acid carboxylase in the liver. This enzyme converts one of the intermediates, 2-amino-3-acroleylfumaric acid into a branch path and so competes with the main pathway. The levels of this enzyme are comparatively low in the domestic fowl but are much higher in the duck, which fits with the duck having a nicotinamide requirement about double that of the domestic fowl (Scott et al., 1982). 

Pyridoxine or vitamin Bg is an important dietary requirement. The aldehyde form is called pyridoxal and its phosphate ester is implicated in many enzyme catalyzed reactions of amino acids and amines. The reactions are numerous and pyridoxal phosphate (pyridoxal-P) is surely one of nature s most versatile catalysts or coenzymes. The chemistry that will be emphasized here is one of proton transfer. In transamination (equation 7-1), a process of central importance in nitrogen metabolism, it is converted to pyridoxamine. 

The amount of each element required in daily dietary intake varies with the individual bioavailabihty of the mineral nutrient. BioavailabiUty depends both on body need as deterrnined by absorption and excretion patterns of the element and by general solubiUty, and on the absence of substances that may cause formation of iasoluble products, eg, calcium phosphate, Ca2(P0 2- some cases, additional requirements exist either for transport of substances or for uptake or binding. For example, calcium-binding proteias are iavolved ia calcium transport an intrinsic factor is needed for vitamin cobalt,... 

One method of treatment is to inject calcitonin, which decreases blood Ca " concentration and increases bone calcification (33). Another is to increase the release of calcitonin into the blood by increasing the blood level of Ca " ( 4). This latter treatment is accompHshed by increasing Ca " absorption from the intestine requiring dietary calcium supplements and avoidance of high phosphate diets. The latter decrease Ca " absorption by precipitation of the insoluble calcium phosphate. 

The development and maintenance of healthy bone depends in part on an adequate supply of dietary calcium. Although phosphate is absolutely required for the mineralization of bone, it is abundant in the diet. If one gets enongh to eat, one gets enough phosphate. The same is not necessarily trne for calcinm and many people snpplement their diet with a preparation of calcium, frequently calcinm glnconate or calcium citrate. More follows below about the chemistry of bone when we get to a consideration of calcium. 

Resorption of the required mineral substances from food usually depends on the body s requirements, and in several cases also on the composition of the diet. One example of dietary influence is calcium (see p. 342). Its resorption as Ca is promoted by lactate and citrate, but phosphate, oxalic acid, and phytol inhibit calcium uptake from food due to complex formation and the production of insoluble salts. 

The RUA for calcium in adults over the age of 24 is 0.8 g. The RIM for w omcn during pregnancy and lactation is 1.2 g. The increased level of 1.2 g is required to supply the fetus with the 30 g of calcium present in the newborn and to provide the 0.24 g ol calcium secreted in the milk each day. The RDA for persons from 11 to 24 years of age is 1,2 g the Rl3As for children (0.8 g) and infanta (0.6 g) are lower. Eggs supply about 30% of dietary calcium and 30% of dietary phosphate for the overall population in the United States. Meat, poultry, and ifish supply 20-257n of our phosphate, but only 107q of our calcium. Milk and dairy products supply 20-25"n of our phosphate, and 50% of our calcium (Calvo and Park, 1996), A dietary deficiency in calcium is quite rare, though calcium nutririon receives much attention because of mainstream health concerns related to calcium, such as osteoporosis, hypercalcemia, and hypocalcemia. 

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