Ascorbic acid dietary requirement

Considerable data are available on the insect s requirement for l- ascorbic acid. Dietary vitamin C is needed for normal growth, molting, and fertility of many insects, and vitamin C, or another compound with similar biological properties, is probably an essential growth... 

One study reported by Pal et al. (1975) indicated that lead toxicity impaired vitamin C synthesis in the rat. This finding is of no apparent significance to human nutrition because ascorbic acid is required in the human diet. Fox et al. (1980) have demonstrated that the addition of vitamin C to the diets of Japanese quail protected against high toxic levels of cadmium and decreased the concentration of cadmium in the liver and kidneys as compared to unsupplemented controls fed very low dietary levels of cadmium (Fox et al., 1970). Vitamin C increased the absorption of iron in the animal studies this increase probably was responsible for the observed protection against cadmium. 

Vitamins are chemically unrelated organic compounds that cannot be synthesized by humans and, therefore, must must be supplied by the diet. Nine vitamins (folic acid, cobalamin, ascorbic acid, pyridoxine, thiamine, niacin, riboflavin, biotin, and pantothenic acid) are classified as water-soluble, whereas four vitamins (vitamins A, D, K, and E) are termed fat-soluble (Figure 28.1). Vitamins are required to perform specific cellular functions, for example, many of the water-soluble vitamins are precursors of coenzymes for the enzymes of intermediary metabolism. In contrast to the water-soluble vitamins, only one fat soluble vitamin (vitamin K) has a coenzyme function. These vitamins are released, absorbed, and transported with the fat of the diet. They are not readily excreted in the urine, and significant quantities are stored in Die liver and adipose tissue. In fact, consumption of vitamins A and D in exoess of the recommended dietary allowances can lead to accumulation of toxic quantities of these compounds. 

The active form of vitamin C is ascorbate acid (Figure 28.8). The main function of ascorbate is as a reducing agent in several different reac lions. Vitamin C has a well-documented role as a coenzyme in hydroxy lation reactions, for example, hydroxylation of prolyl- and lysyl-residues of collagen (see p. 47). Vitamin C is, therefore, required for the mainte nance of normal connective tissue, as well as for wound healing. Vitamin C also facilitates the absorption of dietary jron from the intestine. 

Ascorbic acid (vitamin C) is a water-soluble vitamin but is not part of the B group. It is a metabolic requirement for all species but is a dietary requirement only for those that lack the enzyme for its synthesis (primates, guinea pigs, certain birds, fish). Therefore, it is not required in poultry diets. It is involved in the formation and maintenance of intercellular tissues having collagen or related substances as basal constituents. 

The Average Requirement of ascorbic acid is 30 mg/day. The Population Reference Intake is 45 mg/day for adults. The Lowest Threshold Intake, for which considerable evidence exists, is 12 mg/day (1). These estimates have been supported by the relevant committee of the European Union. A communication from the US National Academy of Sciences, as part of the revision of US Dietary Reference Intakes, while estimating rather higher average requirements of ascorbic acid than the EU committee, does (100 mg/day) also proposed a tolerable upper intake... 

Manduca sexta dietary requirement (0.5 mM) dehydroascorbic acid, Mg 2-0-phosphonoascorbate, Na 6-0-myristoylascorbate fully active D-ascorbic acid 50% active 6-bromoascorbic acid 20% active D-isoascorbic acid 10% active K 2-0-sulfoascor-bate, L-isoascorbic acid, L-gu-lonic acid y-lactone inactive 5... 

Many beneficial claims have been made for ascorbic acid, especially for the intake of ascorbic acid far in excess of that required to prevent scurvy. Whether vitamin C will increase resistance to disease, promote better health, or even cure certain diseases remains to be determined. The practical dietary advice offered depends upon how well the mode of action of this vitamin is understood, and understanding this mode of action in turn depends on how similar the model system used is to humans. 

Vitamins are any organic dietary substance necessary for life, health, and growth that do not function by supplying energy. They usually function as coenzymes. Vitamins for one species may not be vitamins for another. Only humans, monkeys, and guinea pigs lack the ability to synthesize ascorbic acid (vitamin C), for instance. Indeed, bacteria in the gut synthesize some essential vitamins, which are absorbed in amounts sufficient to meet daily requirements. The administration of antibiotics for a long period of time could thus result in a vitamin deficiency of the bacterial host. 

Because wounds fail to heal in scurvy, ascorbic acid has been administered routinely to surgical cases, usually in the absence of any evidence that the tissues of the patients were depleted of the vitamin. Yet the observations of Crandon et al. (1940) make it clear that it requires a really severe dietary deficiency of the vitamin to produce any impmrment in the healing of wounds. If any measurable amount of ascorbic acid is present in the plasma, it is most unlikely that doses of ascorbic acid wUl make any difference to the rate of healing or the strength of the scar. The present writer disagrees, in this matter only, with the views of Hunt (1941). 

Two points deserve early emphasis (a) although most animals can synthesize ascorbic acid, a human cannot, and he Is totally dependent upon dietary intake to satisfy all his requirements (b) ascorbic acid, known to be essential for the structural Integrity of the intercellular matrix, is closely related to glucuronic acid, an essential building block of the principal matrix structures. 

Vitamins (Latin vita + amine) substances present in the animal diet in only small amounts, and indispensable for growth and maintenance of the organism. A dietary requirement is implicit in the definition of a V. Most of the substances that are V. in animals are essential for the metabolism of all living organisms, but plants and microorganisms can synthesize them (some fat-soluble V., however, may have metabolic roles unique to animals). The dietary re quirement in the animal results from the evolutionary loss of this biosynthetic ability. Animals differ in their ability to synthesize certain V., and they therefore display difierent dietary requirements for V. For example, ascorbic acid (V.C) is a V. only for primates and a few other animals (e.g. guinea pig) most animals can synthesize it, and for them it is therefore not a V. Some V. can be synthesized from provitamins in the diet. In addition some of the V. requirement of humans and higher animals is supplied by the intestinal flora, e.g. most of the V.K required by humans is supplied in this way. 

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