Vitamin A Retinoids and Carotenoids

Vitamin A deficiency is a serious problem of public health nutrition, second only to protein-energy malnutrition worldwide, and is probably the most important cause of preventable blindness among children in developing countries. Marginal deficiency is a significant factor in childhood susceptibility to infection, and hence morbidity and mortality, in developing countries; even in developed countries, vitamin A (along with iron) is the nutrient most likely to be supplied in marginal amounts. In addition to primary deficiency of the vitamin, secondary (functional) vitamin A deficiency results fromprotein-energy malnutrition, because of impaired synthesis of the plasma retinol binding protein (RBP) that is required for transport of the vitamin from liver reserves to its sites of action.

The main physiologically active forms of vitamin A are retinaldehyde and retinoic acid, both of which are derived from retinol. Retinaldehyde functions in the visual system as the prosthetic group of the opsins, which act as the signal transducers between reception of light in the retina and initiation of the nervous impulse.

Retinoic acid modulates gene expression and tissue differentiation, acting by way of nuclear receptors. Historically, there was confusion between the effects of deficiency of vitamins A and D; by the 1950s, it was believed that the confusion had been resolved. Elucidation of the nuclear actions of the two vitamins has shown that, in many systems, the two act in concert, forming retinoid-vitamin D heterodimeric receptors; hypervitaminosis A can antagonize the actions of vitamin D.

In vitro, and in experimental animals, vitamin A has anticancer action related to its role in modulating gene expression and tissue differentiation. It retards the initiation and growth of some experimental tumors. However, it only shows these effects at toxic levels, and a number of synthetic analogs, collectively known as retinoids, have been developed for use as anticancer drugs and in dermatology.

Preformed vitamin A is found only in animals and a small number of bacteria. A number of the carotenoid pigments in plants can be cleaved oxidat-ively to yield retinol; f-carotene is quantitatively the most important of these provitamin A carotenoids. Although preformed retinol is both acutely and chronically toxic in excess, carotene is not, because there is only a limited capacity to cleave it to retinol.

In addition to its provitamin A role, f - carotene is a radical trapping antioxidant and may be nutritionally important in its own right both as an antioxidant and possibly also through direct actions that are independent of retinoids. Other carotenoids that occur in foods, and circulate in the bloodstream, also have free radical trapping activity, and, hence, potential metabolic significance, whether or not they are metabolic precursors of vitamin A.

2.1 VITAMIN A VITAMERS AND UNITS OF ACTIVITY

The term vitamin A can include any compound with the biological activity of the vitamin: provitamin A carotenoids, retinol, and its active metabolites.

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