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Fat Soluble Vitamins

Vitamin A

Terms for Fat-Soluble Vitamins

Vitamin E

Vitamin A is a term used to describe a family of essential, fat-soluble compounds structurally related to, and sharing the biological activity of, the lipid alcohol retinol. Vitamin A includes provitamin A carotenoids that are dietary precursers of retinol.

Function

Vitamin A is vital to normal vision. In the retina, vitamin A is responsible for the transduction of light into neural signals. Vitamin A has an important role in cellular differentiation, responsible for the integrity of epithelial tissues. Vitamin A is involved in embryonic development. It is thought that vitamin A has a role in immune function, both cell-mediated immunity and humoral immunity. Vitamin A deficiency is associated with a decrease in resistance to infection. Interventions with vitamin A have demonstrated an association with decreased severity of measles and diarrhea.

Absorption and excretion

Most of dietary preformed vitamin A is absorbed into the intestine, and absorption remains high with increased intake. Vitamin A is packaged along with lipids into chylomicrons for transport through the lymph and plasma to the liver. The liver is the principal storage site of vitamin A and the majority of retinol oxidation and catabolism takes place here. Too little dietary fat and factors that interfere with lipolysis or emulsification may decrease absorption of vitamin A.

Clinical conditions

A deficiency of vitamin A leads to epithelial keratinization, appetite changes resulting in poor growth, and xerophthalmia. Each year approximately 3 to 10 million children in developing countries become xerophthalmic and 250,000 to 500,000 become blind.

Overconsumption of vitamin A by ten times the RDA results in a condition called hypervitaminosis A. Symptoms include headache, vomiting, diplopia, alopecia, dry mucous membranes, desquamation, bone and joint pain, liver damage, hemorrhage, and coma. Too much dietary vitamin A may be teratogenic. Spontaneous abortions and birth defects have occurred with the consumption of the 13-cis RA form of vitamin A, such as found in the acne drug Accutane.

Recommended intake

The units of measure for vitamin A are micrograms retinol equivalents (mcg RE). This is equivalent to 1 mcg of retinol. International unit (IU) is used as a unit of measure also. One IU equals 0.3 mcg retinol. The Recommended Dietary Allowances (RDA) set by the National Academy of Sciences are as follows: 375 mcg RE for infants, 400-700 mcg for children, 1000 mcg for adult males, 800 mcg for adult females and pregnant women, and 1200-1300 mcg for lactating women.

Food Sources

Plants synthesize provitamin A carotenoids, which humans convert to retinol. Preformed vitamin A is found in animal foods only. The best sources of vitamin A include liver, yellow and green leafy vegetables, eggs, and whole milk products.

Figure %: Vitamin A Content of Selected Foods

Supplementation

The safe upper limit of vitamin A is 8000 to 10000 IU, or 3000 mcg RE. Supplements of 50,000 to 200,000 IU have been utilized to protect children from developing xerophthalmia for a period of four to six months. Retinoids are used in topical agents for hyperkeratoic disorders, acne, and skin cancer, due to their influence on epithelial cell proliferation. Vitamin A has also been used in cancer prevention. Epidemiological studies have shown an association of total vitamin A to reduced risk of epithelial cancer.

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