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.
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.
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.
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.
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.
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.
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.