Vitamin A is a fat-soluble vitamin that occurs in nature either preformed Vitamin A or as provitamin or precursor Vitamin A. Preformed Vitamin A, such as retinol and retinal in eggs, is ready to be used by the body directly from food sources.

The carotenoids and carotenes are forms of provitamin or precursor Vitamin A. Carotenes (including beta-carotene) are a group of fat-soluble pigments found in orange, dark yellow, and dark green vegetables and fruits. Provitamin A can be converted into Vitamin A once ingested.

Researchers have identified over 600 active carotenoids, of which only 30-50 can be converted into Vitamin A. Beta- carotene is readily converted into Vitamin A and has many functions, including some which are independent of Vitamin A. Retinol, retinal and retinoic acid are fat-soluble Vitamin A derivatives vital to eye and retina function, that protect the mucous membranes of the mouth, nose, throat and lungs from damage, and reduce risk of infection and cancer. Low levels correlate with increased lung, larynx, esophagus, mouth, stomach, colon, prostate and cervix cancers.

Increased dietary intake of carotene-containing foods is associated with a lower incidence of certain lung, digestive tract, and other cancers. A study monitoring 8,000 men over a five-year period found that those with the lowest intake of beta- carotene had the greatest risk of getting lung cancer. Another long-term study of 2,000 men demonstrated that smokers with the lowest intake of beta-carotene were several times more likely to develop lung cancer than smokers with the highest intake of beta-carotene. (Hendler).

Because Vitamin A is a fat-soluble vitamin, it may be toxic at high levels. Deficiency of Vitamin A and carotenoids, however, are a much greater concern than taking these nutrients in excess. Extensive medical and pharmaceutical research have done with Vitamin A and carotene-derived substances, as evidenced by thousands of articles in numerous scholarly journals, and numerous diverse health care by products of that research, ranging from acne medication (Retin A) to anti-cancer compounds.

Food sources of Vitamin A include animal and fish liver, eggs, milk and butter. While you can overdose on fat-soluble Vitamin A supplements, large doses of water-soluble beta carotene, found in carrots, broccoli, spinach, cabbage, orange and yellow fruits, are non-toxic and remain an extremely potent source of exogenous antioxidant activity.

Individuals deficient in Vitamin A are more susceptible to infectious diseases and have higher mortality rates. Vitamin A stores are severely depleted during infection. Infectious conditions associated with Vitamin A deficiency include the measles, pneumonia, chicken pox, AIDS, chronic nephritis, and respiratory syncytial virus.

Vitamin A and its analogues inhibit cancer in numerous ways. They prevent tumor initiation and proliferation, attack and destroy cancer cells, and may actually reverse precancerous lesions. In addition to their anti-cancer properties, the potent anti-inflammatory and antioxidant properties of Vitamin A and carotenoids reduce some of the more toxic effects of radiotherapy and chemotherapy.

Pregnant women should not use high doses of Vitamin A; instead, use beta-carotene. Since Vitamin A is a fat-soluble vitamin stored in the body, toxicity is possible. Beta carotene or plant sources contain precursor Vitamin A, and cannot convert quickly enough to cause a toxic condition. A nonharmful temporary skin yellowing may occur when large amounts of carotene-rich foods, such as carrots or tomatoes are consumed.

Adults who consume an excess of 50,000 I.U. of Vitamin A per day for several years may develop toxicity. Smaller daily doses may result in toxicity if there are malfunctions in storage and transport of Vitamin A, which occurs in liver cirrhosis, hepatitis, inadequate protein consumption, and in children and adolescents. Symptoms of Vitamin A toxicity include dry, fissured skin, brittle nails, cracks in the corners of the mouth and chapped lips, fatigue, nausea, irritability, gingivitis, alopecia, and anorexia.

Selected References

Buring JE et al. beta-carotene and cancer chemoprevention. J Cell Biochem Suppl 22:226-30, 1995.

Dorgan JF et al. Relationships of serum carotenoids, retinol, alpha- tocopherol, and selenium with breast cancer risk: results from a prospective study in Columbia, Missouri. Cancer Causes Control 9(1):89-97, 1998.

Niles RM. Vitamin A and cancer. Nutrition 16(7-8):573-6, 2000.

Norrish AE, et al. Prostate cancer and dietary carotenoids. Am J Epidemiol 151(2):119-23, 2000.

Albanes D et al. Effects of supplemental alpha-tocopherol and beta-carotene on colorectal cancer: results from a controlled trial. Cancer Causes Control 11(3):197-205, 2000.

Close Window