VITAMINS: Vitamin A (and Beta Carotene)

Functions of vitamin A
Deficiency of vitamin A
People with an increased risk of vitamin A deficiency
Recommended dietary allowance for vitamin A
Food sources of vitamin A
Clinical uses of vitamin A
Toxicity of vitamin A
References

Vitamin A (and Beta Carotene)

Functions of vitamin A

Necessary for growth & repair of body tissues.
Helps maintain smooth, soft disease-free skin.
Helps protect the mucous membranes of the mouth, nose , throat & lungs, thereby reducing susceptibility to infections.
Protects against air pollutants.
Counteracts night-blindness & weak eyesight; aids in bone and teeth formation.
Beta Carotenes are precursors of vitamin A. Unlike Vitamin A, Beta Carotene is non-toxic even in high doses. Current medical research shows that foods rich in Beta Carotene will help reduce the risk of lung cancer & certain oral cancers. Current clinical trials in prevention of cervix cancer and cancer of the lung and breast are using beta carotene. This may also be helpful in the prevention of colon cancer and melanoma.

Deficiency of vitamin A

Night blindness is one of the early signs of vitamin A deficiency.
Bacterial invasion and permanent scarring of the cornea of the eye (xerophthalmia) is a symptom of more profound deficiency.
Profound vitamin A deficiency also results in altered appearance and function of skin, lung, and intestinal tissues.

Children are most at risk of vitamin A deficiency because they have not yet developed adequate vitamin A stores. It has been estimated that 0.5 million children in the world become blind each year, 70% of these due to vitamin A deficiency.

People with an increased risk of vitamin A deficiency

Drinkers
Smokers
Women on the pill

Recommended dietary allowance for vitamin A

Food sources of vitamin A

Vitamin A is present in liver, eggs and fortified foods.
Beta-carotene is found in green leafy vegetables as well as in orange and red fruits and vegetables.

Clinical uses of vitamin A

Some derivatives are used to treat acne and skin wrinkling (trade name Accutane, also known as isotretinoin).
Other derivatives are used to treat breast cancer (4-HPR, Fenretinide). No one should consume vitamin A in quantities exceeding the RDA without a doctor's advice because of the dangers of toxicity.

Toxicity of vitamin A

Acute toxicity (>200 mg RE in adults) can cause nausea, vomiting, headache, and increased cerebrospinal pressure. Symptoms are generally transient.
Chronic toxicity (e.g., >10x RDA) can cause hair loss, bone and muscle pain, headache, liver damage, and increased blood lipid concentrations. A particular danger in pregnant women is teratogenesis (birth defects).
On the other hand, Beta Carotenes as a source of vitamin A are not toxic, even with very high intakes.

References

Ross, C. A. (1999) Vitamin A and retinoids. In: Modern Nutrition in Health and Disease (Shils, M. E., Olson, J. A., Shike, M. & Ross, C. A., eds.), 9th ed., pp. 305-328. Williams & Wilkins, Baltimore, MD.
Sporn, M. B., Roberts, A .B. & Goodman, D.S., eds. (1994) The Retinoids, 2nd ed. Raven Press, New York, NY.