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What is vitamin A?

Vitamin A is one of the four fat-soluble vitamins, so identified because they need fat or bile in order to be absorbed from the intestinal tract. These fat-soluble vitamins are also stored in fatty tissue (usually in the liver), providing a reserve for times when they may not be consumed in the diet.

There are two major forms of vitamin A — the retinols, which come mostly from animal products, and carotenoids (such as beta carotene), which are derived from plants. The retinols are referred to as the active form of vitamin A because the body can readily utilize them. In contrast, the carotenoids are provitamins or precursors that must be converted into the active form before they carry out the essential roles of vitamin A.

The body stores vitamin A in the liver and draws upon it for many functions, including prevention of night blindness.

Why is vitamin A important to nutrition?

First and foremost, vitamin A is the nutrient that is needed for normal vision. Night blindness is a common consequence of vitamin A deficiency. In some third world countries, vitamin A deficiency is a common cause of childhood blindness. Other important functions of vita­min A include:

• Forming normal teeth and bones.
• Fighting infection.
• Building and maintaining
  healthy skin, mucous membranes, and epithelium, the tissue that lines most organs.
• Facilitating reproduction.
  Recent research also indicates that vitamin A may help prevent cancers that originate in the epithelial cells. These include cancers of the prostate, lungs, and urinary tract.

How much vitamin A do I need?
The 1989 Recommended Dietary Allowances (RDAs) for Vitamin A, in retinol equivalents (RE), are:

• 375 for infants.
• 400 to 700 for ages 1 to 10.
• 1,000 for males over 11.
• 800 for females over 11.
• 800 during pregnancy.
• 1,300 during the first 6 months of breast feeding.
• 1,200 during the second 6 months of breast feeding. Many nutrition labels list

Many nutrition labels list vitamin A content in international units (IU) rather than retinol equivalents, a common source of confusion. In general, 1 RE from animal sources equals 3.5 IU's, and 1 RE from plant sources (beta carotene) equals 10 IU.

What happens if I don't get enough vitamin A?
Vitamin A deficiency is rare in the United States but relatively common in developing countries, especially in Africa. Night blindness and other visual defects are the most common consequences of vitamin A deficiency. Other manifestations of vitamin A deficiency include:

• Reduced growth.
• Lowered resistance to infection.
• Dried mucous membranes and reduced out-put of secretions such as saliva, tears, seminal fluid, and vaginal secretions.
• Thickened, rough skin that cracks easily.

What happens if I get too much vitamin A?
Since vitamin A is stored in the body, it can build to toxic levels — a condition called

Foods high in vitamin A.

hypervitaminosis A. It is almost impossible to develop hypervitaminosis from diet alone, but consuming high-dose retinols, either as a dietary supplement or in the form of the acne medication isotretinoin (Accutane), can lead to toxicity.

Signs of vitamin A toxicity include headache, blurred vision, diarrhea, joint and bone pain, rashes, dry skin, hair loss, itchiness, fatigue, and loss of appetite. These symptoms are associated with many illnesses, but if they occur when you are taking Accutane or a vitamin A supplement, you should stop the drug and consult your doctor. Dangers of vitamin A toxicity include:

• Serious birth defects or fetal death. Brain deformities, heart defects, ear and facial malformations, and endocrine abnormalities are among the birth defects associated with excessive vitamin A.
• Liver damage. Vitamin A is stored in the liver; thus, excessive intake can cause serious liver problems.
• Death. Perhaps the most celebrated example involved early explorers to the North Pole, who consumed large amounts of polar bear liver — the richest dietary source of vitamin A — and who are believed to have died of hypervitaminosis.

These dangers are associated only with retinols or other active forms of vitamin A. Large amounts of beta carotene or other vitamin A precursors may cause a yellowing of the skin, but they do not cause toxicity.

Should I take vitamin A supplements?
Active forms of vitamin A, including Accutane, should be taken only under careful medical supervision. Beta carotene is promoted as a possible cancer preventive, but more research is needed to determine whether it actually has this property. In the meantime, nutrition scientists advise that vitamin A, including beta carotene, should be obtained from the diet rather than from pills.