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Core Knowledge

Nutrition

A Guide to Vitamin A

Overview

Vitamin A is an essential, fat-soluble nutrient needed for the health of many parts of your body, including the teeth, eyes, bones, skin, cells, and body systems (respiratory, urinary and intestinal). Perhaps as much as any substance in our diet, vitamin A is a disease-fighting, infection-blocking powerhouse that goes after viruses and bacteria.

Martin Harvey / Getty Images

Vitamin A is widely available in the American diet. It comes in two forms, preformed vitamin A and provitamin A carotenoid, and from two sources, animals and plants. Vitamin A deficiencies are rarely seen in the U.S., but in developing countries it's been estimated that 500,000 preschool-age children worldwide go blind each year as a result of vitamin A deficiency, while millions of others suffer from night blindness. Estimates suggest that more than 100 million children worldwide suffer from vitamin A inadequacy without showing clinical signs of acute deficiency.

Vitamin A by the Numbers

3,000 IUs
The Recommended Dietary Allowance for vitamin A (adult men)

2,310 IUs
The Recommended Dietary Allowance for vitamin A (adult women)

35–50
The percentage of adults in the U.S. who regularly take vitamin and mineral supplements

How Vitamin A Works

Vitamin A isn't just one substance, but rather a combination of compounds identified by different names. The type of vitamin A that comes from animals is called preformed vitamin A, and it's absorbed in the form of retinol. It helps the linings of several body systems, the skin, and the mucus membranes by keeping them from breaking down, making it harder for disease agents to penetrate the body’s lines of defense.

Colorful fruits and vegetables contain a type of vitamin A known as provitamin A carotenoid, which can also be made into retinol. Common carotenoids include substances such as beta-carotene, alpha-carotene and beta-cryptoxanthin. Of the three, beta-carotene is most efficiently transformed into retinol.

The Institute of Medicine recommends including plenty of carotenoid-containing fruits and vegetables in your diet. There is some evidence, but it isn't conclusive, that carotenoids function as antioxidants, which protect cells against the damage caused by free radicals. Free radicals can damage organs as a by-product of oxygen metabolism.

How Much Vitamin A

The Recommended Dietary Allowance for vitamin A is 900 mcg (3,000 IUs) a day for adult men and 700 mcg (2,310 IUs) for adult women. Tolerable Upper Intake Levels for both male and female adults are 3,000 mcg (10,000 IUs). The recommended amounts for pregnant and lactating women are slightly higher. Toxicities of vitamin A relate only to preformed vitamin A, usually labeled as retinol, retinyl acetate, or retinyl palmitate. There is no RDA specifically for beta-carotene or other provitamin A carotenoids.

Too much vitamin A can cause birth defects and vitamin A poisoning, known as hypervitaminosis, which could happen among adults who regularly take more the 25,000 IUs a day. Children are even more susceptible to this issue. Although ingestion of too much preformed vitamin A (retinol) can be toxic, excessive intake of beta-carotene isn't known to induce vitamin A toxicity. Negative feedback mechanisms in the body prevent the over-conversion of beta-carotene to retinol. However, high levels of beta-carotene in the diet can induce hypercarotenosis, a benign condition characterized by a yellowing of the skin. Symptoms can be reversed when dietary intake is reduced. You should be aware of how much beta-carotene you're consuming.

In an October 2009 study in The FASEB Journal, researchers at Sloan-Kettering Institute for Cancer Research found that retinol, the key player in vitamin A, is “essential for the metabolic fitness of mitochondria and acts as a nutritional sensor for the creation of energy in cells.” Mitachondria are the main source of energy within cells, converting nutrients into energy. When the body gets or produces too much or too little vitamin A, the mitachondria don't function normally and begin damaging our organs.

Vitamin A Sources

Animal Sources

Vitamin A-rich animal sources include the foods below. Keep in mind that all of the foods below (except skim milk) are high in cholesterol and saturated fat. 

  • Beef
  • Chicken liver
  • Whole milk
  • Fortified skim milk
  • Cheddar cheese
  • Egg substitutes

Plant Sources

About a quarter of vitamin A consumed by men and a third consumed by women in America comes in the form of provitamin A carotenoid. The foods below can be processed into retinol. 

  • Carrots
  • Spinach
  • Kale
  • Vegetable soup
  • Cantaloupe
  • Apricots
  • Papaya
  • Mango
  • Oatmeal
  • Peas
  • Peaches

Vitamin A Supplements

Healthy adults have plenty of vitamin A stored away in their bodies and shouldn't need to take a vitamin A supplement, although many over-the-counter multivitamins contain them. Long-term problems absorbing fat, according to the National Institute of Health Office of Dietary Supplements, may result in a deficiency. In this case, your doctor may recommend added vitamin A.

People who don't eat eggs and dairy products may need a provitamin A carotenoid supplement, and they should include at least five servings of fruits and vegetables in their diet every day, as well as regular servings of dark green leafy vegetables and orange or yellow fruits.

Vitamin A and Athletic Performance

Other than consuming all vitamins and minerals for general well-being, there is no evidence supporting vitamin A supplements for athletic performance. Athletes who don't get enough calories, are involved in severe weight loss programs, or who eliminate certain food groups from their diets should check with a sports dietitian or physician about vitamin supplements.

Take-Home Advice

The formula for maintaining adequate amounts of vitamin A is relatively simple. The recommended intake is about 2,300 to 3,000 IUs of vitamin A daily, and the fat-soluble vitamin is widely available from both animal and plant sources in the American diet. Brightly colored fruits and vegetables are packed with vitamin A, while dairy products are among the foods that come from animal sources. If your multivitamin contains vitamin A, make sure that it's in the form of beta carotene. Dark leafy greens and fruits and vegetables that have an orange or yellow hue are your best sources of vitamin A. Aim to get three colors coming from fruits and vegetables at each meal.


Tags: Supplements, Vitamins, Health

References

  1. American Journal of Clinical Nutrition
  2. Canadian Journal of Dietetic Practice and Research
  3. Dietary Supplement Fact Sheet: Vitamin A and Carotenoids (from the National Institute of Health)
  4. eMedicine Endocrinology
  5. http://www.usana.com/media/File/dotCom/company/science/components/Beta_Carotene.pdf
  6. Institute of Medicine
  7. Integrative Medicine
  8. Journal of the American Dietary Association
  9. Medical News Today
  10. Medline Plus
  11. National Institutes of Health (Office of Dietary Supplements)
  12. Wellness and Prevention
     

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