Articles

Vitamin A

Introduction

This vital nutrient wasn’t even known about until 1913, when scientists discovered it could prevent night blindness. Then, in 1932 it was discovered that beta-carotene, also known as pro-vitamin A, was the precursor to vitamin A. When we consume beta-carotene, vitamin A is produced naturally by enzymes in the digestive tract that break beta-carotene down. The fat-soluble vitamin A is then stored in the liver, where it can remain for long periods of time.

Vitamin A occurs only in animal products like liver, kidney, butter, egg yolks, whole milk and fortified skim milk. Meanwhile, beta-carotene is found in yellow fruits and vegetables like carrots, sweet potatoes, squash, apricots and cantaloupe, and in dark leafy vegetables like collards and spinach. (See section on beta-carotene)

Dosage Info

Dosage Range

2,000-35,000 IU daily. For some conditions, doses greater than 100,000IU daily are used for short periods of time, but high doses such as these should be supervised by a physician.

Most Common Dosage

5,000IU daily.

1mcg vitamin A = 3.33IU vitamin A

1mcg all-trans retinol (dietary)=
1RE (retinol equivalent) =
6mcg of all-trans beta-carotene (dietary)

1mcg retinol =
1RAE (retinol activity equivalents) =
2mcg beta-carotene (supplement) =
12mcg beta-carotene (dietary)

Dosage Forms

Tablets, gel capsules, emulsified liquid drops, micellized liquid drops, and injectable (Rx only).

Interactions and Depletions

Interactions

Depletions

Reported Uses

The use of vitamin A supplements supports vision and prevents drying of the cornea. It also plays an important role in maintaining the integrity of epithelial tissues, which consist of the mucous membrane-secreting cells that line all the glands and organs of the body. Many studies report that adequate intake of vitamin A is associated with reduced risk of various epithelial-cell cancers (mouth, skin, lungs, bladder, breast, stomach, cervix, etc.) (1) Because it helps maintain healthy epithelial cells, vitamin A also helps the body create effective barriers to infection, thereby boosting overall immunity. (2) Finally, it is essential for the growth of bones, teeth and soft tissues. A deficiency of Vitamin A can contribute to bone loss associated with osteoporosis. (3)

There are a number of acute clinical applications for vitamin A. One should consult with a physician before using vitamin A to treat any disease, however. Patients with acne have reported improvement in studies when vitamin A is given in high doses and over extended periods of time. (4) Vitamin A supplementation in AIDS patients may help increase life expectancy for patients with vitamin A deficiencies. (5) Possible benefits for women include the lessening of risk for cervical dysplasia, easing of heavy menstrual flow, and relief from PMS symptoms. (6) , (7) , (8) Other studies have suggested that vitamin A can lessen the severity of measles and Crohn’s disease. (9) , (10) .

Of concern are some findings regarding vitamin A in The Nurses' Health Study. The Nurses' Health Study has evaluated the health over 72,000 postmenopausal women 34 to 77 years old for a variety of reasons. One group of investigators evaluated the relationship between high vitamin A intake from foods and supplements and the risk of hip fracture among 72,337 postmenopausal women. Women in the highest group of vitamin A intake had higher risk of a hip fracture than the women in the group with the lowest intake of vitamin A. Women using estrogen decreased this risk and the use of beta-carotene did not significantly increase the fracture risk. (11) Women are not the only individuals effected by high doses of vitamin A. Researchers have found that the risk of bone fractures was more prevalent among men with higher serum levels of vitamin A. (12)

Toxicities & Precautions

Introduction

[span class=alert]Be sure to tell your pharmacist, doctor, or other health care providers about any dietary supplements you are taking. There may be a potential for interactions or side effects.[/span]

General

Since vitamin A is fat soluble, excesses can accumulate in fatty tissues to toxic levels. (13) Consult a doctor if you have been taking high doses of vitamin A over a long period to check for possible symptoms.

Pregnancy / Breast Feeding

Doses greater than 10,000 IU have caused birth defects, especially during the first seven weeks of pregnancy. (14) Women who could potentially become pregnant should limit their daily vitamin A levels to less than 10,000 IU daily.

This dietary supplement should not be used if you are breast-feeding an infant without first consulting a physician.

Age Limitations

To date, the medical literature has not reported any adverse effects specifically related to the use of this dietary supplement in children. Vitamins and minerals are an essential part of proper growth and development. Talk to your healthcare professional about the appropriate use of vitamins and minerals in children. Do not use any vitamin or mineral in children under 2 years of age unless first discussed with your healthcare professional.

References

  1. View Abstract: Basu TK. Vitamin A and Cancer of Epithelial Origin. J Hum Nutr. Feb1979;33(1):24-31.
  2. View Abstract: Hof H. Vitamin A: The ‘Anti-infective’ Vitamin? MMW Munch Med Wochenschr. Nov1976;118(46):1485-88.
  3. View Abstract: Advani S, Wimalawansa SJ. Bones and nutrition: common sense supplementation for osteoporosis. Curr Womens Health Rep. 2003 Jun;3(3):187-92.
  4. View Abstract: Kligman AM, et al. Oral Vitamin A in Acne Vulgaris. Preliminary Report. Int J Dermatol. May1981;20(4):278-85.
  5. View Abstract: Semba RD, et al. Increased Mortality Associated with Vitamin A Deficiency during Human Immunodeficiency Virus Type 1 Infection. Arch Intern Med. Sep1993;153(18):2149-54.
  6. View Abstract: Wylie-Rosett JA, et al. Influence of Vitamin A on Cervical Dysplasia and Carcinoma in Situ. Nutr Cancer. 1984;6(1):49-57.
  7. View Abstract: Lithgow DM, et al. Vitamin A in the Treatment of Menorrhagia. S Afr Med J. Feb1977;51(7):191-93.
  8. Block E. The Use of Vitamin A in Premenstrual Tension. Acta Obstet Bynecol Scand. 1960;39:586-92.
  9. View Abstract: Wylie-Rosett JA, et al. Influence of Vitamin A on Cervical Dysplasia and Carcinoma in Situ. Nutr Cancer. 1984;6(1):49-57.
  10. Dvorak AM. Vitamin A in Crohn's Disease. Lancet. Jun1980;1(8181):1303-04.
  11. View Abstract: Feskanich D, Singh V, Willett WC, Colditz GA. Vitamin A intake and hip fractures among postmenopausal women. JAMA. Jan2002;287(1):47-54.
  12. View Abstract: Michaëlsson K, et al. Serum Retinol Levels and the Risk of Fracture. New Eng J Med. Jan2003;348(4):287-294.
  13. View Abstract: Russell RM. The vitamin A spectrum: from deficiency to toxicity. Am J Clin Nutr. Apr2000;71(4):878-84.
  14. View Abstract: Rothman KJ, et al. Teratogenicity of High Vitamin A Intake. N Engl J Med. Nov1995;333(21):1369-73.