Vitamin A is a fat-soluble vitamin necessary for maintaining vision, epithelial (mucous) membranes, bone growth, and immunity. Most commonly known for its importance in preventing "night-blindness," vitamin A is an important part of the visual pigment molecules present in all retinal cells (both rods and cones) and plays a vital role in both day and night vision. Vitamin A has long been known as the "anti-infective" vitamin and plays an important role in the proper functioning of the immune system. Vitamin A maintains the integrity of all mucous membranes, which is the body's first line of defense against infection. When the body is deficient in vitamin A, mucus-producing cells are replaced by keratin-producing cells. The secretion of mucus diminishes and the mucous membranes become tough and relatively inflexible, leaving the body defenseless against invading organisms. Vitamin A stimulates white blood cell function and increases antibody response.

Vitamin A is required during the growth of cell membranes and also for repair of cell membranes during wound healing. Other functions of vitamin A include glycogen synthesis, protein metabolism, hormone synthesis, and as a coenzyme in the skin, bone, retina, liver, and adrenal glands.

Vitamin A is only found in foods of animal origin. Beef, calf, and chicken liver are the richest sources. Vitamin A is also found naturally in whole milk and butter. Lowfat and skim milk is fortified to replace the vitamin A that is lost when the fat is removed from the milk. Dairy products supply about half of the vitamin A consumed by most individuals. The other half is supplied by fruits and vegetables that are rich in beta-carotene, the vitamin A precursor that is readily converted to vitamin A in the body. Beta-carotene is abundant in orange and dark green leafy vegetables and fruits. Refer to the section on beta-carotene for a full listing of good food sources.

There are three different forms of vitamin A: retinol, retinal, and retinoic acid. Retinal and retinoic acid appear to be the active forms of retinol. Retinal is primarily involved with vision and reproduction, while retinoic acid is important for growth and cell differentiation.

Natural vitamin A is available as retinol or retinyl palmitate. All forms are easily absorbed. Supplemental vitamin A is available in tablets or capsules of 10,000 IU, 25,000 IU, or 50,000 IU.

• Treatment of skin disorders, primarily acne and psoriasis
• Immune system enhancement, especially against viral infections
• Reduces infant mortality from measles
• Treatment of severe burns and other wounds
• Night blindness; hyperkeratosis

As of 1980, vitamin A is measured as retinol equivalents or RE. One RE corresponds to the biological activity of 1 mcg of retinol. However, supplemental vitamin A is still most commonly expressed in international units or IU. One RE is equal to 3.33 IU of retinol.

The current RDAs expressed in RE are 1,000 mcg RE for men, and 800 mcg RE for women. However, for convenience in determining supplement dosages, the following RDA information is expressed in both RE and IU.

• Infants up to 1 year:1,875 IU400 mcg RE
• 1 to 3 years:2,000 IU400 mcg RE
• 4 to 6 years:2,500 IU500 mcg RE
• 7 to 10 years:3,500 IU700 mcg RE
• Males over 10 years: 5,000 IU1,000 mcg RE
• Females over 10 years:4,000 IU800 mcg RE
• Pregnant women:5,000 IU1,000 mcg RE
• Lactating women:6,500 IU1,200 mcg RE

To reverse vitamin A deficiency or to prevent deficiency during acute viral infection, a single oral dose of 50,000 IU for one or two days is safe even for infants. Adults and children over 8 years can tolerate up to 50,000 IU daily for up to two weeks. In general, daily doses of up to five times the RDA (25,000 IU for adults and 12,000 IU for children) have been found to be safe. However, a health care provider must continue to monitor any high dose therapy. This applies especially to treatments for skin disorders. For other health issues, beta-carotene is the preferred supplement, as it may be administered in significantly higher doses without the same risk of toxicity. Pregnant women must avoid any type of vitamin A supplements (beyond what is found in prenatal vitamins), due to the higher risk of birth defects.

Vitamin A toxicity (hypervitaminosis A) has been widely studied. Tolerance varies among individuals and may be related to liver function status as well as other variables such as body weight. Doses of 1 million IUs taken daily for five years have been tolerated by some people, while others show symptoms of toxicity with doses as low as 25,000 IU per day. Children are most susceptible to vitamin A toxicity, especially if they take multiple vitamins along with fortified foods. Vitamin A toxicity in the fetus can occur if pregnant women take more than 6,000 IU per day.

Overdoses or chronic excesses affect the same body systems that are affected by vitamin A deficiency, and some of the symptoms of toxicity and deficiency are the same. The most common symptom of toxicity is increased intracranial pressure exhibited as chronic headache. Other symptoms of vitamin A toxicity and deficiency are as follows: abdominal pain, muscle and joint pain, fatigue, dry, cracking skin and lips, dry eyes, conjunctivitis, alopecia, anorexia, nausea, diarrhea, leukocytosis, and bone fractures.

Supplementation with vitamin A over 6,000 IU/day is associated with birth defects, and pregnant women should avoid any vitamin A treatments or supplements beyond what is contained in prenatal vitamins.

Patients must be closely monitored by their provider for any symptoms of toxicity during high dose therapy for skin conditions using isotretinoin (Accutane).

Most multivitamins contain the RDA of 5,000 IU of vitamin A. Dairy products and fortified foods contribute an additional 2,500 IU/day or more. Patients should be careful of using additional fish liver oil supplements or products formulated for "eye health," "immune system enhancement," "skin formulas," "acne formulas," or "bone or joint repair." Any of these formulas are likely to contain additional vitamin A. Always check the labels for vitamin A content, and be careful that daily intake does not exceed the recommended safety levels.

Vitamin A is contraindicated for use in individuals with chronic kidney or liver disease.

Overuse of alcohol makes vitamin A toxicity more likely to occur.

A multi-center, randomized, prospective study involving 60 patients with chronic gastric ulcers evaluated the effects of vitamin A supplementation on ulcer healing (Patty et al. 1983). Patients were divided into three groups and treated for four weeks with antacids only, antacids plus vitamin A (50,000 IU po tid), or antacids, vitamin A, and cyproheptadine (4 mg po tid). Endoscopic analysis revealed that gastric ulcers were completely healed in 15 out of 40 patients who received vitamin A. The reduction in ulcer size was greater in the group receiving vitamin A plus antacids after both two and four weeks of use. This study suggests that the combination of vitamin A and antacids may be more effective than antacids alone to heal ulcers.

HMG-CoA reductase inhibitors may increase blood vitamin A levels (Muggeo et al. 1995).

In one study involving 17 patients with severe cystic acne, subjects received isotretinoin (0.5mg/kg/day) for 3 months; eight patients continued to receive isotretinoin (0.75 mg/kg/day) for another 3 months (Rollman and Vahlquist 1986). After 3 months of therapy, epidermal retinol content increased by 53%, while dehydroretinol decreased by 79%. These findings were corroborated in another study with 22 subjects (Vahlquist et al. 1990). Administration of isotretinoin (1 mg/kg/day) for four months led to a marked increase in retinol and a decrease in dehydroretinol content in the skin. Individuals should avoid consuming high doses of vitamin A while taking tretinoin unless prescribed by a physician.

In a double-blind study in five healthy males, study participants consumed a standard meal containing vitamin A (300,000 IU) followed by either neomycin sulfate (2 g) or placebo (Barrowman et al. 1972). The rise in plasma vitamin A levels was significantly less after neomycin treatment in all study subjects. Neomycin interferes with the absorption of vitamin A when delivered in a very large bolus dose. The significance of this interaction on daily supplementation of vitamin A is unknown.

Omeprazole significantly suppressed the blood response to beta-carotene (120 mg po) in healthy subjects (Tang et al. 1996). Serum beta-carotene levels were significantly greater during the low gastric pH phase of the study compared to the high gastric pH phase. It is not known if omeprazole or lansoprazole adversely affect carotenoid absorption from foods.

Oral contraceptives increase the levels of vitamin A in women (Tyrer 1984).

Barrowman J, Broomhall J, Cannon A, et al. Impairment of vitamin A absorption by neomycin. Clin Sci. 1972;42:17P.

Eades MD. The Doctor's Complete Guide to Vitamins and Minerals. New York, NY: Dell Publishing; 1994:48.

Fawzi WW. Vitamin A supplementation and child mortality. JAMA. 1993;269:898-903.

Fawzi WW, Mbise RL, Hertzmark E, et al. A randomized trial of vitamin A supplements in relation to mortality among human immunodeficiency virus-infected and uninfected children in Tanzania. Pediatr Infect Dis J. 1999;18:127-133.

Fortes C, Forastiere F, Agabiti N, et al. The effect of zinc and vitamin A supplementation on immune response in an older population. J Am Geriatr Soc. 1998;46:19-26.

Futoryan T, Gilchrest BA. Retinoids and the skin. Nutr Rev. 1994;52:299-310.

Kindmark A, Rollman O, Mallmin H, et al. Oral isotretinoin therapy in severe acne induces transient suppression of biochemical markers of bone turnover and calcium homeostasis. Acta Derma Venereol. 1998;78:266-269.

Melhus H, Michaelsson K, Kindmark A, et al. Excessive dietary intake of vitamin A is associated with reduced bone mineral density and increased risk for hip fracture. Ann Intern Med. 1998;129:770-778.

Muggeo M, Zenti MG, Travia D, et al. 1995. Serum retinol levels throughout 2 years of cholesterol-lowering therapy. Metab. 1995;44(3):398-403.

Murray M. Encyclopedia of Nutritional Supplements. Rocklin, Calif: Prima Publishing; 1996.

Nursing '93 Drug Handbook. Springhouse, Pa: Springhouse Corporation; 1993.

Patty I, Benedek S, Deak G, et al. Cytoprotective effect of vitamin A and its clinical importance in the treatment of patients with chronic gastric ulcer. Int J Tissue React. 1983;5:301-307.

Rollman O, Vahlquist A. Oral isotretinoin (13-cis-retinoic acid) therapy in severe acne: drug and vitamin A concentrations in serum and skin. J Invest Dermatol. 1986;86(4):384-389.

Semba RD. Vitamin A, immunity and infection. Clin Infect Dis. 1994;19:489-499.

Tang G, Serfaty-Lacrosniere C, Camilo ME, Russell RM. Gastric acidity influences the blood response to a beta-carotene dose in humans. Am J Clin Nutr. 1996;64(4):622-626.

Tyrer LB. Nutrition and the pill. J Reprod Med. 1984;29(7 Suppl):547-550.

Vahlquist A, Rollmann O, Hollan D, Cunliffe, W. Isotretinoin treatment of severe acne affects the endogenous concentration of vitamin A in sebaceous glands. J Invest Dermatol. 1990;94:496-498.

Whitney E, Cataldo C, Rolfes S. Understanding Normal and Clinical Nutrition. St. Paul, Minn: West Publishing Company; 1987.