encyclopedia

Vitamin A

Natural Sources:
Cod liver oil, liver, egg yolk, fish, whole milk, butter and cheese

Forms:
Standardized vitamin A capsules, tablets and liquid supplements; multivitamin pills containing vitamin A.

Therapeutic Uses:
– Acne

– AIDS

– Aging Disorders

– Alzheimer’s Disease

– Antinflammatory

– Antioxidant

– Cataracts

– Cellular Regeneration

– Cleansing

– Detoxification

– Emphysema

– Eyesight Disorders

– Heart Health Maintenance

– HIV Infection

– Immune System

– Leukemia

– Lupus Nephritis

– Macular Degeneration

– Premenstrual Syndrome (PMS)

– Skin Hyper-pigmentation

– Skin Problems

– Stroke Prevention

– Sunburn

– Ulcers

– Vascular Disorders

– Vitamin A Deficiency (RDA=2.3-7.8mg/day)

– Wrinkles

Overview:

Vitamin A, a fat-soluble vitamin otherwise known as retinol, is most well known for its benefits for the eyes. Vitamin A is also involved in the formation and maintenance of skin, hair, and mucous membranes. Vitamin A specifically helps us to see in dim light and is necessary for proper bone growth, tooth development, and reproduction. Vitamin A and its analogues influence differentiation and proliferation of cells, and also strengthen immune responses.

Chemistry:

Vitamin A, also known as retinol and axerophthol, is a fat-soluble vitamin described in chemical terms as a hydrocarbon (polyene) chain terminated by a cyclic end-group, specifically a b-ionone ring, and terminating in an alcohol group at the other end of the conjugated hydrocarbon chain. Recent research has identified several different forms of retinol. Vitamin A can be obtained in the diet in two principal ways: 1) as retinol, which is found only in animal sources, and 2) via certain carotenoids (provitamins), which are found only in plant sources. Carotenoids are the compounds that give many fruits and vegetables their yellow and orange colour. The most abundant and best known of the carotenoids is beta-carotene. Beta-Carotene is a precursor of vitamin A, or “provitamin A”, because its vitamin A activity occurs only upon conversion to retinol within the body. One molecule of beta-carotene can be cleaved by a specific intestinal enzyme into two molecules of vitamin A (note, however, that carotenoids are not as readily absorbed as retinol so actually six molecules of beta-carotene are equivalent to 1 molecule of retinol).

New International Measurements:

Until recently, vitamin A activity in foods was expressed as international units (IU). This is still the measurement generally used on food and supplement labels. However, it is difficult to calculate the total vitamin A activity in the diet in terms of IU, because both the absorption and conversion of carotenoids, as compared with retinol, are variable. In order to standardize vitamin A measurement, it has now been internationally agreed to state vitamin A activity as a new unit called retinol equivalents or RE. By definition, one retinol equivalent is equal to: 1 RE=1 ΅g retinol=6 ΅g beta-carotene=12 ΅g other provitamin A carotenoids=3.33 IU vitamin A activity from retinol=10 IU vitamin A activity from beta-carotene. [Equivalents information from: F. Hoffmann-La Roche Ltd. 2000. Vitamin A Information Sheet]. Note: To make these conversion less confusing, remember that although 1 molecule of beta-carotene is cleaved into two molecules of retinol, that beta-carotene is poorly absorbed by the gastrointestinal tract and therefore you need 6 molecules of beta-carotene to be equivalent to one molecule of retinol.

Suggested Amount:
Experts recommend getting preformed vitamin A (retinol) in dosages no greater than 5000 to 10,000 IU daily and getting additional fortification from beta-carotene and other carotenoids. The U.S. Recommended Dietary Allowance (RDA) for vitamin A for adults is 1000 RE for men and 800 RE for women. Mothers who are breastfeeding are recommended to take an additional 400-500 RE per day. Generally, dietary intakes of up to 10 times the RDA are considered safe. During pregnancy, daily administration of vitamin A of 10 000 IU should not be exceeded. Infants and children, due to their smaller body size, have a proportionately lower RDA than adults. Note: Fat-soluble vitamins, including vitamins A, E, D and K, are usually absorbed with the help of foods that contain fat. The RDA for beta-carotene as a source of provitamin A is 2.3-7.8mg daily. Natural food sources of beta-carotene include: Carrot juice (24.2mg/cup), sweet potatoes (10mg per medium potato), dried apricots (6.2mg in 10 halves), carrots (5.7mg per medium carrot), cooked spinach (4.9mg per cup), cantaloupe (4.0mg per 1/8th), cooked pumpkin (3.7mg per half cup), and cooked broccoli (1.0mg per half cup).

Drug Interactions:
None known.

Contraindications:
None known. In pregnancy daily administration of vitamin A of 10 000 IU should not be exceeded. Dosages for infants, young children and women of childbearing age should be respected in order to avoid potential toxic overdoses of vitamin A.

Side Effects:

Excessive vitamin A in the diet can cause liver damage and other toxic effects. Because vitamin A (as retinol) is stored in the liver, large amounts taken over a period of time can exceed the liver’s storage capacity, build up in the blood and produce many adverse effects. Many multi-vitamin pills supply beta-carotene because it is much safer than pre-made vitamin A (retinol) in this respect. Successful nutrition intervention programs in countries where vitamin A deficiency is prevalent indicate that single oral doses of 200 000 IU vitamin A in children and 400 000-500 000 IU in adults are safe. However, these are prophylactic doses, given at quite high levels in order to replenish low body stores for at least six months. In well-nourished people, vitamin A toxicity can occur acutely following very high doses (over 500 000 IU) taken over a period of a few days, or as a chronic condition from high doses (over 50 000 IU) taken over a long period of time. Beta-carotene is considered a safe form of vitamin A because the body converts it only as needed. Beta-carotene is poorly absorbed from the gastrointestinal tract, and its conversion to retinol becomes progressively less efficient as vitamin A status improves. High intakes (over 30 mg/day) of beta-carotene, however, may result in an orange-yellow coloration of the skin, which is reversible upon cessation of beta-carotene intake. Italian studies using 90mg/day of beta-carotene have shown no significant signs of toxicity. Hundreds of animal studies using very high dosages of beta-carotene have also detected virtually no toxicity. Note: When using beta-carotene as a source of vitamin A, it is best to stick with natural sources of carotenoids because synthetic beta-carotene supplements (100% All-trans isomer) do not confer all of the same health benefits. At least two different clinical trials of supplemental beta-carotene (using synthetic 100% all-trans isomer) for helping smokers to prevent lung cancer have been stopped prior to completion due to a 27% increased risk of death from lung cancer among participants.

Vitamin A Deficiency Symptoms:

Vitamin A deficiency is common among the elderly and urban poor, people who abuse laxatives, and alcoholics. Deficiency of vitamin A is also still a major cause of premature death in developing nations, particularly among children. One of the early symptoms is night blindness, the inability to see in dim light. If the deficiency persists, total blindness (xerophthalmia) can occur. The appearance of skin lesions (follicular hyperkeratinosis) has also been used as an early indicator of inadequate vitamin A status. In children with xerophthalmia, concurrent problems such as stunted growth, respiratory diseases, diarrhea, and parasitic and infectious diseases are common. Diseases themselves may induce vitamin A deficiency, most notably liver and gastrointestinal diseases which interfere with the absorption and utilization of vitamin A. Poor vitamin A status is also thought to be involved in the development of cancer, although the precise mechanisms are not yet known.

References:

Burg G, Dummer R. 2000. Historical perspective on the use of retinoids in cutaneous T-cell lymphoma (CTCL). Clin Lymphoma. 2000 Nov;1 Suppl 1:S41-4. Review.

Kang S, Leyden JJ, Lowe NJ, Ortonne JP, Phillips TJ, Weinstein GD, Bhawan J, Lew-Kaya DA, Matsumoto RM, Sefton J, Walker PS, Gibson JR. 2001. Tazarotene cream for the treatment of facial photodamage: a multicenter, investigator-masked, randomized, vehicle-controlled, parallel comparison of 0.01%, 0.025%, 0.05%, and 0.1% tazarotene creams with 0.05% tretinoin emollient cream applied once daily for 24 weeks. Arch Dermatol. 2001 Dec; 137(12): 1597-604.

Mao JT, Goldin JG, Dermand J, Ibrahim G, Brown MS, Emerick A, McNitt-Gray MF, Gjertson DW, Estrada F, Tashkin DP, Roth MD. 2002. A pilot study of all-trans-retinoic acid for the treatment of human emphysema. Am J Respir Crit Care Med. 2002 Mar 1; 165(5): 718-23.

Tallman MS, Nabhan C, Feusner JH, Rowe JM. 2002. Acute promyelocytic leukemia: evolving therapeutic strategies. Blood. 2002 Feb 1; 99(3): 759-67. Review.

Villamor E, Mbise R, Spiegelman D, Hertzmark E, Fataki M, Peterson KE, Ndossi G, Fawzi WW. 2002. Vitamin A supplements ameliorate the adverse effect of HIV-1, malaria, and diarrheal infections on child growth. Pediatrics. 2002 Jan; 109(1): E6.

Additional Information:

The Roles of Vitamin A:

Maintaining adequate levels of vitamin A within the body is essential for supporting many cellular and bodily functions and particularly for: 1) vision, 2) for adequate growth, 3) for tissue differentiation 4) for proper glycoprotein synthesis that affects important cell membrane functions, and 5) for strengthening immunity. 1. The rod cells in the retina of the eye contain a light-sensitive pigment called rhodopsin, which is a complex of the protein opsin and vitamin A. When a rod cell is exposed to light, the rhodopsin disintegrates, releasing electrical charges to the brain. These stimuli are then translated into a composite picture that we “see.” At the same time, new rhodopsin is formed in the visual cells from opsin and vitamin A. Thus, vitamin A acts as a sensor of light – and the eyes require a constant supply. 2. Vitamin A also plays an important role in normal growth and development. One of the primary signs of vitamin A deficiency in animals is loss of appetite followed by growth retardation. 3. Vitamin A is also necessary for normal differentiation of epithelial cells (cancers most often originate from epithelial cell layers where rapid cell division occurs). Epithelial cells include all the tissues lining the body, such as skin, mucous membranes, blood vessel walls and the cornea. In vitamin A deficiency, cells lose their ability to differentiate properly. Because there are many different types of cells in the body that perform highly specialized functions, it is vital that the process whereby cells and tissues become “programmed” to carry out their special functions (called differentiation) is properly supported nutritionally by adequate dietary levels of vitamin A. 4. Research shows that vitamin A is also involved in glycoprotein synthesis. Animals deficient in vitamin A produce abnormal glycoproteins and it is now understood that retinyl phosphate (a metabolite of retinal) reversibly binds to mannose, which is used in the synthesis of glycoproteins – compounds important for proper cell membrane functioning, inter-cellular communication and cell surface receptors. 5. Vitamin A supplementation also strengthens the body’s immune system. For example, in one study, the administration of 400 000 IU of vitamin A to children with measles complications (without signs of vitamin A deficiency), decreased mortality by over 50% and significantly lowered morbidity [Microsoft Encarta; and F. Hoffmann-La Roche Ltd. 2000. Vitamin A Information Sheet].

Positive Clinical Findings:

Since 1995, extensive clinical studies and practical use of a retinol-based emollient cream (0.05%) has demonstrated the safety and efficacy of vitamin A in the treatment of fine facial wrinkles, mottled hyper-pigmentation and skin roughness. Two controlled clinical studies were conducted to evaluate the safety and efficacy of a lower concentration formulation (0.02%) in the treatment of moderate-to-severe facial photo-damage (i.e. sunburn). Results show statistically significant improvement in fine wrinkling, coarse wrinkling and yellowing with the use of the cream after 24 weeks compared with placebo. Recent groundbreaking studies have also shown that all-trans retinoic acid induces differentiation of leukemic cells into mature granulocytes and represents an important advance in cancer treatment. Because of this discovery, acute pro-myelocytic leukemia is now the most curable subtype of acute myeloid leukemia in adults. For many suffers, supplementation with vitamin A brings a high complete remission rate, leads to a rapid resolution of the characteristic life-threatening blood coagulation abnormalities, and, most importantly, decreases the relapse rate compared with treatment with chemotherapy alone. Clinical studies have also shown the efficacy of retinoids in treating various types of cutaneous T-cell lymphomas. Vitamin A supplementation has also been shown to help emphysema patients repair damaged lung tissue, previously thought to be impossible. Clinical studies on vitamin A supplementation also document a protective effect for infants who are infected with HIV and malaria against stunting of growth.