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Vitamin A


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Vitamin A is a generic term for a large number of related compounds. Retinol (an alcohol) and retinal (an aldehyde) are often referred to as preformed vitamin A. Retinal can be converted by the body to retinoic acid, the form of vitamin A known to affect gene transcription. Retinol, retinal, retinoic acid, and related compounds are known as retinoids. Beta-carotene and other carotenoids that can be converted by the body into retinol are referred to as provitamin A carotenoids. Hundreds of different carotenoids are synthesized by plants, but only about 10% of them are provitamin A carotenoids (1). The following discussion will focus mainly on preformed vitamin A and retinoic acid.

Function

Vision

The retina is located at the back of the eye. When light passes through the lens, it is sensed by the retina and converted to a nerve impulse for interpretation by the brain. Retinol is transported to the retina via the circulation and accumulates in retinal pigment epithelial cells (diagram). Here, retinol is esterified to form a retinyl ester, which can be stored. When needed, retinyl esters are broken apart (hydrolyzed) and isomerized to form 11-cis-retinol, which can be oxidized to form 11-cis-retinal. 11-cis-retinal can be shuttled across the interphotoreceptor matrix to the rod cell where it binds to a protein called opsin to form the visual pigment, rhodopsin (also known as visual purple). Rod cells with rhodopsin can detect very small amounts of light, making them important for night vision. Absorption of a photon of light catalyzes the isomerization of 11-cis-retinal to all-trans-retinal and results in its release. This isomerization triggers a cascade of events, leading to the generation of an electrical signal to the optic nerve. The nerve impulse generated by the optic nerve is conveyed to the brain where it can be interpreted as vision. Once released, all-trans retinal is converted to all-trans-retinol, which can be transported across the interphotoreceptor matrix to the retinal epithelial cell, thereby completing the visual cycle (2). Inadequate retinol available to the retina results in impaired dark adaptation, known as "night blindness."

Regulation of gene expression

Isomers of retinoic acid (RA) act as hormones to affect gene expression and thereby influence numerous physiological processes. All-trans-RA and 9-cis-RA are transported to the nucleus of the cell bound to cytoplasmic retinoic acid-binding proteins (CRABP). Within the nucleus, RA binds to retinoic acid receptor proteins (diagram). Specifically, all-trans-RA binds to retinoic acid receptors (RAR) and 9-cis-RA binds to retinoid X receptors (RXR). RAR and RXR form RAR/RXR heterodimers; these heterodimers bind to regulatory regions of the chromosome called retinoic acid response elements (RARE). A dimer is a complex of two protein molecules. Heterodimers are complexes of two different proteins, while homodimers are complexes of two of the same protein. Binding of all-trans-RA and 9-cis-RA to RAR and RXR respectively allows the complex to regulate the rate of gene transcription, thereby influencing the synthesis of certain proteins. RXR may also form heterodimers with thyroid hormone receptors (THR) or vitamin D receptors (VDR). In this way, vitamin A, thyroid hormone, and vitamin D may interact to influence gene transcription (3). Through the stimulation and inhibition of transcription of specific genes, retinoic acid plays a major role in cellular differentiation, the specialization of cells for highly specific physiological roles. Many of the physiological effects attributed to vitamin A appear to result from its role in cellular differentiation.

Immunity

Vitamin A is commonly known as the anti-infective vitamin, because it is required for normal functioning of the immune system (4). The skin and mucosal cells (cells that line the airways, digestive tract, and urinary tract) function as a barrier and form the body's first line of defense against infection. Retinol and its metabolites are required to maintain the integrity and function of these cells (5). Vitamin A and retinoic acid (RA) play a central role in the development and differentiation of white blood cells, such as lymphocytes, which play critical roles in the immune response. Activation of T-lymphocytes, the major regulatory cells of the immune system, appears to require all-trans-RA binding of RAR (3).

Growth and development

Both vitamin A excess and deficiency are known to cause birth defects. Retinol and retinoic acid (RA) are essential for embryonic development (4). During fetal development, RA functions in limb development and formation of the heart, eyes, and ears (6). Additionally, RA has been found to regulate expression of the gene for growth hormone.

Red blood cell production

Red blood cells, like all blood cells, are derived from precursor cells called stem cells. Stem cells are dependent on retinoids for normal differentiation into red blood cells. Additionally, vitamin A appears to facilitate the mobilization of iron from storage sites to the developing red blood cell for incorporation into hemoglobin, the oxygen carrier in red blood cells (2, 7).

Nutrient interactions

Zinc

Zinc deficiency is thought to interfere with vitamin A metabolism in several ways: (1) zinc deficiency results in decreased synthesis of retinol binding protein (RBP), which transports retinol through the circulation to tissues (e.g., the retina) and also protects the organism against potential toxicity of retinol; (2) zinc deficiency results in decreased activity of the enzyme that releases retinol from its storage form, retinyl palmitate, in the liver; and (3) zinc is required for the enzyme that converts retinol into retinal (8, 9). At present, the health consequences of zinc deficiency on vitamin A nutritional status in humans are unclear (10).

Iron

Vitamin A deficiency may exacerbate iron-deficiency anemia. Vitamin A supplementation has beneficial effects on iron deficiency anemia and improves iron nutritional status among children and pregnant women. The combination of supplemental vitamin A and iron seems to reduce anemia more effectively than either supplemental iron or vitamin A alone (11). Moreover, studies in rats have shown that iron deficiency alters plasma and liver levels of vitamin A (12, 13).

Deficiency

Vitamin A deficiency and vision

Vitamin A deficiency among children in developing nations is the leading preventable cause of blindness (14). The earliest evidence of vitamin A deficiency is impaired dark adaptation or night blindness. Mild vitamin A deficiency may result in changes in the conjunctiva (corner of the eye) called Bitot's spots. Severe or prolonged vitamin A deficiency causes a condition called xerophthalmia (dry eye), characterized by changes in the cells of the cornea (clear covering of the eye) that ultimately result in corneal ulcers, scarring, and blindness (4, 9).

Vitamin A deficiency and infectious disease

Vitamin A deficiency can be considered a nutritionally acquired immunodeficiency disease (15). Even children who are only mildly deficient in vitamin A have a higher incidence of respiratory disease and diarrhea as well as a higher rate of mortality from infectious disease compared to children who consume sufficient vitamin A (16). Vitamin A supplementation has been found to decrease both the severity and incidence of deaths related to diarrhea and measles in developing countries, where vitamin A deficiency is common (17). The onset of infection reduces blood retinol levels very rapidly. This phenomenon is generally believed to be related to decreased synthesis of retinol binding protein (RBP) by the liver. In this manner, infection stimulates a vicious cycle, because inadequate vitamin A nutritional status is related to increased severity and likelihood of death from infectious disease (18). However, a recent review of four studies concluded that vitamin A supplementation is not beneficial in reducing the mother-to-child transmission of HIV (19). One study found that HIV-infected women who were vitamin A deficient were three to four times more likely to transmit HIV to their infants (20).

The Recommended Dietary Allowance (RDA)

The RDA for vitamin A was revised by the Food and Nutrition Board (FNB) of the Institute of Medicine in 2001. The latest RDA is based on the amount needed to ensure adequate stores (four months) of vitamin A in the body to support normal reproductive function, immune function, gene expression, and vision (21). The table below lists the RDA values in both micrograms (mcg) of Retinol Activity Equivalents (RAE) and international units (IU). For more information on these units, see the section on RAE.

Recommended Dietary Allowance (RDA) for Vitamin A as Preformed Vitamin A (Retinol Activity Equivalents)
Life Stage  Age  Males: mcg/day (IU/day) Females: mcg/day (IU/day) 
Infants (AI 0-6 months  400 (1,333 IU) 400 (1,333 IU)
Infants (AI 7-12 months  500 (1,667 IU)  500 (1,667 IU)
Children  1-3 years  300 (1,000 IU) 300 (1,000 IU)
Children  4-8 years  400 (1,333 IU) 400 (1,333 IU)
Children  9-13 years  600 (2,000 IU) 600 (2,000 IU)
Adolescents  14-18 years  900 (3,000 IU) 700 (2,333 IU)
Adults  19 years and older  900 (3,000 IU) 700 (2,333 IU)
Pregnancy  18 years and younger  750 (2,500 IU)
Pregnancy  19 years and older 770 (2,567 IU)
Breast-feeding  18 years and younger  1,200 (4,000 IU)
Breast-feeding  19 years and older  1,300 (4,333 IU)

 

Disease Prevention

Cancer

Studies in cell culture and animal models have documented the capacity for natural and synthetic retinoids to reduce carcinogenesis significantly in skin, breast, liver, colon, prostate, and other sites (2). However, the results of human studies examining the relationship between the consumption of preformed vitamin A and cancer are less clear.

Lung cancer

At least ten prospective studies have compared blood retinol levels at baseline among people who subsequently developed lung cancer and those who did not. Only one of those studies found a statistically significant inverse association between serum retinol and lung cancer risk (22). The results of the Beta-Carotene And Retinol Efficacy Trial (CARET) suggest that high-dose supplementation of vitamin A and beta-carotene should be avoided in people at high risk of lung cancer (23). About 9,000 people (smokers and people with asbestos exposure) were assigned a daily regimen of 25,000 IU of retinol and 30 milligrams of beta-carotene, while a similar number of people were assigned a placebo. After four years of follow-up, the incidence of lung cancer was 28% higher in the supplemented group compared to the placebo group. A possible explanation for such a finding is that the oxidative environment of the lung, created by smoke or asbestos exposure, gives rise to unusual carotenoid cleavage products, which are involved in carcinogenesis. Presently, it seems unlikely that increased retinol intake decreases the risk of lung cancer, although the effects of retinol may be different for nonsmokers than for smokers (22).

Breast cancer

Retinol and its metabolites have been found to reduce the growth of breast cancer cells in vitro, but observational studies of dietary retinol intake in humans have not confirmed this in vivo (24). The majority of epidemiological studies have failed to find significant associations between retinol intake and breast cancer risk in women (25-28), although one large prospective study found that total vitamin A intake was inversely associated with the risk of breast cancer in premenopausal women with a family history of breast cancer (29). Blood levels of retinol reflect the intake of both preformed vitamin A and provitamin A carotenoids like beta-carotene. Although a case-control study found serum retinol levels and serum antioxidant levels to be inversely related to the risk of breast cancer (30), two prospective studies did not observe significant associations between blood retinol levels and subsequent risk of developing breast cancer (31, 32). Presently, there is little evidence in humans that increased intake of preformed vitamin A or retinol reduces breast cancer risk.

Disease Treatment

Pharmacologic doses of retinoids (see also Upper Level)

Retinoids are used at pharmacologic doses to treat several conditions, including retinitis pigmentosa, acute promyelocytic leukemia, and various skin diseases. It is important to note that treatment with high doses of natural or synthetic retinoids overrides the body's own control mechanisms; therefore, retinoid therapies are associated with potential side effects and toxicities. Additionally, all of the retinoid compounds have been found to cause birth defects. Thus, women who have a chance of becoming pregnant should avoid treatment with these medications. Retinoids tend to be very long acting: side effects and birth defects have been reported to occur months after discontinuing retinoid therapy (2). The retinoids discussed below are prescription drugs and should not be used without medical supervision.

Retinitis pigmentosa

Retinitis pigmentosa describes a broad spectrum of genetic disorders that result in the progressive loss of photoreceptor cells (rods and cones) in the eye's retina (33). Early symptoms of retinitis pigmentosa include impaired dark adaptation and night blindness, followed by the progressive loss of peripheral and central vision over time. The results of a randomized controlled trial in more than 600 patients with common forms of retinitis pigmentosa indicated that supplementation with 4,500 mcg (15,000 IU)/day of preformed vitamin A (retinol) significantly slowed the loss of retinal function over a period of 4-6 years (34). In contrast, supplementation with 400 IU/day of vitamin E increased the loss of retinal function by a small but significant amount, suggesting that patients with common forms of retinitis pigmentosa may benefit from long-term vitamin A supplementation but should avoid vitamin E supplementation at levels higher than those found in a typical multivitamin. Up to 12 years of follow-up in these patients did not reveal any signs of liver toxicity as a result of excess vitamin A intake (35). High-dose vitamin A supplementation to slow the course of retinitis pigmentosa requires medical supervision and must be discontinued if there is a possibility of pregnancy (see Safety).

Acute promyelocytic leukemia

Normal differentiation of myeloid stem cells in the bone marrow gives rise to platelets, red blood cells, and white blood cells that are important for the immune response. Altered differentiation of those stem cells results in the proliferation of immature leukemic cells, giving rise to leukemia. A mutation of the retinoic acid receptor (RAR) has been discovered in patients with a specific type of leukemia called acute promyelocytic leukemia (APL). Treatment with all-trans-retinoic acid or with high doses of all-trans-retinyl palmitate restores normal differentiation and leads to improvement in some APL patients (2, 18).

Diseases of the skin

Both natural and synthetic retinoids have been used as pharmacologic agents to treat disorders of the skin. Etretinate and acitretin are retinoids that have been useful in the treatment of psoriasis, while tretinoin (Retin-A) and isotretinoin (Accutane) have been used successfully to treat severe acne. Retinoids most likely affect the transcription of skin growth factors and their receptors (2). Use of pharmacological doses of retinoids by pregnant women causes birth defects (see Safety in pregnancy).

Sources

Retinol activity equivalents (RAE)

Different dietary sources of vitamin A have different potencies. For example, beta-carotene is less easily absorbed than retinol and must be converted to retinal and retinol by the body. The most recent international standard of measure for vitamin A is retinol activity equivalents (RAE), which represent vitamin A activity as retinol. Two micrograms (mcg) of beta-carotene in oil provided as a supplement can be converted by the body to 1 mcg of retinol giving it an RAE ratio of 2:1. However, 12 mcg of beta-carotene from foods are required to provide the body with 1 mcg of retinol, giving dietary beta-carotene an RAE ratio of 12:1. Other provitamin A carotenoids in foods are less easily absorbed than beta-carotene, resulting in RAE ratios of 24:1. The RAE ratios for beta-carotene and other provitamin A carotenoids are shown in the table below (21). An older international standard, still commonly used, is the international unit (IU). One IU is equivalent to 0.3 mcg of retinol.

Retinol activity equivalents (RAE) ratios for beta-carotene and other provitamin A carotenoids
Quantity Consumed Quantity Bioconverted to Retinol  RAE ratio 
1 mcg of dietary or supplemental vitamin A  1 mcg of retinol*  1:1 
2 mcg of supplemental beta-carotene  1 mcg of retinol  2:1 
12 mcg of dietary beta-carotene  1 mcg of retinol  12:1 
24 mcg of dietary alpha-carotene  1 mcg of retinol  24:1 
24 mcg of dietary beta-cryptoxanthin 1 mcg of retinol  24:1

*One IU is equivalent to 0.3 microgram (mcg) of retinol, and one mcg of retinol is equivalent to 3.33 IU of retinol.

Food sources

Free retinol is not generally found in foods. Retinyl palmitate, a precursor and storage form of retinol, is found in foods from animals. Plants contain carotenoids, some of which are precursors for vitamin A (e.g., alpha-carotene, beta-carotene, and beta-cryptoxanthin). Yellow and orange vegetables contain significant quantities of carotenoids. Green vegetables also contain carotenoids, though the pigment is masked by the green pigment of chlorophyll (1). A number of good food sources of vitamin A are listed in the table below along with their vitamin A content in micrograms of retinol activity equivalents (mcg RAE). In those foods where retinol activity comes mainly from provitamin A carotenoids, the carotenoid content and the retinol activity equivalents are presented. You may use the USDA food composition database to check foods for their content of several different carotenoids, including lycopene, lutein, and zeaxanthin. The vitamin A IU listings in the USDA database, however, do not take into account bioavailability of the various carotenoids. To obtain a more accurate estimate of the number of IUs of vitamin A in carotenoid-containing foods, multiply the RAE by 3.33.

Food Serving Vitamin A, RAE
Vitamin A, IU Retinol, mcg Retinol, IU
Cod liver oil 1 teaspoon 1,350 mcg 4,500 IU 1,350 mcg 4,500 IU
Fortified breakfast cereals 1 serving 150-230 mcg 500-767 IU 150-230 mcg 500-767 IU
Egg 1 large 91 mcg 303 IU 89 mcg 296 IU
Butter 1 tablespoon 97 mcg 323 IU 95 mcg 317 IU
Whole milk 1 cup (8 fl oz.) 68 mcg 227 IU 68 mcg 227 IU
2% fat milk (vitamin A added) 1 cup (8 fl oz) 134 mcg 447 IU 134 mcg 447 IU
Nonfat milk (vitamin A added) 1 cup (8 fl oz.) 149 mcg 497 IU 149 mcg 497 IU
Sweet potato, canned 1/2 cup, mashed 555 mcg 1,848 IU 0 0
Sweet potato, baked 1/2 cup 961 mcg 3,203 IU 0 0
Pumpkin, canned 1/2 cup 953 mcg 3,177 IU 0 0
Carrot (raw) 1/2 cup, chopped 538 mcg 1,793 IU 0 0
Cantaloupe 1/2 medium melon 467 mcg 1,555 IU 0 0
Mango 1 fruit 79 mcg 263 IU 0 0
Spinach 1/2 cup, cooked 472 mcg 1,572 IU 0 0
Broccoli 1/2 cup, cooked 60 mcg 200 IU 0 0
Kale 1/2 cup, cooked 443 mcg 1,475 IU 0 0
Collards 1/2 cup, cooked 386 mcg 1,285 IU 0 0
Squash, butternut  1/2 cup, cooked 572 mcg 1,907 IU 0 0

 

Supplements

The principal forms of preformed vitamin A (retinol) in supplements are retinyl palmitate and retinyl acetate. Beta-carotene is also a common source of vitamin A in supplements, and many supplements provide a combination of retinol and beta-carotene (36). If a percentage of the total vitamin A content of a supplement comes from beta-carotene, this information is included in the Supplement Facts label under vitamin A (see example supplement label). Most multivitamin supplements available in the U.S. provide 1,500 mcg (5,000 IU) of vitamin A, which is substantially more than the current RDA for vitamin A. This is due to the fact that the Daily Values (DV) used by the FDA for supplement labeling are based on the RDA established in 1968 rather than the most recent RDA, and multivitamin supplements typically provide 100% of the DV for most nutrients. Because retinol intakes of 5,000 IU/day may be associated with an increased risk of osteoporosis in older adults (see Safety), some companies have reduced the retinol content in their multivitamin supplements to 750 mcg (2,500 IU).

Safety

Toxicity

The condition caused by vitamin A toxicity is called hypervitaminosis A. It is caused by overconsumption of preformed vitamin A, not carotenoids. Preformed vitamin A is rapidly absorbed and slowly cleared from the body. Therefore, toxicity from preformed vitamin A may result acutely from high-dose exposure over a short period of time or chronically from a much lower intake (2). Acute vitamin A toxicity is relatively rare, and symptoms include nausea, headache, fatigue, loss of appetite, dizziness, dry skin, desquamation, and cerebral edema. Signs of chronic toxicity include dry itchy skin, desquamation, loss of appetite, headache, cerebral edema, and bone and joint pain. Also, symptoms of vitamin A toxicity in infants include bulging fontanels.Severe cases of hypervitaminosis A may result in liver damage, hemorrhage, and coma. Generally, signs of toxicity are associated with long-term consumption of vitamin A in excess of ten times the RDA (8,000 to 10,000 mcg/day or 25,000 to 33,000 IU/day). However, more research is necessary to determine if subclinical vitamin A toxicity is a concern in certain populations (37). There is evidence that some populations may be more susceptible to toxicity at lower doses, including the elderly, chronic alcohol users, and some people with a genetic predisposition to high cholesterol (8). In January 2001, the Food and Nutrition Board (FNB) of the Institute of Medicine set the tolerable upper intake level (UL) of vitamin A intake for adults at 3,000 mcg (10,000 IU)/day of preformed vitamin A (21).

Tolerable Upper Intake Level (UL) for Preformed Vitamin A (Retinol)
Age Group  UL in mcg/day (IU/day)
Infants 0-12 months  600 (2,000 IU) 
Children 1-3 years  600 (2,000 IU) 
Children 4-8 years  900 (3,000 IU) 
Children 9-13 years  1,700 (5,667 IU) 
Adolescents 14-18 years  2,800 (9,333 IU) 
Adults 19 years and older  3,000 (10,000 IU)

 

Safety in pregnancy

Although normal fetal development requires sufficient vitamin A intake, consumption of excess preformed vitamin A (retinol) during pregnancy is known to cause birth defects. No increase in the risk of vitamin A-associated birth defects has been observed at doses of preformed vitamin A from supplements below 3,000 mcg/day (10,000 IU/day) (21). Since a number of foods in the U.S. are fortified with preformed vitamin A, pregnant women should avoid multivitamin or prenatal supplements that contain more than 1,500 mcg (5,000 IU) of vitamin A. Vitamin A from beta-carotene is not known to increase the risk of birth defects. Etretinate and isotretinoin (Accutane), synthetic derivatives of retinol, are known to cause serious birth defects and should not be taken during pregnancy or if there is a possibility of becoming pregnant (38). Tretinoin (Retin-A), another retinol derivative, is prescribed as a topical preparation that is applied to the skin. Because of the potential for systemic absorption of topical tretinoin, its use during pregnancy is not recommended.

Do high intakes of vitamin A increase the risk of osteoporosis?

Results of some studies indicate that vitamin A intake is not associated with detrimental effects on bone mineral density (BMD) or fracture risk (39-41). However, results of some prospective studies suggest that long-term intakes of preformed vitamin A in excess of 1,500 mcg/day (5,000 IU/day) are associated with increased risk of osteoporotic fracture and decreased BMD in older men and women (42-44). Although this level of intake is greater than the RDA of 700-900 mcg/day (2,300-3,000 IU/day), it is substantially lower than the UL of 3,000 mcg/day (10,000 IU/day). Only excess intakes of preformed vitamin A (retinol), not beta-carotene, were associated with adverse effects on bone health. Although these observational studies cannot provide the reason for the association between excess retinol intake and osteoporosis, limited experimental data suggest that excess retinol may stimulate bone resorption (45) or interfere with the ability of vitamin D to maintain calcium balance (46). In the U.S., retinol intakes in excess of 5,000 IU/day can be easily attained by those who regularly consume multivitamin supplements and/or fortified foods, including some breakfast cereals. At the other end of the spectrum, a significant number of elderly people have insufficient vitamin A intakes, which have also been associated with decreased BMD. One study of elderly men and women found that BMD was optimal at vitamin A intakes close to the RDA (43). Until supplements and fortified foods are reformulated to reflect the current RDA for vitamin A, it makes sense to look for multivitamin supplements that contain 2,500 IU of vitamin A or multivitamin supplements that contain 5,000 IU of vitamin A, of which at least 50% comes from beta-carotene (see example supplement label).

Drug Interactions

Chronic alcohol consumption results in depletion of liver stores of vitamin A, and may contribute to alcohol-induced liver damage (47). However, the liver toxicity of preformed vitamin A (retinol) is enhanced by chronic alcohol consumption, thus narrowing the therapeutic window for vitamin A supplementation in alcoholics (48). Oral contraceptives that contain estrogen and progestin increase retinol binding protein (RBP) synthesis by the liver, increasing the export of RBP-retinol complex in the blood. Whether this increases the dietary requirement of vitamin A is not known. Retinoids or retinoid analogs, including acitretin, all-trans-retinoic acid, bexarotene, etretinate and isotretinoin (Accutane), should not be used in combination with vitamin A supplements, because they may increase the risk of vitamin A toxicity (36).

Linus Pauling Institute Recommendation

The RDA for vitamin A (2,300 IU/day for women and 3,000 IU/day for men) is sufficient to support normal gene expression, immune function, and vision. However, following the Linus Pauling Institute’s recommendation to take a multivitamin/mineral supplement daily could supply as much as 5,000 IU/day of vitamin A as retinol, the amount that has been associated with adverse effects on bone health in older adults. For this reason, we recommend taking a multivitamin/multimineral supplement that provides no more than 2,500 IU (750 mcg) of preformed vitamin A (usually labeled vitamin A acetate or vitamin A palmitate) and no more than 2,500 IU of additional vitamin A as beta-carotene. High potency vitamin A supplements should not be used without medical supervision due to the risk of toxicity.

Older adults (> 50 years)

Presently, there is little evidence that the requirement for vitamin A in older adults differs from that of younger adults. Additionally, vitamin A toxicity may occur at lower doses in older adults than in younger adults. Following the Linus Pauling Institute’s recommendation to take a multivitamin/mineral supplement daily could supply as much as 5,000 IU/day of retinol, the amount that has been associated with adverse effects on bone health in older adults. For this reason, we recommend taking a multivitamin/mineral supplement that provides no more than 2,500 IU (750 mcg) of preformed vitamin A (usually labeled vitamin A acetate or vitamin A palmitate) and no more than 2,500 IU of additional vitamin A as beta-carotene. High potency vitamin A supplements should not be used without medical supervision due to the risk of toxicity.

References


Written in December 2003 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in November 2007 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University

Reviewed in November 2007 by:
Robert M. Russell, M.D., Senior Scientist and Director
Jean Mayer USDA Human Nutrition Research on Aging
Tufts University

Copyright 2000-2014  Linus Pauling Institute


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