carrots

Vitamin A

Vitamin A does much more than help you see in the dark. It stimulates the production and activity of white blood cells, takes part in remodeling bone, helps maintain the health of endothelial cells (those lining the body’s interior surfaces), and regulates cell growth and division. This latter role had researchers exploring for years the relationship between vitamin A and cancer. Specifically, researchers looked at whether people could reduce their cancer risk by taking supplements of beta-carotene, one of several precursor compounds that the body can transform into vitamin A, or by taking the active form of vitamin A (also called retinol or preformed vitamin A). Several studies and randomized trials have dashed this hypothesis.

The Institute of Medicine’scurrent recommended intake of vitamin A is 900 micrograms of retinolfor men (equivalent to 3,000 IU) and 700 micrograms of retinol forwomen (equivalent to 2,333 IU). The upper limit for vitamin A intake from retinol is 3,000 micrograms, but intakes this high may increase the risk of hip fracture or interfere with the beneficial actions of vitamin D.

Food sources: Many breakfast cereals, juices, dairy products, and other foods arefortified with retinol (also known as preformed vitamin A). Many fruits and vegetables, andsome supplements, also contain beta-carotene and other vitamin Aprecursors, which the body can turn into vitamin A. It’s best to choose a multivitamin supplement that has all or the vast majority of its vitamin A in the form of beta-carotene.

Although it’s possible to get too little vitamin A, it’s easy to get too much preformed vitamin A (retinol) from supplements. Intake of up to 3,000 micrograms of preformed vitamin A, more than three times the current recommended daily level, is thought to be safe. However, there is some evidence that this much preformed vitamin A might increase the risk of hip fracture (1-3) or some birth defects. (4) Another reason to avoid too much preformed vitamin A is that it may interfere with the beneficial actions of vitamin D.

In contrast to preformed vitamin A, beta-carotene is not toxic even at high levels of intake. The body can form vitamin A from beta-carotene as needed, and there is no need to monitor intake levels, as there is with preformed vitamin A. Therefore, it is preferable to choose a multivitamin supplement that has all or the vast majority of its vitamin A in the form of beta-carotene; many multivitamin manufacturers have already reduced the amount of preformed vitamin A in their products. Smokers should avoid high-dose single supplements of beta-carotene, since some randomized trials in smokers have linked high dose supplementation with increased lung cancer risk. (5-7) There is no strong reason for anyone to take separate beta-carotene supplements.

References

1.Feskanich D, Singh V, Willett WC, Colditz GA. Vitamin A intake and hip fractures among postmenopausal women. JAMA. 2002; 287:47-54.

2.Michaelsson K, Lithell H, Vessby B, Melhus H. Serum retinol levels and the risk of fracture. N Engl J Med. 2003; 348:287-94.

3.Penniston KL, Tanumihardjo SA. The acute and chronic toxic effects of vitamin A. Am J Clin Nutr. 2006; 83:191-201.

4.Azais-Braesco V, Pascal G. Vitamin A in pregnancy: requirements and safety limits. Am J Clin Nutr. 2000; 71:1325S-33S.

5.Omenn GS, Goodman GE, Thornquist MD, et al. Effects of a combination of beta carotene and vitamin A on lung cancer and cardiovascular disease. N Engl J Med. 1996; 334:1150-5.

6.Albanes D, Heinonen OP, Taylor PR, et al. Alpha-Tocopherol and beta-carotene supplements and lung cancer incidence in the alpha-tocopherol, beta-carotene cancer prevention study: effects of base-line characteristics and study compliance. J Natl Cancer Inst. 1996; 88:1560-70.

7.Virtamo J, Pietinen P, Huttunen JK, et al. Incidence of cancer and mortality following alpha-tocopherol and beta-carotene supplementation: a postintervention follow-up. JAMA. 2003; 290:476-85.

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