Vitamin A is a generic term for several related compounds. Retinol is the form often called preformed vitamin A because it is the one readily available for use by the body. Beta-carotene and other carotenoids are called provitamin A because the body must convert them into retinol before they can be used. Retinol is found in animal-based foods while beta-carotene and other carotenoids are found in plants.
Vitamin A is an antioxidant that promotes overall eye health and normal cell reproduction, which is important for preventing precancerous changes in cells. Vitamin A also stimulates the immune system and protects the digestive, urinary, and respiratory tracts against infection, and also plays a role in the formation of bone, growth hormones, and proteins.
There is limited research regarding use of vitamin A and beta-carotene in prostate health, and specifically prostate cancer. A recent (March 2010) review study from Mayo Clinic College of Medicine in Rochester, Minnesota, that looked at vitamins A, B, C, D, and E and their anti-cancer activities found that although lab tests in animals show these vitamins may have some ability to fight prostate cancer, it remains to be “intensively studied” as to whether they are effective in humans for this purpose. (Donkena 2010)
One of the more surprising findings regarding beta-carotene was seen in the Beta-Carotene and Retinol Efficacy Trial (CARET). In that study, researchers found that daily supplementation with beta-carotene (30 mg) and vitamin A (as retinyl palmitate, 25,000 IU) by individuals at high risk for lung cancer because of smoking or asbestos exposure had a 28 percent increased risk of lung cancer, a 17 percent increased risk of death, and a higher rate of cardiovascular disease mortality when compared with individuals not at high risk for lung cancer. A six-year follow-up study noted these risks persisted even after the individuals stopped taking the supplements, although the degree of risk declined. (Goodman 2004)
Another study in the International Journal of Cancer reported that a review of nine randomized controlled trials showed no effect of beta-carotene supplementation on the incidence of several cancers, including prostate, colorectal, breast, melanoma, and pancreatic cancers, but an increase regarding lung and stomach cancers in smokers and asbestos workers. (Druesne-Pecollo 2009) The International Agency for Research on Cancer (IARC) also notes that “There is inadequate evidence with regard to the cancer-preventive activity of beta-carotene at the usual dietary levels,” as well as at high doses. (IARC)
That said, the body needs vitamin A/beta-carotene for all the reasons already mentioned above. The RDA for men 19 years and older is 3,000 IU (925 mcg). Many vitamin A supplements provide both retinol and beta-carotene. Because retinol can cause side effects while beta-carotene typically does not, health practitioners typically suggest people get their vitamin A from beta-carotene rather than from retinol. The Linus Pauling Institute recommends taking a supplement that provides no more than 2,500 IU of vitamin A or one that provides 5,000 IU vitamin A, and that at least half of this dose comes from beta-carotene.
Vitamin A is fat-soluble, which means it attaches to fatty tissue and can accumulate in the body. Therefore, taking too much can cause side effects, including headache, nausea, vomiting, blurry vision, dizziness, liver abnormalities, and reduced bone mineral density. Deficiency is rare. Rich food sources of vitamin A include liver, eggs, and milk fortified with the vitamin, while beta-carotene can be found in a wide variety of fruits and vegetables and are the preferred sources.
Donkena KV et al. Vitamins and prostate cancer risk. Molecules 2010 Mar 12; 15(3): 1762-83.
Druesne-Pecollo N et al. Beta-carotene supplementation and cancer risk: a systemic review and metanalysis of randomized controlled trials. Int J Cancer 2009 Oct 28.
Goodman GE et al. The beta-carotene and retinol efficacy trial: incidence of lung cancer and cardiovascular disease mortality during 6-year follow-up after stopping beta-carotene and retinol supplements. J Natl Cancer Inst 2004 Dec 1; 96(23): 1743-50
IARC: International Agency for Research on Cancer, World Health Organization, IARC handbook of cancer prevention: carotenoids. Lyon, France: 1998:278.
Linus Pauling Institute: http://lpi.oregonstate.edu/infocenter/vitamins/vitaminA/