Note:All the significant positive evidence for beta-carotene applies to food sources, not supplements.
Beta-carotene belongs to a family of natural chemicals known as carotenoids . Widely found in plants, carotenoids along with another group of chemicals, bioflavonoids, give color to fruits, vegetables, and other plants.
Beta-carotene is a particularly important carotenoid from a nutritional standpoint, because the body easily transforms it to vitamin A . While vitamin A supplements themselves can be toxic when taken to excess, it is believed (although not proven) that the body will make only as much vitamin A out of beta-carotene as it needs. Assuming this is true, this built-in safety feature makes beta-carotene the best way to get your vitamin A.
Beta-carotene is also often recommended for another reason: it is an antioxidant, like vitamin E and vitamin C . In observational studies , high intake of carotenoids from food has been associated with reduced risk of various illnesses (including heart disease and cancer). However, observational studies are inherently unreliable, as described below . In intervention trials , beta-carotene supplements have not been found to offer any benefits; in fact, when taken in high doses for a long period of time, beta-carotene supplements might slightly increase the risk of heart disease and some forms of cancer.
Although beta-carotene is not a required nutrient, vitamin A is essential for health, and beta-carotene is converted into vitamin A in the body. The exact conversion factor varies with the circumstances; in general, 2 mcg of beta-carotene in supplement form is thought to be equivalent to 1 mcg of vitamin A. See the article on vitamin A for requirements based on age and sex.
Dark green and orange-yellow vegetables are good sources of beta-carotene. These include carrots, sweet potatoes, squash, spinach, romaine lettuce, broccoli, apricots, and green peppers.
Note: Plant sterols , used to treat high cholesterol , may impair absorption of beta-carotene. ^
We are not sure at the present time whether it is advisable to take dosages of beta-carotene supplements much higher than the recommended allowance for nutritional purposes, which is about 1.5 to 1.8 mg daily in adults. Rather than taking doses higher than this, it is probably more advisable to increase your intake of fresh fruits and vegetables.
What Is the Scientific Evidence for Beta-Carotene?
The story of beta-carotene and cancer prevention is full of apparent contradictions. It starts in the early 1980s, when the cumulative results of many studies suggested that people who eat a lot of fruits and vegetables are significantly less likely to get cancer. ^ A close look at the data pointed to carotenoids as the active ingredients in fruits and vegetables. It appeared that a high intake of dietary carotene might significantly reduce the risk of lung cancer, ^ bladder cancer, ^ breast cancer, ^ esophageal cancer, ^ and stomach cancer. ^ However, observational studies cannot prove cause and effect. It is always possible that individuals who consume a great deal of carotenoids in the diet are different in other ways; for example, they might exercise more or have healthier lifestyles in other regards.
This is not a purely theoretical issue. For example, based primarily on observational studies, hormone replacement therapy was promoted as a heart-protective treatment for postmenopausal women. However, when placebo-controlled studies were performed, hormone replacement therapy was shown to slightly increase the risk of heart disease. One possible explanation for this discrepancy is that the apparent benefits of hormone replacement therapy were due to the fact that women who used it tended to belong to a higher socioeconomic class than those who did not. (For a variety of reasons, some of which are not known, higher income is associated with improved health.)
Something similar appears to be the case with beta-carotene. Although individuals who consume foods high in beta-carotene appear to obtain some protection from heart disease and cancer, when researchers gave beta-carotene supplements to study participants, there was no protective effect.
Most studies enrolled people in high-risk groups, such as smokers, because it is easier to see results when you look at people who are more likely to develop cancer to begin with.
The anticancer bubble burst for beta-carotene in 1994 when the results of the Alpha-Tocopherol, Beta-Carotene (ATBC) study became available. ^ These results showed that beta-carotene supplements did not prevent lung cancer, but actually increased the risk of getting it by 18%. This trial had followed 29,133 male smokers in Finland who took supplements of about 50 IU of vitamin E (alpha-tocopherol), 20 mg of beta-carotene (more than 10 times the amount necessary to provide the daily requirement of vitamin A), both, or placebo daily for 5 to 8 years. (In contrast to the results for beta-carotene, vitamin E was found to reduce the risk of cancer, especially prostate cancer.)
In January 1996, researchers monitoring the Beta-Carotene and Retinol Efficacy Trial (CARET) confirmed the prior bad news with more of their own: the beta-carotene group had 46% more cases of lung cancer deaths. ^ This study involved smokers, former smokers, and workers exposed to asbestos. Alarmed, the National Cancer Institute ended the $42 million CARET trial 21 months before it was planned to end.
At about the same time, the 12-year Physicians' Health Study of 22,000 male physicians was finding that 50 mg of beta-carotene (about 25 times the amount necessary to provide the daily requirement of vitamin A) taken every other day had no effect—good or bad—on the risk of cancer or heart disease. In this study, 11% of the participants were smokers and 39% were ex-smokers. ^ Similarly, another study of beta-carotene supplements failed to find any effect on the risk of cancer in women. ^ And, in a final indictment of beta-carotene’s safety and effectiveness, researchers, who combined the results of 12 recent placebo-controlled trials investigating the association between antioxidant supplementation and cancer, found that beta-carotene use was associated with an increased incidence of cancer among smokers. ^ But the story doesn’t end there. In yet another careful analysis of 4 randomized trials involving 109,394 smokers and former-smokers, researchers found that current smokers who consumed between 20-30 mg of beta-carotene were at a significantly greater risk of developing lung cancer. There was no such risk among former smokers. ^ There are several possible explanations for these apparently contradictory findings. As noted above, it is possible that intake of carotenoids as such is unrelated to cancer, and that some unrelated factor common to individuals with a high carotene diet is the cause of the benefits seen in observational trials.
Another possibility is that beta-carotene alone is not effective, and the other carotenoid found in fruits and vegetables may be more important for preventing cancer than beta-carotene. One researcher has suggested that taking beta-carotene supplements depletes the body of these other beneficial carotenoids, and thereby causes a harmful effect. ^ In support of this theory, a large study found that consumption of fruits and vegetables is generally associated with lower lung cancer risk, but when beta-carotene is taken, this preventive effect disappears. ^
Heart Disease Prevention
The situation with beta-carotene and heart disease is rather similar to that of beta-carotene and cancer. Numerous studies suggest that carotenoids as a whole can help prevent heart disease . ^ However, isolated beta-carotene may not help prevent heart disease and could actually increase your risk.
The same double-blind intervention trial involving 29,133 Finnish male smokers (mentioned under the discussion of cancer and beta-carotene) found 11% moredeaths from heart disease and 15% to 20% morestrokes in those participants taking beta-carotene supplements. ^ Similar poor results with beta-carotene were seen in another large, double-blind study of smokers. ^ Beta-carotene supplementation was also found to increase the incidence of angina in smokers. ^
A high dietary intake of beta-carotene is associated with a significantly slower progression of osteoarthritis , according to a study in which researchers followed 640 individuals over a period of 8 to 10 years. ^ However, as with heart disease and cancer, we don't know whether beta-carotene is responsible for this effect. ^
One small, double-blind study suggested that beta-carotene supplements might raise white blood cell count in people with HIV . ^ However, two subsequent larger controlled trials found no significant differences between those taking beta-carotene or placebo in white blood cell count, CD4+ count, or other measures of immune function. ^ Evidence from observational studies suggests that higher intakes of vitamin A or beta-carotene may be helpful; however, caution is in order regarding dosage. ^ Researchers generally linked higher intake of vitamin A or beta-carotene to lower risk of AIDS and lower death rates, with an important exception: people with the highest intake of either nutrient (more than 11,179 IU per day of beta-carotene or more than 20,268 IU per day of vitamin A) did worse than those who took somewhat less.
Macular Degeneration and Cataracts
Despite promising results from observational studies, intervention trials of beta-carotene for these eye conditions have generally not shown benefit. Beta-carotene proved ineffective for preventing cataracts in one large study, ^ and in another large study, beta-carotene supplements combined with vitamin E and C failed to prevent either macular degeneration or cataracts. ^ On a more positive note, one large study found that beta-carotene supplements helped prevent cataracts in study participants who smoked; nonetheless, no benefit was seen in the group as a whole. ^
According to a 2-year, double-blind, placebo-controlled study of 141 women with mild cervical dysplasia (a precancerous condition of the cervix), beta-carotene, taken at a dosage of 30 mg daily along with 500 mg of vitamin C , does not help to reverse the dysplasia. ^ Negative results were seen in other trials of beta-carotene as well. ^
A double-blind, placebo-controlled trial of 1,484 individuals with intermittent claudication found no benefit from beta-carotene (20 mg daily), vitamin E (50 mg daily), or a combination of the two. ^
In a very large study involving over 29,000 male smokers, researchers failed to find benefit of beta-carotene (20 mg/day), alpha-tocopherol (50 IU/day), or the two taken together for the prevention of type 2 diabetes over a 5 to 8 year period. ^