Your doctor is probably right. At least, he or she is following conventional medical wisdom, which states that smokers, former smokers and asbestos workers shouldn't take beta-carotene supplements. Despite convincing scientific evidence that people who eat a lot of fruits and vegetables containing beta-carotene are less likely to develop cancer and heart disease, the results from two well-designed clinical trials yielded surprising results. One, conducted in Finland by the U.S. National Cancer Institute (NCI) and the National Public Health Institute of Finland, found 18 percent more lung cancers and eight percent more overall deaths among 29,133 male smokers given a supplement containing 20 mg of beta carotene and 50 mg of vitamin E. A second study, also funded by NCI testing the effects of beta carotene supplements found 28 percent more lung cancers and 17-percent more deaths among participants taking supplements of beta-carotene and vitamin E. This study included 14,254 men and women between the ages of 50 and 69 who were smokers or former smokers as well as 4,060 men between 45 and 69 who had been exposed to asbestos at work.
A third study, involving 22,071 male U.S. physicians, 11 percent of them smokers, showed neither benefit nor harm in the group that took beta-carotene.
None of these results suggest that the beta-carotene supplements cause cancer, but they do indicate that they don't help to prevent it among smokers. One theory holds that under certain circumstances, antioxidants such as beta-carotene can promote - rather than control - free radicals and that smoking might increase this effect.
Despite these findings, no one suggests that smokers and former smokers should avoid fruits and vegetables rich in beta-carotene. The best thing you can do to protect yourself from lung cancer is to stop smoking immediately. The second best is to make sure your diet contains lots of fruits and vegetables, especially peaches, melons, mangoes, sweet potatoes, squash, pumpkins, tomatoes and dark leafy greens.
What these studies really point out is the folly of confusing an isolated element from a whole food with health benefits. It is probably the whole family of carotenoid pigments that works together to reduce cancer risks, not any one of them. A compromise, for those who can't or won't eat enough fruits and vegetables is to take a supplement providing mixed carotenoids in roughly the proportions they occur in those foods.
Andrew Weil, MD
Dosage Update, October, 2004
In order to provide the most up-to-date health information, I may change my recommendations from time to time. Due to compelling new research on carotene, I now suggest that you take 15,000 IU of mixed carotenes. (My previous recommendation was 25,000 IU; please adjust your dosage accordingly.)