Lung cancer kills more Americans each year than any other type of cancer. In 2004, an estimated 157,000 men and women will die of the disease.

In the early 1980s, studies began to suggest that beta-carotene, a nutrient that the body converts into vitamin A, could help prevent lung cancer. Two large studies (known as CARET and ATBC) involving almost 50,000 participants tested the effects of a beta-carotene supplement on men and women at high risk for lung cancer. Both trials were stopped early when researchers determined that beta-carotene supplements did not prevent lung cancer; quite the contrary, they increased the risk of lung cancer and overall death in current and former smokers.

Though all study participants stopped taking the beta-carotene supplements at the early conclusion of the studies, the question remained whether the beta-carotene already taken would have any lasting health effects.

In a study published in the December 1, 2004 issue of the Journal of the National Cancer Institute , researchers report that six years after ending CARET, the risk of lung cancer and overall death had decreased, but was still higher in study subjects who had taken the beta-carotene supplement than in those who had taken a placebo. Among the beta-carotene takers, women and former smokers were at the greatest risk for lung cancer. Risk of death from cardiovascular disease, which was also studied in the original CARET trial, decreased quickly after the study ended.

About the Study

Original CARET Study

In the original CARET study, researchers recruited 18,314 men and women who were at increased risk for lung cancer. This included current smokers and recent ex-smokers, as well as men who had been exposed to asbestos, a cancer-causing substance. The study participants were given either a combination of 30 milligrams (mg) beta-carotene and 25,000 International Units (IU) of retinol or a placebo pill to be taken every day. Researchers recorded cases of lung cancer, deaths due to cardiovascular disease, and deaths from all causes.

The original CARET trial ended in January 1996, at which time all of the study participants were asked to stop taking their daily pills.

Post-Intervention Trial

For the next six years, the researchers followed both groups of study participants—the beta-carotene group and the placebo group—and recorded cases of lung cancer, deaths due to cardiovascular disease, and death from any cause. This data formed the basis of the current study.

The Findings

Six years after taking their last beta-carotene pill, study subjects who had taken the supplement were still at increased risk from lung cancer and overall death, compared to those who had taken a placebo. The risk was lower than it had been during the original study and was no longer statistically significant.

After the study subjects stopped taking the supplements, the increased risk of cardiovascular death dropped quickly, and there was no significant effect of beta-carotene on long-term cardiovascular health compared to placebo.

A serious limitation of this study was that the researchers did not track changes in smoking status after the beginning of the original study. Cigarette smoking is a strong risk factor for lung cancer, cardiovascular disease, and death from any cause, so changes in smoking status (quitting or resuming the habit) could have affected the results of this study.

How Does This Affect You?

In this follow-up to the CARET study, researchers found that although the harmful effects of beta-carotene decreased over the years, the risk of lung cancer and overall death was still higher six years after the study subjects had stopped taking beta-carotene.

CARET, as well as the other beta-carotene study (ATBC), looked at the effects of large doses of supplements, which were taken for a specific health benefit. In these studies, the amount of beta-carotene taken far exceeded the amount of these substances that you could get from food.

No one is suggesting that the amounts of beta-carotene in carrots, sweet potatoes, pumpkins and other foods rich in the nutrient are harmful to your health. In fact, carotenoids play an essential role in vision, and are important for bone growth and immune function. What this study does suggest, however, is that supplementing your diet with high doses of beta-carotene, particular if you smoke, may cause lasting harm.

Eating a wide variety of fruits and vegetables will ensure that you consume an array of vitamins and minerals, each with their own important health benefits. But be wary of supplements: just because a substance is found in food does not automatically make it safe in high doses.