Beta-carotene is a carotenoid , which is a pigment found in certain foods that can be converted to vitamin A. Beta-carotene is abundant in darkly colored fruits and vegetables, including carrots, cantaloupes, sweet potatoes, and spinach.

A number of observational studies have suggested that people who consume more beta-carotene in their diet have a lower risk of many types of cancer, including lung cancer . These findings led researchers to design intervention trials to see if beta-carotene supplements could help reduce the incidence of lung cancer in high-risk individuals, namely smokers.

Two landmark trials, the Alpha-Tocopherol Beta-Carotene (ATBC) and the Cancer Prevention Study and Carotene and Retinol Efficacy Trial (CARET), had unexpected findings. The ATBC trial studied the effects of beta-carotene supplements in more than 29,000 male smokers and found that the men who were taking the supplement had an 18% higher risk of developing lung cancer than those not taking the supplement. Subsequently, CARET tested the effects of beta-carotene supplements in preventing lung cancer in smokers and/or asbestos workers. This trial was stopped early after researchers discovered the participants taking beta-carotene had a 46% increased risk of dying from cancer.

A new study in the September 21, 2005 issue of the Journal of the National Cancer Institute addressed this issue in women. Researchers found that nonsmokers who consumed more beta-carotene—through diet or supplementation—were less likely to develop tobacco-related cancers, but smokers who consumed more beta-carotene were more likely to develop these cancers.

About the Study

This study included almost 60,000 women who were participating in a French trial addressing risk factors for cancer in women.

The women filled out questionnaires that assessed their medical history, dietary intake, dietary supplement intake, and smoking status. The researchers used the dietary history questionnaires to estimate daily intake of beta-carotene.

The participants’ level of beta-carotene intake was classified into four groups: low, medium and high consumers of dietary beta-carotene, plus users of beta-carotene supplements (considered the highest intake group).

The researchers followed the women for an average of 7.4 years, and determined how beta-carotene intake was associated with the risk of developing tobacco-related cancers (i.e., cancers of the head-and-neck, urinary tract, digestive system, lung, and cervix ) in nonsmokers compared with smokers.

The researchers controlled for factors that may affect cancer risk, including alcohol intake, body mass index (a measure of weight), physical activity, family history, diet, and level of cigarette smoking (i.e., cigarettes smoked per day, years of smoking, and time since quitting smoking, if applicable).

The Findings

In the women who had never smoked, the risk of tobacco-related cancer decreased with increasing beta-carotene intake. In smokers, however, increasing beta-carotene intake was associated with increased risks of cancer. For every 10,000 women, high beta-carotene intake was associated with about 100 fewer cancers in nonsmokers over 10 years compared with low beta-carotene intake. In smokers, however, high beta-carotene intake was associated with about 200 more cancers.

These results do have certain limitations. Few women in the study (2%) reported using beta-carotene supplement use, so the data on supplement use was not strong. Furthermore, the beta-carotene intake level was based on questionnaires, which are subject to error. Finally, the study was limited to women who provided information on smoking status and whether they were taking beta-carotene supplements. Participants who respond more thoroughly to questionnaires tend to be highly motivated and may not represent the general population.

How Does This Affect You?

These findings support previous studies that suggest beta-carotene has different effects on nonsmokers and smokers. In nonsmokers, the effects of beta-carotene seem to be beneficial in preventing certain cancers. But in smokers, the opposite is true; increased beta-carotene intake actually increases the risk of these same cancers.

Whereas other studies have primarily looked at the effects of beta-carotene supplementation on lung cancer risk, this study looked at the effects of real-world beta-carotene intake, from both diet and supplements, on a range of cancers. The findings suggest that in future dietary studies, researchers should analyze smokers and nonsmokers separately, since it seems the same nutrient can affect each group very differently.

This study does not indicate that smokers should avoid beta carotene-rich foods in order to avoid tobacco-related cancers. Foods that are rich in beta-carotene, namely colorful fruits and vegetables, have numerous health benefits, which likely outweigh any risk posed by beta-carotene content. However, smokers should avoid taking beta-carotene supplements.

The women in the study who did not smoke and consumed the highest levels of beta-carotene were the least likely to develop cancer, reinforcing the recommendation of not smoking and eating a variety of fruits and vegetables for good health.