Beta-carotene has been studied for its potential antioxidant effects, including cancer prevention. But scientific data on its efficacy have been mixed. Some studies, in fact, have even shown it to be potentially harmful. Two large studies published in the 1990s surprised many people by showing that beta-carotene supplementation was associated with an increased risk for lung cancer, particularly in people who used tobacco and drank alcohol. Now a new study finds that beta-carotene supplementation in people who smoke and drink may increase risk for colorectal cancer.

Studying recurrence of colon polyps

Subjects for this study were adults who had had at least one benign colorectal polyp removed in the past three months and were polyp-free at the study's start. A total of 864 people were randomly assigned to receive one of four treatments: beta-carotene (25 mg/day); vitamin C (1000 mg/day) and vitamin E (400 mg/day); beta-carotene, vitamin C, and vitamin E; or placebo.

The study subjects filled out detailed questionnaires about lifestyle factors, including diet, smoking, and alcohol intake. From this information, the researchers categorized the subjects as "current," "former," or "never" smokers and as "non-drinkers," "light drinkers" (1 drink a day or less, on average), or "moderate/heavy drinkers" (more than 1 drink a day, on average).

At the 1-year and 4-year marks from the study's start, the participants (707 of the original 864) underwent colonoscopy to detect recurrence of colon polyps. The researchers then performed statistical analyses to determine if beta-carotene supplementation was associated with protection from recurrent polyps. They compared results between subjects with histories of alcohol and tobacco use and those without.

Beta-carotene plus alcohol and tobacco: a harmful mix?

Beta-carotene was, in fact, associated with a decreased risk for recurrent polyps in people who neither smoked nor drank. These subjects had a 44% lower risk compared with their counterparts who took a placebo. On the other hand, however, subjects taking beta-carotene who also smoked and took more than one drink a day had double the risk for recurrent polyps.

In doing the statistical calculations, the researchers adjusted for other factors that could influence risk, including age, sex, and diet.

There are, however, limitations to this study, including the fact that the researchers had to rely on the subjects' own reports of their alcohol and tobacco use, introducing the possibility for error. And it's important to keep in mind that the primary endpoint for this study was recurrent benign colon polyps -- not colorectal cancer. Although polyps are a risk factor for colorectal cancer, it cannot be concluded from this study that beta-carotene necessarily protects against colorectal cancer in nonsmokers and nondrinkers, or that it causes colon cancer in smokers and drinkers.

What should you do?

There's not enough data to warrant widespread beta-carotene supplementation for the prevention of colorectal cancer. But these results add to existing evidence that people with histories of alcohol and tobacco use may have higher risks associated with taking beta-carotene.

While research continues to elucidate what supplements, if any, may be protective against colorectal cancer, you can still, and you should, eat for colon health with a balanced, high-fiber diet rich in fruits, vegetables, and whole grains.

Also important in the effort to prevent colon cancer is to get regular screenings as recommended by your doctor. Early detection is key in preventing and treating colorectal cancer.