Beta-carotene has been studied for its potential to prevent different types of cancer. Results have been mixed. While some research has shown beta-carotene to have a protective effect, others have not. Some, in fact, showed a harmful effect: The results of two landmark studies published in the 1990s surprised many researchers by showing that beta-carotene supplementation was actually associated with increased risk for lung cancer, particularly in subjects who had histories of tobacco and alcohol use.

The purpose of the present study was to determine the association between beta-carotene supplementation and recurrence of colorectal adenomas (benign colon tumors considered a precursor to colorectal cancer ) in subjects who smoked and drank compared with those who did not.

The study, published in the May 21, 2003 Journal of the National Cancer Institute , found that smokers and drinkers who took beta-carotene supplements to help prevent cancer may actually have increased their risk of colorectal adenomas.

About the study

Researchers studied 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 subjects were randomly assigned to receive one of four treatments: (1) beta-carotene (25 mg/day), (2) vitamin C (1000 mg/day) plus vitamin E (400 mg/day), (3) beta-carotene plus vitamin C plus vitamin E, or (4) placebo.

At the beginning of the study the subjects filled out detailed questionnaires about lifestyle factors such as 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/day, on average), or moderate/heavy drinkers (> 1 drink/day, on average).

To determine how many subjects experienced recurrence of colon polyps during the course of the study, the researchers performed colonoscopies on the subjects at years one and four of the study (a total of 707 subjects completed these examinations). The researchers then determined whether beta-carotene use was associated with protection against recurrent polyps and if its effects were influenced by tobacco and alcohol use.

The findings

The researchers found that beta-carotene was, in fact, associated with decreased risk for recurrent colon polyps in people who neither smoked nor drank. Among these subjects, beta-carotene was associated with a 44% lower risk for recurrent polyps compared with placebo. But among subjects who smoked and drank more than one alcoholic drink per day, beta-carotene was associated with double the risk for recurrent polyps.

To minimize confounding effects, the researchers statistically adjusted for other factors that could influence risk of recurrent polyps, including age, sex, and diet. Still, the study was limited by the fact that the researchers had to rely on the subjects’ own reports of their alcohol and tobacco use, introducing the possibility for error.

How does this affect you?

In this study, beta-carotene supplementation appeared to lower the risk of colorectal polyps in non-smokers and non-drinkers in high-risk subjects who have had polyps in the past. In light of previous research, however, these findings are not enough to warrant the routine use of beta-carotene for the prevention of colorectal cancer in high risk or low risk individuals.

And since beta-carotene has now been linked to an increased risk of two cancers—colorectal and lung—people who use tobacco and alcohol would do well to avoid taking beta-carotene as a supplement. This does not mean that smokers and drinkers should avoid fresh vegetables and other foods rich in beta-carotene. On the contrary, there is fairly convincing evidence that a healthful diet can lower the risk of colorectal cancer in everyone.

Perhaps the most important thing you can do to reduce your risk for colorectal cancer is to get regular screenings. Identifying and removing polyps before they become cancerous has been shown to reduce mortality from this disease. Most authorities now recommend a sigmoidoscopy or colonoscopy at age 50 unless there are other factors, like family history, that may increase the risk of polyps at a younger age. See your doctor to determine what your risk factors may be and what type and frequency of screening tests you may need.