In previous trials, supplementation with beta-carotene was found to increase the risk of lung cancer. The subjects in these studies were predominantly cigarette smokers, and adverse effects were concentrated in those participants who also drank alcohol.
Beta-carotene supplementation appeared not to increase the risk of cancer generally. However, it is not clear whether smoking and/or alcohol use alters the effect of beta-carotene on carcinogenesis outside the lung.
In this study, researchers from the United States assessed the effect of beta-carotene supplementation on colorectal adenoma recurrence.
|Beta-carotene conferred a modest increase in risk of recurrence among those who either smoked or drank.|
|Journal of the National Cancer Institute|
They performed a multicenter double-blind, placebo-controlled clinical trial of antioxidants for the prevention of colorectal adenomas.
The team included 864 subjects, who had had an adenoma removed and were polyp-free, in the study. Subjects were randomized to receive either beta-carotene (25mg) or placebo, and/or vitamins C (1000mg) and E (400g) in combination or placebo.
The participants were followed with colonoscopy for adenoma recurrence 1 year and 4 years after the qualifying endoscopy.
The researchers found that a total of 707 subjects had 2 follow-up examinations, and provided smoking and alcohol data.
The team used adjusted multivariate risk ratios (RRs), and 95% confidence intervals, (CIs) to assess the effects of beta-carotene on adenoma recurrence.
They found that in subjects who neither smoked nor drank, beta-carotene was associated with a marked decrease in the risk of 1 or more recurrent adenomas (RR = 0.56).
However, beta-carotene supplementation conferred a modest increase in the risk of recurrence among those who either smoked (RR = 1.36), or drank (RR = 1.13).
Furthermore, in participants who smoked cigarettes, and drank more than 1 alcoholic drink each day, beta-carotene doubled the risk of adenoma recurrence (RR = 2.07).
Dr John Baron's team concluded, "Alcohol intake and cigarette smoking appear to modify the effect of beta-carotene supplementation on the risk of colorectal adenoma recurrence".