The study, published in today's issue of the Journal of the National Cancer Institute (Vol. 97, No 18), seems to support earlier evidence from intervention studies that suggest it may increase the risk of lung and digestive cancers in smokers.
Researchers from the Institut Gustave Roussy studied dietary and lifestyle data collected from a cohort of 59,910 French women, who were followed over an average of 7.4 years. Of these, 700 women developed forms of cancer associated with smoking, such as lung cancer and head and neck cancers.
Women who had the highest beta-carotene intake, derived from dietary supplements, were seen to have the least risk of developing cancer if they had never smoked. But amongst women who were smokers at some point in their lives, their risk of developing cancer significantly increased with beta-carotene intake.
"Beta-carotene intake was inversely associated with risk of tobacco-related cancers among nonsmokers with a statistically significant dose-dependent relationship, whereas high beta-carotene intake was directly associated with risk among smokers," concluded the researchers.
Although the cohort studied included only women, the researchers project that the same effects would be seen in men. They are planning to do a follow-up involving both sexes.
In an accompanying editorial, Susan Mayne (Yale University School of Medicine) and Scott Lippman (University of Texas) said that the findings "reinforce the conclusion that future studies of micronutrients and their derivatives should include stratification by smoking status".
Despite this value, they drew attention to several limitations of the new study - namely that, despite its overall size, only two percent of the women reported taking beta carotene supplements, and amongst these only five women who had never smoked and 12 women who had smoked at some time in their lives developed cancer. The final risk assessment was based on this small group.
Moreover, there is no information on the dose of beta-carotene that the supplement users were taking.
Mayne and Lippman argued against smokers curtailing their dietary intake of beta carotene.
Dietary sources consist mainly of fruit and vegetables, such as carrots, cantaloupes, and others with natural red, orange or yellow pigment.
However no evidence has linked these foodstuffs to a higher risk of cancer in either smokers or non-smokers. On the other hand, they have been linked to a reduced risk of cardiovascular disease in both groups.
"The new research on the interaction between the nutrients and smoking should not alter current policy recommendations with regard to nutrients and cancer risk," they wrote.
The study authors also concede that nutrients such as vitamins C and E in fruit and vegetables could counteract the effect of beta-carotene in smokers.