When can a supplement do more harm than good? When it's taken by smokers, in some cases. At least, that's what the latest data suggest about beta-carotene. Beta-carotene is a compound found in a wide variety of fruits and vegetables. In the early 1990s, some animal studies and small, observational human studies suggested that beta-carotene, as an antioxidant, might have cancer-preventive properties. Then, in 1994, a large study of heavy smokers suggested that beta-carotene slightly increased their risk of lung cancer (1). More recent research is helping to clarify the apparent contradiction.
A study reported in the Journal of the National Cancer Institute (2) followed the health and lifestyle characteristics of nearly 60,000 French women for a median of 7.4 years. The researchers found that for never-smokers, the highest level of beta-carotene use was associated with a significantly decreased risk of several cancers (including cancer of the lung, lower intestine, thyroid, ovary, and cervix). Conversely, among ever-smokers (current as well as ex-smokers), high intake of beta-carotene was associated with an increased risk of such cancers. In both groups of women, those who consumed the greatest amounts of beta-carotene intake obtained them from dietary supplements.
In an accompanying editorial, Drs. Scott M. Lippman and Susan T. Mayne, from MD Anderson Cancer Center and the Yale University Medical School, point out that the study did have limitations. For example, the use of beta-carotene supplements was relatively uncommon in the population studied (only 2% used them), and very few of the supplement-users developed one of the cancers of interest -- only five in the never-smoking group and twelve in the ever-smoking group. These small numbers make the reliability of the results less robust. In spite of such caveats, they called the result "provocative" and cite similar information related to interactions of other carotenes and tobacco.
For example, in one study they cited, the levels of carotenes in participants' plasma was directly correlated with mortality risk in smokers but inversely correlated with risk in non-smokers (3).
It is important to note that in these studies of the effects of carotenes on either mortality or cancer risk, the high doses of the nutrients were typically derived from supplements, not from foods. There is no evidence that high fruit and vegetable consumption, the main food sources of the carotenes, increases cancer risk, even in smokers.
Studies such as this one indicate that, as we have previously pointed out, high intake of some nutrients in the form of supplements, will not necessarily improve health. Some nutrients, such as the carotenoids, increase the health risk from smoking, which is already extremely high.
(1) Alpha-tocopherol beta carotene cancer prevention study group. The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers. N Engl J Med 1994;330:1029-1035.
(2) Touvier M, Kesse E, Clavel-Chapelon F, and Boutron-Ruault, M-C. Dual association of beta-carotene with risk of tobacco-related cancers in a cohort of French women. JNCI 2005; 97(18):1388-1344.
(3) Mayne ST, Cartmel B, Lin H et al. Low plasma lycopene concentration is associated with increased mortality in a cohort of patients with prior oral, pharynx or larynx cancers. J Am Coll Nutr 2004;23:34-42/
For more information about smoking risks, log onto the ACSH smoking website, http://TheScooponSmoking.org/.