Beta-carotene is an orange-red pigment found in foods such as carrots, sweet potatoes, pumpkins and spinach. Its antioxidant properties make it an attractive supplement, as the free-radical-scavenging activity of these compounds make it useful in the treatment of macular degeneration of the eye as well as a host of skin and systemic conditions; beta-carotene may also reduce the risk of certain cancers. However, beta-carotene supplements are, owing to the results of observations made in numerous human and animal studies, now generally contraindicated in smokers, according to MedlinePlus, a publication of the National Institutes of Health.
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For many years, the medical consensus was that a diet high in beta-carotene-rich foods, a high serum level of beta-carotene, or both were associated with a reduced risk of certain types of cancer, notably lung cancer. However, intervention studies in the mid-1990s involving beta-carotene supplements and smokers demonstrated an increased risk of lung cancer. Further experiments involving ferrets -- which metabolize carotene very similarly to the way humans do -- showed that cigarette smoke exposure and high-dose beta-carotene each produced higher malignancy-like effects in lung tissue and that these exposures combined to produce more marked such effects than did either factor alone. The cause appears to be reduced levels of tumor-suppressor genes as well as a greater expression of oncogenes, or "cancer genes," in the exposed animals, according to a 2002 paper published in "Pure and Applied Chemistry."
Dietary Versus Pharmacologic Beta-Carotene
The Alpha-Tocopherol, Beta-Carotene Cancer Prevention Trial, conducted in Finland between 1985 and 1993, showed that male smokers aged 50 to 69 who took 20mg of beta-carotene per day had an 18 percent increased incidence of lung cancer. Dr. Robert Russell and his colleagues, however, reported in the August 2002 issue of "Pure and Applied Chemistry" that while pharmacologic, or supplement-level, doses of beta-carotene had damaging effects on lung tissue, physiologic -- that is, dietary-level -- doses did not. Therefore, beta-carotene intake in and of itself does not appear to explain the increased risk of lung cancer, and in fact the amount ingested in a normal diet may be protective in this regard.
Exposure to asbestos and cigarette smoking have each been found to increase the risk of lung cancer, and, as described by Dr. Peter Lee in the July 2002 issue of "Occupational and Environmental Medicine," asbestos exposure multiplies the carcinogenic effect in smokers. In addition, as Dr. Gary Goodman states in the December 2004 issue of the "Journal of the National Cancer Institute," beta-carotene supplementation was found to increase the risk of lung cancer in those with a history of occupational exposure to asbestos. Taken together, this suggests that taking beta-carotene supplementation is especially contraindicated in smokers with a history of asbestos exposure.