Chemoprevention of cancer: lessons to be learned from beta-carotene trials

Our plain-English summary of Chemoprevention of cancer: lessons to be learned from beta-carotene trials, by H. Vainio (Toxicology Letters, 2000).

The study

• Author: H. Vainio
• Journal: Toxicology Letters
• Published: 2000;112-113:513-7
• Type of article: Review
• Cited by: 25 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Observational studies indicate a lower incidence of chronic diseases, including various cancers and cardiovascular disease, related to higher intakes of carotenoid containing foods (fruits and vegetables). Beta-carotene, one of the large number of naturally occurring carotenoids, thus appears to actively participate in health. However, recent intervention trials indicate that beta-carotene supplements are not efficacious in the prevention of cardiovascular disease and major cancers occurring in well-nourished populations. In fact, supplemental beta-carotene appears to increase, rather than to reduce, lung cancer incidence and deaths from cardiovascular disease in current smokers and in asbestos exposed workers. In order to resolve these paradoxes, we need to better understand the underlying biology, identify interactions, develop mechanistic hypotheses and test them in clinical trials in humans. Until that time, we should confine any premature enthusiasm for chemopreventive supplementation.

In plain English

This article is a commentary and review by the author, not a new study. It discussed cancer chemoprevention, meaning taking drugs, nutrients or other compounds to lower cancer risk, using beta-carotene as its main example. The author noted that earlier observational studies had suggested beta-carotene might protect against cancers of the lung, oral cavity and pharynx. Three large intervention trials were started in the 1980s to test this. According to the article, in the two largest trials beta-carotene significantly increased the risk of lung cancer among smokers and/or asbestos-exposed workers within three to six years of the start. The third trial, among mostly non-smoking US physicians, showed no increase in lung cancer risk. The author wrote that no clear-cut mechanism had been found to explain this. Research had turned to how products of cigarette smoking and heavy asbestos exposure interact with high blood levels of beta-carotene. The article did not name asbestos fiber types. The author concluded that premature enthusiasm for chemopreventive supplementation should be held back until the biology is better understood and tested in human trials.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Commentary / narrative review of published observational studies and intervention trials of beta-carotene and other chemopreventive agents; no new data
• Who was studied: No participants of its own. The author reviewed published research, chiefly three large beta-carotene intervention trials started in the 1980s. These involved smokers and/or asbestos-exposed workers, plus a trial of US physicians who were primarily non-smokers.
• Exposure: Beta-carotene supplements given in intervention trials, with attention to current smokers and asbestos-exposed workers. Asbestos fiber types are not named. The article also discusses alpha-tocopherol (vitamin E).
• Main results:
  • Earlier observational studies, as described by the author, suggested beta-carotene had cancer-preventive effects against cancers of the lung, oral cavity and pharynx.
  • In the two largest trials reviewed (cited as ATBC 1994, Albanes et al. 1996, Omenn et al. 1996), beta-carotene significantly increased lung cancer risk among smokers and/or asbestos-exposed workers within three to six years after the start.
  • The third trial (Hennekens et al., 1996), among US physicians who were primarily non-smokers, showed no increase in lung cancer risk.
  • The author stated that the observational findings on beta-carotene clearly conflicted with the randomized trial results.
  • No clear-cut mechanism had been found to explain the trial findings. Research had switched to the interaction between products of cigarette smoking, heavy asbestos exposure and high blood beta-carotene.
  • Use of beta-carotene had been stopped in all ongoing chemoprevention trials except one.
  • In the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study, smokers randomly assigned to daily alpha-tocopherol (vitamin E) had a 32% reduction in prostate cancer incidence and a 41% reduction in deaths from it.
  • In the same study, men taking vitamin E had a 50% increase in cerebral haemorrhage.
• The authors' conclusion: The author concluded that the underlying biology must be better understood and tested in human trials. Until then, premature enthusiasm for chemopreventive supplementation should be confined.

Topics

Medical terms in this study, explained in our Lexicon: Lung Cancer, Clinical Trial.

Related studies

• Carotenoid actions and their relation to health and disease — N.I. Krinsky, E.J. Johnson (2005)
• The enigma of beta-carotene in carcinogenesis: what can be learned from animal studies — R.M. Russell (2004)
• The Beta-Carotene and Retinol Efficacy Trial: incidence of lung cancer and cardiovascular disease mortality during 6-year follow-up after stopping beta-carotene and retinol supplements — G.E. Goodman, M.D. Thornquist, et al. (2004)
• Risk factors for lung cancer and for intervention effects in CARET, the Beta-Carotene and Retinol Efficacy Trial — G.S. Omenn, G.E. Goodman, et al. (1996)
• Effects of a combination of beta carotene and vitamin A on lung cancer and cardiovascular disease — G.S. Omenn, G.E. Goodman, et al. (1996)
• Beta-carotene and other carotenoids as antioxidants — S.A. Paiva, R.M. Russell (1999)

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