Beta-carotene and other carotenoids as antioxidants

Our plain-English summary of Beta-carotene and other carotenoids as antioxidants, by S.A. Paiva, R.M. Russell (Journal of the American College of Nutrition, 1999).

The study

• Authors: S.A. Paiva, R.M. Russell
• Journal: Journal of the American College of Nutrition
• Published: 1999;18(5):426-33
• Type of article: Review
• Cited by: 303 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Carotenoids are natural pigments which are synthesized by plants and are responsible for the bright colors of various fruits and vegetables. There are several dozen carotenoids in the foods that we eat, and most of these carotenoids have antioxidant activity. Beta-carotene has been best studied since, in most countries it is the most common carotenoid in fruits and vegetables. However, in the U.S., lycopene from tomatoes now is consumed in approximately the same amount as beta-carotene. Antioxidants (including carotenoids) have been studied for their ability to prevent chronic disease. Beta-carotene and others carotenoids have antioxidant properties in vitro and in animal models. Mixtures of carotenoids or associations with others antioxidants (e.g. vitamin E) can increase their activity against free radicals. The use of animals models for studying carotenoids is limited since most of the animals do not absorb or metabolize carotenoids similarly to humans. Epidemiologic studies have shown an inverse relationship between presence of various cancers and dietary carotenoids or blood carotenoid levels. However, three out of four intervention trials using high dose beta-carotene supplements did not show protective effects against cancer or cardiovascular disease. Rather, the high risk population (smokers and asbestos workers) in these intervention trials showed an increase in cancer and angina cases. It appears that carotenoids (including beta-carotene) can promote health when taken at dietary levels, but may have adverse effects when taken in high dose by subjects who smoke or who have been exposed to asbestos. It will be the task of ongoing and future studies to define the populations that can benefit from carotenoids and to define the proper doses, lengths of treatment, and whether mixtures, rather than single carotenoids (e.g. beta-carotene) are more advantageous.

In plain English

This article was a review, not a new study. The authors summarized published laboratory, animal, observational and clinical trial research on beta-carotene and other carotenoids (the natural pigments in colored fruits and vegetables) as antioxidants that might prevent heart disease and cancer. Asbestos enters through one trial they described, CARET in the United States. It enrolled men and women at high risk of lung cancer from cigarette smoking or occupational asbestos exposure and gave 30 mg of beta-carotene plus 25,000 IU of retinyl palmitate or placebo. The review reported a 17% increase in relative risk of lung cancer death in supplemented heavy smokers versus placebo. It gave no separate figure for asbestos-exposed participants. It reported that three of four high-dose beta-carotene trials showed no protection against cancer or cardiovascular disease. It also reported that the high-risk population (smokers and asbestos workers) in these trials showed an increase in cancer and angina cases. The authors concluded that carotenoids appear to promote health at dietary levels but may have adverse effects in high doses in people who smoke or have been exposed to asbestos.

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: Narrative review of laboratory, animal, epidemiologic and intervention-trial research on carotenoids; it reports no new data of its own
• Who was studied: No study population of its own. Among the published studies reviewed is the CARET trial (United States), which enrolled men and women at high risk of lung cancer because of smoking histories or occupational asbestos exposure. The review gives no participant numbers or dates for CARET.
• Exposure: The subject is beta-carotene and other dietary carotenoids, not asbestos. In the CARET trial described, participants received 30 mg beta-carotene plus 25,000 IU retinyl palmitate (vitamin A) or placebo. Asbestos exposure was a reason for enrollment, not the treatment studied. No fiber types are named.
• Main results:
  • CARET trial (as reported by the review): relative risk of death from lung cancer increased by 17% in beta-carotene-supplemented heavy smokers compared with placebo. No separate figure is given for asbestos-exposed participants.
  • ATBC trial in Finland, male heavy smokers (as reported by the review): an 8% increase in relative risk of death among smokers given beta-carotene for five to eight years
  • Physicians Health Study (as reported by the review): beta-carotene supplementation over twelve years was neither beneficial nor harmful for malignant neoplasms, cardiovascular disease or death from all causes
  • Linxian trial in China (as reported by the review): a combination supplement including 15 mg beta-carotene was followed by a 9% reduction in cancer mortality and a 21% reduction in stomach cancer mortality over five years
  • Overall, three of the four high-dose beta-carotene intervention trials showed no protective effect against cancer or cardiovascular disease
  • The authors characterized the high-risk population (smokers and asbestos workers) in these trials as showing an increase in cancer and angina cases
  • Observational epidemiologic studies reviewed showed an inverse relationship between various cancers and dietary carotenoids or blood carotenoid levels
  • The authors hypothesized that high-dose beta-carotene and its breakdown products accumulating in lung tissue might, together with cigarette smoke and/or asbestos exposure, promote rather than prevent cancer. This is presented as a hypothesis, not a finding.
• The authors' conclusion: The authors concluded that carotenoids, including beta-carotene, appear to promote health at dietary levels but may have adverse effects when taken in high doses by people who smoke or who have been exposed to asbestos. They said future studies are needed to define who benefits, the proper doses and the length of treatment.
• Limitations the authors state:
  • The authors note that most laboratory animals used do not absorb or metabolize carotenoids as humans do, so the animal results are equivocal
  • The authors note that, in epidemiologic studies, carotenoids may have been markers for other protective factors in fruits and vegetables rather than acting themselves
  • The authors suggest the negative trial results could reflect the wrong dose, the wrong duration, or treatment started too late, when cancers were already present in heavy smokers

Related studies

• Carotenoid actions and their relation to health and disease — N.I. Krinsky, E.J. Johnson (2005)
• The role of carotenoids in human health — E.J. Johnson (2002)
• 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, vitamin A, and lung cancer chemoprevention: results of an intermediate endpoint study — J.W. McLarty, D.B. Holiday, et al. (1995)
• Beta-carotene and the application of transcriptomics in risk-benefit evaluation of natural dietary components — J. Keijer, A. Bunschoten, et al. (2005)
• Chemoprevention of cancer: lessons to be learned from beta-carotene trials — H. Vainio (2000)

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