Very high doses of beta carotene supplements may be cancer causing, as two earlier, and controversial studies had concluded. Diets that are rich in foods containing beta carotene do not carry the same risks.
The two studies the Beta Carotene Cancer Prevention Trial (ATBC) and the Beta Carotene and Retinol Efficacy Trial (CARET) had both shown the carcinogenic effects of the supplement, but controversially had included smokers in the trials.
A research team from Tufts University School of Medicine in Boston, Massachusetts, decided to investigate the significance of smoking to the results of such studies, by looking at the lungs of ferrets animals which metabolise beta carotene in much the same way as humans.
Researchers found that all the animals given the supplement had squamous metaplasia in their lungs, although this pre cancerous change was more marked in the animals that were also exposed to smoke. But it's wise to remember that animal studies, like this one, may not apply to humans (Lancet, 1999; 353: 215).