Findings from observational studies in the 1980s led researchers to wonder whether supplements of beta carotene and vitamin A (also called retinol) might reduce the incidence of cancer, particularly lung cancer. The body converts beta carotene, which is found in plants, to vitamin A, and vitamin A is known to play a part in preventing the uncontrolled growth of cells. The phase III Beta Carotene and Retinol Efficacy Trial (CARET) was designed to test whether supplements of these nutrients could prevent lung cancer in people at high risk for the disease.

CARET was a clinical trial because it was both experimental and prospective. It was experimental in that it was studying the effects of an intervention – in this case, vitamin supplements. And it was prospective in that researchers tracked the health of participants forward in time, after they enrolled in the trial.

The multicenter trial was both controlled and randomized. Between 1983 and 1994, 18,314 men and women who were smokers, former smokers, or workers exposed to asbestos were randomly assigned to one of two groups: the control group received dummy pills (placebos) each day; the intervention group received daily supplements of beta carotene and vitamin A.

The trial was also double-blinded, meaning neither participants nor their doctors knew who was taking the supplements and who was taking the dummy pills.

In January 1996, researchers reported that a preliminary analysis found 28 percent more lung cancer cases and 17 percent more lung cancer deaths in the intervention group than in the placebo group. In other words, there was “clear evidence of no benefit and substantial evidence of possible harm” with regard to the supplements.

Though researchers had planned to continue the experiment for another two years, the trial had progressed far enough and had enrolled enough participants for these early findings to be statistically significant. Researchers immediately told participants to stop taking both the supplements and the placebos, but kept following the participants' health for the next several years. The initial findings were subsequently published in the November 6, 1996, issue of the Journal of the National Cancer Institute. (See the journal abstract of the study.)

The CARET results confirmed the findings of an earlier phase III, randomized, controlled clinical trial, the Alpha-Tocopherol and Beta Carotene (ATBC) trial, published in 1994 (follow-up data were published in 2003; see the journal abstract). The ATBC trial, conducted in Finland, involved more than 29,000 male smokers. Participants taking beta carotene supplements experienced 16 percent more cases of lung cancer than those taking either a vitamin A supplement or a placebo.

On the strength of the ATBC and CARET findings, current medical consensus is that taking beta carotene supplements does not help to prevent lung cancer, and may in fact be harmful.

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