After nearly a decade of intense study, researchers at Harvard Medical School are beginning to understand how beta carotene, the simple chemical found in carrots, apricots and other vegetables, may hold the potential to stave off fatal diseases in humans.

These clinical scientists are now hypothesizing that this simple organic molecule may ward off cancer and heart disease by scooping up charged oxygen atoms, dangerous particles which wreak havoc as they roam through the body's circulatory system.

Many--but not all--of those scientists examining beta carotene have joined together on the Physicians' Health Study, a massive project involving as subjects 22,071 male physicians between 40 and 84 years old. The study was designed to test the effect of aspirin on heart disease and beta carotene on cancer.

Researchers cut short the aspirin half of the study in 1988, after preliminary data showed incontrovertably that aspirin has a beneficial effect on heart disease. Overseers of the study said they felt that to continue giving half of the subjects placebo pills instead of aspirin was unethical, as was withholding the clear scientific evidence of aspirin's positive benefits.

But for all their successes so far, leaders of the Physicians' Health Study are still coming up short on conclusive data about the mysterious vegetable extract beta carotene, composed of a backbone of carbon atoms flanked by two 6-atom carbon "rings."

"I must tell you," says Charles Hennekens, who heads the study, "the beta carotene aspect was supposed to end in December 1990." Lacking sufficient data, however, Hennekens and his colleagues have applied to the National Institutes of Health (NIH) for extended funding through 1996. Ninety-two percent of the physicians involvedas subjects in the study have agreed to continuetaking their pills, Hennekens says.

Meanwhile, clinical researchers around thecountry await the outcome of the Harvard study."We may have to be patient," says E. RobertGreenberg of Dartmouth College's Norris CottonCancer Center in Hanover, N.H. "You're talkingabout a process that may take decades."

"Wonder Drug"

In recent years, some of beta carotene'sproponents have heralded the chemical as a "wonderdrug," a biological police officer patrolling thebody and eliminating tumors and fatty buildupswherever they occur.

In particular, two compelling pieces ofscientific evidence point to beta carotene'santi-carcinogenic effect. For one, clinicalresearchers have shown that cancer victims have,in general, consumed less of the chemicalthroughout their lives: "You basically see thatthose individuals who have cancer report a dietlow in beta carotene," says Hennekens.

In addition, some clinicians have frozen bloodsamples from large samples of healthy subjects andseveral years later compared levels of betacarotene. Those who contracted cancer had,researchers found, a significantly lower level ofbeta carotene in their blood than the healthypopulation.

Promising though the chemical may look, Harvardscientists are not ready to claim victory yet.Researchers are quick to point out thatstatistical studies such as these only show that alow intake of beta carotene and a high risk ofcancer go together, but not necesarily that thechemical actually reduces risk.

Instead of a cure-all, scientists say, betacarotene might just be a "marker"--a biologicalred herring which does little for the human body,yet always shows up in a healthy diet high invegetables and fiber.

"The analogy I like to use is...the majority ofpeople with lung cancer have a yellow finger,"from smoking cigarettes, says Hennekens. "Bypreventing yellow finger, you're not going to doanything about [cancer]."

Conclusive proof of a link between betacarotene and cancer, says Hennekens, will comeabout only through the completion of a large-scalecomparison like the Physicians Health Trial, inwhich researchers carefully isolate the effect ofbeta carotene from all other substances with whichit naturally occurs.