Dietary Supplement Beta-Carotene Ineffective against Diabetes

Sept. 15, 1999 (Seattle) -- Beta-carotene, a widely touted dietary supplement made from carrots, does not prevent diabetes, contrary to previous evidence supporting such a protective role. This finding is the latest disappointment for beta-carotene, which already has been shown to be ineffective for preventing cancer and heart disease.

Liu says his study was designed to detect even a very small benefit from beta-carotene. The study compared nearly 11,000 doctors who took supplements with an equal number of doctors who took placebos. Liu says that during the 12-year study, doctors who took the supplements were no less likely than the other doctors to get diabetes.

"A lot of people won't be happy about this, because there was a lot of excitement about this compound," says Simin Liu, MD, the Harvard Medical School researcher who lead the study. The study was published in the of The Journal of the American Medical Association.

Liu tells WebMD that researchers have been intrigued by beta-carotene because it is an "antioxidant" -- a type of substance believed to prevent damage to cells. Laboratory experiments have shown that antioxidants can neutralize molecules called oxygen free radicals, which are created by normal chemical processes in the body and which can harm cells much the way rust harms metal. Many scientists believe that fruits and vegetables protect people from disease because these foods contain antioxidants.

In the past decade, a series of studies found that people who consume large amounts of beta-carotene in food are less likely to get heart disease, some cancers, and the type of diabetes that usually occurs in overweight adults. Companies that sell beta-carotene supplements have suggested their products can prevent a range of illnesses. However, experiments that have given people extra beta-carotene haven't found that the supplements offer any protection.

Liu says the results mean there's no reason to take beta-carotene supplements as a way to head off diabetes. But he says people who eat carrots and other vegetables containing beta-carotene may still lower their risk. That's because these foods contain many compounds that act as antioxidants and may offer protection through mechanisms that scientists still don't understand, Liu says.

Earl Ford, MD, a nutrition researcher at the CDC, tells WebMD that the Harvard study shows how difficult it is to isolate a single ingredient in foods that protects against disease. Ford says beta-carotene is just one of a group of similar compounds called carotenoids. "The thing with the carotenoids is that there are hundreds of them, so there is no reason to believe that just one is going to protect against everything," he says.

Ford says it's possible that another carotenoid will prove to be the "magic bullet" against diabetes that scientists hope to find. But he says it's more likely that large numbers of these substances act together somehow to prevent disease.

Liu says he still believes that free radicals play a role in causing diabetes and that scientists will eventually identify certain antioxidants that can help prevent the disease. In the meantime, he says, "eating a diet with lots of fruits and vegetables is not a bad idea."

The research was supported by grants from the National Institutes of Health.