One of the mantras of modern health advice is that a diet high in fruits and vegetables is key to preventing cancer. So far, though, studies have been unable to pinpoint the responsible agents. For example, recent clinical trials of supplemental beta-carotene, pegged as one of the most promising anticancer nutrients, failed to show any benefit .

A study in the March 20 Journal of the National Cancer Institute may help clarify the relationship between some nutrients and cancer prevention. Researchers at the State University of New York at Buffalo interviewed nearly 300 premenopausal women with breast cancer and a similar number of cancer-free women about their consumption of 172 foods over the previous two years. Looking specifically at fruits, vegetables, and several of their nutritional components, the researchers found a strong protective effect against breast cancer for vegetables in general as well as for diets rich in vitamins C and E, folic acid, alpha- and beta-carotene, and dietary fiber. Fruit consumption didn't have as strong an effect. Interestingly, these components appeared to provide protection only when eaten as part of a regular diet; supplements offered no benefit.

No one nutrient will be a magic bullet

This study suggests that if you want to reduce your risk of cancer, the best approach is to eat more fruits and vegetables. The value of supplements is still in question, at least in warding off cancer.