A new study suggests eating fruits and vegetables does more to prevent heart disease than cancer. Researchers at the Harvard School of Public Health found that people who ate at least 5 servings a day of fruits and vegetables had a 28% lower risk of heart disease than people who ate less than 1.5 servings per day. Cancer risk, however, was not affected by the amount of fruits and vegetables eaten.

But that doesn't necessarily mean a healthy diet has no impact on your cancer risk. The American Cancer Society and other health organizations -- including the National Cancer Institute and the American Heart Association -- recommend eating at least 5 servings of fruits and vegetables a day.

"When you eat fruits and vegetables, you're meeting your calorie needs with healthy food, as opposed to meeting them with sugar, fat, or low-nutrient foods," said Jeanne Calle, director of analytic epidemiology for ACS. "Making good food choices is going to directly protect you from heart disease, but it's also going to protect you from weight gain, and that's going to protect you from cancer."

Calle published a study last year that showed being overweight or obese can substantially raise a person's risk of dying from cancer.

The Harvard study, published in the Journal of the National Cancer Institute (Vol. 96, No. 21:1577-1584), followed more than 100,000 participants for more than 10 years, periodically asking them what they ate and whether they had developed cancer, or had a stroke or heart attack.

Its findings support the 5-a-day recommendation, said senior author Walter Willett, MD.

"Our study means that everyone should still try to eat 5 or more servings of fruit and vegetables per day, but that the benefit will be mainly for cardiovascular disease," he said.

It's possible that fruits and vegetables do protect against cancer, he said, but the benefit is not as great as the public may believe. "Not smoking, avoiding [becoming] overweight, and staying physically active will be more effective in preventing cancer."

Why didn't Willett's study show a bigger impact on cancer from these healthy foods? There could be a number of reasons.

One possibility raised in an editorial accompanying the study is that the food questionnaires used to gauge people's diets may not be entirely accurate. In this study, as in many others, participants were asked how often they had eaten particular foods over the past year; if their recollections were flawed, the study results may be, too.

If that's the case, then it's possible that the protective effect on cardiovascular disease is even greater than the study showed, and that there actually is an effect on cancer that the study couldn't find.

The time frame of the study may also have disguised an effect of fruits and veggies on cancer risk. Because cancer can take decades to develop, it may simply take longer follow-up to find a benefit.

Or, Calle said, it may be that what people ate more recently has more of an impact on heart disease, while diet at a younger age has more of an impact on cancer. The Harvard researchers only tracked what participants ate during the course of the study, not during earlier periods of life.

Another possibility, Calle said, is that the study masked any protective effect on cancer by looking at all cancers combined, rather than specific cancers.

"Cancers are very different from one another, and risk factors for cancer are very different," she said. "If you looked at individual cancers you might see things that you don't see with all cancers combined."

Willett also noted that some fruits and vegetables may have an effect on some types of cancer.

"I think it is plausible that there are some components of fruits and vegetables that may modestly reduce the risk of some cancers, but lumping all fruits and vegetables together obscures the benefit," he said. "For example, we have seen evidence that a higher intake of tomato-based products may reduce the risk of prostate cancer."

In addition, the researchers found a protective association for cruciferous vegetables (such as cauliflower, cabbage, broccoli, and even mustard and collard greens, for instance) and cancer, but only in men. Whether the types of cancers occurring in men are more responsive to these types of vegetables compared to cancers in women remains to be determined.

The bottom line, Calle said, is that studying the effects of foods on disease is a very complex process.

"While the data don't really indicate a reduction in risk for all cancers combined, we're not really ready to believe there's no reduction for individual cancer sites," she said. "Fruits and vegetables are healthy choices whether we can directly show this impact on all cancers combined or not."