Hopes that a diet low in fat and chock full of fruits and vegetables could prevent the return of breast cancer were dashed Tuesday by a large, seven-year experiment involving more than 3,000 women.

The U.S. government study found no benefit from a mega-veggies-and-fruit diet over the U.S. recommended servings of five fruits and vegetables a day — more than most Americans get. Canada's Food Guide recommends seven to 10 servings a day for adults between 19 and 50 years of age.

Researchers noted that none of the breast cancer survivors lost weight on either diet. That led some experts to suggest that weight loss and exercise should be the next frontier for cancer prevention research. 

'This should really lift some of the guilt if women are feeling, "I'm just not doing enough."' —Researcher Marcia Stefanick of Stanford University

The study appears in Wednesday's Journal of the American Medical Association.

"It sends us back to the drawing board," said Susan Gapstur of Northwestern University's Feinberg School of Medicine, who wasn't involved in the new study, but co-wrote an accompanying editorial in the journal.

"Should we really have focused on dietary components like fruits, vegetables and fat?" Gapstur asked. "Or should we be focusing, in addition to diet, on lifestyle factors including physical activity and weight?"

For now, the message for breast cancer survivors is that they don't need to go overboard on veggies, researchers said.

"This should really lift some of the guilt if women are feeling, 'I'm just not doing enough,'" said study co-author Marcia Stefanick of Stanford University.

New study more rigorous

The research was kicked off by a $5-million US grant from the late Wal-Mart heir John Walton, and got an additional $30 million US from the National Cancer Institute.

Walton wanted to support a scientific study so cancer survivors wouldn't have to "rely on folklore," said John Pierce, head of cancer prevention at University of California, San Diego, who led the research.

Earlier research on whether a healthy diet prevents breast cancer has shown mixed results. The new study was designed to be more rigorous.

In this experiment, all the women had been successfully treated for early-stage breast cancer. Their average age was 53 when the study began.

In one group, 1,537 women were randomly assigned to a daily diet that included five vegetable servings, three fruit servings, 16 ounces (about 500 ml) of vegetable juice and 30 grams of fibre. In most cases, a serving equalled a half-cup (125 ml). French fries and iceberg lettuce couldn't be counted as vegetables.

The women were allowed to eat meat, but were told to get no more than 15 per cent to 20 per cent of their calories from fat, a goal they ultimately were unable to achieve.

"That's a tough diet," said Pierce, who ate that way himself along with his staff and the women in the study.

As a comparison, another 1,551 women were assigned to get educational materials about the importance of eating five servings of fruits and vegetables a day.

The women in both groups kept food diaries regularly, but not daily, throughout the study.

During the next seven years, the cancer returned in about the same proportion of women in both groups: 256 women (16.7 per cent) of the women on the special diet and 262 women (16.9 per cent) in the comparison group. During that time, about 10 per cent of the subjects in both groups died, most of them from breast cancer.

It didn't matter whether the breast cancer was the most common type — fuelled by hormones — or not; the special diet didn't prevent the cancer from returning. Those results run counter to a previous study by different researchers that suggested low-fat diets may help prevent the return of the type of breast cancer that is not linked to hormones.

Easy to monitor veggie intake

In the mega-veggies group, the women changed their eating habits substantially, mostly by increasing fruits and vegetables to as much as 11 servings a day. They failed to meet the fat target, but did eat 13 per cent less in fat calories than did the comparison group.

After one year, women on the high-vegetable diet had 73 per cent higher blood levels of carotenoids (pigments found in fruits and vegetables) than the other women. That indicates they were truthful about how many fruits and vegetables they ate, Pierce said.

But they may not have been so honest about the calories they ate. The super-veggie group gained 1.3 pounds and the comparison group gained 0.88 pound, on average.

"There's no question they were under-reporting on calories, especially the heavier women," Pierce said, or they would have lost weight.