Eating double the amount of veggies and fruits recommended by general dietary guidelines doesn't reduce the likelihood of breast cancer recurrence among women whose cancers were treated at an early stage of the disease, says a researcher at the Stanford University School of Medicine.
"For breast cancer survivors who are meeting the '5-a-day' dietary goals set by the National Cancer Institute, there is no benefit to further increasing their vegetable and fiber intake in terms of preventing breast cancer recurrence," said Marcia Stefanick, PhD, professor of medicine at the Stanford Prevention Research Center and the senior author of a study that will be published in the July 18 issue of the Journal of the American Medical Association.
The multicenter study documents the effect of diet on nearly 3,100 women previously treated for early-stage breast cancer. Half of the women were randomly assigned to follow the dietary guidelines from the NCI and the U.S. Department of Agriculture that promote eating five servings of fruits and vegetables daily. The other half were asked to eat almost double that amount of vegetables and fruits while reducing their fat intake.
After following the women for an average of 7.3 years, the researchers found that the breast cancer recurrence and mortality rates were nearly identical for both groups of women.
"I was really surprised and, frankly, a little disappointed by the results," Stefanick said. "We expected the two-fold increase in vegetables and fruits, plus the increased fiber and reduced fat to make a difference in the recurrence rates."
Although the recurrence rates for both groups were the same, Stefanick noted that rate for the control group was much lower than the researchers expected at the outset. About 17 percent of the women in both groups had a recurrence of breast cancer during the course of the study, compared with the 30 percent researchers anticipated in the control group, based on previous studies. The complex nature of cancer makes it difficult to know how much of the lower rate in the control group might be attributed to diet - especially because many of the women were consuming a healthier diet than most Americans at the study's outset, Stefanick said. At the time they enrolled, 75 percent of the participants reported that they were already consuming at least five servings of fruit and vegetables daily, having changed their diets after being diagnosed with breast cancer.
"I would certainly hope that people don't interpret these results as evidence that eating a lot of vegetables doesn't make a difference in breast cancer," she said. "What it shows is that getting more than the recommended amounts doesn't change the recurrence rate for women who have already completed treatment for early-stage breast cancer."
The notion that eating more fruits and vegetables might stave off cancer stems from animal studies showing that plant-derived foods contain anti-carcinogens. There have also been studies indicating that high-fat diets may be linked to an increased risk for cancer.
But the association between diet and cancer in humans is not definitive, Stefanick said, which prompted researchers to launch the Women's Health Eating and Living trial to assess how diet might affect breast cancer recurrence rates. The study is the largest-ever randomized clinical trial to assess the connection between diet and breast cancer recurrence.
Stanford was one of seven sites for the trial. Researchers recruited women who had been diagnosed with early stage breast cancer while they were between the ages of 18 and 70, and who had successfully completed treatment for the disease.
The women assigned to the intervention group made enormous changes in their diets, Stefanick said. First, they were asked to limit the amount of fat in their diet to 15-20 percent of their total calories. In addition, they were expected to consume five servings of vegetables plus 16 oz. of vegetable juice, three servings of fruit and 30 grams of fiber on a daily basis. While that sounds like a lot of veggies, Stefanick pointed out that a serving size amounts to about a half-cup of chopped or shredded vegetables.
But it couldn't be just any vegetable. "We were really strict about what counted as a vegetable," she said. "Iceberg lettuce and white potatoes didn't count. We emphasized nutrient-dense vegetables, such as dark, leafy greens, sweet potatoes and carrots."
The women in the control group were asked to follow national guidelines that call for eating five daily servings of fruits and vegetables, 20 grams of fiber and getting less than 30 percent of their calories from fat.
All of the women completed both telephone and written questionnaires during the course of the study to assess their adherence to the diets and their overall health. In addition, they were weighed and had their blood drawn annually during the first four years of the study and again at the six-year mark to validate their compliance.
Although many of the women weren't able to meet their dietary targets completely, Stefanick said the women in the intervention group achieved major dietary changes. At the end of the first four years of the study, the women in the intervention group were consuming 65 percent more vegetables, 25 percent more fruit, 30 percent more fiber and 13 percent less fat than the women in the control group. Additionally, blood tests at the four-year mark showed that a biomarker linked to vegetable and fruit consumption was 43 percent higher for women in the intervention group.
But when the researchers compared the recurrence and mortality rates among the women, there was little difference. The study found that 16.7 percent of the women on the high-veggie diet experienced a recurrence of the disease compared with 16.9 percent of those in the control group. Similarly, 10.1 percent of the women in the intervention group died vs. 10.3 percent of the control group.
The women weren't restricted in the number of calories they could consume, which might explain why-despite the decrease in fat and increase in fruit and veggie consumption - neither group of women lost weight over the course of the study, Stefanick said.
Additionally, the study didn't address whether consuming a high vegetable/fruit/fiber and low-fat diet earlier in life would alter the risk of developing breast cancer, or whether a combination of dietary changes, increased exercise and weight loss are required in order to reduce the recurrence rate. "Diet alone might not be enough, but it would be interesting to explore the effect of a combined approach," Stefanick said.
The lead author of the study is John Pierce, PhD, a professor at the Moores Cancer Center at the University of California-San Diego. In addition to Stanford and UCSD, the other sites participating in the trial were the University of California-Davis, M.D. Anderson Cancer Center in Houston, the University of Arizona and Kaiser Permanente in Oakland, Calif., and in Portland, Ore. Some of the participants in the Stanford portion of the trial were seen at a satellite site at UC-San Francisco.
The study was funded by the Walton Family Foundation and the National Cancer Institute.
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