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"Breast cancer mortality was less [in the low-fat group], but not significantly," said Dr. Rowan Chlebowski, a medical oncologist at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center in Torrance, Calif.
Chlebowski plans to present the findings Monday at the American Association for Cancer Research annual meeting, in New Orleans. Studies presented at medical meetings are viewed as preliminary until they are published in a peer-reviewed journal.
In the new analysis, researchers looked at whether following a diet that limited fat intake to no more than 20 percent of calories helped women survive longer if they developed breast cancer. They were compared to women following a typical Western diet that had fat intake of more than 32 percent of total calories.
The low-fat diet won out.
"There were 20 percent fewer deaths after breast cancer in the low-fat group than the usual diet or control group" during the follow-up period, Chlebowski said.
The study is a secondary analysis of part of a U.S. study known as the Women's Health Initiative. In that part of the study, researchers enrolled nearly 49,000 women, all past menopause, at 40 different clinical centers across the United States from 1993 to 1998.
The researchers assigned those women, all cancer-free at the start of the study, to follow a low-fat diet or their usual eating plan.
During the course of the eight-year study, about 1,700 women developed breast cancer. Those in the low-fat group were somewhat less likely to develop the disease and somewhat less likely to die from it, but the results were not dramatic.
Even so, the researchers continued to follow the women, looking to see if those on the low-fat plan were more likely to survive after a breast cancer diagnosis.
Ten years after diagnosis, the researchers found that women on the low-fat plan had a survival advantage when all causes of death were examined. Some of the reduction in all causes of death was due to fewer deaths from heart disease in the low-fat group, the researchers explained.
During the study, the low-fat group went to group sessions led by registered dietitians to learn how to limit their fat intake. The women were told they could eat the same foods, but to prepare them differently, Chlebowski said.
The findings can't prove cause and effect, he said, but rather should guide future research about the value of low-fat diets. The results also suggest that women need to stay on that low-fat diet to maintain the survival benefit, Chlebowski said.
Dr. Courtney Vito is an assistant clinical professor of surgical oncology at the City of Hope, in Duarte, Calif. She endorsed the study, adding the message can help empower women who have had breast cancer.
Doctors have long known that women who are past menopause and overweight are at higher risk for getting breast cancer, Vito said. That is believed to be partly explained by higher levels of circulating estrogen, as fat cells produce estrogen, she said.
"So it would make sense that someone eating a lower-fat diet will overall be healthier," Vito said.
Lowering fat intake to just 20 percent of calories is not easy, Vito acknowledged. For a typical 2,000-calorie-a-day diet, that would mean getting just 400 calories from fat, or 44 total fat grams. For perspective, a single 3.5-ounce serving of caramel chunk ice cream has 16 grams of fat.
Even so, it is a goal worth striving for, Vito said. "It empowers women who want to know what they can do to reduce the chance of their breast cancer coming back," she added.
Her advice for women?
"I would say do your best to aim for that 20 percent," Vito said, because falling short of that goal is better than not trying.
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SOURCES: Rowan Chlebowski, M.D., Ph.D., medical oncologist, Los Angeles Biomedical Research Institute, Harbor-UCLA Medical Center, Torrance, Calif.; Courtney Vito, M.D., director, fellow education, and assistant clinical professor, surgical oncology, City of Hope, Duarte, Calif.; April 18, 2015, presentation, American Association for Cancer Research annual meeting, New Orleans