Low-fat diet reduces risk of breast cancer and subsequent death

Women’s Health Initiative study indicates benefit to low-fat, fruit and veggie-rich diet
healthy food
Postmenopausal women who ate a low-fat, fruit and veggie-rich diet were 35 percent less likely to be diagnosed with breast cancer and then die from any cause. Fred Hutch file

Eating less fat and more fruits and vegetables reduces postmenopausal women’s chances of being diagnosed with breast cancer and then dying — from any cause, according to a new analysis of data from a Women’s Health Initiative study presented Monday at the American Association for Cancer Research conference.

In the mid-1990s, almost 50,000 postmenopausal women were enrolled in a study designed to look at the effect of a low-fat, high fruit and vegetable diet on breast cancer risk. When the researchers focused their analysis on breast cancer followed by death within the planned eight-year period during which study participants changed their diets, they saw that women on the low-fat diet were 35 percent less likely to be diagnosed with breast cancer and then die from any cause. They also found that consuming a low-fat diet boosted breast cancer survival by 20 percent.

“[The study] indicates that [a diet with] substantially reduced fat content as well as increased fruits and vegetables improved breast cancer–related outcomes,” said Dr. Ross Prentice, the Fred Hutchinson Cancer Research Center epidemiologist and statistician who co-led the analysis.

Does diet influence breast cancer risk?

For the original randomized trial — designed to examine the association between a diet low in fat and high in fruits and vegetables and breast and colorectal cancer risk — 48,835 postmenopausal women were assigned to one of two groups. Forty percent worked to reduce their dietary fat intake from about 32 percent to 20 percent of their calories and increase their consumption of fruits, vegetables, and grains. They underwent regular nutritional and behavioral counseling in a small group setting. The other 60 percent were not asked to modify their diet and received diet-related educational materials but no counseling. On average, women in the first group spent around eight years sticking closely to their diet goals and lost an average of about six pounds in the first year.

The study, part of the WHI, was led by Prentice and Dr. Garnet Anderson, principal investigator of the Fred Hutch–based WHI Clinical Coordinating Center, along with other WHI investigators including medical oncologist and breast cancer specialist Dr. Rowan Chlebowski at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center.

The first analysis was published in 2006 in The Journal of the American Medical Association and looked at all breast cancer diagnosed during the eight-year study period. Though Prentice, Chlebowski, and Anderson saw a trend toward lower breast cancer risk on the low-fat diet, the results at that time were not quite statistically significant.

A deeper dive into the data

Once the study ended, the women involved no longer received dietary counseling, though they continued to be followed through 2014. The researchers knew that their 2006 analysis drew from data, including the national mortality database, that was likely incomplete at the time. When they were able to include data through 2014, they didn’t see a reduction in breast cancer risk for the low-fat diet group beyond the study’s eight-year dietary "intervention" period.

Chlebowski and his colleagues realized that the diet would likely have little effect on women’s health years after they’d reverted to higher-fat habits. So the research team decided to further analyze the data, zeroing in on breast cancer-related outcomes during the eight-year intervention period.

The researchers looked at various outcomes, including breast cancer diagnosis and death. Some women were diagnosed with breast cancer and subsequently passed away during the intervention period — and that’s where the team, including WHI Coordinating Center data analyst Aaron Aragaki, saw a link between a low-fat diet and a better outcome. A low-fat diet buffered women against falling into the camp of those who received a breast cancer diagnosis and also died, reducing their risk by 35 percent.

“It’s the first time in this dietary modification trial that we see a clear benefit. It’s exciting,” said Prentice.

When they examined breast cancer survival through 2014, the team saw that 82 percent of breast cancer patients assigned to the low-fat diet group survived their disease, compared to 78 percent of patients with breast cancer who didn’t change their dietary habits. Prentice cautions that these results aren’t drawn from two randomized groups, making them less solid.

Chlebowski views the new finding with optimism. “It’s a 4 percent change in absolute survival. There have been sweeping changes in breast cancer treatment, such as switching from aromatase inhibitors to tamoxifen, based on similar numbers,” he said.

One-chance-only trial

A randomized trial of this size “will never be repeated,” said Prentice. There’s no national infrastructure in place to support huge trials of diet and lifestyle and their possible links to chronic disease prevention.

Most studies draw on participants’ own assessment or recollection of their diet and lifestyle — which may be mischaracterized or misremembered. The WHI itself has demonstrated that sometimes-popular hypotheses based on these types of studies, known as observational studies, can fall apart when examined more rigorously, said Chlebowski.

For example, the WHI study of combined estrogen plus progestin hormone replacement showed an unexpected and significant increase in coronary heart disease — contrary to prior observational results. These findings, as well as increases in breast cancer and stroke risk, led to a decrease in hormone therapy use, resulting in 15,000 to 20,000 fewer cases of breast cancer each year in the U.S.

Because the current results arise from a secondary analysis, Chlebowski notes that some may regard them as “hypothesis generating” rather than definitive findings that would be used to guide health care recommendations.

Research questions arising from the latest results include whether the modest weight loss seen in the low-fat diet group influenced their breast cancer-related outcomes. Perhaps outcomes differed between women who adhered to the low-fat diet and lost weight and those who ate a low-fat diet but didn’t lose pounds. Chlebowski said he has already heard from researchers who, based on these findings, are considering redesigning studies that will look at the link between diet or weight loss and breast cancer outcomes.

Tumors can grow for decades before producing symptoms, responding to cues from environmental and lifestyle factors, including nutrition and physical activity. The team saw that women who were diagnosed with breast cancer early and on the low-fat diet longest during treatment seemed to do better.

“I think all this prevention business is actually early therapy,” said Chlebowski.

Sabrina Richards is a staff writer at Fred Hutchinson Cancer Research Center. She has written about scientific research and the environment for The Scientist and OnEarth Magazine. She has a Ph.D. in immunology from the University of Washington, an M.A. in journalism and an advanced certificate from the Science, Health and Environmental Reporting Program at New York University. Reach her at srichar2@fredhutch.org.

Solid tumors, such as those of the breast, are the focus of Solid Tumor Translational Research, a network comprised of Fred Hutchinson Cancer Research Center, UW Medicine and Seattle Cancer Care Alliance. STTR is bridging laboratory sciences and patient care to provide the most precise treatment options for patients with solid tumor cancers.

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