CHICAGO — Women who consumed a balanced diet and reduced fat intake did not significantly lower their risk of developing breast cancer but were less likely to die of breast cancer, data from the Women’s Health Initiative (WHI) showed.
As previously reported, women assigned to a low-fat diet had 8% fewer breast cancers as compared with women who consumed a more traditional high-fat diet, but the difference did not achieve statistical significance. A trend toward fewer breast cancer deaths emerged during the intervention period, which had a median duration of 8.5 years. Among patients who developed breast cancer, all-cause mortality was significantly lower in the intervention group (HR 0.64, 95% CI 0.45-0.95), reported Rowan Chlebowski, MD, PhD, of Harbor-UCLA Medical Center in Torrance, California.
After almost 20 years of follow-up, data for the 49,000 study participants showed a statistically significant 21% reduction in the risk of dying of breast cancer in the intervention group versus the control group, whose participants consumed a traditional high-fat diet. Among women who developed breast cancer, those assigned to the low-fat diet had a 15% lower risk of death from any cause during the follow-up period, also a significant difference.
The risk of breast cancer death declined by 69% among women with risk factors for metabolic syndrome and who were randomized to the low-fat diet, the authors noted.
“A dietary change can favorably influence a woman’s risk of dying from breast cancer,” Chlebowski said during a press briefing prior to the American Society of Clinical Oncology (ASCO) annual meeting, which begins here May 31. “It’s a dietary change that we think can be achievable by many because it represents dietary moderation, which was achieved by 19,000 participants.”
“To our review, this is the only study providing randomized clinical trial evidence that an intervention can reduce a woman’s risk of dying from breast cancer,” he added.
The study provided clear evidence that “what we put on the plate matters,” said Lidia Schapira, MD, of Stanford University Medical Center in California.
“Primary care doctors and cancer doctors for a long time have been trying to answer these questions, which are front and foremost on the minds of many women who have had breast cancer or are at risk,” said Schapira, an ASCO expert in breast cancer. “This was a prevention study, so it does not help us with counseling women after breast cancer, but it helps us in general by saying that it is worth approaching our patients to put fruit, vegetables, and grains on their plates.”
“This is not easy,” Schapira added. “As Dr. Chlebowski said, they did not accomplish reducing the dietary fat as much as they intended to, but even at the level that they did, they showed that there was a health advantage.”
In a statement, ASCO President Monica Bertagnolli, MD, of Dana-Farber Cancer Institute in Boston, said the study “makes clear there are no downsides, only upsides, to a healthier diet, and it adds to a growing volume of studies showing similar positive effects across cancer types.”
The rationale for the study had its genesis in prior observational data showing an inconsistent association between dietary fat and breast cancer. The WHI comprised several randomized trials, as well as an observational component. One of the randomized studies assessed the impact of dietary modification on breast cancer risk among postmenopausal women. The trial also assessed the impact of a low-fat diet on colorectal cancer and heart disease, which was reported separately.
Eligible women were ages 50-79, with no history of breast cancer, a baseline daily fat intake ≥32% of calories, and normal mammograms. Investigators at 40 centers in the U.S. enrolled 48,835 women during 1993-1998, and randomized them to continue with their usual diet or to a dietary intervention that emphasized consumption of fruits, vegetables, and whole grains and limited fat intake to 20% of their total calories. The intervention group also included 18 small-group sessions and quarterly follow-up, whereas the control group had one follow-up visit per year.
The randomized phase continued for a median duration of 8.5 years and median total follow-up of 19.6 years. The trial had a primary endpoint of breast cancer incidence during the intervention period. Breast cancer mortality and death from any cause after breast cancer diagnosis were secondary endpoints.
The study population had a median age of 62, >80% were white, median BMI was 28, >40% had undergone hysterectomy, and median daily fat intake was 32%. Chlebowski said the intervention group reached a median fat intake of 24.5% before “drifting back up” to 29% during the intervention period.
The updated follow-up data showed that the intervention group had a significantly lower risk of breast cancer mortality (HR 0.79, 95% 0.64-0.97) and death from any cause after the development of breast cancer (HR 0.85, 95% CI 0.74-0.96).
A subgroup analysis identified women with risk factors for metabolic syndrome as deriving greater benefit from the dietary intervention. Participants with one or two components of the syndrome had a nonsignificant 20% lower risk of breast cancer mortality, and those with three or four components had a statistically significant 69% reduction in relative risk (P=0.01), although the latter subgroup comprised only 42 patients.
The study was supported by the NIH.
Chlebowski disclosed relevant relationships with Novartis, Pfizer, Genentech, Amgen, and AstraZeneca.