Adhering to a low-fat diet may significantly reduce the risk of death from breast cancer in postmenopausal women, as well as lower the incidence of breast cancer in these women. These are among the long-term results from the Women’s Health Initiative Dietary Modification (WHI DM) clinical trial, which were presented during a poster session (abstract 520) at the 2019 American Society of Clinical Oncology (ASCO) Annual Meeting.
The study marks the first evidence from a randomized clinical trial to confirm these benefits. Previous observational studies of dietary fat intake and breast cancer have had inconsistent findings.
“When we think about cancer prevention and survivorship, we know that obesity is a really big issue in terms of the risk of getting cancer, and once somebody has cancer, the risk for other malignancies, even recurrence and other chronic illnesses, is high,” Patricia Ganz, MD, director of Cancer Prevention and Control Research at UCLA’s Jonsson Comprehensive Cancer Center, told Cancer Network. Ganz was not involved in the study. “The only issue is, do we have the resources to have patients see a dietician and do the kind of intensive intervention that was done in the study?”
The WHI DM trial was conducted at 40 centers in the United States. A total of 48,835 postmenopausal women aged 50 to 79 years with no history of breast cancer and a 32% or higher dietary fat intake were enrolled in the study. Participants were randomized to either a usual diet or dietary intervention group, which aimed to reduce fat to 20% of total energy intake and increase consumption of vegetables, fruits, and grains.
Compared with the usual diet group, those in the low-fat diet group successfully increased fruit, vegetable, and grain intake, and averaged a small weight loss of 3% (all P < .001). The intervention lasted a total of 8.5 years, and the researchers noted 8% fewer breast cancers in this group compared with those who did not alter their diet.
In earlier reports, deaths from breast cancer trended lower, but did not reach statistical significance. However, in this study, all-cause mortality after breast cancer diagnosis decreased during intervention (hazard ratio [HR], 0.65; 95% CI, 0.45–0.95), as well as over a median follow-up of 16.1 years.
The latest data—which increased long-term follow-up to a median of 19.6 years, with a total of 3,374 breast cancer cases recorded—confirms the reduction in all-cause mortality after diagnosis (HR, 0.85; 95% CI, 0.74–0.96). It also showed, for the first time, a reduction in mortality attributed to breast cancer (HR, 0.79; 95% CI, 0.64–0.97).
Ganz attributed the potential mechanism behind these effects to primarily to weight loss, or at least weight maintenance in the face of the metabolic slowdown that can cause women to put on a few extra pounds each year once they reach menopause. “For older women, weight gain is a risk factor for breast cancer,” she said.