Does a Low-Fat Diet Reduce the Risk of Dying From Breast Cancer?

Results from the Women’s Health Initiative Trial suggest that postmenopausal women who reduced the amount of fat they ate seem to have a 21% lower risk of dying from breast cancer.

Results were reported at a media briefing for the 2019 American Society of Clinical Oncology (ASCO) Annual Meeting on May 15, 2019. Read the abstract of “Low-fat dietary pattern and long-term breast cancer incidence and mortality: The Women’s Health Initiative randomized clinical trial.”

Diet and breast cancer

Diet is thought to be partly responsible for about 30% to 40% of all cancers. But diet alone is unlikely to be the “cause” or the “cure” of cancer. In the 1970s, the theory that a high-fat diet increased breast cancer risk became popular, and some people still believe that theory. Still, studies done since that time have offered mixed results.

For example, previously reported results from a study called the WHEL (Women’s Healthy Eating and Living) study was designed to see if a strict diet that was low in fat and high in vegetables, fruit, and fiber made a difference in survival or breast cancer recurrence (the breast cancer coming back) in women who had been diagnosed. The results were published in 2007 and showed that the diet had no effect on survival or recurrence, which surprised many people.

The Women’s Health Initiative Trial

This research is part of the very large Women’s Health Initiative Clinical Trial and the Women’s Health Initiative Observational Study. Both studies are commonly called the WHI. Together, the two studies include information from more than 161,608 postmenopausal women who were ages 50 to 79 when they joined between 1993 to 1998. The WHI wants to find any links between health, diet, and lifestyle factors and health problems such as cancer.

The researchers started with information from 48,835 postmenopausal women who enrolled in the WHI between 1993 and 1998. When they enrolled in the study, none of the women had been diagnosed with breast cancer and all reported eating a diet that was more than 32% fat. The women were randomly assigned to one of two diet plans:

19,541 women (40%) were told to eat a low-fat diet (fat intake was supposed to be only 20% of the diet); these women also participated in group sessions led by a nutritionist to teach the women how to reduce their fat intake

29,294 women (60%) were told to eat their usual diet and received educational materials on healthy eating

The women followed the diet plans for about 8.5 years. The women in both groups did about the same amount of exercise.

Most of the women in the low-fat diet group reduced their fat intake to 25% of their diet. This means that most of the women didn’t meet the goal of having fat intake be 20% of their diets.

In 2017, results from the Women’s Health Initiative Trial suggested that postmenopausal women who ate a low-fat diet after a breast cancer diagnosis were less likely to die from any cause compared to women who ate a diet that was higher in fat. In 2018, a secondary analysis of the data suggested that postmenopausal women who ate a low-fat diet after being diagnosed with breast cancer were more likely to be alive 10 years after diagnosis compared to women who ate a diet that was higher in fat after being diagnosed.

For these 2019 results, the women have been followed for nearly 20 years.

Overall, 3,374 cases of breast cancer were diagnosed in the women between 1993 and 2013.

Compared to women who ate their usual diet and were diagnosed with breast cancer, women in the low-fat diet group who were diagnosed with breast cancer had:

a 21% lower risk of dying from breast cancer

a 15% lower risk of dying from any cause

"A dietary change can favorably influence a woman's risk of dying from breast cancer," Rowan Chlebowski, M.D., Ph.D., of the Harbor-University of California-LA Medical Center, said during the media briefing. "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."

Chlebowski is the WHI’s principal investigator.

What this means for you

While these results are encouraging, it’s important to keep several things in mind:

The study relied on the women accurately reporting what they ate and then estimating the fat content in the food. Sometimes people don’t remember everything they ate or how much they ate, which would affect the results of the study.

The study didn’t look to see if the women stuck to their breast cancer treatment plans completely. Stopping a treatment, such as hormonal therapy, early would affect the study results.

The study only looked at postmenopausal women diagnosed with breast cancer. The results can’t be applied to premenopausal women with breast cancer.

While the women in the study had no history of breast cancer, it's not completely clear if the researchers accounted for differences in family history of breast cancer. This could possibly affect the results of the study.

If you’re a postmenopausal woman who’s been diagnosed with breast cancer, it makes sense to make healthy diet and lifestyle choices to keep your risk of recurrence as low as it can be and your overall health the best it can be, including:

eating a diet low in added sugar and processed foods

eating a diet rich in unprocessed, nutrient-dense foods (foods that have the most vitamins, minerals, and healthy compounds)

exercising regularly at the highest intensity level you’re comfortable with

avoiding alcohol

not smoking

It also makes sense to stick to your treatment plan. Treatments such as chemotherapy and radiation therapy can require trips to the hospital or doctor’s office for several months. You also may need to take hormonal therapy medicines for 5 or 10 years after surgery. You get the best results when you follow your treatment plan completely and on schedule.