New story in Health from Time: Losing Weight May Reduce Your Risk of Breast Cancer

Excess body weight is an established risk factor for breast cancer. Research suggests that’s because too much body fat can elevate levels of sex hormones like estrogen, especially among postmenopausal women. But despite knowing there is a correlation between extra weight and breast cancer, it’s been difficult to study how losing that weight could affect an individual woman’s chance of developing cancer.

Now, a new paper published in the Journal of the National Cancer Institute provides encouraging evidence that, for women 50 and older, virtually any amount of sustained weight loss translates to a reduction in breast cancer risk.

“We’re so thankful to be able to say it’s not too late to lower your risk if you’ve previously gained weight, even after age 50,” says study co-author Lauren Teras, scientific director of epidemiology research at the American Cancer Society.

The research drew on data collected through the Pooling Project of Prospective Studies of Diet and Cancer, an international set of studies that seeks to elucidate the relationship between diet and cancer among women without a history of the disease. For the new paper, researchers used data from about 180,000 women living in the U.S., Australia and Asia, all of whom were 50 or older and cancer-free when the studies began. Each woman also provided researchers with data about her weight and body mass index, as well as lifestyle and demographic characteristics.

The researchers monitored participants’ weight changes for 10 years after they joined the study, examining survey data collected every few years to see if their weight had gone up, gone down or remained stable. (Most studies used self-reported height and weight data.) After that decade, they tracked the women for another eight years or so to see how many developed breast cancer. Almost 7,000 did.

After adjusting for other factors that can affect breast cancer risk, such as exercise habits and use of hormone replacement therapy, the researchers found that the more weight a woman lost, the lower her risk of breast cancer became. Among women not using hormone therapy (which is sometimes used to replace hormones lost during menopause, and has been linked to breast cancer risk), losing about 4.5 pounds—and keeping it off—seemed to be enough to drive risk down by around 18%, compared to a woman of similar starting weight who did not lose any. Sustained weight loss of 20 pounds and up corresponded to a roughly 32% lower risk.

Given the relationship between excess body weight and cancer risk, the effects of weight loss appeared to be much stronger among women who started the study overweight. That’s an important takeaway, Teras notes, since about 70% of American adults are considered either overweight or obese. “Women who are at a healthy weight don’t need to lose weight,” Teras says.

Teras’ study could not prove cause and effect, only pick up on associations between weight loss and breast cancer. For that reason, she says, it’s not entirely possible to say why dropping pounds could reduce cancer risk, though it likely reverses some of the hormonal activity sparked by weight gain.

While breast cancer is virtually always caused by multiple factors, the research highlights actionable advice for women seeking to minimize their risk—not to mention their chances of developing other chronic illnesses such as diabetes, heart disease and other types of cancer.