African-American women with breast cancer are more likely to die from the disease than are white women. Some scientists believe that obesity is a major reason why, but new research gives less weight to the role of women’s body fat in survival disparities.

Yani Lu and colleagues say obesity does not spur the higher mortality seen among African-Americans with breast cancer. (Photo by Alicia Di Rado)

According to the most recent available data, about 90 percent of white women in the U.S. diagnosed with breast cancer were still alive five years after diagnosis — far more than the 78 percent of African-American women who survived five years.

Women who are obese before their breast cancer diagnosis are more likely to die of the disease, and obesity is more common among African-American women. So scientists thought obesity was the likely culprit behind the difference in survival.

But research from the Women’s Contraceptive and Reproductive Experiences Study calls that into question.

The findings show that white women who are obese before diagnosis have a greater chance of dying from breast cancer than normal-weight white women — but weight has no effect on African-American women’s likelihood of beating the disease.

“It appears that obesity itself does not contribute to the higher mortality seen among black women with breast cancer,” said Leslie Bernstein, Ph.D., director of City of Hope’s Division of Cancer Etiology. “Other factors, like health-care access and quality, psychological and behavioral issues or biological characteristics of the tumors, may be at play.”

The researchers studied more than 1,600 African-American and more than 2,900 white women with breast cancer living in the Atlanta, Detroit, Los Angeles, Philadelphia and Seattle areas. Participants were between ages 35 and 64 when they were diagnosed with invasive cancer between 1994 and 1998.

Researchers interviewed women about a variety of factors potentially linked to cancer including their height and their weight five years before diagnosis, reproductive history, use of hormone therapy and contraceptives, physical activity and alcohol consumption.

Women were deemed to be obese if they had a body mass index (BMI) of at least 30 kilograms per square meter. BMI is a calculation that incorporates height and weight; as an example, a 5-foot-6-inch woman would need to weigh 186 pounds to have a BMI of 30.

In the eight to nine years following diagnosis, obese women in the study had a 20 percent greater risk of dying than did women with normal BMI.

Then the researchers dug deeper.

Among white women, those who were obese had a 54 percent greater risk than those with normal BMI of dying of any cause.

But obese African-American women showed little difference in mortality risk compared to African-American women with normal BMI — so higher BMI could not account for the poorer outcomes among African-American women.

“We did find that black women were more likely than white women to have other characteristics linked to increased risk of mortality,” said Yani Lu, Ph.D., assistant research professor in the Division of Cancer Etiology. These characteristics include lower education levels and other accompanying medical problems.

Also, more black women had advanced cancers or cancers that do not rely on estrogen to grow, and these are more challenging to treat successfully.