It is still a mystery why black women are more likely to die from breast cancer than whites, according to a new study that shows the racial disparity can't be chalked up to obesity differences.

As a group, black women in the U.S. tend to be heavier than whites and researchers had thought that might explain why only 78 percent survive five years after diagnosis, compared to 90 percent of white women.

"This has been an important question," said Susan M. Gapstur of the American Cancer Society, who wasn't involved in the new work. Several studies have tied obesity to poorer survival after breast cancer, but only a few small ones have tested whether that relationship varies by race.  The new study, published in the Journal of Clinical Oncology, is the most detailed and well-designed so far, Gapstur, formerly of Northwestern U., told Reuters Health.

Yani Lu of City of Hope in Duarte, California, and colleagues used data collected for an earlier breast cancer study on more than 4,500 women living in major American cities.  About a third of the women were black and the rest were white, and all were between 35 and 64 years old when they were diagnosed with breast cancer. After more than eight years, 14 percent of the white women and 25 percent of the black women had died of breast cancer.

The women had been interviewed about their weight five years before their cancer diagnosis and more than twice as many black women as white were obese (27 percent versus 12 percent).
Obese white women had a 46-percent higher chance of dying of breast cancer than their normal-weight white peers and the increased risk remained after taking other diseases and education into account.  But there was no such link for blacks.

The researchers did find a hint that extra poundage might be related to cancer death in black women with advanced disease, but Gapstur said those results would need to be replicated by additional studies in other groups."It was surprising that this study shows a positive relationship between obesity and breast cancer mortality in white women and not in black women," she said. "It raises important questions about other possible reasons."

While there aren't any bulletproof answers yet, researchers believe differences in tumor biology as well as health care access could be at play.  Last year, for instance, one study found that black and Hispanic women wait longer to get drug treatment after breast cancer surgery than whites.    Previous research has also found that black women are more likely to be diagnosed with breast cancers at later stages than white women.

"We don't yet have a clear picture," Gapstur said.  Still, she added, the message to black breast cancer survivors is not that they shouldn't worry about their weight.
"It is always important to maintain a healthy weight, for a variety of reasons," she told Reuters Health.

SOURCE: Reuters Health and  Journal of Clinical Oncology, online July 25, 2011.