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 United States tend to be heavier than whites and researchers had thought that might explain why only 78% survive five years after diagnosis, compared to 90% of white women. 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 is the most detailed and well-designed so far. Researchers 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% of the white women and a quarter 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% vs 12%). Obese white women had a 46% 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 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. While there aren't any bulletproof answers yet, researchers believe that differences in tumor biology could be at play.