Breast cancer patients who are overweight may have somewhat poorer survival odds than leaner women with the disease. The findings, from a study of breast cancer patients in China, mirror those of some studies in Western countries, where excess weight has been linked to a greater risk of dying from the disease.

Not all studies have found such a relationship, however, and the importance of weight in a woman’s chances of surviving breast cancer is not entirely clear.

Still, the authors of the new study conclude, the evidence overall suggests that maintaining a healthy weight could help improve breast cancer survival.

The implication is that obesity prevention should be a “high priority” not only for the general population, but for breast cancer patients as well, said study co-author Dr. Xiao-Ou Shu, of Vanderbilt University in Nashville, Tennessee.

Shu and colleagues from the U.S. and China report their findings in the current issue of the American Journal of Epidemiology.

The study included 1,455 Chinese women who were followed for roughly 5 years after being diagnosed with breast cancer. While most women were still alive after 5 years, those who were overweight at the time of diagnosis or soon afterward had a poorer survival rate.

Eighty percent of overweight women were still alive versus 86.5 percent among the leanest women and 84 percent among those who were slightly heavier but still in the normal weight range.

The link between body weight and survival remained when the researchers considered important factors in death risk, such as age, tumor size at diagnosis and the type of treatment patients received.

It’s not fully clear why excess weight might lower a woman’s chances of surviving breast cancer, but Shu’s team points to a number of possibilities. Some studies have found that obese women tend to be diagnosed at later stages in the disease - though, in this study, overweight women had poorer survival even when their cancer was caught early.

Another theory is that excess body fat, by boosting levels of estrogen, testosterone and other hormones, helps speed the growth and spread of breast tumors. Some studies have also linked excess weight to a higher risk of developing breast cancer in the first place, and the hormonal effects of extra body fat are suspected of playing a role.

These latest findings, the researchers conclude, “support the idea that interventions directed at weight control may have a substantial effect on not only breast cancer incidence but also on breast cancer survival.”

More research is needed to define the best ways for women with breast cancer to control their weight, Shu said, but in general, moderate exercise and a diet rich in fruits, vegetables and whole grains are key.

SOURCE: American Journal of Epidemiology, January 15, 2006.