SATURDAY, June 4 (HealthDay News) -- Breast cancer patients who were overweight or obese before and after the diagnosis have a lower chance of surviving, a new study suggests.

Researchers found the link between dying from breast cancer and obesity only in estrogen receptor-positive forms of the disease, or cases in which the hormone estrogen fuels the breast cancer.

"This relationship between dying and being obese or overweight may depend on whether the type of breast cancer is hormonally dependent," said the study's lead author, Christina Dieli-Conwright, assistant research professor at City of Hope National Medical Center in Duarte, Calif., in a news release.

Women who are obese or overweight tend to have higher levels of estrogen in their blood, which may explain the heightened risk of death, the research showed.

The findings are slated to be presented on Saturday at The Endocrine's Society's 93rd annual meeting in Boston.

The study included nearly 4,000 women in the California Teachers Study who were diagnosed with invasive breast cancer from 1995 to 2006. Of those, 262 died.

Researchers looked at their body mass index (or BMI, a measure of height and weight) at age 18 and at the time of diagnosis. Obesity is defined as a BMI of 30 and up, while being overweight is defined as a BMI of 25 to 29.

Women who were obese when diagnosed had a 69 percent higher risk of dying from the breast cancer than women who were a healthy weight. The same increased risk applied to those who were overweight when they were 18 years old.

"What we know now is that there is a strong link between dying from breast cancer and being obese," said Dieli-Conwright. "And it's not just your BMI near the time you're diagnosed that's important."

Moreover, researchers found that for women with estrogen-dependent cancer, the higher their body mass index, the greater their risk of dying of the disease. This link was not found among women with estrogen-negative breast cancer.

Because this study was presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.

The study's authors noted their findings suggest maintaining a healthy weight through diet and exercise is crucial to the prevention and treatment of cancer.

More information

The National Cancer Institute has more information on the causes and risk factors of breast cancer.