British researchers looked at 54 postmenopausal women with estrogen receptor-positive breast cancer, meaning that the tumor may grow in the presence of estrogen. More than three-quarters of breast cancers require estrogen to grow, so blocking the production or action of estrogen is one of the main ways to treat the disease.

The researchers found that obese breast cancer patients had higher levels of estrogen than women of normal weight.

The women in the study also were compared according to their body-mass index (BMI). BMI is a measure of obesity based on height and weight, and a BMI of 30 is considered the threshold for obesity. Women with a BMI of 30 to 35 had about three times higher levels of estrogen in their blood than those with a BMI of less than 25.

After treatment with hormone-suppressing drugs, estrogen levels in the obese women dropped significantly, but still remained at more than double the levels seen in women of normal weight.

The researchers emphasized that women undergoing breast cancer treatment should not be concerned by the findings. They also said the study results may lead to improvements in doctors' ability to select the best treatment for overweight and obese breast cancer patients.

"Our findings are based on laboratory studies, so we would need to carry out clinical trials to tell us whether women with a higher BMI would benefit from changes to their treatment," study senior author Mitch Dowsett, a team leader in the Breakthrough Breast Cancer Research Centre at the Institute of Cancer Research in London, said in an institute news release.