Among women who have been treated for breast cancer, heavier women are more likely to have their disease come back and more likely to die of cancer, according to a new study.
That could be because certain hormones that are linked to body weight may also fuel tumor growth in the most common form of the disease, known as estrogen receptor-positive cancer.
Previous studies have tied obesity to a higher chance of getting breast cancer - and worse outcomes in women who have already been diagnosed.
But these findings make the post-diagnosis picture clearer, said lead researcher Dr. Joseph Sparano, associate chairman of medical oncology at the Montefiore Einstein Center for Cancer Care in the Bronx, New York.
"Obesity seemed to carry a higher risk of breast cancer recurrence and death - even in women who were healthy at the time that they were diagnosed, and despite the fact that they received the best available chemotherapy and hormone therapy," he said.
Data for the new study came from trials sponsored by the National Cancer Institute (NCI) of women with stage I, II and III breast cancer who were given standardized treatment, with drug doses adjusted based on weight.
Out of close to 5,000 women treated for cancer, about one-third were obese and another one-third were overweight.
Over the next eight years, one in four women had their cancer come back and 891 died - including 695 from breast cancer.
Sparano and his colleagues found that compared to women of normal weight, obese women were 40 percent more likely to have a breast cancer recurrence over the study period and 69 percent more likely to die from breast cancer or any other cause.
Even among overweight but not obese women, there was also a general trend toward a higher risk of recurrence and death with increasing weight, according to findings published Monday in the journal Cancer.
The link was especially strong for women with estrogen receptor positive cancer, which accounts for two-thirds of breast cancers.
But weight wasn't clearly linked to breast cancer outcomes for women with other types of cancer not dependent on estrogen for growth.
ESTROGEN, INSULIN, OR SOMETHING ELSE?
Although the new study can't prove that extra weight and fat have a direct impact on certain breast cancers, Sparano said that was "biologically plausible."
"There may be factors that are fueling the growth of the estrogen receptor positive tumors," he said - such as estrogen itself. Women carrying extra fat have been shown to make more estrogen.
In addition, Sparano added, "Insulin levels are known to be higher in patients who are obese because they develop insulin resistance... (and) insulin can stimulate the growth of breast cancer cells."
Dr. Massimo Cristofanilli, head of medical oncology at the Fox Chase Cancer Center in Philadelphia, said it's possible that anti-estrogen drugs such as tamoxifen can't do enough to overcome the extra-high estrogen levels in obese women.
"Maybe obese women require much longer treatment because their risk of recurrence remains over time," Cristofanilli, who has studied the link between weight and breast cancer outcomes but wasn't involved in the new study, told Reuters Health.
According to the NCI, one in eight women in the U.S. will be diagnosed with breast cancer at some point, but the risks vary greatly over the course of a woman's life.
Whether women with the disease can improve their long-term outlook by losing weight hasn't been proven, researchers said.
"The highest priority is just getting through the chemotherapy if chemotherapy is necessary and taking their endocrine therapy," Sparano told Reuters Health.
"But for those who are obese or overweight, there may be additional benefits that one can achieve through diet and through weight reduction that may produce a reduction in the risk of recurrence that's just as significant as the reduction that they get from the standard therapies," he said.
Cristofanilli agreed on the benefits of weight loss and said "it's never too late" for women to become healthier through diet and other lifestyle changes, even after a cancer diagnosis.