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 US study.
Previous studies have tied obesity to a higher chance of getting breast cancer and a worse outcome in women who have already been diagnosed.
But the current study - which appeared in the journal Cancer - makes the post-diagnosis picture clearer, said lead researcher 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.
The relationship with weight may be because certain hormones that are linked to body weight may also fuel tumour growth in the most common form of the disease, known as estrogen receptor-positive cancer.
Data for the study came from trials sponsored by the US National Cancer Institute 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 per cent more likely to have a breast cancer recurrence over the study period and 69 per cent 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.
The link was especially strong for women with estrogen receptor-positive cancer, which accounts for two-thirds of breast cancers.
Although the new study can't prove that extra weight and fat have a direct impact on certain breast cancers, Sparano said it was "biologically plausible."
Women carrying extra fat have been shown to make more estrogen, meaning that this may fuel the growth of the estrogen receptor-positive tumours, he said.
"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," he added.
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 therapies," Sparano said.
"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 added.