Researchers have discovered why being obese can increase a woman's risk of developing breast cancer.
Some women have a family history of breast cancer
It was known that eating a high-fat diet raised a woman's risk of developing the cancer.
But US researchers say obesity is dangerous in postmenopausal women because it raises the level of female hormones in the blood.
It can increase a woman's risk of breast cancer by about a fifth, researchers say.
Body Mass Index
In an international study, blood samples from almost 3,000 women who did not have breast cancer in eight centres in the US, Europe and Asia.
None of the women were using hormone replacement therapy.
They were followed for between two and 12 years.
Six hundred and twenty-four women developed breast cancer.
Hormones in their blood were compared with those in the blood of cancer-free women.
Obesity was measured using the Body Mass Index (BMI), calculated by dividing a person's weight by their height squared.
The healthy BMI range is between 18.5 and 25. A BMI between 25 and 30 is overweight and over 30 is obese.
The women were divided into five BMI groups; under 22.5, between 22.5 and 24.9, between 25 and 27.4, 27.5 to 29.9 and over 30.
It was found that as BMI climbed so did oestrogen levels and breast cancer risk.
Women with a BMI of 30 had an 18% higher risk of developing breast cancer than women with a BMI of 25.
The results also showed that a type of oestrogen called oestradiol was particularly associated with increased risk.
Researchers, including a team from Cancer Research UK, say obesity is a risk factor women can control through a sensible diet and exercise, unlike factors over which they have no control, such as a family history of the disease.
Dr Tim Key of the Cancer Research UK Epidemiology Unit at Oxford University, who led the research, said: "Women's risk is affected by many fixed factors - a family history of the disease, the number of children they have, the age they have their children, when they start their periods and when they stop.
"But obesity is something that women have a level of control over.
"Put simply, maintaining a healthy weight avoids extra breast cancer risk for these women."
He added: "We know that hormonal factors are central to the development of breast cancer.
"This study helps us to better understand the role obesity and certain hormones play in the mechanism that increases risk of the disease.
"Obesity may be the principal contributing factor for a substantial number of breast cancer cases. And with the obesity rate climbing it is imperative that we understand its dangers.
Dr Lesley Walker, director of cancer information for Cancer Research UK, said: "Breast cancer risk increases with age, and obesity after the menopause further adds to this risk while causing a wide range of other health problems."
The research is published in the Journal of the National Cancer Institute.