Older women who are obese have a much higher risk of breast cancer because their fat cells release too much estrogen.
The international study comparing obese women to women of normal weight confirms what doctors have long suspected - that fat cells release the hormone into the blood, allowing it to help turn normal cells cancerous.
A woman'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.
Researchers in Britain, Italy, Japan and the United States studied eight different groups of women who were past menopause - when the risk of breast cancer rises dramatically. None of the women had cancer and none were taking hormone replacement therapy when their blood samples were first taken. The researchers then watched the women for between two and 12 years to see which ones developed breast cancer.
The more the women weighed, the higher their risk of cancer. And the more the women weighed, the higher their levels of a form of estrogen called estradiol. A woman who was obese, with a body mass index of 30 or more, had an 18 percent higher chance of developing breast cancer than a woman with a BMI of 25 - just on the border of being overweight. Body mass index compares weight to height, giving a broad range of healthy weights.
A BMI of 22 is considered optimal, while over 25 is overweight, 30 is obese, meaning the risk of diseases is greatly increased, and 40 is morbidly obese.
Researchers have found in other studies that a low-fat diet - especially a diet low in animal and other saturated fats - can reduce the risk of breast cancer.
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