Aug. 19, 2003 -- Obese women may be more likely to develop breast cancer largely because their bodies produce more estrogen than thin women do.
New research suggests that the increase in breast cancer risk faced by obese postmenopausal women may largely be due to higher levels of estrogens circulating in their bodies. High levels of estrogen are known to stimulate certain types of breast cancer tumors to grow and develop.
The study, published in the Journal of the National Cancer Institute, shows that the average concentration of estrogens in obese women was between 50% and 219% higher than in thin women, and the risk of breast cancer increased by about 18% with each increase in body mass index (a measurement of weight and height used to indicate obesity).
Researchers say that previous studies have also shown that breast cancer risk among postmenopausal women increases with increasing BMI or obesity, but until now the reasons for the increase in health risk associated with obesity and breast cancer have not been clearly understood.
Obesity and Breast Cancer Risk
"The association between obesity and breast cancer risk is important because obesity may be the principal contributing factor for a substantial number of cases of breast cancer and because the prevalence of obesity is high and increasing," write researcher Timothy J. Key, DPhil, of the Endogenous Hormones and Breast Cancer Collaborative Group, and colleagues.
For example, researchers say the prevalence of obesity in the U.S. among women 60-79 years old increased from 29% in 1988-1994 to 40% in 1999-2000.
In addition, obese women with breast cancer tend to fare worse than thin women.
For the study, researchers analyzed information on 624 postmenopausal women with breast cancer and 1,669 similar healthy women.
They found obese women with a BMI of 30 or above had estrogen concentrations between 60% and 219% higher than thin women, and the risk of breast cancer increased as BMI increased at an average rate of about 18% per 5-point increase in BMI.
Estrogens May Explain Risk
But when researchers adjusted for specific types of estrogen, such as one known as free estradiol, the additional breast cancer risk associated with each increase in BMI dropped from 19% to 2%.
Researchers say the finding strongly suggests that "the increased breast cancer risk in obese postmenopausal women is largely due to the associated increase in bioavailable [free] estradiol."
Researchers say the results are important because obesity is a modifiable risk factor for breast cancer while several of the other known risk factors are either genetic or non-modifiable, such as the age at which menstruation began, number of pregnancies, age at pregnancies, and age at menopause.