For many years, obesity has been suspected of playing a role in breast cancer risk. Most studies find little association between obesity and breast cancer risk in pre-menopausal women but a significantly increased risk of breast cancer in post-menopausal women. Recent research suggests that measures of central obesity such as waist circumference and the waist-to-hip ratio may be more accurate predictors of breast cancer risk than BMI alone.
A study, published in 1990 compared the body measurements of 216 women recently diagnosed with breast cancer with the measurements of 432 healthy controls. The researchers found a significant correlation between the waist-to-hip ratio (WHR) and risk of developing breast cancer. Women with a WHR greater than 0.80 were 6.46 times more likely to develop breast cancer than women with a WHR less than 0.73.
A second study, conducted by Finnish researchers and published in the International Journal of Cancer, found no association between BMI and breast cancer but a strong positive correlation between WHR and breast cancer risk. Pre-menopausal women with a WHR greater than 0.89 were 4.6 times more likely to develop breast cancer than women with a WHR less than 0.79. In post-menopausal women, a WHR greater than 0.89 led to a 2.6 fold increase in breast cancer risk compared to a WHR less than 0.79.
Finally, a 1998 study also published in the International Journal of Cancer found a 2.63 RR of breast cancer for women in the top quartile of WHR compared to the bottom quartile although the increased risk was only found in post-menopausal women.
Several mechanisms are responsible for the increased risk of breast cancer observed in obese women. Numerous studies have reported an increased risk of ER positive breast cancer and circulating levels of estrogen hormones. Estrogen raises the rate of mammary cell division increasing the probability of a potentially cancerous mutation occuring. Overweight and obese women tend to have higher levels of estrogen because fat cells are involved in estrogen production. This is particularly true in post-menopausal women who are not receiving hormone replacement therapy because once the ovaries stop producing estrogen, body fat becomes the primary estrogen source.
A 1995 study entitled Alcohol, Height, and Adiposity in Relation to Estrogen and Prolactin Levels in Postmenopausal Women, found that levels of estradiol, an estrogen hormone, were on average 53% lower in post-menopausal women with a BMI less than 21 compared to women with a BMI greater than 29. Some of the results of that study are presented in the graph to the right.
The hormones leptin and adiponectin may also play a role in breast cancer. Leptin is believed to exhibit a pro-carcinogenic effect with respect to breast cancer while adiponectin is thought to suppress tumour proliferation. Leptin levels tend to be elevated and adiponectin levels reduced in obese individuals.
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