According to the National Cancer Institute, approximately one in eight women in the United States will develop breast cancer during their lifetime. Obesity is thought to be a principal contributing—and modifiable—factor for a substantial number of cases of breast cancer. Unfortunately, the prevalence of obesity is on the rise (it jumped from 29% in the years between 1988 and 1994 to 40% in 1999-2000 among 60-74 year old women).

Although the exact mechanism for the relationship between obesity and breast cancer risk has not been established, it is believed to result from an increase in the blood serum concentrations of estrogen, due both to its production in fat tissue and to blood factors that maintain its concentration in the serum.

In a new study published in the August 20, 2003 issue of the Journal of the National Cancer Institute , researchers examined whether sex hormone levels could explain the relationship between body mass index (BMI), a measure of body fat based on height and weight, and breast cancer risk in postmenopausal women. They found that BMI was positively and significantly associated with both breast cancer risk and concentration of estrogens in the blood.

About the Study

The researchers analyzed data from eight prospective studies done in four countries (the U.S., Italy, Japan, and the United Kingdom) on healthy postmenopausal women who were followed to identify those who developed breast cancer. The studies included data on blood samples drawn that measured concentrations of various forms of estrogen, androgen, and sex-hormone binding globulin (a protein that helps regulate the body’s levels of estrogen).

Women who were using hormone replacement therapy (HRT) or other exogenous sex hormones were excluded from the analysis.

Body mass index (BMI), measured as weight in kilograms divided by height in meters squared, was available for 630 case subjects (who developed breast cancer) and 1,704 control subjects (who did not develop breast cancer).

The Findings

The researchers found breast cancer risk increased significantly with increased BMI. Compared to the women with a BMI of less than 22.5 (a BMI in the range of 20-24 is considered healthy; below that is underweight; 25-29 is considered overweight; and 30 and above is considered obese), women with a BMI of 22.5-24.9 had a 10% increased risk, women with a BMI of 25.0-27.4 had a 45% increased risk, women with a BMI of 27.5-29.9 had a 62% increased risk, and women with a BMI greater than or equal to 30 had a 36% increased risk of breast cancer.

In addition, the researchers found that each of the forms of estrogen was strongly and significantly positively associated with BMI. When the researchers adjusted for serum estrogen concentrations, the increase in relative risk of breast cancer was reduced substantially. There were no statistically significant associations between any of the androgens and BMI.

The results of this study are limited by the fact that they were based on single measurements of hormones for each woman. Although they are likely to be random, such measurements are subject to error. It is also possible that the association detected between BMI and breast cancer risk could be partly explained by other biochemical variables not included in the study, such as other hormones like insulin.

How Does This Affect You?

Women who are overweight at or after the time of menopause have a greater chance of developing breast cancer than women who are thinner. The results from this study suggest that, although risk did not increase further when BMI exceeded 30, the increase in breast cancer risk with increasing BMI among postmenopausal women is in fact largely the result of the associated increase in estrogens.

The prevalence of obesity in this country is high and only getting worse. The good news, however, is that obesity is a modifiable risk factor for breast cancer. Although losing weight is not easy, the benefits are clearly worth it.