Women who are obese continue to have higher levels of estrogen despite treatment with aromatase inhibitors such as Arimidex® (anastrozole) and Femara® (letrozole)—and these higher estrogen levels could affect their response to treatment, according to the results of a study published in the Journal of Clinical Oncology.

The majority of breast cancers are hormone receptor-positive. These cancers are stimulated to grow by the circulating female hormones estrogen and/or progesterone. Treatment of hormone receptor-positive breast cancer often involves hormonal therapy with aromatase inhibitors, which suppress the production of estrogen in postmenopausal women.

Researchers from London conducted a study to investigate the relationship between body mass index (BMI) and estrogen levels. The study involved 54 postmenopausal women with estrogen-receptor positive early breast cancer.

BMI is a measure of obesity based on height and weight. Individuals with a BMI of 30 or higher are considered obese. Researchers calculated the BMI and estrogen levels for all women in the study at baseline and found that obese women had higher estrogen levels than women of normal weight. In fact, those with a BMI ranging between 30 and 35 had circulating estrogen levels that were three times higher than women with a BMI of less than 25.

The women in the study underwent treatment with Arimidex for 3 months followed by Femara for 3 months or the opposite sequence and then researchers analyzed correlations between estrogen suppression and BMI for each drug. They found that after treatment with the hormone-suppressing drugs, estrogen levels did drop significantly in the obese women; however, their estrogen levels were still double those of women of normal weight. Suppression of estrogen was greater with Femara across the full range of BMIs in the study.

The researchers concluded that BMI does have an impact on the level of estrogen suppression achieved with aromatase inhibitors. Obese women continue to have higher estrogen levels, despite treatment with one of the drugs.

The results of this study should not change compliance with treatment—in other words, obese women should continue to take an aromatase inhibitor if one has been prescribed by their doctor; however, the data indicates that future research may need to focus on individualized treatment options that address such differences.


Folkerd EJ, Dixon JM, Renshaw L, et al. Suppression of plasma estrogen levels by letrozole and anastrozole is related to body mass index in patients with breast cancer. Journal of Clinical Oncology. Published early online July 16, 2012. doi: 10.1200/JCO.2012.42.0273

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