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Department of Psychology pres.

EHAP Speaker Series: Hormonal Contraceptives and Breast Cancer: A Case of Evoluntionary Mismatch

Beverly I. Strassman, Professor of Anthropology & Faculty Associate, Institute for Social Research, University of Michigan

Strassman Strassman
In the evolutionary past, women of reproductive age rarely menstruated as they were usually pregnant or breast-feeding. In modern societies, the evolutionarily novel pattern of frequent menses, and the associated increase in endogenous hormonal exposure, is a risk factor for breast cancer. It is unclear, however, whether oral contraceptives further increase or actually decrease hormonal exposure. My collaborators and I examined variation in hormonal exposure across frequently prescribed oral contraceptive (OC) formulations with the goal of providing a quantitative comparison of endogenous and exogenous hormonal exposure. Endogenous data came from 12 published studies of serum estradiol (E2) and progesterone (P4) in European or American women. Exogenous data came from pharmacokinetic package insert data for seven different OC formulations. We found that, with the exception of one formulation, median ethinyl estradiol (a synthetic estrogen) exposure over one menstrual cycle was similar to median E2 exposure. However, median exposure from progestins (synthetic progesterone) was 4-fold higher than the median endogenous exposure from P4. Given that breast cancer risk has a dose-response relationship to hormonal exposure, these findings are cause for concern. Not all formulations produce the same exposures, making these findings also pertinent to contraceptive choice. Our results are discussed in the light of a recent Danish study on hormonal contraceptives and breast cancer risk.

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