Endometriosis: Being Lean May Increase Risk

Larry Hand

May 14, 2013

Women with a low body mass index (BMI) may be at greater risk of developing endometriosis than women who are morbidly obese, according to a large prospective study published online May 14 in Human Reproduction.

Divya K. Shah, MD, clinical assistant professor of obstetrics and gynecology at the University of Iowa Hospitals and Clinics, Iowa City, and colleagues analyzed data from the Nurses' Health Study II, which involved 116,430 female registered nurses aged 25 to 42 years between 1989 and 2011. Among the study population, the researchers identified 5504 cases of endometriosis during 1,299,349 woman-years (385/100,000 woman-years). They included only laparoscopically confirmed cases.

Morbidly obese women (BMI > 40 kg/m2) had a 39% (95% confidence interval [CI], 0.50-0.75) lower rate of endometriosis compared with low-normal-weight women (BMI, 18.5 - 22.4 kg/m2).

The inverse relationship was even stronger in infertile women (P < .0001). Obese infertile women with BMIs of 35 to 39.9 kg/m2 had a 55% lower risk (95% CI, 0.30 - 0.67), and obese infertile women with BMIs of 40 kg/m2 or higher had a 62% lower risk (95% CI, 0.23 - 0.62), compared with infertile women with low-normal weights.

The associations were not confounded by factors including race, birthweight, age at menarche, and menstrual cycle length and pattern, nor by factors including age at first birth, alcohol use, and smoking status. Adjusting for BMI at age 18 years closed the gap on endometriosis risk somewhat, but morbidly obese women still had a 25% lower rate (95% CI, 0.60 - 0.95) of endometriosis than low-normal weight women.

Although the study "provides strong evidence" of a relationship between BMI and endometriosis, "inferences regarding causation or the pathophysiologic processes underlying these relations cannot be made," the researchers write. The medical literature has established an inverse relationship, but "there is no consensus as to whether a lean body type is the cause of endometriosis or a result of the disease."

A strength of the study is its prospective design. However, the community prevalence of endometriosis is low, and the Nurses' Health Study II cohort is mostly white, which means the results might not be generalizable to ethnically diverse populations.

"The study does not suggest that the morbidly obese women are, in some way, healthier than the lean women and that is the reason for their lower risk of endometriosis," Dr. Shah said in a news release. "It is more likely that factors related to infertility, which is more common among the very obese, are linked to the reduced risk of endometriosis."

She continued, "Our finding that lean women have a higher risk is useful information for doctors when making a diagnosis. It also means that future research can focus on these women to discover the causes, so that we can design treatments that could help prevent the condition developing."

This study was supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Development. The Nurses' Health Study II is supported by the Public Health Service of the National Cancer Institute. The authors have disclosed no relevant financial relationships.

Hum Reprod. Published online May 14, 2013. Abstract

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