Published: May 14, 2013 | Updated: May 15, 2013
By Cole Petrochko , Staff Writer, MedPage Today

Action Points

• Thin women may have a greater risk of developing endometriosis than women who are morbidly obese.
• Note that the association between body mass index (BMI) and endometriosis was strongest in infertile women, with a significantly lower rate of endometriosis among the infertile currently morbidly obese versus those with a low to normal BMI.

Thin women had a greater risk of developing endometriosis than women who were morbidly obese, a large prospective study showed.

Among a cohort of more than 100,000 women, the risk of endometriosis was 39% lower in those with a body mass index (BMI) greater than 40 kg/m2compared with women with a BMI in the low to normal range (18.5 to 22.4 kg/m2), according to Divya Shah, MD, of the University of Iowa Hospitals and Clinics in Iowa City, and colleagues.

In addition, BMI at age 18 and BMI at the time of follow-up had a significant inverse association with incidence of endometriosis (P<0.0001), the authors wrote online in the journal Human Reproduction.

The authors also noted that the association was strongest in infertile women — defined as those who had been trying to become pregnant for more than a year — with a significantly lower rate of endometriosis (62%) among the infertile currently morbidly obese versus those with a low to normal BMI.

While the link between body weight and endometriosis was strong, the authors cautioned that “inferences regarding causation or the pathophysiologic processes underlying these relations cannot be made” based on their study.

Endometriosis affects an estimated one-in-10 reproductive-age women and may account for over $49 billion in healthcare expenditures in the U.S., they explained.

Additionally, prior research has shown inverse associations between BMI and endometriosis, though few studies that have reached that conclusion were designed to examine that association “and none has been able to account for changes in an individual’s weight and associated covariates over time,” they wrote.

They studied the relationship between BMI, body shape, and endometriosis through a prospective cohort of 116,430 female nurses enrolled in the Nurses’ Health Study II who were ages 25 to 42 at baseline.

Participants identified outcomes of laparoscopically-confirmed, physician-diagnosed endometriosis via questionnaire, and were followed-up at 2-year intervals. The questionnaires also addressed weight, height, and waist and hip circumference, as well as confounders such as age at first birth, time since last birth, smoking status, infertility history, parity, alcohol use, and oral contraceptive use. These data were also evaluated at baseline and follow-up.

Women in the study were white (93%), and those reporting higher current BMI were mostly older, had younger age at menarche, and reported a longer menstrual cycle length.

The participants reported 5,504 incident cases of endometriosis during 1,299,349 patient-years for an incidence rate of 385 per 100,000 years.

The highest two categories of BMI with obesity were significantly and inversely associated with endometriosis, which grew stronger with infertile status (P<0.0001).

Among participants without reported infertility, underweight women had a rate ratio of 1.31 (95% CI 1.07-1.60) and morbidly obese women had an RR of 0.70 (95% CI 0.55-0.88) for developing endometriosis. Risks were not confounded by covariates other than BMI at age 18, which attenuated risks, though the protective affect for morbidly obese women was still significant.

Compared with infertile women with low to normal BMI, infertile women with obesity and a BMI of 40 kg/m2 or greater (RR 0.38, 95% CI 0.23-0.62) had significantly reduced risk of endometriosis.

Risks for endometriosis were also significantly associated with BMI at age 18 (P<0.0001) for women with and without infertility, “but was again noted to be stronger among women with infertility.”

Magnitude of weight change from age 18 was also inversely associated with endometriosis risk (P=0.0009).

The associations were not confounded by variables such as smoking, parity, and oral contraceptive use.

The authors speculated on possible reasons for the association including the possibility that BMI at a younger age may influence health later in life.

Also, “polycystic ovarian syndrome is more common among obese women and the effect it has on menstruation and hormones might play a role in reducing or slowing the growth of endometrial lesions,” they suggested.

Finally, among women with the smallest waist-to-hip ratio of less than 0.60, there was nearly a three-fold risk of endometriosis (RR 2.78, 95% CI 1.38-5.60) versus those with waist-to-hip ratios between 0.70 and 0.79, although the authors cautioned that the sample size was small.

Other study limitations included a mostly-white patient population and a low community prevalence of asymptomatic endometriosis.

The authors called for more research to understand the biological mechanisms underlying the associations.

Shah cautioned in a statement that “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. 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.”

The data on the higher risk for leaner women may help doctors when making a diagnosis of endometriosis, she added.