Body size and endometriosis: results from 20 years of follow-up within the Nurses' Health Study II prospective cohort

• Published in: Human reproduction (Oxford) Vol. 28; no. 7; pp. 1783 - 1792
• Main Authors: Shah, Divya K., Correia, Katharine F., Vitonis, Allison F., Missmer, Stacey A.
• Format: Journal Article
• Language: English
• Published: England Oxford University Press 01-07-2013
• Subjects: Adolescent; Adult; Body Size; Endometriosis - epidemiology; Endometriosis - pathology; Female; Follow-Up Studies; Humans; Incidence; Nurses; Odds Ratio; Original; Risk Factors; Waist-Hip Ratio; USA; endometriosis; body mass index; body fat
• ISSN: 0268-1161; 1460-2350
• DOI: 10.1093/humrep/det120

STUDY QUESTION
Is there a relationship between body mass index (BMI), body shape and endometriosis?
SUMMARY ANSWER
Endometriosis is inversely associated with early adult BMI and may correlate with a peripheral body fat distribution.
WHAT IS KNOWN ALREADY
The literature suggests an inverse relation between endometriosis and BMI, although few studies have specifically explored this association in depth.
STUDY DESIGN, SIZE, DURATION
Prospective cohort study using data collected from 116 430 female nurses from September 1989 to June 2011 as part of the Nurses' Health Study II cohort.
PARTICIPANTS/MATERIALS, METHODS AND SETTING
Cases were restricted to laparoscopically confirmed endometriosis. Weight at age 18 and height were reported at baseline, and current weight was updated every 2 years. Waist and hip measurements were first taken in 1993 and updated in 2005. Rate ratios (RR) and 95% confidence intervals (CI) were calculated using Cox proportional hazards models with time-varying covariates.
MAIN RESULTS AND THE ROLE OF CHANCE
A total of 5504 incident cases of endometriosis were reported during 1 299 349 woman-years (incidence rate = 385 per 100 000 woman-years). BMI at age 18 and current BMI were each significantly inversely associated with endometriosis (P-value, test for linear trend <0.0001). Both associations were stronger among infertile women. Obese infertile women with current BMIs of 35–39.9 kg/m2 and ≥40 kg/m2 had a 55% (95% CI 0.30–0.67) and a 62% (95% CI 0.23–0.62) lower risk of endometriosis, respectively, compared with the low-normal BMI referent (18.5–22.4 kg/m2). Rates of endometriosis were nearly 3-fold higher in women with waist-to-hip ratios <0.60 (RR = 2.78, 95% CI 1.38–5.60) compared with those with waist-to-hip ratios between 0.70 and 0.79, although the sample size for this category was very small.
LIMITATIONS AND REASONS FOR CAUTION
Although women with undiagnosed endometriosis certainly remain in the comparison population even in this prospective cohort study, the community prevalence of endometriosis in an asymptomatic population is very low. Moreover, the characteristics of this small proportion of undiagnosed cases are diluted among the >90 000 women accurately defined as being endometriosis-free and are, therefore, unlikely to impact on effect estimation. Although geographically diverse, the NHS II cohort is overwhelmingly Caucasian, which may limit generalizability to more ethnically diverse populations.
WIDER IMPLICATIONS OF THE STUDY
The results of this study suggest that endometriosis is inversely associated with early adult BMI and may correlate with a peripheral body fat distribution.
STUDY FUNDING/COMPETING INTEREST
This study was supported by research grants HD48544 and HD52473 and HD57210 from the Eunice Kennedy Shriver National Institute of Child Health and Human Development. The Nurses' Health Study II is supported by the Public Health Service grant CA50385 from the National Cancer Institute, NIH, U.S. Department of Health and Human Services. None of the authors has a conflict of interest to disclose.