Women with a lean body shape have a greater risk of developing endometriosis than women who are morbidly obese, a new study has revealed.
The study found that the risk of endometriosis was 39 percent lower in morbidly obese women - those with a body mass index (BMI) greater than 40 kg/m2 - compared with women with a current BMI in the low normal range (18.5-22.4 kg/m2).
When the researchers looked back at the women's BMIs when they were 18, they found that women who were morbidly obese at that point in their lives had a 41 percent lower risk of developing endometriosis than women with low normal BMI.
The association was strongest in the group of women who were infertile - those who had been trying to become pregnant for more than a year: there was a significantly lower rate of endometriosis (62 percent) among the currently morbidly obese compared with those with a low normal BMI, while it was 77 percent lower among women who were morbidly obese at age 18 compared with those with a low normal BMI at 18.
The authors of the study stress that although their findings establish firm evidence of a link between endometriosis and BMI, it does not show that low BMI causes endometriosis.
"It is important to note that despite the strength of the evidence underlying the association between body weight and endometriosis, inferences regarding causation or the pathophysiologic process underlying these relations cannot be made," they said.
The mechanisms that might be responsible for the link between BMI and risk of endometriosis are unclear, but the researchers point to the possibility that BMI at a younger age may have an influence on health in later life, especially as it is known to have an effect on other diseases; and also that polycystic ovarian syndrome (PCOS) 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.
First author of the study, Clinical Assistant Professor, Divya Shah (MD), at the University of Iowa Hospitals and Clinics (Iowa City, USA), said that further research is needed to understand the biological mechanisms underlying the associations that they have seen in our study.
"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," she noted.
"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," she added.
The study has been published online in Europe's leading reproductive medicine journal Human Reproduction.
