Relationship between relative fat mass and infertility: A cross-sectional study.

Infertility is closely related to obesity. Relative fat mass (RFM) is a newer index for assessing percentage of body fat, which reflects the amount of body fat better than body mass index (BMI), but its relationship with infertility needs further study. The purpose of this study was to assess whether there was an association between RFM and infertility in women aged 20 to 44 years in the United States. The corresponding participants were selected from the National Health and Nutrition Examination Survey (NHANES) 2013 to 2018. RFM was used as the independent variable and infertility as the dependent variable. Multiple logistic regression and generalized additive models were used to explore the association between RFM and infertility, 2-stage linear regression models were used to calculate threshold effects, and subgroup analyses and tests of interactivity were used to find sensitive populations. A total of 2328 women aged 20 to 44 years were included. In the fully adjusted model, the risk of infertility increased by 6% for each increase in RFM (OR = 1.06, 95% CI: 1.00-1.12). There was a nonlinear relationship between RFM and infertility with 2 breakpoints of K1 = 31.04 and K2 = 48.4. There was a positive association between RFM and infertility on the right side of K1 and, on the left side of K2 (OR = 1.08, 95% CI: 1.01-1.16; OR = 1.07, 95% CI: 1.01-1.14). In contrast, no statistically significant association between RFM and infertility was found on the left side of K1 as well as on the right side of K2. There was a nonlinear relationship between RFM and infertility, with a positive association with infertility when RFM was in the range of 31.04 to 48.4.This suggests that RFM may be an alternative to BMI in the management of obese infertile women, but this needs to be further confirmed by prospective studies.