High body mass index (BMI) in both women and men is associated with subfertility in observational studies. This relationship is further substantiated by a few small randomized controlled trials of weight reduction and success of assisted reproduction. Women with low BMI also have lower conception rates with assisted reproduction technologies.
The results of a study published in Human Reproduction support a causal role of body mass index (BMI) on subfertility.
The study included 28,341 women (average age 30, average BMI 23.1 kg/m2) and 26,252 men (average age 33, average BMI 25.5 kg/m2) from the Norwegian Mother, Father and Child Cohort Study.
Participants had available genotype data and provided self-reported information on time-to-pregnancy and BMI.
The results show that 10 per cent of couples were sub-fertile (time-to-pregnancy ≥12 months). Women and men who were sub-fertile were older, had a lower educational level, were more likely to be current or former smokers, were more likely to be trying for a first pregnancy, and had, on average, a greater BMI.
After multivariable and Mendelian randomisation analyses, the researchers found that BMI has a J-shaped association with subfertility in both women and men. Participants with BMI values <20.0 kg/m2 and ≥30.0 kg/m2 had an increased risk of subfertility. The findings also expand the current evidence by indicating that individuals with BMI values <20 kg/m2 may have an increased risk of subfertility. These results suggest that BMI values between 20 and 25 kg/m2 are optimal for a minimal risk of subfertility.
The findings support a causal effect of obesity on subfertility. They indicate that a genetically predicted BMI of 23 25 kg/m2 and 25 kg/m2 is linked to the lowest subfertility risk in women and men, respectively.
A 1 kg/m2 greater genetically predicted BMI was linked to 18% greater odds of subfertility in obese women (≥30.0 kg/m2) and 15% lower odds of subfertility (-24% to -2%) in women with BMI <20.0 kg/m2.
A 1 kg/m2 higher genetically predicted BMI was linked to 26% greater odds of subfertility (8-48%) among obese men.
Low genetically predicted BMI values were also related to greater subfertility risk in men at the lower end of the BMI distribution.
The main limitations of the study were that the researchers did not know whether the subfertility was driven by the women, men or both, the exclusive consideration of individuals of northern European ancestry; and the limited amount of participants with obesity or BMI values <20.0 kg/m2.
Read more: https://pubmed.ncbi.nlm.nih.gov/34668019/