To investigate the shape of the causal relation between body mass index (BMI) and mortality.
Linear and non-linear mendelian randomisation analyses.
Nord-Trøndelag Health (HUNT) Study (Norway) and UK Biobank (United Kingdom).
Middle to early late aged participants of European descent: 56?150 from the HUNT Study and 366?385 from UK Biobank.
All cause and cause specific (cardiovascular, cancer, and non-cardiovascular non-cancer) mortality.
12?015 and 10?344 participants died during a median of 18.5 and 7.0 years of follow-up in the HUNT Study and UK Biobank, respectively. Linear mendelian randomisation analyses indicated an overall positive association between genetically predicted BMI and the risk of all cause mortality. An increase of 1 unit in genetically predicted BMI led to a 5% (95% confidence interval 1% to 8%) higher risk of mortality in overweight participants (BMI 25.0-29.9) and a 9% (4% to 14%) higher risk of mortality in obese participants (BMI =30.0) but a 34% (16% to 48%) lower risk in underweight (BMI