Sun YQ, et al. – Via conducting linear and non-linear mendelian randomization analyses, researchers studied the shape of the causal relation between body mass index (BMI) and mortality in middle to early late aged participants of European descent: 56,150 from the HUNT Study and 366,385 from UK Biobank. They noted a J shaped relation between genetically predicted BMI and the risk of all-cause mortality, with the lowest BMI risk of around 22-25. Subgroup analyses stratified by smoking status suggested a J shaped relationship in ever-smokers, but an ever-increasing BMI-mortality relationship in never-smokers. A J shaped relation was only found for non-cardiovascular non-cancer mortality outcomes in analyses split by cause of mortality (cardiovascular vs cancer vs non-cardiovascular non-cancer). An increased risk of underweight mortality was only noted in ever-smokers.