Obesity is one of the most significant causes of all-cause and cause-specific mortality across the globe. Body Mass Index (BMI) is a globally accepted parameter for measuring obesity in individuals. The objective of this study is to evaluate the causal relationship between body mass index and mortality

This is linear and non-linear mendelian randomization analyses based on Nord-Trøndelag Health (HUNT) Study (Norway) and UK Biobank (United Kingdom). The analysis included a total of 56,150 participants from the HUNT study and 366,385 from UK biobank. All the participants were middle to early-late aged, and the primary outcome of the analysis was all-cause and cause-specific mortality.

A total of 12,015 and 10,334 participants died during a median follow-up of 18.5 years and 7.0 years in the HUNT Study and UK Biobank, respectively. Post linear mendelian randomization, the researchers found an overall positive association between genetically predicted BMI and the risk of all-cause mortality. An increase in 1 unit of BMI was linked to a 5% increase in all-cause mortality. Non-linear mendelian randomization predicted 22-29 to be the lowest-risk BMI. In the case of underweight individuals (BMI<18.5), the higher risk of mortality was associated only in smokers.

The research concluded that the increase in BMI showed a positive association with the risk of all-cause mortality. In underweight individuals, the risk of mortality was high in smokers.

Ref: https://www.bmj.com/content/364/bmj.l1042