Does the obesity paradox apply to congenital heart disease?

The linked paper by Brida et al 1 is an observational study on a prevalence cohort consisting of people with adult congenital heart disease (ACHD). Observational studies are appropriate for generating hypotheses and for examining safety. The hypotheses should then be examined by randomised controlled trials (RCTs), or using randomised Mendelian techniques.2 Incidence cohorts give information about the natural history of a disease, but may lack statistical power for looking at mortality as the number of events initially is small. In prevalence cohorts, there is not a delay in events occurring and hence often the number of subjects studied is relatively small.

This present study looks at 3086 people. The numbers become quite small when looking at body mass index (BMI) groups: only 6.2% were underweight. The follow-up length was short: 2.54 years (IQR: 0.65–5.31 years).

They report a U-shaped relationship between BMI and mortality. Mortality is highest in those with low and high BMI.

Critically the lowest mortality was found at a BMI of 34.1 kg/m², much higher than anticipated.

As the authors state, the U or J relationship between BMI and mortality has …