Researchers may have finally put an end to a chicken and egg style debate on whether being underweight is a cause or a consequence of respiratory disease.

Writing in Thorax public health researchers from the University College London said research from the Prospective Studies Collaboration had shown that every 5-unit decrease in body mass index from 25 to 15 kg/m2 was associated with a 1.7-fold increase in respiratory mortality.

However the clinical importance of the findings was uncertain because it was unclear whether the association was causal or a consequence of illness-induced weight loss.

The likelihood that death is caused by an existing weight-modifying condition is highest in the earliest stages of follow-up and progressively diminishes over time, the researchers said.

They hypothesised that if the strength of the under- weight–mortality association attenuates when deaths occurring in earlier years of follow-up are removed from the analyses, reverse causation was a likely explanation.

Analysing data from the Whitehall Study spanning 45 years the researchers discovered the age- adjusted and smoking-adjusted risk was 1.55- fold higher among underweight compared with normal weight participants, but attenuated in a stepwise manner to 1.14 after serial exclusions of deaths during the first 5 to 35 years of follow-up.

“The observed association between underweight and elevated respiratory mortality is mainly attributable to reverse causation, such that underweight is a consequence and not a cause of respiratory disease or its risk factors,” they concluded.

The Prospective Studies Collaboration excluded participants who died within the first 15 years of follow-up while other studies typically excluded mortality within the first 3 or 5 years, they noted.

This suggested the exclusion periods were too short, and the confounded effect of underweight persists much longer, they said.