The authors note that the increased risk of mortality observed in underweight people could at least partly be caused by residual confounding from pre-diagnostic disease. Also, lack of exclusion of ever smokers, people with prevalent and preclinical disease, and early follow-up could bias the results towards a more U shaped association.
A related editorial discusses this research, as well as a recently published study investigating life trajectory of body weight and mortality risk. It puts this research in the context of current understanding of optimal body weight and of how weight gain over the life course, is likely to influence health and mortality. It highlights that major challenges remain in finding effective ways to prevent weight gain, support weight loss, and prevent weight re-gain, in both individuals and populations.