Are the underweight more ill than those of acceptable weight or overweight?

A recent paper has created considerable furore by concluding that those who are underweight or obese are at greater risk of mortality than those of acceptable weight or those who are overweight (1). Critics of the paper have claimed that individuals in the underweight category may have underlying chronic disease, be smokers or that the overweight lead a healthy active lifestyle and carry more muscle mass (2). A review of the literature on the ‘underweight’ has indicated a dearth of research on the characteristics of individuals in the underweight category, although a ‘J’-shaped relationship between BMI and many factors has been noted. This uncertainty makes it difficult to determine whether to include or exclude these individuals when estimating the health and mortality impacts of BMI. (1st paragraph)