Underweight and overweight related to all-cause mortality
In the European cohorts of the SCS middle-aged men with a body mass index (BMI) <18.5 kg/m2 had a 2-fold greater all-cause mortality after 15 years of follow-up compared to those in the reference category (BMI 18.5-25 kg/m2). Overweight (BMI 25-30 kg/m2) was not related to higher mortality. Obese (BMI >30 kg/m2) men who never smoked had an approximately 50% higher risk of all-cause mortality. BMI was not associated with excess all-cause mortality however in obese men who smoked.
The excess all-cause mortality among the lowest BMI may be the result of a low lean body mass rather than a low fat mass. The reason for a lack of association of excess mortality with a BMI greater than 30 kg/m2 among smokers may be due to their high absolute risk of all-cause mortality. Smoking is such a dominantly determining risk factor that obesity adds only a small additional absolute risk.