Mortality risk associated with underweight: a census-linked cohort of 31,578 individuals with up to 32 years of follow-up

Abstract

Background: In contrast to obesity, information on the health risks of underweight is sparse. We examined the long-term association between underweight and mortality by considering factors possibly influencing this relationship.

Methods: We included 31,578 individuals aged 25-74 years, who participated in population based health studies between 1977 and 1993 and were followed-up for survival until 2008 by record linkage with the Swiss National Cohort (SNC). Body Mass Index (BMI) was calculated from measured (53% of study population) or self-reported height and weight. Underweight was defined as BMI < 18.5 kg/m2. Cox regression models were used to determine mortality Hazard Ratios (HR) of underweight vs. normal weight (BMI 18.5- < 25.0 kg/m2). Covariates were study, sex, smoking, healthy eating proxy, sports frequency, and educational level.

Results: Underweight individuals represented 3.0% of the total study population (n = 945), and were mostly women (89.9%). Compared to normal weight, underweight was associated with increased all-cause mortality (HR: 1.37; 95% CI: 1.14-1.65). Increased risk was apparent in both sexes, regardless of smoking status, and mainly driven by excess death from external causes (HR: 3.18; 1.96-5.17), but not cancer, cardiovascular or respiratory diseases. The HR were 1.16 (0.88-1.53) in studies with measured BMI and 1.59 (1.24-2.05) with self-reported BMI.

Conclusions: The increased risk of dying of underweight people was mainly due to an increased mortality risk from external causes. Using self-reported BMI may lead to an overestimation of mortality risk associated with underweight.

Figure 1

Hazard ratios (all-cause mortality) with 95% confidence interval for interactions with BMI category. Interaction with the type of BMI assessment (A), age group (B), smoking status (C) and sex (D) and adjustment for age at baseline, age², sex, study waves, smoking status. Reference categories are participants with BMI between 20 and <22.5 kg/m² and either measured BMI (A), aged between 25 and 45 years (B), non-smokers (never and former) (C) or women (D). 31,528 participants (25–74 years at baseline) of the Swiss MONICA, NRP 1A, SOMIPOPS and SHS 92/93. Number of deaths in the underweight group: A) measured BMI: 56, self-reported BMI: 72; B) aged between 25 and 45 years: 28, aged between 46 and 74 years: 100; C) non-smokers: 70, current smokers: 58; D) men: 24, women: 104.