Background: Diabetes increases susceptibility to infectious respiratory diseases, but the impact of underweight status on mortality from these conditions remains unclear. We evaluated the association between low body mass index (BMI) and respiratory infection-related mortality in a large nationwide cohort of individuals with diabetes.

Methods: We conducted a population-based cohort study using linked Korean national health and mortality data, including 2 508 409 adults with type 2 diabetes. Participants were stratified by BMI and followed for cause-specific mortality from influenza/pneumonia, tuberculosis and COVID-19. Cox proportional hazards models were used to estimate adjusted hazard ratios (aHRs), accounting for demographics, lifestyle factors, comorbidities and competing risks.

Results: Over a median follow-up of 6 years, 18 024 (0.72%) participants died due to respiratory infections. Compared to individuals with BMI ≥ 18.5 kg/m2, the aHRs for respiratory mortality were 7.27 (95% CI, 6.31-8.38), 4.79 (4.25-5.41) and 3.15 (2.93-3.38) for BMI < 16, 16-16.9 and 17-18.4 kg/m2, respectively (all p < 0.001) with a progressive risk gradient. Underweight status amplified the risk of tuberculosis-related mortality most markedly (aHR, 9.93 for BMI < 16 kg/m2, p < 0.001). Mortality risks in underweight individuals exceeded those for BMI ≥ 35 kg/m2 relative to the reference group (25-29.9 kg/m2). Associations were more pronounced in individuals under 65 years and those without cardiovascular or kidney disease.

Conclusions: Underweight status is a strong, independent predictor of respiratory infection-related mortality in individuals with diabetes. Recognition of low BMI as a marker of vulnerability may improve targeted preventive strategies in diabetes care.