Underweight and Mortality in Type 2 Diabetes: A Nationwide Retrospective Cohort Study.

BACKGROUND: Being underweight is an underrecognized risk factor for mortality among individuals with type 2 diabetes (T2D). This study aimed to evaluate the associations of underweight status with mortality among individuals with T2D. METHODS: This nationwide, retrospective, population-based cohort study used data from the Korean National Health Insurance Service. We included 1 788 996 adults with T2D who underwent baseline screening between 1 January 2015 and 31 December 2016, with follow-up through December 31, 2022. Underweight was defined as body mass index (BMI) < 18.5 kg/m2 and further categorized as mild (17.0-18.4 kg/m2), moderate (16.0-16.9 kg/m2), and severe (< 160 kg/m2). The primary outcome was all-cause mortality analysed using multivariable Cox proportional hazards regression. RESULTS: During a median follow-up of 6.96 years, 176 056 deaths occurred. Compared with the non-underweight group (BMI ≥ 18.5 kg/m2), all-cause mortality increased stepwise across underweight categories, with adjusted hazard ratios (aHRs) of 2.037 (95% CI, 1.982-2.094) for mild underweight, 2.719 (2.587-2.857) for moderate underweight, and 3.876 (3.646-4.119) for severe underweight. Cause-specific mortality showed similar gradients. For diabetes-related mortality, the aHRs were 2.265 (2.073-2.474), 3.306 (2.845-3.843) and 5.136 (4.300-6.134) across mild, moderate and severe underweight, respectively; for cardiovascular mortality, 1.881 (1.721-2.055), 2.087 (1.751-2.487) and 2.825 (2.267-3.519); and for cerebrovascular mortality, 2.100 (1.881-2.344), 2.300 (1.846-2.866) and 3.501 (2.702-4.537). Notably, the severe underweight group (BMI < 16.0 kg/m2) showed significantly higher all-cause mortality than the severe obesity group (BMI ≥ 35.0 kg/m2) when compared to the BMI 25.0-29.9 kg/m2 reference group (aHR [95% CI]: 5.168 [4.857-5.499] vs. 1.504 [1.418-1.595]). CONCLUSIONS: Among individuals with T2D, underweight status was associated with substantially elevated mortality risks, with severe underweight exhibiting greater risk than severe obesity.