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

ABSTRACT 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/m 2 and further categorized as mild (17.0–18.4 kg/m 2 ), moderate (16.0–16.9 kg/m 2 ), and severe (< 160 kg/m 2 ). 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/m 2 ), 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/m 2 ) showed significantly higher all‐cause mortality than the severe obesity group (BMI ≥ 35.0 kg/m 2 ) when compared to the BMI 25.0–29.9 kg/m 2 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.