Body mass index and all-cause mortality in patients with newly diagnosed type 2 diabetes mellitus in South Korea: a retrospective cohort study

Objectives: The lower risk of death in overweight or obese patients, compared with normal-weight individuals, has caused confusion for patients with diabetes and healthcare providers. This study investigated the relationship between body mass index (BMI) and mortality in patients with type 2 diabetes.

Design: A retrospective cohort study.

Setting: We established a national population database by merging the Korea National Health Insurance (KNHI) claims database, the National Health Check-ups Database and the KNHI Qualification Database of South Korea.

Participants: A total of 53 988 patients who were newly diagnosed with type 2 diabetes (E11 in International Classification of Diseases, 10th Edition) in 2007, had available BMI data, lacked a history of any serious comorbidity, received diabetes medication and did not die during the first 2 years were followed up for a median of 8.6 years.

Primary outcome measures: All-cause mortality.

Results: The mean BMI was 25.2 (SD 3.24) kg/m², and the largest proportion of patients (29.4%) had a BMI of 25-27.4 kg/m². Compared with a BMI of 27.5-29.9 kg/m² (the reference), mortality risk continuously increased as BMI decreased while the BMI score was under 25 (BMI <18.5 kg/m²: adjusted HR (aHR) 2.71, 95% CI 2.24 to 3.27; BMI 18.5-20.9 kg/m²: aHR 1.94, 95% CI 1.70 to 2.22; BMI 21-22.9 kg/m²: aHR 1.51, 95% CI 1.34 to 1.70; and BMI 23-24.9 kg/m²: aHR 1.14, 95% CI 1.01 to 1.28). For patients aged ≥65 years, the inverse association was connected up to a BMI ≥30 kg/m² group (aHR 0.76, 95% CI 0.59 to 0.98). However, the associations for men, patients aged <65 years and ever smokers resembled a reverse J curve, with a significantly greater risk of death in patients with a BMI ≥30 kg/m².

Conclusions: This study suggests that, for patients with type 2 diabetes at a normal weight, distinct approaches are needed in terms of promoting muscle mass improvement or cardiorespiratory fitness, rather than maintaining weight status. Improved early diagnosis considering the inverse association between BMI and mortality is also needed.