Increased mortality risk with lower versus higher BMI in older adults with T2DM: a Shanghai diabetes management cohort study

This study aims to investigate the relationship between body mass index (BMI) and all-cause mortality among older adults with type 2 diabetes mellitus (T2DM) in Shanghai, seeking to determine the optimal BMI threshold and safe range, as well as to analyze the asymmetric effects of low and high BMI on mortality risk. This large-scale community cohort study is based on the diabetes management cohort from the Shanghai Municipal Center for Disease Control and Prevention. A total of 372,829 T2DM patients age ≥ 60 were included, with a follow-up period from May 1, 2020, to Dec 31, 2024, and a median follow-up of 19 months. BMI was calculated using standardized measurements and self-reported height and weight (kg/m2). All-cause mortality data were matched from the Shanghai Death Registry System. Covariates included sex, age, marital status, education level, smoking, alcohol, and physical activity. Statistical analysis utilized Restricted Cubic Spline (RCS) analysis to explore the dose–response relationship between BMI and mortality risk, identifying the optimal cutoff point. BMI groups were further compared using Cox regression and Kaplan–Meier (KM) survival curves to assess risk differences. BMI intervals of ± 5% and ± 10% were used to verify risk gradients, with adjustments for covariates. The significance level was set at α = 0.05. The RCS curve revealed a U-shaped relationship between BMI and all-cause mortality, identifying a BMI of 22.68 as the point of lowest mortality risk (covariates adjusted model, P for nonlinear 23.81 (+ 5% interval) had survival probabilities that were not significantly different from those of the reference group on the KM curve. Similarly, BMI 24.95 (+ 10% interval) showed no statistically significant difference compared to the reference group on the KM curve. There is an obesity survival paradox among T2DM older adults, with low BMI significantly increasing mortality risk, while the protective effects of high BMI may partially counterbalance its metabolic harms, resulting in a lower effect size of risk. These findings suggest that weight management strategies for older adults with T2DM should prioritize the avoidance of low BMI over aggressive weight reduction.