When Metabolically Healthy Is Not Low Risk: BMI Thresholds for Incident Diabetes in East Asian Adults

Aims To determine whether metabolic health attenuates body mass index (BMI)‐related risk of diabetes in East Asian adults and to identify BMI thresholds at which risk increases among metabolically healthy individuals. Materials and Methods We analysed 125 843 adults from two longitudinal cohorts, including 113 354 Chinese and 12 489 Japanese participants. Metabolic health was stringently defined as the absence of elevated blood pressure, dyslipidaemia and impaired fasting glucose. Participants were categorised by BMI and metabolic health status. Cox proportional hazards models estimated hazard ratios (HRs) and 95% confidence intervals (CIs) for incident diabetes, using metabolically healthy normal weight (MHNW) as the reference. Restricted cubic splines assessed non‐linear BMI–diabetes associations by metabolic health status. Results During 352 036 Chinese and 89 483 Japanese person‐years, 1483 and 312 participants, respectively, developed diabetes. In the primary model adjusted for age, gender, smoking, and drinking, compared with MHNW, metabolically healthy overweight was associated with incident diabetes in Chinese (HR: 2.33, 95% CI: 1.63–3.33) and Japanese participants (HR: 3.32, 95% CI: 1.50–7.35). Metabolically healthy obesity (MHO) was also associated with a higher risk (Chinese: HR: 12.40, 95% CI: 6.77–22.68; Japanese: HR: 27.89, 95% CI: 8.42–92.41). The reported change points were 22.2 kg/m 2 in Chinese and 21.8 kg/m 2 in Japanese metabolically healthy participants. Conclusions Metabolic health did not eliminate BMI‐related diabetes risk in East Asian adults. Diabetes risk increased at BMI levels below conventional overweight thresholds, supporting joint assessment of BMI and metabolic health for diabetes risk stratification.