Insulin resistance and incident prediabetes in underweight individuals: a 5-year Chinese cohort study

To examine the association between surrogate insulin-resistance (IR) indices and incident prediabetes across BMI categories, with particular attention to underweight individuals. This retrospective cohort study included 99,016 Chinese adults with normoglycaemia at baseline. Four IR indices—triglyceride-glucose (TyG) index, TyG-BMI, atherogenic index of plasma (AIP) and metabolic score for IR (METS-IR)—were calculated. Participants were classified as underweight, normal-weight or overweight/obese. The primary outcome was new-onset prediabetes (fasting plasma glucose 5.6–6.9 mmol/L). Variable importance was ranked using the random-forest-based Boruta algorithm, and multivariable Cox proportional hazards models were employed to estimate risk associations. Over a median follow-up of 3.12 years, 12,152 (12.3%) individuals developed prediabetes. All IR indices were significantly and positively associated with prediabetes risk (all P < 0.0001). The strongest associations were observed in the underweight group; for example, TyG-BMI yielded an HR of 1.28 (95% CI 1.12–1.46), higher than in other BMI categories. Boruta identified the IR indices as top-ranking predictors, and ROC analysis showed that TyG-BMI achieved the best discriminative performance for prediabetes. IR indices are powerful predictors of incident prediabetes, with the most pronounced associations seen in underweight individuals. These findings challenge the notion that low BMI guarantees metabolic health and underscore the need to screen for insulin resistance across the entire BMI spectrum, including underweight persons, to enable more precise diabetes prevention.