Association between the sarcopenia index and diabetes mellitus in middle-aged and older Chinese adults

The association between the sarcopenia index (SI), calculated as serum creatinine-to-cystatin C ratio * 100, and diabetes risk remains incompletely elucidated. This study aimed to evaluate the association between SI and diabetes risk in middle-aged and older Chinese adults. Data from the China Health and Retirement Longitudinal Study (CHARLS) in 2015 (cross-sectional) and 2018 (longitudinal) were analyzed. The exposure variable was SI, and the outcome was self-reported diabetes. Multivariable logistic regression and Cox proportional hazards models were used to assess the SI-diabetes association in cross-sectional and longitudinal analyses, respectively. The cross-sectional analysis included 9,609 eligible participants (46.1% male; mean age 59.6 ± 7.6 years). Multivariable-adjusted logistic regression revealed a significant inverse association between SI and diabetes prevalence. The longitudinal cohort comprised 7,942 participants with a median follow-up of 3.0 years, during which 526 incident diabetes cases (6.62%) were identified. In the fully adjusted Cox model, lower SI levels were significantly associated with higher diabetes risk (adjusted HR: 0.82, 95% CI: 0.74–0.92). Compared with participants in Q1, the HR values for diabetes in Q2 to Q4 showed a graded reduction: 0.74 (95% CI: 0.58–0.94), 0.69 (95% CI: 0.51–0.87), and 0.60 (95% CI: 0.45–0.80). Subgroup analysis demonstrated significant effect modification by BMI ( P for interaction = 0.023), with the strongest inverse association observed in participants with BMI < 24 kg/m². Higher SI was independently associated with a lower risk of incident diabetes in middle-aged and older Chinese adults, particularly among those with BMI < 24 kg/m².