Residual β-cell function is associated with continuous glucose monitoring metrics in patients with type 1 diabetes mellitus

Abstract

Background To evaluate the relationship between residual islet β-cell function assessed by fasting C-peptide (FCP) and continuous glucose monitoring (CGM) metrics in patients with type 1 diabetes mellitus (T1DM), and to clarify the impact of residual islet β-cell function on glycemic control in T1DM. Methods A retrospective study was conducted including 112 patients with T1DM [68 classic T1DM, 44 latent autoimmune diabetes in adults (LADA)] hospitalized from January 2023 to December 2025. All patients wore CGM devices for ≥7 days. Participants were stratified into four groups by FCP quartiles. Spearman correlation, restricted cubic spline (RCS) regression, and multivariable linear regression with progressive adjustment for confounders were used to evaluate associations between FCP and CGM-derived metrics, including time in range (TIR), time above range (TAR), and mean glucose (MG). Subgroup and interaction analyses were performed by diabetes subtype. Sensitivity analyses included quartile-based categorization, exclusion of LADA patients, and exclusion of those with diabetes duration ≥10 years. Results In this retrospective study, higher FCP levels were significantly associated with favorable CGM-derived metrics, including increased TIR, decreased TAR, and lower MG (all P < 0.001). These associations persisted after multivariable adjustment for sex, age, disease duration, BMI, insulin dosage, and glucose coefficient of variation (CV). RCS analysis revealed significant nonlinear dose-response relationships, with pronounced glycemic improvements at lower FCP concentrations and plateau effects at higher levels. LADA patients exhibited higher FCP levels, longer TIR, and lower TAR compared with classic T1DM (all P < 0.05). Subgroup analyses demonstrated consistent FCP-CGM associations in both subtypes without significant interaction (all P for interaction > 0.05). Sensitivity analyses confirmed robust associations after excluding LADA patients or those with disease duration ≥10 years. Shorter disease duration and lower daily insulin requirements correlated with higher FCP levels. Conclusion Residual β-cell function is independently associated with improved CGM-derived metrics (increased TIR, decreased TAR, and lower MG) in both classic T1DM and LADA. Preserved C-peptide secretion correlates with shorter disease duration and lower daily insulin requirements. These findings highlight the importance of protecting residual β-cell function to achieve glycemic stability and reduce exogenous insulin dependence in T1DM.