Maintenance of endogenous pancreatic β-cell function could be an important goal in the management of type 1 diabetes. However, the impact of stimulated C-peptide level on overall glycemic control is unknown. The relationship between C-peptide and parameters of glucose control was therefore characterized in a cohort with rapidly changing β-cell function following islet transplantation.

Standardized mixed-meal tolerance test was undertaken in 12 consecutive islet recipients at 1–6-month intervals, with graft function determined by 90-min stimulated C-peptide. Continuous glucose monitoring was undertaken in the week preceding each assessment and the relationship between C-peptide and glucose control evaluated by mixed Poisson regression.

Recipients completed 5 (1–14) [median (range)] clinical assessments over 18 (1–51) months posttransplant encompassing a wide range of stimulated C-peptide levels (7–2,622 pmol/L). Increasing β-cell function across predefined C-peptide groups was associated with reduced insulin dose, HbA_1c, mean glucose (low [<200 pmol/L] 10.7 vs. excellent [>1,000 pmol/L] 7.5 mmol/L), and glucose SD (low, 4.4 vs. excellent, 1.4 mmol/L). Highly statistically significant continuous associations between stimulated C-peptide and mean interstitial glucose (lower by 2.5% [95% CI 1.5–3.5%] per 100 pmol/L higher C-peptide), glucose SD, time outside glucose target range, and measures of hyper-/hypoglycemia risk were confirmed.

Repeated assessment of islet transplant recipients has enabled modeling of the relationship between endogenous β-cell function and measures of glycemic control providing quantitative estimates of likely impact of an acute change in β-cell function in individuals with type 1 diabetes.