The majority of patients with long-duration type 1 diabetes are insulin microsecretors and have functioning beta cells

Aims/hypothesis Classically, type 1 diabetes is thought to proceed to absolute insulin deficiency. Recently developed ultrasensitive assays capable of detecting C-peptide under 5 pmol/l now allow very low levels of C-peptide to be detected in patients with long-standing type 1 diabetes. It is not known whether this low-level endogenous insulin secretion responds to physiological stimuli. We aimed to assess how commonly low-level detectable C-peptide occurs in long-duration type 1 diabetes and whether it responds to a meal stimulus. Methods We performed a mixed-meal tolerance test in 74 volunteers with long-duration (> 5 years) type 1 diabetes, i.e. with age at diagnosis 16 (9-23) years (median [interquartile range]) and diabetes duration of 30 (19-41) years. We assessed fasting and stimulated serum C-peptide levels using an electrochemiluminescence assay (detection limit 3.3 pmol/l), and also the urinary C-peptide: creatinine ratio (UCPCR). Results Post-stimulation serum C-peptide was detectable at very low levels (> 3.3 pmol/l) in 54 of 74 (73%) patients. In all patients with detectable serum C-peptide, C-peptide either increased (n = 43, 80%) or stayed the same (n = 11) in response to a meal, with no indication of levels falling (p < 0.0001). With increasing disease duration, absolute C-peptide levels fell although the numbers with detectable C-peptide remained high (68%, i. e. 25 of 37 patients with > 30 years duration). Similar results were obtained for UCPCR. Conclusions/interpretation Most patients with long-duration type 1 diabetes continue to secrete very low levels of endogenous insulin, which increase after meals. This is consistent with the presence of a small number of still functional beta cells and implies that beta cells are either escaping immune attack or undergoing regeneration.