Introduction
Type 1 diabetes is defined as a disease of progressive autoimmune destruction of beta cells, leading to absolute insulin deficiency. The decline in insulin production in patients diagnosed with type 1 diabetes is variable. Most (92–97%) patients at more than 5 years from diagnosis have a stimulated serum C-peptide value <200 pmol/l [
1]. Enduring endogenous insulin production has a protective effect on microvascular complications and hypoglycaemia [
2].
The prevalence of detectable C-peptide in type 1 diabetic patients depends on several factors including duration of diabetes, age at diagnosis, the type and timing of sample collection (e.g. after a stimulus), and by the sensitivity of assays used [
3‐
5]. There have been recent improvements in the sensitivity of C-peptide assays. Thus Wang et al reported that, using an ultrasensitive ELISA, C-peptide can be detected in fasting blood samples of 43% of patients with type 1 diabetes after a median disease duration of 15 years [
6]. In keeping with this finding, insulin-containing beta cells have long been identified in pancreatic autopsy specimens of type 1 diabetic patients and more recently have been demonstrated to be present in up to 88% of patients with long-duration type 1 diabetes [
7‐
10]. It is not clear whether the very low levels of C-peptide detected and the beta cells visible in autopsy specimens reflect functioning beta cells that can respond to a physiological stimulus.
The urinary C-peptide:creatinine ratio (UCPCR) is a recently described method of assessing C-peptide production that involves using a spot urine test. It is a reliable, sensitive and specific method for assessment of insulin secretion in type 1 diabetes, with test material remaining stable for 3 days at room temperature, allowing outpatients to easily be tested [
11,
12]. No studies have assessed the ability of UCPCR to detect low-level insulin production.
We aimed to assess the prevalence of detectable C-peptide in patients with long-duration type 1 diabetes and determine whether low-level C-peptide rises after a meal stimulus. This was done using serum and urine samples to assess their relative sensitivity.
Discussion
We found persistent C-peptide secretion, which increased after a meal stimulus, in the majority of patients with type 1 diabetes and a disease duration of more than 5 years. We propose that patients with a detectable stimulated C-peptide value of <30 pmol/l, which would not have been detected with older assay technologies, should be called insulin microsecretors.
Our finding that 73% of patients with type 1 diabetes of more than 5 years duration (median 29 years) had detectable C-peptide is consistent with previous studies. In a recent study, Wang et al demonstrated that 43% of patients with type 1 diabetes had detectable fasting C-peptide after a median disease duration of 15 years [
6]. In Joslin medallists (diabetes duration >50 years), 67% had a detectable random C-peptide (>30 pmol/l). Indeed, it was suggested that this enduring C-peptide may have contributed to the patients’ survival [
8].
An important finding is the response of very low-level C-peptide to a mixed meal. This strongly supports the notion that the residual beta cells are functional and excludes the possibility that very low levels of C-peptide are the result of analytical noise. Other studies have not directly addressed the responsiveness of these patients with very low C-peptide levels. In Joslin medallists, the response to a mixed meal was only studied in those with C-peptide over 100 pmol/l [
8], but we report here that a response occurred in the majority of patients with a C-peptide concentration <30 pmol/l. Wang et al reported a correlation between glucose levels and low-level C-peptide in three patients with serial random C-peptide measurements over 3 months, but did not look at the direct response to a meal [
6].
Our results provide clear evidence that the infrequent beta cells seen in histological studies of long-duration type 1 diabetes are still functional. Occasional beta cells have been noted in histological studies of patients with long-standing type 1 diabetes since 1965 [
9,
10]. These were shown recently to be present in the pancreases of most patients [
7,
8]. The received wisdom was that these beta cells were not functional, as older assays could not detect C-peptide in long-standing cases of type 1 diabetes. Our finding that low-level C-peptide in people with type 1 diabetes increases after a meal establishes that these rare isolated islets are functional.
We detected low-level C-peptide production not only in analyses of serum, but also of urine. This is an important point for future studies. The stability and non-invasiveness of single sample UCPCR means tests can be done at home. This could facilitate large community/cohort studies and provide a way to easily identify insulin microsecretors for future investigation.
Our study has limitations. The patients studied were not randomly selected from a population-based sample of patients with type 1 diabetes as would be the case for a true epidemiological cohort. We cannot rule out the possibility that some patients with non-type 1 diabetes were inadvertently included, although this is unlikely to be a major source of error, as a tight definition of type 1 diabetes was used and the absolute levels of C-peptide found were far lower than those described in type 2 diabetes or MODY [
15]. The lack of assay standardisation makes it difficult to compare the prevalence of persistent C-peptide production when assessed by different assays [
16,
17]; our comparison of the methods showed that the Roche assay detects low-level C-peptide in more patients than the Mercodia assay.
Further work is needed to establish the clinical significance and cause of insulin microsecretion. The benefits of residual insulin microsecretion in terms of hypoglycaemia, microvascular complications and long-term survival are unknown, but any impact is likely to be small so that further studies will need to have larger numbers of participants. The immunological and genetic characterisation of patients with and without residual C-peptide may help to explain why some people with type 1 diabetes have enduring endogenous insulin secretion and some do not.
In conclusion, we were able to detect enduring low-level insulin production in the majority of patients with type 1 diabetes, even in those with very long disease duration. The increase in C-peptide levels after a meal provides convincing evidence of functionally responsive beta cells. This implies that beta cells are either escaping immune attack or undergoing regeneration.
Acknowledgements
We thank S. Hammersley (Paediatric Diabetes Specialist Nurse, University of Exeter Medical School) and J. Ludvigsson (Professor of Paediatrics, Linköping University, Sweden), who recruited patients and provided samples for the study.
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