In most cases, type 1 diabetes is the late consequence of a β-cell autoimmune destruction leading to absolute insulin deficiency (1). At onset of clinical diabetes, β-cell mass is thought to be reduced by 80–90% (2), and 73% of adult patients have ketosis or ketoacidosis at diagnosis (3). Patients with type 1 diabetes have major defects in insulin sensitivity at diagnosis of overt diabetes (4–8). However, the fact that pre-diabetic patients can maintain normal blood glucose levels in spite of dramatically low insulin secretory capacities suggests that their insulin sensitivity is normal.
Here we report that insulin sensitivity, measured by the glucose clamp method in nine patients, remains normal even during the very late preclinical stage of type 1 diabetes. None of these patients had clinical symptoms of overt diabetes, spontaneous weight loss, or ketosis before or at the time of the study. Mean age at entry in the study was 27 years (range 20–41). The mean BMI was 20.9 kg/m2 (range 16.3–23.2). All patients were islet cell antibody positive, and seven of eight tested had at least one susceptibility HLA haplotype. Mean fasting glucose level was 7.2 ± 1.3 mmol/l (range 5–9.6). Of the eight patients who met the criteria for diabetes, four had fasting blood glucose level <7 mmol/l; in the ninth patient, oral glucose tolerance test disclosed impaired glucose tolerance. The mean HbA1c level was 6.4 ± 0.8% (range 5.3–7.8; normal values 5.0 ± 0.5%). The sum of 1- and 3- min plasma insulin levels after the intravenous glucose tolerance test was 15.3 ± 5.8 mU/l (range 11–25).
Four patients were treated with insulin immediately after the metabolic explorations reported here. Four others have initiated permanent insulin therapy within 24 months of follow-up. The last patient was still non-insulin dependent when last seen 1 year after the study.
During clamp studies, the insulin dose-response curve of preclinical type 1 diabetic subjects was superimposable to that of 20 healthy control subjects. Maximal glucose infusion rate, at a plasma insulin level of 1,400 ± 120 mU/l, was 17.2 ± 1.6 mg · kg−1 · min−1 (Fig. 1). It was 16.4 ± 0.6 mg · kg−1 · min−1 in 20 control subjects at a similar plasma insulin level: 1,500 ± 100 mU/l. By contrast, the maximal glucose infusion rate was significantly lower in 15 patients with symptomatic diabetes of recent onset (11 ± 1.4 mg · kg−1 · min−1, P = 0.003 by Mann-Whitney U test). These results confirm, at an even later stage of the natural history of the disease, those observed in a substudy of the Diabetes Prevention Trial-1 (9). Thus, the onset of overt clinical diabetes may be triggered by the addition of insulin resistance to insulin deficiency; the triggers may be external or internal factors. In this respect, the seasonal peaks of the incidence of diabetes (10) suggest that viral infections may play a role in the very final act of the preclinical phase by triggering insulin resistance and metabolic storm. The respective parts of insulin deficiency and insulin resistance may be determinants for the onset and duration of the honeymoon period.
References
M.E. is currently affiliated with the UFR de Physiologie, Faculté des sciences, nutrition et endocrinologie, Errachidia, Morocco; J.T. and C.B. are currently affiliated with the Service d’immunologie-diabétologie, hôpital Cochin, Assistance Publique-Hôpitaux de Paris, Paris, France; R.B. is currently affiliated with UMR 5018 CHU Rangueil, Toulouse, France.