Realising the long-term promise of insulin therapy: the DCCT/EDIC study

The introduction of insulin in the treatment of juvenile-onset, now type 1, diabetes mellitus transformed a rapidly fatal disease into a chronic degenerative one. During the insulin-treatment era, long-term microvascular and cardiovascular complications proved to be the bane of existence for people with type 1 diabetes, leading to blindness, kidney failure, amputations, cardiovascular disease (CVD) and premature mortality. The nascent understanding of the link between non-physiologically regulated glucose levels and these complications led to the development of new treatment tools in the 1970s and 1980s that facilitated the delivery of insulin to achieve glucose levels closer to non-diabetic levels. These therapeutic advances set the stage for definitive testing of the glucose hypothesis. The Diabetes Control and Complications Trial (DCCT), supported by the National Institute of Diabetes Digestive and Kidney Diseases, National Institutes of Health (NIH), definitively established the benefits and risks of intensive therapy that substantially lowered mean blood glucose levels, measured by HbA_1c, over a mean 6.5 years of therapy. Intensive therapy in the DCCT, resulting in a mean HbA_1c of ~7% (53 mmol/mol), reduced the development and progression of early microvascular and neurological complications associated with diabetes by 34–76% compared with the conventional-treatment group, which maintained an HbA_1c of ~9% (75 mmol/mol). Intensive therapy was also associated with weight gain and a threefold increased risk for hypoglycaemia. At the end of the DCCT, a long-term observational follow-up study, the Epidemiology of Diabetes Interventions and Complications (EDIC) study, commenced. Despite the convergence of HbA_1c levels between the two groups during EDIC, owing to the adoption of intensive therapy by the original DCCT conventional-treatment group and the return of all participants to their own healthcare providers for diabetes care, the development and progression of complications continued to be substantially less in the original intensive-treatment group vs the conventional-treatment group; this phenomenon was termed ‘metabolic memory’. The DCCT demonstrated a major reduction in early-stage complications with intensive therapy and the metabolic memory phenomenon during EDIC contributed to a substantially lower burden of advanced complications over time. These included a 57% lower risk of CVD events and 33% lower rate of mortality in the original intensive-treatment group compared with the conventional-treatment group. DCCT/EDIC has ushered in the intensive-treatment era, which has been universally adopted and includes the goal of achieving HbA_1c levels less than 7% (53 mmol/mol) for most patients. Although the challenge of making intensive therapy (with the aim of achieving normoglycaemia) as widely accessible and safe as possible remains, continuing improvements in insulin therapy 100 years after its introduction promise a brighter future for people with type 1 diabetes.