Definition and classification of diabetes


The term Diabetes Mellitus defines a group of metabolic diseases sharing the common feature of chronic hyperglycemya. This condition is caused by a defect of insulin secretion, insulin action or both. After a meal, food is broken down into glucose, a sugar, which is carried by blood to peripheral cells throughout the body. With the action of insulin, a hormone secreted by the pancreas, cells can remove the excess of sugar from the blood; sugar is thus stored and released when needed. When the production of insulin becomes insufficient or when peripheral cells become insensitive to the action of insulin, the sugar level in the blood rises resulting in hyperglycemia and thus diabetes. The metabolic abnormalities generated by chronic hyperglycemya cause secondary damages in multiple organ systems, in particular, in the cardiovascular system, in the kidney, in the eyes, and in the peripheral neural system. Commonly, diabetes is classified into three types, each one with different causes and risk factors:

  1. Type I diabetes is an autoimmune disease characterized by the destruction of the pancreatic β-cells (the specialized cells that in the pancreas secrets insulin) and by absolute insulin deficiency. It usually develops in the childhood or early adulthood and results in the need of daily insulin injections. The cause is unknown but it is related to a combination of genetic susceptibility and environmental factors. The most common symptoms at presentation are thirst and polyuria, vomiting, fatigue, blurry vision, and weight loss. The treatment of type I diabetes rely on insulin injection.
  2. Type II diabetes is caused by a combination of a reduced sensitivity to insulin action in the peripheral cells (insulin resistance) and of an inadequate insulin secretion by pancreatic β-cells. It mostly develops in adulthood and is highly associated to obesity, hyper caloric diet and lack of physical activity. Genetic factors are also present. Type 2 diabetes develops slowly and may also remain asymptomatic till the development of complications. A correct screening of patients at risk may help to identify affected patients. This form is also called noninsulin-dependent diabetes because patients can initially control the glycemic level with diet and oral medications, however, insulin injections become often necessary when the disease progress.
  3. Gestational diabetes is a condition that is specifically associated to the pregnancy and can result in birth defects and in increased rate of miscarriage. Moreover, children may be affected by jaundice and hypoglycemia at birth. Hyperglycemia usually disappears after delivery. Women affected by gestational diabetes are at higher risk for type II diabetes development. Gestational diabetes is usually treated by diet, however, insulin injections may be required.
  4. Maturity onset diabetes of the young (MODY) It is an hereditary form of diabetes caused by just a single aberrant gene that disrupt insulin production. MODY is a monogenic form of diabetes, to separate it from the polygenic forms type 1 or type 2, which involve multiple genes and environmental factors. The most common forms are GCK-MODY (MODY2) and HNF1α-MODY (MODY3) triggered by mutations in the GCK  and  HNF1A  genes, respectively. MODY runs strongly in families because is highly heritable bu, however, not everyone who inherits a MODY gene develops diabetes.
  5. Latent Autoimmune Diabetes of Adults (LADA) is a form of type 1 diabetes, with slower progression to insulin dependence in later life. Occurs in individuals with a clinical phenotype resembling type 2 diabetes but immunologically LADA is considered a form of type 1 diabetes, characterized by detection of autoantibodies directed against glutamic decarboxylase (GADA), islet cells (ICA), insulinoma-associated antigen (IA2A), insulin (IAA) and Zinc transporter 8 (ZnT8). People with LADA do not require insulin for the first 3 to 6 months following diagnosis, but up to 80% will require insulin within the next five years.

[Last update: July 2018]