Diabetes mellitus is a group of chronic disorders of metabolism characterized by elevated blood glucose, resulting from insufficient production or ineffective action of insulin. Persistent high glucose can damage blood vessels, nerves, eyes, kidneys, and the heart. The principal forms are type 1 diabetes, type 2 diabetes, and gestational diabetes; they share abnormal glucose regulation but differ in their causes and clinical course. (niddk.nih.gov)
Biological basis
Insulin is a hormone produced by beta cells in the pancreas. It helps regulate blood glucose and enables tissues to use glucose for energy. When insulin production or action is inadequate, glucose accumulates in the bloodstream, producing hyperglycemia. Diabetes therefore involves disrupted glucose regulation rather than simply excessive sugar consumption. (niddk.nih.gov)
In insulin resistance, muscle, fat, and liver tissues respond inadequately to insulin. The pancreas may initially compensate by secreting more insulin, but glucose rises when beta cells cannot maintain sufficient secretion. In severe insulin deficiency, increased use of fat for energy can produce excessive ketones, contributing to acute metabolic complications. (www2.niddk.nih.gov)
Classification and causes
Type 1 diabetes usually results from autoimmunity: the immune system attacks insulin-producing beta cells. The resulting insulin deficiency requires insulin replacement for survival once clinical disease develops. Although frequently diagnosed in children and young adults, it can appear at any age. Age alone therefore does not determine diabetes type. (niddk.nih.gov)
Type 2 diabetes combines impaired insulin action with inadequate insulin secretion and accounts for the large majority of cases worldwide. Its development involves genetic and environmental factors. Associated risk factors include family history, increasing age, overweight or obesity, and physical inactivity. It also occurs in children and adolescents. Neither body weight nor a single lifestyle characteristic establishes the diagnosis. (ncbi.nlm.nih.gov)
Gestational diabetes develops during pregnancy. Glucose levels often return to normal after delivery, but affected individuals have an increased subsequent risk of type 2 diabetes. Diabetes detected during pregnancy can also represent previously unrecognized diabetes rather than gestational diabetes. (niddk.nih.gov)
Less common forms include monogenic diabetes, caused by a change in a single gene, and diabetes associated with pancreatic damage or removal. Identifying the underlying form matters because treatment can differ. Prediabetes describes glucose levels above the normal range but below diabetes diagnostic thresholds; it is associated with increased risk of type 2 diabetes and cardiovascular disease. (niddk.nih.gov)
Symptoms and diagnosis
Common symptoms include increased thirst, frequent urination, fatigue, unexplained weight loss, and blurred vision. Type 2 diabetes may develop gradually over several years, with mild or absent symptoms. Some cases become apparent only when complications develop. Gestational diabetes also commonly produces no noticeable symptoms. (niddk.nih.gov)
Diagnosis relies on laboratory blood tests. Common diagnostic thresholds outside pregnancy include:
- Fasting plasma glucose of 126 mg/dL (7.0 mmol/L) or higher.
- Glycated hemoglobin, or HbA1c, of 6.5% or higher.
- Plasma glucose of 200 mg/dL (11.1 mmol/L) or higher two hours after an oral glucose tolerance test.
- Random plasma glucose of 200 mg/dL (11.1 mmol/L) or higher in the presence of characteristic symptoms.
A second test is usually used to confirm the diagnosis. Pregnancy involves different testing protocols and thresholds. HbA1c reflects average glucose over approximately three months, but some blood disorders and pregnancy-related changes can affect its reliability. Glucose tests establish abnormal glycemia but do not, by themselves, identify diabetes type; autoantibody or genetic testing can assist classification. (niddk.nih.gov)
Complications
Acute complications include diabetic ketoacidosis, in which insufficient insulin leads to excessive ketone production. It occurs most commonly in type 1 diabetes but can also affect people with type 2 diabetes. It can involve vomiting, abdominal pain, dehydration, and abnormal breathing and constitutes a medical emergency. (niddk.nih.gov)
Long-term complications include retinal disease and vision loss, kidney damage, peripheral nerve damage, heart disease, and stroke. Nerve damage and impaired circulation can contribute to foot ulcers and lower-limb amputation. Complications are risks rather than inevitable outcomes; glucose management and detection and treatment of associated conditions can reduce or delay them. (who.int)
Hypoglycemia, or abnormally low blood glucose, is another important problem in diabetes care. Severe episodes can cause confusion or loss of consciousness. Management therefore addresses both excessive glucose and the risk of glucose falling too low. (niddk.nih.gov)
Treatment and monitoring
Diabetes care combines glucose management, nutrition, physical activity, medication where appropriate, and monitoring for complications. It also addresses blood pressure and cholesterol because cardiovascular risk extends beyond glucose levels alone. Treatment varies with diabetes type, other health conditions, and access to care. (niddk.nih.gov)
Type 1 diabetes requires insulin, administered through injections or an insulin pump. Type 2 diabetes may be treated with oral or injectable medicines, including insulin. Metformin reduces liver glucose production and improves insulin use. Metabolic surgery is an option for selected patients. (niddk.nih.gov)
Monitoring includes periodic HbA1c measurement and, depending on treatment, glucose meters or continuous glucose monitoring. Continuous monitors estimate glucose throughout the day and night, providing information about fluctuations that an average measurement cannot show. (niddk.nih.gov)
Epidemiology and public health
The World Health Organization estimated that the number of people living with diabetes increased from approximately 200 million in 1990 to 830 million in 2022. Its November 2024 fact sheet reported faster increases in prevalence in low- and middle-income countries and substantial gaps in medication coverage. These figures refer to 2022 estimates, not a count for the publication year. (who.int)
Diabetes is a major public health concern because of its frequency, complications, and continuing treatment needs. Type 2 diabetes can often be prevented or delayed through interventions addressing diet, physical activity, and excess weight. Affordable insulin, other medicines, diagnostic testing, and complication care remain central elements of health-system responses. (who.int)