What is diabetes?

Diabetes mellitus is a group of metabolic diseases defined by elevated blood glucose (hyperglycemia) resulting from defects in insulin secretion, insulin action, or both. Chronic hyperglycemia is associated with long-term damage to organs such as the eyes, kidneys, nerves, heart and blood vessels. Effective management reduces complications and improves quality of life.

Types of diabetes

Type 1 diabetes

Type 1 diabetes is an autoimmune disease in which the immune system destroys insulin-producing beta cells in the pancreas. People with type 1 diabetes require lifelong insulin replacement. It most commonly presents in children and young adults but can occur at any age.

Type 2 diabetes

Type 2 diabetes is characterized by insulin resistance (the body's cells do not respond properly to insulin) and relative insulin deficiency. It is the most common form of diabetes and is strongly associated with overweight/obesity, physical inactivity, aging and genetics. Many patients manage type 2 diabetes with lifestyle measures and oral or injectable medications; some require insulin.

Gestational diabetes

Gestational diabetes develops during pregnancy in women who previously had normal glucose tolerance. It increases the risk of complications during pregnancy and delivery and raises the mother's and child's long-term risk of developing type 2 diabetes.

Other specific types

Less common forms include monogenic diabetes (e.g., MODY), secondary diabetes due to medications or pancreatic disease, and diabetes associated with endocrine disorders or genetic syndromes.

Epidemiology and burden

Diabetes affects hundreds of millions worldwide and its prevalence continues to rise due to population aging, urbanization, dietary changes and rising obesity. Type 2 diabetes accounts for the majority of cases. The global burden includes increased risk of cardiovascular disease, kidney failure, blindness and lower-limb amputations, making diabetes a major public health priority.

Causes and risk factors

Causes differ by type but include genetic predisposition, autoimmune processes, environmental exposures and lifestyle factors.

Risk factors for type 2 diabetes

• Overweight and obesity (especially central/visceral fat)
• Physical inactivity
• Family history of diabetes
• Age (risk increases after age 45)
• Ethnicity (higher risk in some populations)
• History of gestational diabetes or giving birth to a large baby
• High blood pressure and abnormal lipid levels
• Polycystic ovary syndrome (PCOS)

Factors in type 1 diabetes

Type 1 diabetes results from autoimmune destruction of pancreatic beta cells. Genetic predisposition interacts with environmental triggers (viral infections, early-life exposures) in susceptible individuals.

How diabetes develops (basic pathophysiology)

Insulin is the hormone that allows glucose entry into muscle, fat and other tissues and suppresses glucose production by the liver. In insulin deficiency or resistance, blood glucose rises. In type 2 diabetes, insulin resistance often precedes and strains pancreatic beta cells, which can eventually reduce insulin secretion. Chronic hyperglycemia causes glycation of proteins and oxidative stress, damaging tissues over time.

Symptoms and signs

Symptoms range from subtle to severe and can develop gradually (type 2) or rapidly (type 1). Common symptoms include:

• Frequent urination (polyuria)
• Excessive thirst (polydipsia)
• Unintended weight loss (more typical of type 1)
• Increased hunger (polyphagia)
• Fatigue and weakness
• Blurry vision
• Slow-healing wounds and recurrent infections
• Numbness or tingling in the feet or hands (neuropathy)

Some people with type 2 diabetes have no symptoms and are diagnosed through screening or routine tests.

Diagnosis: Tests and criteria

Diagnosis is typically made using blood glucose measurements. Key tests include:

Fasting plasma glucose (FPG)

Measured after an overnight fast. Diagnostic threshold: ≥126 mg/dL (≥7.0 mmol/L).

2-hour plasma glucose during 75g oral glucose tolerance test (OGTT)

Diagnostic threshold for diabetes: 2-hour value ≥200 mg/dL (≥11.1 mmol/L).

Hemoglobin A1c (HbA1c)

Reflects average blood glucose over ~2–3 months. Diagnostic threshold: HbA1c ≥6.5% (48 mmol/mol). HbA1c may be unreliable in certain conditions (hemoglobinopathies, anemia).

Random plasma glucose

In people with classic hyperglycemia symptoms, a random glucose ≥200 mg/dL (≥11.1 mmol/L) suggests diabetes.

When results are borderline, repeat testing or use of a different test is recommended. For gestational diabetes, specific screening protocols in pregnancy (OGTT at 24–28 weeks) are used.

Acute complications

Untreated or poorly controlled diabetes can cause life-threatening acute complications:

• Diabetic ketoacidosis (DKA): mainly in type 1 — marked insulin deficiency leads to hyperglycemia, ketone production, metabolic acidosis; requires emergency care.
• Hyperosmolar hyperglycemic state (HHS): mostly in type 2 — extreme hyperglycemia with dehydration and altered mental status; high mortality if untreated.
• Severe hypoglycemia: low blood glucose (often due to excess insulin or missed meals) causing confusion, seizures or loss of consciousness; prompt treatment with glucose is essential.

Chronic complications and long-term risks

Persistent hyperglycemia increases risk of microvascular and macrovascular complications:

• Microvascular: diabetic retinopathy (vision loss), diabetic nephropathy (progressive kidney disease, possibly requiring dialysis), diabetic neuropathy (painful or numb feet; risk of foot ulcers).
• Macrovascular: accelerated atherosclerosis leading to heart attack, stroke and peripheral arterial disease.
• Other: infections, cognitive decline, and increased risk of certain cancers have been associated with diabetes.

Early detection and tight risk factor control (blood pressure, lipids, smoking cessation, glycemic management) reduce complication risk.

Treatment overview — goals and strategies

Treatment goals focus on normalizing blood glucose as safely as possible, minimizing symptoms, preventing complications and maintaining quality of life. Individualized targets consider age, comorbidities and hypoglycemia risk. Management is multifaceted:

• Lifestyle modifications (nutrition, physical activity, weight management)
• Glucose monitoring (self-monitoring and/or continuous glucose monitoring)
• Medications — oral and injectable including insulin
• Cardiovascular risk control (blood pressure, lipids, antiplatelet therapy when indicated)
• Regular screening for complications

Nutrition and physical activity

Nutrition and exercise are foundational to diabetes care for all types.

Nutrition principles

• Focus on whole foods: vegetables, lean proteins, whole grains, legumes, fruits (in moderation) and healthy fats (olive oil, nuts).
• Limit added sugars and refined carbohydrates.
• Portion control and carbohydrate counting for those on insulin.
• Individualize meal plans with a dietitian, especially for people requiring insulin adjustments.

Exercise

Regular physical activity improves insulin sensitivity, cardiovascular health and weight control. Aim for at least 150 minutes/week of moderate aerobic activity plus resistance training twice weekly when possible. Monitor blood glucose around exercise if on insulin or insulin secretagogues to avoid hypoglycemia.

Medications: oral and injectable options

Medication choice is individualized. Common drug classes for type 2 diabetes include:

• Metformin: first-line agent that reduces hepatic glucose production and improves insulin sensitivity.
• Sulfonylureas and meglitinides: increase insulin secretion (risk of hypoglycemia).
• DPP-4 inhibitors: modest glucose lowering with neutral weight effects.
• GLP-1 receptor agonists: injectable agents that lower glucose, promote weight loss and reduce cardiovascular risk in some patients.
• SGLT2 inhibitors: lower glucose by increasing urinary glucose excretion and have cardiovascular and renal protective effects in many patients.
• Insulin: required for type 1 diabetes and used when type 2 diabetes is severe or uncontrolled. Insulin regimens vary from basal-only to multiple daily injections or pump therapy.

Medication selection considers efficacy, side effects, cardiovascular and kidney benefits, weight effects and cost.

Glucose monitoring and technology

Monitoring allows patients and clinicians to assess control and guide therapy. Options include:

• Self-monitoring of blood glucose (SMBG): fingerstick testing using a glucometer.
• Continuous glucose monitoring (CGM): sensors measure interstitial glucose continuously and provide trends, alerts and time-in-range metrics. CGM is increasingly used in type 1 and selected type 2 patients.
• Insulin pumps and automated insulin delivery (AID) systems: deliver insulin continuously and, when paired with CGM, automate some insulin dosing to improve control and reduce hypoglycemia.

Screening and preventing complications

Regular screening identifies early complications:

• Eye exams: annual dilated retinal exam for diabetic retinopathy.
• Kidney tests: annual urine albumin-to-creatinine ratio and serum creatinine/eGFR.
• Foot exams: annual comprehensive foot exam and daily self-checks to prevent ulcers and infections.
• Cardiovascular risk management: blood pressure and lipid control; aspirin when indicated.

Special considerations: pregnancy, children and elderly

Pregnancy and gestational diabetes

Good glycemic control reduces risks to the mother and fetus. Gestational diabetes is managed with dietary changes, exercise and insulin if needed. Women with prepregnancy diabetes require close management and planning.

Children and adolescents

Type 1 diabetes is common in pediatric practice and requires family-centered education, insulin therapy, monitoring and school plans. Type 2 diabetes is rising in youths linked to obesity; early intervention is essential.

Older adults

Goals may be relaxed in frail elderly patients to avoid hypoglycemia. Focus on maintaining function and preventing symptomatic hyperglycemia.

Prevention — reducing the risk of type 2 diabetes

Type 2 diabetes is often preventable or delayable with lifestyle changes. Evidence-based prevention strategies include:

• Weight loss of 5–10% in overweight individuals reduces progression from prediabetes to diabetes.
• Regular physical activity (150 minutes/week of moderate exercise).
• Healthy dietary patterns: Mediterranean-style or high-fiber diets with reduced refined carbohydrates and sugary drinks.
• Smoking cessation and control of blood pressure and lipids.
• In high-risk patients, medications such as metformin may be considered for prevention under medical guidance.

Living well with diabetes: practical advice

• Education: diabetes self-management education and support (DSMES) is core to effective care.
• Medication adherence: take medicines as prescribed and communicate side effects to your provider.
• Build a care team: primary care, endocrinologist, diabetes nurse educator, dietitian and eye/foot specialists as needed.
• Plan for sick days: know how to adjust medications and when to seek help during illness.
• Emotional health: address stress, depression or diabetes distress—mental health matters for outcomes.

Recent advances and the future of diabetes care

Recent progress includes improved CGM technology, closed-loop insulin delivery systems, new drug classes with cardiovascular and renal benefits (SGLT2 inhibitors, GLP-1 receptor agonists), and research into beta cell replacement and immunotherapies for type 1 diabetes. Telemedicine and digital health tools are expanding access to care and self-management support.

Frequently asked questions (FAQs)