Diabetes mellitus type 2 overview
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Diabetes mellitus type 2 Microchapters |
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Differentiating Diabetes Mellitus Type 2 from other Diseases |
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Diagnosis |
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Treatment |
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Medical therapy |
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Seyedmahdi Pahlavani, M.D. [2],Tarek Nafee, M.D. [3],Basir Gill, M.B.B.S, M.D.[4]
Overview
Diabetes mellitus type 2 (T2DM) (formerly called non insulin-dependent diabetes (NIDDM), obesity related diabetes, or adult-onset diabetes) is a metabolic disorder that is primarily characterized by insulin resistance, relative insulin deficiency, and hyperglycemia. The defective responsiveness of body tissues to insulin almost certainly involves the insulin receptor in cell membranes. In the early stage the predominant abnormality is reduced insulin sensitivity, characterized by elevated levels of insulin in the blood. At this stage hyperglycemia can be managed by engaging in exercise, modifying one's diet and medications that improve insulin sensitivity or reduce glucose production by the liver. As the disease progresses the impairment of insulin secretion worsens, and therapeutic replacement of insulin often becomes necessary. It is rapidly increasing in the developed world, and there is some evidence that this pattern will be followed in much of the rest of the world in coming years. The CDC has characterized the increase as an epidemic.
Historical Perspective
Diabetes mellitus is a well-recognized disease from ancient times. In 1812 diabetes mellitus became a recognized clinical entity in The New England Journal of Medicine and Surgery. In 1889, the pancreas was identified as playing a major role in the pathogenesis of the disease. The discovery of insulin in 1921 was a major turning point in the history of diabetes when Frederick Banting and Charles Best were able to reverse the diabetic state in dogs by injecting the pancreatic isolate from healthy dogs.
Classification
Type 2 diabetes doesn't have any specific classification. Although diabetes mellitus is classified in to 3 main categories:
Pathophysiology
T2DM develops from a combination of insulin resistance and progressive β‑cell dysfunction. Contrary to T1DM, patients with T2DM sufficiently produce insulin; however, cellular response to the circulating insulin is diminished. The mechanism by which the insulin resistance develops is postulated to be influenced by both genetic and environmental factors. Contributing mechanisms include increased hepatic glucose production, impaired incretin response, elevated renal glucose reabsorption, reduced insulin-mediated glucose uptake in muscle/adipose tissue, and dysregulated glucagon secretion. Over 600 genetic variants are associated with T2DM risk, although lifestyle factors remain dominant. Environmental influences on the pathogenesis of T2DM include high glycemic diets, central obesity, older age, male gender, low-fiber diet, and high saturated fat diet.
Causes
The underlying cause of T2DM is insulin resistance. The exact cause of insulin resistance is not known, however several theories exist. Central obesity, aging, and high glycemic diets are most commonly implicated in the development of T2DM.
Differentiating Diabetes mellitus type 2 from other Diseases
T2DM must be differentiated from other disorders that can present with polyuria, polydipsia, weight change, or hyperglycemia. These include other forms of diabetes mellitus such as type 1 diabetes, latent autoimmune diabetes in adults (LADA), and monogenic diabetes (MODY)—as well as secondary causes like pancreatic disease and endocrine disorders including hypothyroidism, Cushing syndrome, Wolfram syndrome, and Alström syndrome. Several medications, particularly glucocorticoids and immunotherapies, may also induce hyperglycemia. Distinguishing these conditions relies on careful clinical evaluation alongside targeted testing: autoantibodies (GAD, IA-2, ZnT8) help identify autoimmune forms, while C-peptide levels assess endogenous insulin production. MODY should be considered in lean young adults with a strong autosomal dominant family history.
T2DM must be differentiated from other disorders that may present with polyuria, polydipsia, weight loss or weight gain. Such disorders may include other forms of diabetes mellitus (e.g.T1DM, MODY) or other endocrine disorders (e.g. hypothyroidism, cushing's syndrome, wolfram syndrome, alstrom syndrome) or drug that can cause hyperglycemia (e.g. glucocorticoids)
Epidemiology and Demographics
Type 2 diabetes mellitus T2DM (DM) is a chronic metabolic disease, accounting for 90–95% of all diabetes cases. Although its prevalence is well studied in the United States and other developed nations, global estimates from 2022–2024 indicate that between 589 and 828 million adults are living with diabetes worldwide, with an overall prevalence of 11–14% of adults. Despite this burden, substantial variation exists across developing countries, particularly in rural regions with limited access to healthcare, where an estimated 50% of adults with diabetes remain undiagnosed. In the United States, T2DM affects roughly 1 in 6 adults and disproportionately affects individuals with overweight/obesity, physical inactivity, older age, family history of diabetes, history of gestational diabetes, and certain ethnic groups, including Hispanic/Latino, Asian, Black, and American Indian populations. Prevalence increases sharply after age 65, though childhood-onset T2DM is rising. Interestingly, the prevalence of type 2 diabetes is 39.2%among patients with kidney failure. Globally, men are more commonly affected than women, and low- to middle-income countries carry the highest burden. The continued rise in T2DM parallels rapid urbanization and lifestyle transitions, underscoring its designation as a global epidemic.
Risk Factors
Common risk factors associated with development of T2DM include: positive family history, certain ethnicity, obesity, smoking, physical inactivity, poor dietary habits, certain drugs (.eg. glucocorticoids) and certain medical conditions that may result in weight gain and inactivity.
Screening
Diabetes screening is recommended for many people at various stages of life, and for those with any of several risk factors. Screening tests are the same tests used for diagnosis. Early diagnosis and treatment can control the complications and result in better clinical outcomes.
| Criteria for testing for diabetes or prediabetes in asymptomatic adults |
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Testing should be considered in overweight or obese (BMI ≥25 kg/m2 or ≥23 kg/m2 in Asian Americans) adults who have one or more of the following risk factors:
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| For all patients, testing should begin at age 35 years. |
| If results are normal, testing should be repeated at a minimum of 3-year intervals, with consideration of more frequent testing depending on initial results (e.g., those with
prediabetes should be tested yearly) and risk status. |
Natural History, Complications and Prognosis
If T2DM left untreated, it may result in hyperosmolar hyperglycemic state (HHS) and in rare circumstances diabetic ketoacidosis (DKA). These are classified as acute complications of diabetes. Chronic complications of diabetes mellitus include microvascular (retinopathy, nephropathy, neuropathy) and macrovascular (myocardial infarction, stroke, cardiovascular death) complications. Updated estimates show that 32% of individuals with T2DM have existing cardiovascular disease. Diabetes is a leading cause of kidney failure, blindness, nontraumatic amputations, and increased mortality. Additional associations include metabolic dysfunction–associated steatotic liver disease (MASLD), colorectal and breast cancers, and dementia. Early diagnosis and prompt treatment of these complications may result in improved prognosis and less long term morbidity and mortalities.
Diagnosis
History and Symptoms
A detailed history must be taken from every person presenting with diabetes symptoms. Classic symptoms of diabetes include: weight loss, polyphagia, polydipsia and polyuria. Less common symptoms include vision changes, tingling or numbness in exterimities, fatigue and skin changes.
Physical Examination
Usually patients with T2DM have normal physical examination findings unless complications develop in these patients. Common physical examination findings include, pigmented skin patches and acanthosis nigricans.
Laboratory Findings
Laboratory findings of T2DM are diagnostic for this disease. Diabetes may be diagnosed based on plasma glucose criteria, either the fasting plasma glucose (FPG) or the 2-hours plasma glucose (2-h PG) value after a 75-g oral glucose tolerance test (OGTT) or A1C criteria. A1c remains the preferred test for most adults, but clinicians should recognize its limitations in conditions such as anemia, hemoglobinopathies, pregnancy, transfusion, dialysis, or altered red cell turnover.
| Criteria for the diagnosis of diabetes |
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| FPG ≥126 mg/dL (7.0 mmol/L). Fasting is defined as no caloric intake for at least 8 hours. |
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| 2-hours Plasma Glucose (PG) ≥200 mg/dL (11.1 mmol/L) during an OGTT. The test should be performed as described
by the WHO, using a glucose load containing the equivalent of 75 g anhydrous glucose dissolved in water. |
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| A1C ≥6.5% (48 mmol/mol). |
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| In a patient with classic symptoms of hyperglycemia or hyperglycemic crisis, a random plasma glucose ≥200 mg/dL (11.1 mmol/L). |
Treatment
Lifestyle modification is fundamental for diabetes management and it's a part of therapy. It includes, diabetes self-management education (DSME), diabetes self-management support (DSMS), nutrition therapy, physical activity, smoking cessation counseling, and psychosocial care. Randomized clinical trials have reported absolute reductions in microvascular disease (3.5%), such as retinopathy and nephropathy, myocardial infarction (3.3%-6.2%), and mortality (2.7%-4.9%), with intensive glucose-lowering strategies (hemoglobin A1C <7%) vs conventional treatment 2 decades after trial completion. The overall objectives of DSME and DSMS are to support informed decision making, self-care behaviors, problem solving, and active collaboration with the health care team to improve clinical outcomes, health status, and quality of life in a cost-effective manner.
Glucose Monitoring
Self‑monitoring of blood glucose (SMBG) provides modest benefit in non‑insulin‑treated T2DM but remains essential for individuals on insulin. Continuous glucose monitoring (CGM) improves HbA1c, reduces hypoglycemia, and increases time‑in‑range. Adults with T2DM on basal‑bolus insulin should be offered CGM. Time‑in‑range goal: >70% between 70–180 mg/dL.
Lifestyle Therapy
Lifestyle modification remains foundational. Clinically meaningful HbA1c reductions occur with Mediterranean, DASH, and low‑carbohydrate dietary patterns. Structured medical nutrition therapy and diabetes self‑management education improve glycemic control. Physical activity targets include ≥150 minutes/week of moderate intensity activity, with evidence showing 0.4–1.0% HbA1c reduction and improves cardiovascular risk factors (i.e., hypertension and dyslipidemia). Weight loss of ≥5–10% improves glycemia, and intensive weight‑loss interventions can induce diabetes remission in select patients.
Medical Therapy
The main goals of treatment are, eliminate hyperglycemic symptoms, control the long term complications and improve the patient's quality of life. T2DM is initially treated by lifestyle modification and weight loss, especially in obese patients. Metformin is the first line pharmacologic therapy prescribed once the diagnosis is confirmed unless contraindications exist. If glycemic goals are not achieved, a second agent must be added to metformin. A wide range of options are available to add as combination therapy based on the patient's condition and comorbidities. DPP‑4 inhibitors, sulfonylureas, and thiazolidinediones remain options but lack cardiorenal benefits and carry class‑specific risks.
Metformin monotherapy is recommended unless there is a contraindication to it. In the following conditions, dual therapy is initiated:
- HbA1C greater than 9 commence dual oral blood glucose lowering agent.
- HbA1C greater than 10 OR blood glucose level greater than 300 mg/dl OR a markedly symptomatic patient; consider combination therapy with insulin.
- . For individuals with T2DM and established ASCVD, heart failure, or chronic kidney disease, guidelines recommend early initiation of SGLT2 inhibitors and/or GLP‑1 receptor agonists—independent of HbA1c.
High‑potency GLP‑1RA and dual GIP/GLP‑1 agonists (e.g., semaglutide, tirzepatide) achieve A1c reductions up to ~2.5% and weight loss >5–10%. Large, randomized trials show SGLT2 inhibitors reduce major adverse cardiovascular events (MACE) by ~10%, heart failure hospitalization by 18–25%, and kidney disease progression by 24–39%. GLP‑1 receptor agonists reduce MACE by 12–26% and promote significant weight loss. These benefits support a comorbidity driven instead of purely HbA1c‑driven therapeutic approach.
Surgery
Pancreas and islet cell transplantation is considered in patients with chronic diabetes who have frequent metabolic complications and are intolerant to exogenous insulin or have failed insulin therapy.
Primary Prevention
Life style modification is the mainstay for diabetes mellitus prevention. Metformin is another adjunctive measure to prevent diabetes in high risk persons. Studies have shown that 7% weight loss within a duration of 6 months in obese individuals is effective for diabetes prevention.
Secondary Prevention
The most important secondary prevention strategy in T2DM is to decrease the macrovascular complications. Lipid control, smoking cessation and treatment of hypertension are the most important secondary preventive measures.