Diabetes mellitus type 2
Diabetes mellitus type 2 (formerly noninsulin-dependent diabetes mellitus (NIDDM) or adult-onset diabetes) is a metabolic disorder that is characterized by hyperglycemia (high blood sugar) in the context of insulin resistance and relative lack of insulin.[2] This is in contrast to diabetes mellitus type 1, in which there is an absolute lack of insulin due to breakdown of islet cells in the pancreas.[3] The classic symptoms are excess thirst, frequent urination, and constant hunger. Type 2 diabetes makes up about 90% of cases of diabetes, with the other 10% due primarily to diabetes mellitus type 1 and gestational diabetes. Obesity is thought to be the primary cause of type 2 diabetes in people who are genetically predisposed to the disease.
Type 2 diabetes is initially managed by increasing exercise and dietary changes. If blood sugar levels are not adequately lowered by these measures, medications such as metformin or insulin may be needed. In those on insulin, there is typically the requirement to routinely check blood sugar levels.Rates of type 2 diabetes have increased markedly since 1960 in parallel with obesity. As of 2010 there were approximately 285 million people diagnosed with the disease compared to around 30 million in 1985.[4][5] Long-term complications from high blood sugar can include heart disease, strokes, diabetic retinopathy where eyesight is affected, kidney failure which may require dialysis, and poor blood flow in the limbs leading to amputations. The acute complication of ketoacidosis, a feature of type 1 diabetes, is uncommon,[6] however hyperosmolar hyperglycemic state may occur.
Signs and symptoms
The classic symptoms of diabetes are polyuria (frequent urination), polydipsia (increased thirst), polyphagia (increased hunger), and weight loss.[7] Other symptoms that are commonly present at diagnosis include a history of blurred vision, itchiness, peripheral neuropathy, recurrent vaginal infections, and fatigue.[3] Many people, however, have no symptoms during the first few years and are diagnosed on routine testing.[3] People with type 2 diabetes mellitus may rarely present with hyperosmolar hyperglycemic state (a condition of very high blood sugar associated with a decreased level of consciousness and low blood pressure).[3]Complications
Main article: Complications of diabetes mellitus
Type 2 diabetes is typically a chronic disease associated with a ten-year-shorter life expectancy.[4] This is partly due to a number of complications with which it is associated, including: two to four times the risk of cardiovascular disease, including ischemic heart disease and stroke; a 20-fold increase in lower limb amputations, and increased rates of hospitalizations.[4] In the developed world, and increasingly elsewhere, type 2 diabetes is the largest cause of nontraumatic blindness and kidney failure.[8] It has also been associated with an increased risk of cognitive dysfunction and dementia through disease processes such as Alzheimer's disease and vascular dementia.[9] Other complications include acanthosis nigricans, sexual dysfunction, and frequent infections.Cause
The development of type 2 diabetes is caused by a combination of lifestyle and genetic factors.[8][10] While some of these factors are under personal control, such as diet and obesity, other factors are not, such as increasing age, female gender, and genetics.[4] A lack of sleep has been linked to type 2 diabetes.[11] This is believed to act through its effect on metabolism.[11] The nutritional status of a mother during fetal development may also play a role, with one proposed mechanism being that of altered DNA methylation.[12]Lifestyle
Main article: Lifestyle causes of diabetes mellitus type 2
A number of lifestyle factors are known to be important to the development of type 2 diabetes, including obesity and overweight (defined by a body mass index of greater than 25), lack of physical activity, poor diet, stress, and urbanization.[4][13]
Excess body fat is associated with 30% of cases in those of Chinese and
Japanese descent, 60-80% of cases in those of European and African
descent, and 100% of cases in Pima Indians and Pacific Islanders.[3] Those who are not obese often have a high waist–hip ratio.[3]Dietary factors also influence the risk of developing type 2 diabetes. Consumption of sugar-sweetened drinks in excess is associated with an increased risk.[14][15] The type of fats in the diet are also important, with saturated fats and trans fatty acids increasing the risk, and polyunsaturated and monounsaturated fat decreasing the risk.[10] Eating lots of white rice appears to also play a role in increasing risk.[16] A lack of exercise is believed to cause 7% of cases.[17] Persistent organic pollutants may also play a role.[18]
Genetics
Main article: Genetic causes of diabetes mellitus type 2
Most cases of diabetes involve many genes, with each being a small
contributor to an increased probability of becoming a type 2 diabetic.[4] If one identical twin
has diabetes, the chance of the other developing diabetes within his
lifetime is greater than 90%, while the rate for nonidentical siblings
is 25–50%.[3] As of 2011, more than 36 genes had been found that contribute to the risk of type 2 diabetes.[19] All of these genes together still only account for 10% of the total heritable component of the disease.[19] The TCF7L2 allele, for example, increases the risk of developing diabetes by 1.5 times and is the greatest risk of the common genetic variants.[3] Most of the genes linked to diabetes are involved in beta cell functions.[3]There are a number of rare cases of diabetes that arise due to an abnormality in a single gene (known as monogenic forms of diabetes or "other specific types of diabetes").[3][4] These include maturity onset diabetes of the young (MODY), Donohue syndrome, and Rabson-Mendenhall syndrome, among others.[4] Maturity onset diabetes of the young constitute 1–5% of all cases of diabetes in young people.[20]
Medical conditions
There are a number of medications and other health problems that can predispose to diabetes.[21] Some of the medications include: glucocorticoids, thiazides, beta blockers, atypical antipsychotics,[22] and statins.[23] Those who have previously had gestational diabetes are at a higher risk of developing type 2 diabetes.[7] Other health problems that are associated include: acromegaly, Cushing's syndrome, hyperthyroidism, pheochromocytoma, and certain cancers such as glucagonomas.[21] Testosterone deficiency is also associated with type 2 diabetes.[24][25]Pathophysiology
Type 2 diabetes is due to insufficient insulin production from beta cells in the setting of insulin resistance.[3] Insulin resistance, which is the inability of cells to respond adequately to normal levels of insulin, occurs primarily within the muscles, liver, and fat tissue.[26] In the liver, insulin normally suppresses glucose release. However, in the setting of insulin resistance, the liver inappropriately releases glucose into the blood.[4] The proportion of insulin resistance versus beta cell dysfunction differs among individuals, with some having primarily insulin resistance and only a minor defect in insulin secretion and others with slight insulin resistance and primarily a lack of insulin secretion.[3]Other potentially important mechanisms associated with type 2 diabetes and insulin resistance include: increased breakdown of lipids within fat cells, resistance to and lack of incretin, high glucagon levels in the blood, increased retention of salt and water by the kidneys, and inappropriate regulation of metabolism by the central nervous system.[4] However, not all people with insulin resistance develop diabetes, since an impairment of insulin secretion by pancreatic beta cells is also required.[3]
Diagnosis
| Condition | 2 hour glucose | Fasting glucose | HbA1c |
|---|---|---|---|
| Unit | mmol/l(mg/dl) | mmol/l(mg/dl) | % |
| Normal | <7.8 (<140) | <6.1 (<110) | <6.0 |
| Impaired fasting glycaemia | <7.8 (<140) | ≥ 6.1(≥110) & <7.0(<126) | 6.0–6.4 |
| Impaired glucose tolerance | ≥7.8 (≥140) | <7.0 (<126) | 6.0–6.4 |
| Diabetes mellitus | ≥11.1 (≥200) | ≥7.0 (≥126) | ≥6.5 |
- fasting plasma glucose ≥ 7.0 mmol/l (126 mg/dl)
- or
- with a glucose tolerance test, two hours after the oral dose a plasma glucose ≥ 11.1 mmol/l (200 mg/dl)
Threshold for diagnosis of diabetes is based on the relationship between results of glucose tolerance tests, fasting glucose or HbA1c and complications such as retinal problems.[4] A fasting or random blood sugar is preferred over the glucose tolerance test, as they are more convenient for people.[4] HbA1c has the advantages that fasting is not required and results are more stable but has the disadvantage that the test is more costly than measurement of blood glucose.[32] It is estimated that 20% of people with diabetes in the United States do not realize that they have the disease.[4]
Diabetes mellitus type 2 is characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency.[2] This is in contrast to diabetes mellitus type 1 in which there is an absolute insulin deficiency due to destruction of islet cells in the pancreas and gestational diabetes mellitus that is a new onset of high blood sugars associated with pregnancy.[3] Type 1 and type 2 diabetes can typically be distinguished based on the presenting circumstances.[30] If the diagnosis is in doubt antibody testing may be useful to confirm type 1 diabetes and C-peptide levels may be useful to confirm type 2 diabetes,[33] with C-peptide levels normal or high in type 2 diabetes, but low in type 1 diabetes.
Screening
No major organization recommends universal screening for diabetes as there is no evidence that such a program would improve outcomes.[34] Screening is recommended by the United States Preventive Services Task Force in adults without symptoms whose blood pressure is greater than 135/80 mmHg.[35] For those whose blood pressure is less, the evidence is insufficient to recommend for or against screening.[35] The World Health Organization recommends only testing those groups at high risk.[34] High-risk groups in the United States include: those over 45 years old; those with a first degree relative with diabetes; some ethnic groups, including Hispanics, African-Americans, and Native-Americans; a history of gestational diabetes; polycystic ovary syndrome; excess weight; and conditions associated with metabolic syndrome.[7]Prevention
Main article: Prevention of diabetes mellitus type 2
Onset of type 2 diabetes can be delayed or prevented through proper nutrition and regular exercise.[36][37] Intensive lifestyle measures may reduce the risk by over half.[8][38] The benefit of exercise occurs regardless of the person's initial weight or subsequent weight loss.[39] Evidence for the benefit of dietary changes alone, however, is limited,[40] with some evidence for a diet high in green leafy vegetables[41] and some for limiting the intake of sugary drinks.[14] In those with impaired glucose tolerance, diet and exercise either alone or in combination with metformin or acarbose may decrease the risk of developing diabetes.[8][42] Lifestyle interventions are more effective than metformin.[8]Management
Further information: Diabetes management
Management of type 2 diabetes focuses on lifestyle interventions,
lowering other cardiovascular risk factors, and maintaining blood
glucose levels in the normal range.[8] Self-monitoring of blood glucose for people with newly diagnosed type 2 diabetes was recommended by the British National Health Service in 2008,[43] however the benefit of self monitoring in those not using multi-dose insulin is questionable.[8][44] Managing other cardiovascular risk factors, such as hypertension, high cholesterol, and microalbuminuria, improves a person's life expectancy.[8]
Intensive blood pressure management (less than 130/80 mmHg) as opposed
to standard blood pressure management (less than 140–160/85–100 mmHg)
results in a slight decrease in stroke risk but no effect on overall
risk of death.[45]Intensive blood sugar lowering (HbA1c<6%) as opposed to standard blood sugar lowering (HbA1c of 7–7.9%) does not appear to change mortality.[46][47] The goal of treatment is typically an HbA1c of less than 7% or a fasting glucose of less than 6.7 mmol/L (120 mg/dL); however these goals may be changed after professional clinical consultation, taking into account particular risks of hypoglycemia and life expectancy.[7] It is recommended that all people with type 2 diabetes get regular ophthalmology examination.[3] Treating gum disease in those with diabetes may result in a small improvement in blood sugar levels.[48]
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