Diabetes mellitus is a disorder in which the body does not produce enough insulin and/or does not respond normally to insulin, causing blood sugar (glucose) levels to be abnormally high.
Symptoms of diabetes may include increased urination and thirst, and people may lose weight even if they are not trying to.
Doctors diagnose diabetes by measuring blood sugar levels.
Diabetes can damage blood vessels and increase the risk of heart attack, stroke, chronic kidney disease, and vision loss.
Diabetes can damage nerves and cause problems with the sense of touch.
People with diabetes need to follow a healthy diet that is low in refined carbohydrates (including sugar), saturated fat, and processed foods. They also need to exercise, maintain a healthy weight, and usually take medications to lower blood sugar levels and promote weight loss if their weight is above a healthy level.
Between 11 and 14% of adults worldwide have diabetes. Doctors often use the full name diabetes mellitus, rather than diabetes alone, to distinguish this disorder from arginine vasopressin deficiency, which used to be called diabetes insipidus (a relatively rare disorder that does not affect blood glucose levels but, just like diabetes mellitus, causes increased urination).
Blood sugar
The 3 major nutrients that make up most food are carbohydrates, proteins, and fats. Sugars are one of 3 types of carbohydrates, along with starch and fiber.
There are many types of sugar. Some sugars are simple, and others are complex. Table sugar (sucrose) is made of 2 simpler sugars called glucose and fructose. Milk sugar (lactose) is made of glucose and a simple sugar called galactose. The carbohydrates in starches, such as bread, pasta, rice, and similar foods, are long chains of different simple sugar molecules. Sucrose, lactose, carbohydrates, and other complex sugars must be broken down into simple sugars by enzymes in the digestive tract before the body can absorb them.
Once the body absorbs simple sugars, it usually converts them all into glucose, which is an important source of fuel for the body. Glucose is the sugar that is transported through the bloodstream and taken up by cells. The body can also make glucose from fats and proteins. Blood "sugar" really means blood glucose.
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Insulin
Insulin, a hormone released from the pancreas (an organ behind the stomach that also produces digestive enzymes), controls the amount of glucose in the blood. Glucose in the bloodstream stimulates the pancreas to produce insulin. Insulin helps glucose to move from the blood into the cells. Once inside the cells, glucose is converted to energy, which is used immediately, or the glucose is stored as fat or the starch glycogen until it is needed.
The levels of glucose in the blood vary normally throughout the day. They rise after a meal, when there is more glucose in the blood, and return to pre-meal levels within about 2 hours after eating. Once the levels of glucose in the blood return to pre-meal levels, insulin production decreases. The variation in blood glucose levels is usually within a narrow range, about 70 to 110 milligrams per deciliter (mg/dL), or 3.9 to 6.1 millimoles per liter (mmol/L) of blood in healthy people. If people eat a large amount of carbohydrates, the levels may increase more. People older than 65 years tend to have slightly higher levels, especially after eating.
If the body does not produce enough insulin to move the glucose into the cells, or if the cells stop responding normally to insulin (called insulin resistance), the resulting high levels of glucose in the blood and the inadequate amount of glucose in the cells together produce the symptoms and complications of diabetes.
Types 1 and 2 Diabetes
There are 2 main types of diabetes mellitus (diabetes), type 1 diabetes, which comprises 5 to 10% of all diagnosed diabetes, and type 2 diabetes, which comprises 90 to 95% of diabetes cases. The remainder of diabetes consists of other, less common, types.
Type 1 diabetes
In type 1 diabetes (formerly called insulin-dependent diabetes or juvenile-onset diabetes), the body's immune system attacks the insulin-producing cells of the pancreas, and most of them are permanently destroyed. The pancreas, therefore, produces little or no insulin. Most people who have type 1 diabetes develop the disease before age 30 years, although it can develop later in life. Scientists believe that an environmental factor—possibly a viral infection or a nutritional factor during childhood or early adulthood—causes the immune system to destroy the insulin-producing cells of the pancreas. A genetic predisposition makes some people more susceptible to that environmental factor.
Type 2 diabetes
In type 2 diabetes (formerly called non–insulin-dependent diabetes or adult-onset diabetes), the pancreas often continues to produce insulin, sometimes even at higher-than-normal levels, especially early in the disease. However, the body develops resistance to the effects of insulin, so there is not enough insulin to meet the body’s needs. As type 2 diabetes progresses, the insulin-producing ability of the pancreas decreases. Type 2 diabetes was once rare in children and adolescents but has become more common. However, it usually begins in people older than 30 years and becomes progressively more common with age.
Certain disorders and medications can affect the way the body uses insulin and can lead to type 2 diabetes. High levels of steroids (sometimes called glucocorticoids or corticosteroids), most commonly due to use of steroid medications, such as prednisone, or and can lead to type 2 diabetes. High levels of steroids (sometimes called glucocorticoids or corticosteroids), most commonly due to use of steroid medications, such as prednisone, orCushing syndrome can result in impaired insulin use.
Prediabetes
Prediabetes is a condition in which blood glucose levels are too high to be considered normal but not high enough to be labeled diabetes. People have prediabetes if their fasting blood glucose level is between 100 mg/dL (5.6 mmol/L) and 125 mg/dL (6.9 mmol/L) or if their blood glucose level 2 hours after a glucose tolerance test is between 140 mg/dL (7.8 mmol/L) and 199 mg/dL (11.0 mmol/L). Prediabetes carries a higher risk of future diabetes as well as heart disease. Decreasing body weight by 5 to 10% through diet and exercise can significantly reduce the risk of developing diabetes.
Other Types and Causes of Diabetes
Other types of diabetes mellitus account for a smaller proportion of cases. Causes include:
Pregnancy (gestational diabetes)
Monogenic diabetes
Latent autoimmune diabetes in adults
Diabetes due to destruction or removal of the pancreas (sometimes called type 3c diabetes)
Other conditions that affect the pancreas such as pancreatitis or hemochromatosis
Post-transplant diabetes
Undernutrition-related diabetes
Endocrine disorders such as Cushing syndrome or acromegaly
Medications, most notably glucocorticoids, beta-blockers, protease inhibitors, atypical antipsychotics, immune checkpoint inhibitors, and calcineurin inhibitors
Diabetes may develop in people with excess production of growth hormone (acromegaly) and in people with certain hormone-secreting tumors. Severe or recurring pancreatitis and other disorders that directly damage the pancreas can lead to diabetes.
Gestational diabetes
Gestational diabetes occurs in some pregnant women because pregnancy causes resistance to the effects of insulin.
Monogenic diabetes
Monogenic forms of diabetes are caused by genetic defects affecting how the pancreas secretes insulin, insulin action in the body, or other processes in cells.
Latent autoimmune diabetes in adults
Latent autoimmune diabetes is a variant of diabetes that develops in adulthood in which one or more autoantibodies is present. It is more slowly progressive than classic type 1 diabetes, and some adults do not need insulin when blood glucose abnormalities first develop. This form of diabetes may initially be diagnosed as type 2 diabetes.
Diagnosis of Diabetes Mellitus
Measuring the level of glucose in the blood, sometimes while fasting or after consuming a standard amount of sugar
The diagnosis of diabetes (or prediabetes) is made when people have abnormally high levels of glucose in the blood. Doctors may do screening tests on people who are at risk of diabetes but have no symptoms, on people who have other conditions often associated with diabetes, or on people with symptoms of diabetes.
Blood glucose measurement
Doctors check blood glucose levels in people who have symptoms of diabetes, such as increased thirst, urination, or hunger. Additionally, doctors may check blood glucose levels in people who have disorders that can be complications of diabetes, such as frequent infections, foot ulcers, and yeast infections.
To accurately evaluate blood glucose levels, doctors usually use a blood sample taken after people have fasted overnight. Diabetes can be diagnosed if fasting blood glucose levels are 126 mg/dL (7.0 mmol/L) or higher. However, it is possible to use blood samples taken without fasting (called a random glucose level). Some elevation of blood glucose levels after eating is normal, but even after a meal the levels should not be very high. Diabetes can be diagnosed if a random (not done after fasting) blood glucose level is higher than 200 mg/dL (11.1 mmol/L).
Hemoglobin A1C
Doctors can also measure the level of a protein, hemoglobin A1C (also called glycosylated or glycolated hemoglobin), in the blood, which reflects the person's long-term trends in blood glucose levels rather than rapid changes.
Hemoglobin is the red, oxygen-carrying substance in red blood cells. When blood is exposed to high blood glucose levels over a period of time, glucose attaches to the hemoglobin and forms glycosylated hemoglobin. The hemoglobin A1C level blood test is reported as the percentage of hemoglobin that is A1C .
Measurements of hemoglobin A1C can be used to diagnose diabetes when testing is done by a certified laboratory (not by instruments used at home or in a doctor's office). People with a hemoglobin A1C level of 6.5% or more have diabetes. If the level is between 5.7 and 6.4, they have prediabetes and are at risk of developing diabetes.
Oral glucose tolerance test
Another kind of blood test, an oral glucose tolerance test, may be done in certain situations, such as screening pregnant women for gestational diabetes or testing older adults who have symptoms of diabetes but normal glucose levels when fasting. However, it is not routinely used for testing for diabetes because the test can be very cumbersome.
In this test, people fast, have a blood sample taken to determine the fasting blood glucose level, and then drink a special solution containing a large, standard amount of glucose. More blood samples are then taken over the next 2 to 3 hours and are tested to determine whether the glucose in the blood rises to abnormally high levels.
Complications of Diabetes
Some complications of diabetes develop quickly as a result of sudden changes in blood glucose levels that lead to other changes in blood chemistry. These include hypoglycemia, diabetic ketoacidosis, and hyperosmolar hyperglycemic state.
Other complications develop more slowly as a result of years of damage caused by high blood glucose levels. Diabetes damages blood vessels, causing them to narrow and therefore restricting blood flow. Because blood vessels throughout the body are affected, people may have many complications of diabetes.
Many organs can be affected, particularly the following:
Eyes (diabetic retinopathy), causing blindness
Kidneys (diabetic nephropathy), causing chronic kidney disease
Nerves (diabetic neuropathy), causing decreased sensation, mainly in the feet and legs
Heart, causing heart attack or heart failure
Brain, causing stroke
Legs, causing peripheral artery disease
High blood glucose levels also cause disturbances in the body's immune system, so people with diabetes mellitus are particularly susceptible to bacterial and fungal infections.
Treatment Considerations in Older Adults and People With Medical Problems
Treatment of people with type 1 and type 2 diabetes is discussed in Type 1 Diabetes - Treatment and Type 2 Diabetes - Treatment. However, there are some considerations for treating older adults, and people with other medical problems, regardless of the type of diabetes.
Older adults and people with many medical problems, particularly serious problems, need to follow the same general principles of diabetes management—education, diet, exercise, and medications—as younger or healthier people. However, risking hypoglycemia (a low blood glucose level) by trying to strictly control blood glucose levels may be harmful for frail people or people with many medical problems.
Education
In addition to learning about diabetes itself, people with many medical problems may have to learn how to fit management of diabetes in with their management of other conditions. Learning about how to avoid complications, such as dehydration, skin breakdown, and circulation problems, and to manage factors that can contribute to complications of diabetes, such as high blood pressure and high cholesterol levels, is especially important. Such problems become more common as people age, whether they have diabetes or not.
Diet
Many older adults have difficulty following a healthy, balanced diet that can control blood glucose levels and weight. Changing long-held food preferences and dietary habits may be hard. Some people have other disorders that can be affected by diet and may not understand how to integrate the dietary recommendations for their various disorders.
Some people cannot control what they eat because someone else is cooking for them—at home or in a nursing home or other institution. When people with diabetes do not do their own cooking, the people who shop and prepare meals for them must also understand the diet that is needed. These people and their caregivers usually benefit from meeting with a dietitian to develop a healthy, feasible eating plan.
Exercise
Some people may have a difficult time adding exercise to their daily life, particularly if they have not been active in the past or if they have a disorder that limits their movement, such as arthritis. However, they may be able to add exercise to their usual routine. For example, people can walk instead of drive or climb the stairs instead of taking the elevator.
Medication
Taking the medications used to treat diabetes, particularly insulin, may be difficult for some people. Poor eyesight may make it hard for people to read dose scales on insulin syringes. People with arthritis or Parkinson disease or who have had a stroke may have problems manipulating the syringe. For those with vision problems or other problems that make accurately filling a syringe difficult, a caregiver can prepare the syringes ahead of time and store them in the refrigerator. People whose insulin dose is stable may purchase prefilled syringes. Prefilled insulin pen devices may be easier for people with physical limitations. Some of these devices have large numbers and easy-to-turn dials.
Monitoring blood glucose levels
Poor vision, limited manual dexterity due to arthritis, tremor, or stroke, or other physical limitations may make monitoring blood glucose levels more difficult for some people. Poor eyesight may make it hard for people to read glucose meters. However, some monitors have large numerical displays that are easier to read. Some provide audible instructions and results. Some monitors read blood glucose levels through the skin and do not require a blood sample. People can consult a diabetes educator to determine which meter is most appropriate.
Hypoglycemia
The most common complication of treating high blood glucose levels is low blood glucose levels (hypoglycemia). The risk is greatest for people who are frail, who are sick enough to require frequent hospital admissions, or who are taking several medications. Of all available medications to treat diabetes, long-acting sulfonylurea medications or insulin are most likely to cause low blood glucose levels in people with severe or many medical problems and especially in older adults. When they take these medications, these people are also more likely to have serious symptoms, such as fainting and falling, and to have difficulty thinking or using parts of the body due to low blood glucose levels.
In older adults, hypoglycemia may be less obvious than in younger people. Confusion caused by hypoglycemia may be mistaken for dementia or the sedative effect of medications. Also, people who have difficulty communicating (as after a stroke or as a result of dementia) may not be able to let anyone know they are having symptoms.
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