Diabetes mellitus is a condition where the body does not produce enough insulin or responds abnormally to insulin, causing blood sugar levels to be high.

Elevated sugar in the blood can cause a range of symptoms.

These include blurry vision, intense thirst, need to urinate often, fatigue, and numbness or tingling in the hands or feet    Diabetes damages blood vessels and increases the risk of heart attack, stroke, chronic kidney disease, and loss of vision.

The three major nutrients that make up most foods are carbohydrates, proteins, and fat. Sugars are one of three types of carbohydrates, along with starch and fiber.

Some sugars are simple, and others are complex. Table sugar is made of two 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.


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, and 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 and return to pre-meal levels 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 of Diabetes


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 future diabetes.

Type 1 diabetes often comes on suddenly. It usually strikes children and teenagers but can appear later in life. It is an autoimmune disease, meaning it happens because the body’s immune system mistakenly attacks and destroys the body’s insulin-making cells. Type 1 diabetes can’t be cured, but it can be managed by taking insulin before eating.

Type 2 diabetes takes longer to develop. It can begin any time from childhood onward. Type 2 diabetes is usually triggered by being overweight or obese and not getting adequate physical activity. Treatment for type 2 diabetes includes weight loss if needed, daily exercise, a healthy diet, and medications.

Obesity is the chief risk factor for developing type 2 diabetes, and 80 to 90% of people with this disorder are overweight or obese. Because obesity causes insulin resistance, obese people need exceptionally large amounts of insulin to maintain normal blood glucose levels.

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 environmental factors.

Certain disorders and drugs can also affect the way the body uses insulin and can lead to type 2 diabetes.

Pregnancy (gestational diabetes)

When the blood glucose level rises above 160 to 180 mg/dL (8.9 to 10.0 mmol/L), glucose spills into the urine. When the level of glucose in the urine rises even higher, the kidneys excrete additional water to dilute the large amount of glucose. Because the kidneys produce excessive urine, people with diabetes urinate large volumes frequently (polyuria). The excessive urination creates abnormal thirst (polydipsia). Because excessive calories are lost in the urine, people may lose weight. To compensate, people often feel excessively hungry.

Sometimes during the early stages of diabetes, the blood glucose level is abnormally low at times, a condition called hypoglycemia.

Because people with type 2 diabetes produce some insulin, ketoacidosis does not usually develop even when type 2 diabetes is untreated for a long time. Rarely, the blood glucose levels become extremely high (even exceeding 1,000 mg/dL [55.5 mmol/L]). Such high levels often happen as the result of some superimposed stress, such as an infection or drug use. When blood glucose levels get very high, people may develop severe dehydration, which may lead to mental confusion, drowsiness, and seizures, a condition called hyperosmolar hyperglycemic state. Many people with type 2 diabetes are diagnosed by routine blood glucose testing before they develop such severely high blood glucose levels.

Complications of diabetes

Diabetes damages blood vessels, causing them to narrow and therefore restrict blood flow. Because blood vessels throughout the body are affected, people may have many complications of diabetes. Many organs can be affected, including the following:

  • Brain, causing stroke,
  • Eyes (diabetic retinopathy), causing blindness.
  • Heart, causing heart attack.
  • Kidneys (diabetic nephropathy), causing chronic kidney disease.
  • Nerves (diabetic neuropathy), causing decreased sensation in feet.

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.


The diagnosis of diabetes is made when people have abnormally high levels of glucose in the blood. Doctors also do screening tests on people who are at risk of diabetes but have no symptoms.

Diabetes can be diagnosed if fasting blood glucose levels are higher than 125 mg/dL (6.9 mmol/L). However, it is possible to use blood samples taken after people have eaten. Some elevation of blood glucose levels after eating is normal, but even after a meal the levels should not be exceedingly high. Diabetes can be diagnosed if a random blood glucose level is higher than 199 mg/dL (11.0 mmol/L).

Hemoglobin A1C

Doctors can also measure the level of a protein, hemoglobin A1C (also called glycosylated or glycolated hemoglobin), in the blood. When blood is exposed to high blood glucose levels over a period, glucose attaches to the hemoglobin and forms glycosylated hemoglobin. The hemoglobin A1C level (reported as the percentage of hemoglobin that is A1C) reflects long-term blood glucose levels.

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.

Oral glucose tolerance test

The oral glucose tolerance test is used for screening pregnant women for gestational diabetes or testing older people who have symptoms of diabetes but normal glucose levels when fasting.


    In type 1 diabetes, insulin injections are the best course of treatment.

    In type 2 diabetes, oral medications and sometimes insulin or other drugs by injection, are used for treatment.

Diet, exercise, and education are the core pillars of diabetes treatments, and weight loss is especially important for patients who are overweight. People who continue to have elevated blood glucose despite lifestyle changes or have very high blood glucose levels, and people with type 1 diabetes, also require medication.

Treatment of high blood pressure and high cholesterol levels, which can contribute to circulation problems, can help prevent some of the complications of diabetes as well. A low dose of aspirin taken daily is recommended in people with risk factors for heart disease. All people with diabetes who are between 40 and 75 years are given a statin (a drug to decrease cholesterol levels) regardless of cholesterol levels. People younger than 40 or older than 75 years and with an elevated risk of heart disease also should take a statin.

Diabetes treatment goals

People should keep their blood glucose levels:

  • Between 80 and 130 mg/dL (4.4 and 7.2 mmol/L) fasting (before meals)
  • Less than 180 mg/dL (10.0 mmol/L) 2 hours after meals
  • Hemoglobin A1C levels should be less than 7%.

Diet for people with diabetes

A controlled diet is especially important in people with both types of diabetes mellitus. People with diabetes can benefit from consulting a dietitian or a diabetes educator to develop an eating plan. This includes avoiding simple sugars and processed foods, increasing dietary fiber, and limiting portions of carbohydrate-rich and saturated fats.

Exercise can also help people control their weight and improve glucose levels. Because blood glucose goes down during exercise, people should be vigilant for hypoglycemia as. well.

Drug treatment of diabetes

There are many drugs used to treat diabetes. People with type 1 diabetes require insulin injections to lower blood glucose levels. Most people with type 2 diabetes require drugs by mouth to lower blood glucose levels, but some also require insulin or other injectable drugs.

A fingerstick glucose test is commonly used to monitor blood glucose. Most glucose meters use only a drop of blood obtained by pricking the tip of the finger with a small lancet.


The most common complication of treating high blood glucose levels is low blood glucose levels

Hypoglycemia in older people may be less obvious than in younger people. Confusion caused by hypoglycemia may be mistaken for dementia or the sedative effect of drugs.


Type 1 diabetes

No treatments prevent the onset of type 1 diabetes mellitus. Some drugs may induce remission of early type 1 diabetes in some people, possibly because they prevent the immune system from destroying the cells of the pancreas. However, these drugs cause side effects that limit their use.

Type 2 diabetes

Type 2 diabetes can be prevented with lifestyle changes. People who are overweight and lose as little as 7 percent of their body weight and who increase physical activity (for example, walking 30 minutes per day) can decrease their risk of diabetes mellitus by more than 50%. Metformin and acarbose, drugs that are used to treat diabetes, may reduce the risk of diabetes in people with impaired glucose regulation.

Resources for Information on Diabetes:

  • American Diabetes Association: Comprehensive information on diabetes, including resources for living with diabetes.
  • JDRF (previously called Juvenile Diabetes Research Foundation): General information on type 1 diabetes mellitus.
  • National Institute of Diabetes and Digestive and Kidney Diseases: General information on diabetes, including on the latest research and community outreach programs


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