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Diabetes Mellitus

Medically reviewed by Last updated on Aug 24, 2022.

What Is It?

Harvard Health Publishing

Diabetes mellitus is a condition defined by persistently high levels of sugar (glucose) in the blood. There are several types of diabetes. The two most common are called type 1 diabetes and type 2 diabetes.

During digestion, food is broken down into its basic components. Carbohydrates are broken down into simple sugars, primarily glucose. Glucose is a critically important source of energy for the body’s cells. To provide energy to the cells, glucose needs to leave the bloodstream and get inside the cells.

An organ in the abdomen called the pancreas produces a hormone called insulin, which is essential to helping glucose get into the body's cells. In a person without diabetes, the pancreas produces more insulin whenever blood levels of glucose rise (for example, after a meal), and the insulin signals the body's cells to take in the glucose. In diabetes, either the pancreas's ability to produce insulin or the cells' response to insulin is altered.

Diabetes Mellitus

Type 1 diabetes is an autoimmune disease. This means it begins when the body's immune system mistakenly attacks other cells in the body. In type 1 diabetes, the immune system destroys the insulin-producing cells (called beta cells) in the pancreas. This leaves the person with little or no insulin in his or her body. Without insulin, glucose accumulates in the bloodstream rather than entering the cells. As a result, the body cannot use this glucose for energy. In addition, the high levels of blood glucose cause excessive urination and dehydration, and damage the body's tissues.

Type 2 diabetes occurs when your body's cells become less responsive to insulin's efforts to drive glucose into the cells, a condition called insulin resistance. As a result, glucose starts to build up in the blood.

In people with insulin resistance, the pancreas "sees" the blood glucose level rising. The pancreas responds by making extra insulin to try to usher the glucose into the cells. At first, this works, but over time, the body's insulin resistance gets worse. In response, the pancreas makes more and more insulin. Finally, the pancreas gets "exhausted." It cannot keep up with the demand for more and more insulin. As a result, blood glucose levels rise and stay high.

Type 2 diabetes is also called adult-onset diabetes. That’s because it almost always used to start in middle or late adulthood. However, more and more children and teens are now developing this condition.

Type 2 diabetes is much more common than type 1 diabetes. It tends to run in families. Obesity also increases your risk of type 2 diabetes. It is truly a different disease than type 1 diabetes, although both types involve a high blood glucose level and the risk of complications associated with it.

Another kind of diabetes, called gestational diabetes, happens in women who have higher-than-expected blood sugar levels during pregnancy. Once it occurs, it lasts throughout the remainder of the pregnancy. Like the other types of diabetes, gestational diabetes happens when the hormone insulin can't efficiently move sugar (glucose) into the body's cells so it can be used as fuel. In gestational diabetes, the body does not respond well to insulin, unless insulin can be produced or provided in larger amounts.

In most women, the disorder goes away when the pregnancy ends, but women who have had gestational diabetes are at increased risk of developing type 2 diabetes later.


Diabetes initially might not cause any symptoms. It can sometimes be caught early with a routine blood test before a person develops symptoms.

When diabetes does cause symptoms, they may include:

  • excessive urination
  • excessive thirst, leading to drinking a lot of fluid
  • weight loss.

People with diabetes also have an increased susceptibility to infections, especially yeast (Candida) infections.

When the amount of insulin in the blood stream is too low, extremely high blood sugar levels can lead to dangerous complications. The body can become too acidic, a condition called diabetic ketoacidosis. Or the blood sugar level gets so high, the person becomes severely dehydrated. It’s called hyperosmolar syndrome.

The symptoms of these complications include confused thinking, weakness, nausea, vomiting, and even seizures and coma. In some cases, diabetic ketoacidosis or hyperosmolar syndrome is the first sign that a person has diabetes.

The treatment of diabetes also can produce symptoms. Too much glucose-lowering medicine, relative to dietary intake, can lead to a blood sugar level that has dropped too low (called hypoglycemia). Symptoms of hypoglycemia include:

  • sweating
  • trembling
  • dizziness
  • hunger
  • confusion
  • seizures and loss of consciousness (if hypoglycemia is not recognized and corrected).

You can correct hypoglycemia by eating or drinking something that has carbohydrates. This raises your blood sugar level.

Long-term diabetes can have other complications, including:

  • Atherosclerosis — Atherosclerosis is fat buildup in the artery walls. This can impair blood flow to all parts of the body. The heart, brain, and legs are affected most often.
  • Retinopathy — Tiny blood vessels in the retina (the part of the eye that sees light) can become damaged by high blood sugar. The damage can block blood flow to the retina, or can lead to bleeding into the retina. Both reduce the retina's ability to see light. Caught early, retinopathy damage can be minimized by tightly controlling blood sugar and using laser therapy. Untreated retinopathy can lead to blindness.
  • Neuropathy — This is another term for nerve damage. The most common type is peripheral neuropathy, which affects nerves in the feet and hands. The nerves to the legs are damaged first, causing pain and numbness in the feet. This can advance to cause symptoms in the legs and hands. Damage to the nerves that control digestion, sexual function, and urination can also occur.
  • Foot problems — Any sores, injuries, or blisters on the feet can lead to the following complications:
    • If peripheral neuropathy causes numbness, a person may not feel any irritation or injury that occurs on the foot. The skin can break down and form an ulcer, and the ulcer can get infected.
    • Blood circulation can be poor, leading to slow healing of any foot injuries. Left untreated, a simple sore can become very large and get infected. If medical treatment cannot heal the sore, an amputation may be required.
  • Nephropathy — This refers to damage to the kidneys. This complication is more likely if blood sugars remain elevated and high blood pressure is not treated aggressively.


Diabetes is diagnosed through blood tests that detect the level of glucose in the blood.

  • Fasting plasma glucose (FPG) test. A blood sample is taken in the morning after you fast overnight. A normal fasting blood sugar level is between 70 and 100 milligrams per deciliter (mg/dL). Diabetes is diagnosed if the fasting blood sugar level is 126 mg/dL or higher.
  • Oral glucose tolerance test (OGTT). Your blood sugar is measured two hours after you drink a liquid containing 75 grams of glucose. Diabetes is diagnosed if the blood sugar level is 200 mg/dL or higher.
  • Random blood glucose test. A blood sugar of 200 mg/dL or greater at any time of day, combined with symptoms of diabetes, is sufficient to make the diagnosis.
  • Hemoglobin A1c (glycohemoglobin). This test measures your average blood glucose level over the prior two to three months. Diabetes is diagnosed if the hemoglobin A1c level is 6.5% or higher.

Expected Duration

Type 1 diabetes is a lifelong illness. Usually, type 2 diabetes is also life-long. However, people with type 2 diabetes can sometimes restore their blood sugar levels to normal just by eating a healthy diet, exercising regularly, and losing weight.

Gestational diabetes usually goes away after childbirth. However, women with gestational diabetes are at high risk for developing type 2 diabetes later in life.

In people with diabetes, aging and episodic illnesses can cause the body's insulin resistance to increase. As a result, additional treatment typically is required over time.


Type 1 diabetes cannot be prevented.

You can decrease your risk of developing type 2 diabetes.

If a close relative—particularly, a parent or sibling—has type 2 diabetes, or if your blood glucose test shows "pre-diabetes" (defined as blood glucose levels between 100 and 125 mg/dL), you are at increased risk for developing type 2 diabetes. You can help to prevent type 2 diabetes by

  • maintaining your ideal body weight.
  • exercising regularly—such as a brisk walk of 1-2 miles in 30 minutes—at least five times a week, even if that does not result in you achieving an ideal weight. That’s because regular exercise reduces insulin resistance even if you don’t lose weight.
  • eating a healthy diet.
  • taking medication. The medication metformin (Glucophage) offers some additional protection for people with pre-diabetes.

If you already have type 2 diabetes, you can still delay or prevent complications by doing the following.

Keep control of your blood sugar. This helps reduce the risk of most complications.

Lower your risk of heart-related complications. Aggressively manage other risk factors for atherosclerosis, such as:

  • high blood pressure
  • high cholesterol and triglycerides
  • cigarette smoking
  • obesity

Visit an eye doctor and a foot specialist every year. This can help you reduce the risk of eye and foot complications.


Type 1 diabetes is always treated with insulin injections.

In most cases, type 2 diabetes treatment begins with weight reduction through diet and exercise. A healthy diet for a person with diabetes is low in total calories, free of trans fats and nutritionally balanced, with abundant amounts of whole grains, fruits and vegetables, and monounsaturated fats.

Most people with type 2 diabetes need drug therapy to control blood sugar. However, it is possible to achieve normal blood sugar levels with weight loss, a healthy diet and regular exercise.

Even if medications are required, diet and exercise remain important for controlling diabetes.

The medications used for type 2 diabetes include pills and injections. They work in many different ways. They include medications that:

  • reduce insulin resistance in the muscles and liver
  • increase the amount of insulin made and released by the pancreas
  • provide additional insulin
  • cause a burst of insulin release with each meal
  • delay the absorption of sugars from the intestine
  • slow your digestion
  • reduce your appetite for large meals
  • decrease the conversion of fat to glucose.

Weight loss surgery may be an option for some obese people with type 2 diabetes.

Treatment options

The following list of medications are in some way related to or used in the treatment of this condition.

When To Call a Professional

If you have diabetes, see your doctor regularly.

People with high blood sugar levels have a higher risk of dehydration. Contact your doctor immediately if you develop vomiting or diarrhea and are not able to drink enough fluids.

Monitor your blood sugar as advised by your health care team. Report any significant deviations in blood sugar levels.


The prognosis in people with diabetes varies. It depends on how well an individual modifies his or her risk of complications. If blood sugar is not well controlled, it can increase a person's risk of heart attack, stroke, and kidney disease, which can result in premature death. Disability due to blindness, amputation, heart disease, stroke, and nerve damage may occur. Some people with diabetes become dependent on dialysis treatments because of kidney failure.

Additional Info

American Diabetes Association

Academy of Nutrition and Dietetics

Diabetes Health Information

National Institute of Diabetes and Digestive and Kidney Diseases


Learn more about Diabetes Mellitus

Treatment options

Care guides

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.