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Type 1 Diabetes Mellitus

Medically reviewed by Last updated on Jan 3, 2024.

What is Type 1 Diabetes Mellitus?

Harvard Health Publishing

Type 1 diabetes is a disease in which the body does not make enough insulin to control blood sugar levels. Type 1 diabetes was previously called insulin-dependent diabetes or juvenile diabetes. 

During digestion, food is broken down into 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 blood and get inside the cells. 

Insulin traveling in the blood signals the cells to take up glucose.  Insulin is a hormone produced by the pancreas. When levels of glucose in the blood rise, like following a meal, the pancreas normally produces more insulin. 

Type 1 diabetes occurs when some or all of the insulin-producing cells in the pancreas are destroyed. This leaves the patient with little or no insulin. Without insulin, sugar 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 glucose that remain in the blood cause excessive urination and dehydration, and damage tissues of the body. 

Type 1 diabetes is an autoimmune disease. This means it begins when the body's immune system attacks cells in the body. In type 1 diabetes, the immune system destroys insulin-producing cells (beta cells) in the pancreas.  

Why the immune system attacks the beta cells remains a mystery. Some people are genetically predisposed to the disease. That does not mean they will necessarily get the disease. It just means that they are more likely to do so.  Something in the environment, such as particular viral infections or something about the diet, may trigger this autoimmune disease in people with a genetic predisposition. 

Type 1 diabetes is not caused by the amount of sugar in a person's diet before the disease develops. 

Type 1 diabetes is a chronic disease. It is diagnosed most commonly between ages 10 and 16. Type 1 diabetes equally affects males and females. 


Initial Symptoms 

Symptoms usually come on suddenly and strongly. Typically the most prominent symptoms are excessive urination and extreme thirst. This is because the increased glucose in the blood causes the kidneys to create more urine than usual. Losing more fluid in the urine makes a person dehydrated. And dehydration leads to great thirst. Children may start to wet the bed again.  

Weight loss, with no loss of appetite, also is common. The weight loss is due in part to dehydration. Water has weight.  Imagine holding a gallon jug of water: it weighs about eight pounds. People with new, uncontrolled type 1 diabetes can lose a gallon of water from dehydration. 

Other common symptoms are weakness, fatigue, confusion, nausea and vomiting. These symptoms can be caused both by dehydration and by a condition called ketoacidosis. 

Ketoacidosis occurs because cells can't use the glucose they need for energy. So the cells have to use something else. In response to low insulin levels, the liver produces an alternative fuel called ketones. Ketones are a kind of acid. When they build up in the blood, it's called ketoacidosis. Ketoacidosis can cause heart problems and affect the nervous system. Within hours, it may put a person at risk of coma or death. 

Chronic Symptoms  

Even after it is diagnosed and treatment is begun, type 1 diabetes can affect all body systems. It is less likely to damage the body, and cause symptoms, if the blood sugar levels are well controlled by treatment. 

The serious and potentially life-threatening complications that can occur with type 1 diabetes include: 

Hypoglycemia can lead to coma if it is not corrected by eating or drinking carbohydrates. Glucagon is a substance that makes the liver release glucose into the bloodstream. An injection of glucagon can also correct hypoglycemia.  


Type 1 diabetes is diagnosed by a combination of symptoms, a person's age and blood tests.  The blood tests include tests for sugar levels and for other substances.   

Fasting plasma glucose (FPG) test. Blood is taken in the morning after fasting overnight. Normally, blood sugar levels remain between 70 and 100 milligrams per deciliter (mg/dL). Diabetes is diagnosed if a fasting blood sugar level is 126 mg/dL or higher.  

Oral glucose tolerance test (OGTT). Blood sugar is measured two hours after drinking 75 grams of glucose. Diabetes is diagnosed if the 2-hour 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 the average glucose level over the prior two to three months. Diabetes is diagnosed if the hemoglobin A1C level is 6.5% percent or higher.

Expected Duration

Type 1 diabetes currently is a lifelong disease.  

People with type 1 diabetes need regular checkups. They must carefully monitor their blood sugar levels every day. They must receive insulin treatment throughout life.  

A small number of people can become exceptions to this rule. Some people with diabetes eventually require kidney transplants. A transplant of the pancreas, or of the insulin-producing cells from the pancreas (called "islets"), sometimes is performed at the same time. Since the new pancreas can make insulin, this can cure the diabetes.  

On unusual occasions, when someone's type 1 diabetes is very hard to control with available treatments, pancreas or islet transplantation may be performed even when kidney transplantation is not necessary. However, this approach is still experimental, and is not generally recommended. 

Scientists have recently reported exciting but still experimental new ways for encouraging the pancreas to again start producing its own insulin-producing beta cells.


There is no proven way to prevent type 1 diabetes.  Vitamin D deficiency, which is very common, may increase the risk of diabetes.  However, correcting the deficiency has not been yet shown to prevent diabetes.  Likewise, avoiding cow's milk during infancy may possibly prevent type 1 diabetes in genetically susceptible infants. But there is no definite proof that this prevents the disease.


Treatment of type 1 diabetes requires daily insulin injections. The injected insulin makes up for the insulin that is not produced by the body. Most people with type 1 diabetes need two to four injections per day.  

People with type 1 diabetes must properly regulate both their dietary intake and their dose of insulin. If a person takes too much insulin relative to their dietary intake, or if they forget to eat, they can develop dangerous hypoglycemia.  If they take too little insulin, or eat too much, they can develop ketoacidosis.  

In order to properly regulate their insulin intake, people with type 1 diabetes need to monitor their blood sugar levels several times per day. They do this by testing a sample of blood. They must prick their finger and place a small drop of blood on a test strip. The test strip is inserted into a device called a glucose monitor. An accurate reading of blood sugar levels is returned within seconds.  

Newer glucose monitors have test strips that take the blood directly from the spot that was pricked. This process requires less blood. Other monitors allow blood to be taken from the forearm, thigh or the fleshy part of the hand. This can be less painful. 

Some people use a syringe for injections. Other patients use semiautomatic injector pens that help to measure precise amounts of insulin.  

An increasing number of patients use insulin pumps. Insulin pumps deliver a regulated dose of insulin through a needle implanted under the skin. The insulin pump is worn in a pack on the body. Some pumps include a sensor that constantly measures the level of blood sugar, and adjusts the dose of insulin accordingly.  Whether devices with such sensors lead to improved health is not yet certain. 

Fast-acting insulin may be taken as needed, depending on the amount of carbohydrates ingested. Your doctor or dietitian will help you determine the best insulin and diet schedule for you or your child. 

A healthy diet and regular exercise are important for everyone—but particularly for people with diabetes.  A healthy diet for someone with type 1 diabetes not only keeps the amount of glucose in the blood relatively constant. It also includes eating "good carbs" instead of "bad carbs", "good fats" instead of "bad fats".  

In order to keep blood sugar levels at a normal and relatively constant level, a person with type 1 diabetes typically is advised to eat, exercise and take insulin at about the same times every day. Regular habits help to keep glucose levels within the normal range.  

People with type 1 diabetes should get regular exercise. Exercise protects the health of the heart and blood vessels in people with diabetes, as in everyone.  In addition, regular exercise helps to control blood sugar by causing muscles to use glucose and by keeping body weight down. Ask your doctor how much and when to exercise to best control your diabetes.  

Treatment options

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

View more treatment options

When To Call a Professional

Call your health care professional if you experience a sudden increase in thirst and urination. Unexplained weight loss always should be reported to a physician. 

If you or your child has type 1 diabetes, see your doctor regularly to make sure that you are keeping good control of your blood sugar. You should also be checked regularly for early signs of complications such as heart disease, eye problems and skin infections.  

Your doctor most likely will suggest that you also visit other specialists regularly. These may include a podiatrist to check your feet and an ophthalmologist to check your eyes for signs of diabetes complications. 


People with type 1 diabetes generally adjust quickly to the time and attention that is needed to monitor blood sugar, treat the disease and maintain a normal lifestyle.  

As time goes on, the risk of complications is substantial. But it can be reduced greatly if you strictly monitor and control your blood glucose levels. 

Additional Info

American Diabetes Association

American Dietetic Association

National Institute of Diabetes & Digestive & Kidney Disorders

Further information

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