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If you have type 1 diabetes, you have too much glucose in your blood. Glucose is a kind of sugar your body uses for energy. But if it builds up in your blood, it can make you sick.

Diabetes doesn't go away. And if you don't treat it, you can get serious health problems. But if you take insulin and watch your condition closely, you can keep your blood sugar under control. You should be able to live a long and healthy life.

• Diabetes is a serious condition. But with the right treatment, you should be able to stay healthy.
• If you have diabetes, you have too much glucose in your blood.
• There are two main types of diabetes: type 1 and type 2. This information is for people with type 1 diabetes. For information about the other type, see Type 2 diabetes.
• If you have type 1 diabetes, you need to control the glucose in your body with shots of a hormone called insulin. You'll also need to eat a healthy diet.

• Over time, too much glucose in your blood can damage your blood vessels. This can lead to problems in your heart, eyes, kidneys and other parts of your body.
• If you keep your blood glucose level as close to normal as possible, you might be able to avoid these problems.

You always need to have some glucose in your blood. It comes from food and it gives your body energy. Every cell in your body needs glucose to work properly.

Normally, the amount of glucose in your blood is carefully controlled by a chemical called insulin. Insulin is a hormone made by your pancreas, a gland that sits behind your stomach. Insulin helps move glucose from your blood into your body's cells. Your cells use the glucose as energy. Insulin keeps your blood level of glucose steady.

Doctors measure how much glucose is in your blood in milligrams per deciliter. The short version is mg/dL.

People who don't have diabetes have between 70 mg/dL and about 120 mg/dL glucose in their blood.¹ Your doctor might talk about your blood glucose level using just the number. For example, your doctor might say, "Your blood glucose is 120."

Normally, your blood glucose level goes up and down throughout the day. Before breakfast, a normal level is less than 100 mg/dL.

To read more, see What is glucose? and How does my body control my blood glucose level?

• There are two main types of diabetes: type 1 and type 2.
• Type 1 diabetes usually starts in adolescence.
• Type 2 comes on gradually, usually you're 40 years or older.
• There's also a condition called impaired glucose tolerance. This isn't diabetes, but it does increase your chances of getting diabetes.
• Some women get a kind of diabetes that only happens while they're pregnant. This is called gestational diabetes.

This information is for people with type 1 diabetes. For more information about other types of diabetes, see:

• Type 2 diabetes
• Other types of diabetes and high blood glucose.

Type 1 diabetes usually starts when you're in your teens or when puberty begins. Between 5 percent and 10 percent of people with diabetes have type 1.²

If you have type 1 diabetes, your body has stopped making insulin. This kind of diabetes used to be called insulin-dependent diabetes mellitus (or IDDM for short) because it can be controlled by taking insulin. You might also hear people call it juvenile-onset diabetes. It was called this because it tends to start in childhood.

If you have diabetes, your body can't control how much glucose is in your blood. Instead of the glucose being gradually used up as fuel by your cells, it builds up in your blood. You'll hear the word hyperglycemia a lot. It means having too much glucose in your blood. To find out how this affects you, see What are the symptoms of type 1 diabetes?

You need insulin to keep your blood glucose level under control. But when you have type 1 diabetes, your body stops making insulin.

Usually, type 1 diabetes is an autoimmune disease. This means that the cells in your immune system, which normally fight germs, attack some of your own cells by mistake. In type 1 diabetes, your immune system attacks the cells making insulin in your pancreas.

You can get type 1 diabetes at any age. But it usually takes years for the immune system to kill all the cells that make insulin.³ Exactly how long it takes depends in part on your genes.

No one knows why the immune system starts killing the cells in the pancreas. The attack might be started by viruses, but no one knows for sure. Some of the things researchers think have a role are:

• The virus that causes German measles (also called rubella)
• The virus that causes hand-foot-and-mouth disease
• Your genes. If your genes give you a low chance of getting diabetes, you might not be affected by these other things.⁴

You probably won't notice when the cells that make insulin start dying. You need only a small number of these cells to keep your blood glucose level under control. But once you lose about four-fifths (or 80 percent) of these cells, you start to get symptoms of diabetes. This happens because glucose starts to build up in your blood.³

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.