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If you have diabetes, you have too much glucose in your blood. Glucose is a kind of sugar your body uses for energy. 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. If you change the way you live, take medication and watch your condition closely, you can keep your glucose levels 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 (see below).
• Many people with diabetes don't know they have it. They might not have any symptoms, or they might think the symptoms they do have aren't important. If you have symptoms like feeling very thirsty, hungry or tired, blurry vision or needing to urinate a lot, don't ignore them.

• Over time, high levels of 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 some of these problems.
• It's not easy to keep your blood glucose under control. And if you don't feel sick, you might not want to think about your glucose level. But keeping it under control is important for staying healthy.

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 levels of glucose steady.

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

In people who don't have diabetes, the amount of glucose in their blood is between 70 mg/dL and about 120 mg/dL.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 when you're 40 or over.
• 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 while they're pregnant. This is called gestational diabetes.

This information is for people with type 2 diabetes. For more information about other types of diabetes, see:

• Type 1 diabetes
• Other types of diabetes and high blood glucose.

Most people who have diabetes get type 2 diabetes.² More than 90 percent of people with diabetes have this type.

It usually starts later in life and is more common in people who are obese. But type 2 diabetes is getting more common in children. This is because more children are overweight today than in the past.

Type 2 diabetes used to be called non-insulin-dependent diabetes mellitus (or NIDDM for short) because people who have type 2 diabetes often don't need to take insulin. It's sometimes called adult-onset diabetes because it tends to start in people over 40.

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 2 diabetes?

You need insulin to keep your glucose level under control. In diabetes, your glucose level can get too high for different reasons.

• Your body may not be making enough insulin. It might make only a little or none at all.
• Your body may be making insulin but the insulin isn't working properly. Scientists think that your cells don't behave as they should when insulin reaches them. Doctors call this insulin resistance.
• One of insulin's jobs is to tell cells in your liver how to use glucose. If there's any extra glucose in your blood, it should be stored in your liver. But if you have type 2 diabetes, your liver cells don't get the right messages. So instead of storing the glucose, your liver pumps more glucose into your blood.

Certain things can increase your chances of getting a disease. Doctors call these things risk factors. Having a risk factor for diabetes doesn't mean you will get the disease for sure. It just means you are more likely to get diabetes than someone who doesn't have that risk factor.

Here are some risk factors for type 2 diabetes.

• Having a relative with type 2 diabetes: You're more likely to get type 2 diabetes if someone in your family has it. Your genes play a role. If both of your parents have type 2 diabetes, you have a 1 in 2 chance of getting it.³ (To find out more about what these numbers mean, see Understanding risks.)
• Being obese: Some people believe that eating too much sugar causes diabetes. They think this extra sugar wears out your pancreas. This isn't true.⁴ What matters is your weight. Obesity is the main risk factor for type 2 diabetes.
• Lack of exercise: About 80 percent of people with type 2 diabetes are overweight and don't get enough exercise. If you're at risk for this type of diabetes, you can reduce your risk if you exercise. For example, if you're overweight and you lose about 5 percent of your body weight, and you also get regular, moderate exercise (such as a brisk walk for 30 minutes, five days a week), your chance of getting diabetes is cut by one-half.⁵
• Your ethnic background: African-Americans, Hispanic Americans, Asian-Americans, American Indians and Pacific Islanders are more likely to get type 2 diabetes than other Americans.⁶ We're not sure why this is.
• Factors related to pregnancy: If you are a woman, you are more likely to get type 2 diabetes if you give birth to a baby weighing more than 9 pounds or have diabetes during pregnancy.⁷ For more information, see Other types of diabetes and high blood glucose.
• Having a condition called the metabolic syndrome: People who have this condition are more likely to get type 2 diabetes. For more, see The metabolic syndrome.

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.