Diabetes Diabetes Mellitus

Authored by , Reviewed by Dr Colin Tidy | Last edited | Meets Patient’s editorial guidelines

Added to Saved items

There are two main types of diabetes mellitus, which are called type 1 diabetes and type 2 diabetes. Type 1 usually first presents in children or in young adults. Type 2 is more associated with being overweight and most often first presents in people over the age of 40 years. However, type 2 diabetes is increasingly diagnosed in children and in young adults.

Both types of diabetes can cause serious complications but these are much less common and less severe with good treatment and regular monitoring. In addition to the treatment from healthcare professionals, understanding your diabetes and being able to manage your own diabetes are very important.

What is diabetes?

Dr Partha Kar

Diabetes mellitus (just called diabetes from now on) occurs when the level of sugar (glucose) in the blood becomes higher than normal. There are two main types of diabetes. These are called type 1 diabetes and type 2 diabetes.

Type 1 diabetes

This type usually develops quite quickly, over days or weeks, as the pancreas stops making insulin. Type 1 diabetes usually first presents in childhood. See the separate leaflet called Type 1 Diabetes.

Type 2 diabetes

This is more common in people who are overweight or obese. Type 2 diabetes tends to develop gradually (over weeks or months). This is because people with type 2 diabetes still make insulin (unlike type 1 diabetes). However, people with type 2 diabetes do not make enough insulin for the body's needs, or the body is not able to use insulin properly. Type 2 diabetes usually starts in middle-aged or elderly adults but is increasingly being seen in children and in young adults. See the separate leaflet called Type 2 Diabetes.

Video Playlist

Diabetes Q&A

What's the difference between type 1 and type 2 diabetes? Can you reverse pre-diabetes? How do you know if you have diabetes? Can diabetes affect your sperm count? All your questions answered.

Watch Now

Gestational diabetes

As well as type 1 and type 2 diabetes, there are other types of diabetes. Pregnant women who have never had diabetes before but who have high blood glucose levels during pregnancy are said to have gestational diabetes. Gestational diabetes affects about 4% of all pregnant women. See the separate leaflet called Diabetes and Pregnancy.

Other types of diabetes

There are also a number of less common forms of diabetes, including:

  • Secondary diabetes: diabetes can be caused by other diseases. Examples include diseases that affect the pancreas, such as cystic fibrosis and chronic pancreatitis. Diabetes can also be caused by other hormone (endocrine) diseases, such as Cushing's syndrome and acromegaly.
  • Latent autoimmune diabetes of adults (LADA): a slow-onset form of type 1 diabetes that occurs in adults, often with a slower onset than the usual type 1 diabetes that starts in childhood.
  • Maturity-onset diabetes of the young (MODY): this includes several forms of diabetes which are caused by genetic defects of the cells in the pancreas that make insulin.
  • Wolfram's syndrome: another genetic cause of diabetes. It is also called DIDMOAD (because it causes diabetes insipidus, diabetes mellitus, optic atrophy and deafness).
  • Other genetic diseases that can cause diabetes: these include Friedreich's ataxia and haemochromatosis.

A medical paper in 2018 suggested that adult-onset diabetes should be classified in a new way. This is because your level of resistance to diabetes can affect your risk of complications.

In the new model, those most resistant to insulin have a significantly higher risk of diabetic kidney disease than individuals with differing types who had been prescribed similar diabetes treatment. People with the greatest insulin deficiency have the highest risk of retinopathy.

However, for now this new model is purely being used for research. 

After you eat, various foods are broken down in your gut (intestine) into sugars. The main sugar, glucose, passes through your gut wall into your bloodstream. However, it's important to remember it's not just sugar that increases blood glucose. Other starchy carbohydrate foods such as potato, rice or breakfast cereals digest down into surprising amounts of glucose too.

To remain healthy, your blood sugar (glucose) level should not go too high. So when your blood glucose level begins to rise - particularly after you eat - the level of a hormone called insulin should also rise. Insulin works on the cells of your body and makes them take in glucose from the bloodstream, thus lowering blood glucose again. Some of the glucose is used by the cells for energy. Any glucose not used up in this way is converted into glycogen in the liver or fat in the liver or abdomen.

When the blood glucose level begins to fall (between meals or when we have no food), the level of insulin falls. Some glycogen or fat is then converted back into glucose. This is released from the cells into the bloodstream to keep the blood glucose level normal.

Over time, if you have more glucose in your system than you need for energy, this can lead to central obesity (weight carried around your tummy) and fatty liver.

Insulin is a hormone that is made by cells called beta cells. These are part of little islands of cells (islets) within the pancreas gland. Hormones are chemicals that are released by glands into the bloodstream and work on various parts of the body.

Diabetes develops if you do not make enough insulin, or if the insulin that you do make does not work properly on the body's cells:

  • In type 1 diabetes, your body does not make any insulin at all.
  • In type 2 diabetes, your levels of insulin may be normal, but your body doesn't respond properly to them. This is called insulin resistance. Over time, your body's ability to produce insulin also drops if you have type 2 diabetes and your blood sugar (glucose) levels remain high.

The symptoms when you first develop diabetes may include:

  • Being very thirsty a lot of the time.
  • Passing a lot of urine. (The reason you make a lot of urine and become thirsty is because blood sugar (glucose) leaks into your urine, which pulls out extra water through the kidneys.)
  • Tiredness, weight loss and feeling generally unwell.

The symptoms tend to develop quite quickly, over a few days or weeks for people with type 1 diabetes.

Type 2 diabetes develops much more slowly and symptoms may not start until you have had type 2 diabetes for a number of years. This means that people with type 2 diabetes may already have complications of diabetes (see below) when diabetes is first diagnosed.

The symptoms of diabetes resolve when you start treatment for diabetes. However, the symptoms may come back if your blood glucose levels are poorly controlled. Without treatment, the blood glucose level becomes very high.


Do I Have Diabetes?

Our quiz will help you discover whether you are exhibiting some of the symptoms and evaluate your risk.

Take the Test

A simple dipstick test can detect sugar (glucose) in a sample of urine. This may suggest the diagnosis of diabetes. However, the only way to confirm the diagnosis is to have a blood test to look at the level of glucose in your blood. If this is high then it will confirm that you have diabetes.

Some people have to have two samples of blood taken and they may be asked to fast (this means having nothing to eat or drink, other than water, from midnight before the blood test is performed). A different blood test which measures a chemical called HbA1c is now also used to diagnose type 2 diabetes but is not suitable for the diagnosis of type 1 diabetes.

Read more in the separate leaflet called Tests for Blood Sugar (Glucose) and HbA1c.

Any person with diabetes needs to follow a healthy lifestyle with a healthy diet, maintaining an ideal body weight, taking regular exercise and not smoking. People with type 1 diabetes also always need treatment with insulin. See the separate leaflet called Insulins.

People with type 2 diabetes sometimes don't need any medicines for diabetes control when the diagnosis is first made. However, most people with diabetes need to start taking one or more medicines if a healthy lifestyle is not enough to control blood sugar (glucose) levels. However, in recent years it has become clear that by losing weight and following a low-carbohydrate diet (or, with close medical supervision, a very low-calorie diet), many people can bring their blood sugar back to normal without medication.

Some people with type 2 diabetes need to use insulin injections if the other medicines don't adequately control the blood glucose levels. See the separate leaflet called Type 2 Diabetes Treatment.

Treating diabetes is not just about blood glucose levels. It is also very important to reduce the risk of the complications of diabetes (see below). It is therefore important to keep blood pressure and cholesterol levels in the normal range. Read more in the separate leaflet called Diabetes and High Blood Pressure.

The treatment for diabetes also includes regular monitoring to diagnose and treat complications at an early stage.

Diabetes in pregnancy is associated with possible problems for the mother and baby. Women with diabetes who become pregnant need very close monitoring and specialist treatment to make sure that the mother and baby remain well with no problems. See the separate leaflet called Diabetes and Pregnancy.

When you are unwell for any reason, even just a sore throat, this may badly affect your blood glucose control. It is therefore very important to know what to do if you are unwell. Read about advice for people with diabetes when unwell, in the separate leaflet called Diabetes and Illness.

What are the aims of treatment?

Although diabetes cannot be cured, it can be treated successfully. If a high blood glucose level is brought down to a normal or near-normal level, your symptoms will ease and you are likely to feel well again.

You still have some risk of complications in the long term if your blood glucose level remains even mildly high - even if you have no symptoms in the short term. However, studies have shown that people who have better glucose control have fewer complications (such as heart disease or eye problems) compared with those people who have poorer control of their glucose level.

Therefore, the main aims of treatment are:

  • To keep your blood glucose level as near normal as possible.
  • To reduce any other risk factors that may increase your risk of developing complications. In particular, to stop smoking, lose weight or lower your blood pressure if needed and to keep your blood lipids (cholesterol and triglyceride) low.
  • To detect any complications as early as possible. Treatment can prevent or delay some complications from becoming worse.

Healthy diet and other lifestyle advice

You should eat a healthy diet. This diet is the same as that recommended for everyone. The idea that you need special foods if you have diabetes is a myth. 'Diabetic foods' still raise blood glucose levels, contain just as much fat and calories and are usually more expensive than non-diabetic foods. Basically, you should aim to eat a diet low in fat, salt and sugar and high in fibre, and with plenty of fruit and vegetables.

However, if you're using insulin, you will need to know how to balance the right amount of insulin for the amount of food that you eat. You may also need to make adjustments to your diet, if you have type 2 diabetes and your diet and lifestyle contributed to your condition. Therefore, you will normally be referred to a dietician for detailed advice. See the separate leaflet called Type 2 Diabetes Diet.

Smoking is a high risk factor for complications. You should see your practice nurse or attend a smoking cessation clinic if you have difficulty stopping smoking. If necessary, medication or nicotine replacement therapy (nicotine gum, etc) may help you to stop.

Regular physical activity also reduces the risk of some complications such as heart and blood vessel disease. If you are able, a minimum of 30 minutes' brisk walking at least five times a week is advised. Anything more vigorous is even better - for example, swimming, cycling, jogging, dancing. The best exercise is vigorous enough to make you at least mildly out of breath and sweaty. You can spread the activity over the day (for example, two fifteen-minute spells per day of brisk walking, cycling, dancing, etc).

Depending on your age and how long you have had diabetes, you may be advised to take a medicine to lower your cholesterol level. This will help to lower the risk of developing some complications such as heart disease and stroke.

Try to lose weight if you are overweight or obese. Excess weight is also a risk factor for heart and blood vessel disease. Getting to a perfect weight is often unrealistic. However, if you are overweight, losing some weight will help.

Many people with diabetes enjoy alcohol and there is no need to give up completely unless you wish to. Whether you have diabetes or not, healthy guidelines in the UK generally recommend a limit of 14 units a week for both men and women (government guidelines have recently been revised for men). See the separate leaflet called Alcohol and Sensible Drinking. However, do be aware that drinking alcohol when you have diabetes can make an episode of hypoglycaemia (a 'hypo') more likely.


You should be immunised against flu (each autumn) and immunised against infection from pneumococcal germs (bacteria). These infections can be particularly unpleasant if you have diabetes.


Anyone with diabetes who needs treatment with insulin should monitor their blood glucose levels. This is usually not necessary for people with type 2 diabetes who do not need insulin.

It is important to have regular checks, as some complications, particularly if detected early, can be treated or prevented from becoming worse. Most GP surgeries and hospitals have special diabetes clinics. Doctors, nurses, dieticians, specialists in foot care (podiatrists), specialists in eye health (optometrists) and other healthcare workers all play a role in giving advice and checking on progress. As well as ongoing advice on diet and lifestyle, regular checks may include:

Checking levels of blood glucose, HbA1c, cholesterol and blood pressure
It is important to keep blood glucose, cholesterol and blood pressure levels as normal as possible. The HbA1c blood test helps to check your blood glucose control. Ideally, the aim is to maintain your HbA1c to less than 48 mmol/mol (6.5%). However, this may not always be possible to achieve and your target level of HbA1c should be agreed between you and your doctor.

Checking for early signs of complications
For example:

If your blood sugar (glucose) levels are not well controlled this may cause a lack of fluid in the body (dehydration), tiredness and drowsiness. This may progress to a serious illness which can be life-threatening, especially if you have type 1 diabetes. A very high blood glucose level sometimes develops if you have other illnesses such as any infections. People with diabetes who use insulin injections may need to adjust the dose of insulin to keep the blood glucose levels normal. See the separate leaflet called Diabetes and Illness for more details.

For people with type 1 diabetes, high blood glucose levels can lead to acids called ketones in the bloodstream (this is called ketoacidosis). Less commonly, people with type 2 diabetes may develop a condition with very high blood glucose levels but no ketones (called hyperosmolar hyperglycaemic coma). Either of these conditions can dangerously reduce the fluid in your body and both conditions need urgent treatment.

Too much insulin can make the blood glucose level go too low (hypoglycaemia, sometimes called a 'hypo'). This can cause you to feel sweaty, confused and unwell; you may lapse into a coma. Emergency treatment of hypoglycaemia is with sugar, sweet drinks, or a glucagon injection (a hormone which has the opposite effect to insulin). Then you should eat a starchy snack such as a sandwich. See the separate leaflet called Hypoglycaemia (Low Blood Sugar).

Diabetes can cause other problems, especially repeated infections. Infections may take longer to get better or be harder to treat. This is particularly the case if your blood glucose levels are poorly controlled.

Diabetes can also cause a lot of psychological difficulties such as depression, anxiety and eating disorders.

Long-term complications

If the blood glucose level is higher than normal, over a long period of time, it can have a damaging effect on the blood vessels.

Even a mildly raised glucose level which does not cause any symptoms in the short term can affect the blood vessels in the long term. This may lead to some of the following complications:

The type and severity of long-term complications vary from case to case. You may not develop any at all. In general, the nearer your blood glucose level is to normal, the less your risk of developing complications. Your risk of developing complications is also reduced if you deal with any other risk factors that you may have such as high blood pressure.

Diabetes burnout

Caring for diabetes is extremely demanding. Diabetes burnout occurs when you feel overwhelmed by diabetes. This can lead to feeling angry, frustrated, defeated and also worried about not taking care of diabetes well enough.

Try not to be a perfectionist - don't be too hard on yourself. Keep a balance in your life so that diabetes doesn't take over completely. Talk with your doctor or nurse to tell them how you're feeling. Just talking about how you feel can help. Burnout can also respond well to talking therapies such as cognitive behavioural therapy (CBT).

Although diabetes is associated with serious complications, these complications can be prevented or greatly reduced in severity. A healthy lifestyle, regular monitoring and taking medicines to keep your blood sugar (glucose), blood pressure and cholesterol levels as normal as possible are all very important.

Some people with type 2 diabetes are able to achieve 'diabetes in remission'. This means their blood glucose stays within normal levels without medication. The most successful way to achieve this is with lifestyle changes, including losing weight and either a medically supervised very low-calorie diet or a low-carbohydrate diet. You can find out more details from the separate leaflet Type 2 Diabetes.

There is currently no known way to prevent type 1 diabetes although many studies are looking into a number of different possibilities.

Type 2 diabetes can be prevented by following a healthy lifestyle, such as a healthy diet, regular exercise and not being overweight. This is very important for everyone. However, it is particularly important for people who are at increased risk, including those who have pre-diabetes (impaired glucose tolerance).

Further reading and references