Diabetes mellitus

Diabetes mellitus is a common disorder resulting in high levels of sugar in the blood. It affects a large number of people, with many more people remaining undiagnosed.

Alternative names for diabetes mellitus

Diabetes; type 2 diabetes; type 1 diabetes; sugar diabetes; T2DM; T1DM; insulin-dependent diabetes mellitus; IDDM; non-insulin-dependent diabetes; juvenile-onset diabetes

What is diabetes mellitus?

Diabetes mellitus is a condition in which the body does not produce enough of the hormone insulin, or the insulin produced does not work effectively, resulting in high levels of sugar in the blood. There are several different types of diabetes; the most common forms are type 1 (lack of insulin) or type 2 diabetes (insulin which does not work effectively). Gestational diabetes occurs during the latter part of pregnancy and is covered in a separate article. Diabetes can also be caused by diseases which stop insulin working well or reduce insulin production by damaging the the pancreas, e.g. Cushing's syndromeacromegaly, and some rare genetic forms e.g. Maturity-Onset Diabetes of the Young (MODY).

High blood sugar can cause damage to large blood vessels (called macrovascular complications e.g. cardiovascular- heart arteries, cerebrovascular- brain arteries, and peripheral vascular disease – arteries to the legs), and to small blood vessels (called microvascular complications e.g. small blood vessels in the eye, kidney, and nerves). Therefore, Diabetes Mellitus is linked with an increased risk of heart attacks, strokes, poor blood circulation to the legs, as well as damage to the eyes (retinopathy), nerves to feet (neuropathy) and kidneys (nephropathy). Early diagnosis and tight control of blood sugar, blood pressure, and cholesterol levels, can help to prevent or delay complications associated with diabetes. Maintaining a healthy lifestyle (regular exercise, eating healthily and having a healthy weight) is important in reducing the risk of developing and treating type 2 diabetes.

What causes diabetes mellitus?

Insulin is a hormone produced by the beta cells of the pancreas in response to an increase in blood sugar following the intake of food. Insulin acts to lower blood sugar (glucose) levels by allowing cells in the muscle, liver, and fat, to take up sugar from the blood so that these tissues can use it for energy or store it away. In type 1 diabetes (previously called insulin-dependent diabetes mellitus), the insulin-producing beta cells of the pancreas are destroyed and are no longer able to produce insulin. This means that sugar in the blood is not accessed by tissues giving rise to abnormally high sugar levels. This, in turn, causes dehydration and thirst (called polydipsia because the high glucose ‘spills over’ into the urine and pulls water out of the body). To exacerbate the problem, because the body is not making insulin and it is unable to use sugar as a source of energy; it ‘thinks’ that it is starving so it does everything it can to release even more stores of energy from other sources (like fat) into the bloodstream. If left untreated, patients become increasingly unwell, very dehydrated, lose weight and develop a condition called diabetic ketoacidosis (also known as DKA), which is due to the excessive acidic by-products of those other energy stores and causes severe changes to how energy is used and stored in the body.

In ‘type 2 diabetes’ (previously called non-insulin-dependent diabetes mellitus), which accounts for 90% of all diabetes, the beta cells do not stop making insulin, but the insulin produced does not work effectively to reduce blood sugar levels. As a consequence, the pancreas has to produce even more insulin to compensate for this reduction in insulin function. This is called ‘insulin resistance’ and is commonly associated with excess body fat and obesity. This type of diabetes is seen more commonly over the age of 40 years but can occur at any age.  

What are the signs and symptoms of diabetes mellitus?

There are a range of different symptoms in people with diabetes. They may feel thirsty, pass a large amount of urine, wake up overnight to pass urine, lose weight and have blurred vision. Patients are vulnerable to infections such as skin infections, or thrush. Particularly in type 2 diabetes, patients may not be aware of their diabetes for several years and a diagnosis may only be made when they seek treatment for diabetes-related complications such as foot, eye or kidney problems. Some patients may become severely ill and be taken into hospital with an infection and/or very high blood sugar levels.

How common is diabetes mellitus?

Diabetes mellitus is a public health problem around the world. In 1980, 108 million adults worldwide had diabetes (4.7% of the global population). By 2021 this had risen to 537 million adults (10.5% of the global population). By 2045, the number is expected to be 784 million adults. In 2021, it is estimated that over 4.9 million people live with diabetes in the UK. Type 2 diabetes accounts for more than 90% of all patients with diabetes. 

Is diabetes mellitus inherited?

This depends on the type of diabetes. Type 2 diabetes, and to a lesser extent type 1 diabetes, runs in families. If a parent has diabetes, their children will not necessarily get it but they are at an increased risk. In type 2 diabetes, lifestyle factors such as being overweight or having obesity, and a lack of exercise can increase the risk of developing diabetes. Some rarer types of diabetes mellitus may be inherited.

How is diabetes mellitus diagnosed?

Diabetes can be indicated by testing a urine sample for sugar, but for a diagnosis, a blood sample is required. This may be a simple measurement of the sugar level, usually fasting (fasting blood glucose). Sometimes, particularly in pregnancy, an ‘oral glucose tolerance test’ (OGTT) is performed. This involves blood tests before and 2 hours after a sugary drink.

Alternatively, a test called a ‘glycated haemaglobin’ (HbA1c) can be used to estimate sugar levels over the past 2-3 months.

If someone has typical symptoms of diabetes, only a single abnormal test is required. Where there are no symptoms, a second confirmatory test is required.


How is diabetes mellitus treated?

The aim of treatment in diabetes is to:

  • reduce blood glucose levels to as normal a level as possible (whilst avoiding low blood sugar levels) to avoid the long-term complications of diabetes such as eye, nerve, and kidney damage.
  • avoid acutesymptoms of high blood sugar levels such as thirst, fatigue, infections, and weight loss.

While control of blood glucose levels is the primary aim of treatment, other aspects that increase risk including control of blood pressure and blood cholesterol levels are also very important in the management of diabetes and the prevention of long-term health consequences. 

Type 1 diabetes is always treated with insulin, a life-saving treatment. Patients will need to self-administer insulin several times a day for the rest of their lives. Insulin is usually given through injections under the skin, normally two to four times a day. An increasing number of patients with type 1 diabetes are being treated with ‘insulin pumps’, which provide a continuous supply of insulin.

Patients with diabetes try to ensure that their blood glucose levels are kept as normal as possible so that delicate tissues in the body (especially blood vessels in the eyes, kidneys and peripheral nerves) are not damaged by high glucose levels over a long period of time. To achieve this, patients should measure their glucose levels regularly and be shown how to adjust their insulin doses to control their glucose levels. Good control of diabetes helps to minimise the risk of long-term diabetes complications, as well as short-term symptoms (such as thirst).

Patients with type 2 diabetes can still make insulin, but it may not function well enough to control their glucose levels. Type 2 diabetes is therefore initially treated with a combination of lifestyle changes (weight loss, healthy diet, and exercise), which increase the body’s response to insulin and therefore lowers glucose levels.

If this is insufficient to achieve good glucose control, a range of tablets are available:

  • Metformin is the first-line treatment for type 2 diabetes, which makes insulin work better (reduces insulin resistance) and helps reduce the risk of diabetes-related complications.
  • Pioglitazone improves the function of insulin (reduces insulin resistance).
  • Sulphonylureas (e.g. gliclazide) stimulate insulin secretion.
  • DPP4 inhibitors (e.g sitagliptin) prevent the breakdown of the naturally occurring hormone GLP-1.
  • Oral GLP-1receptor agonists (e.g. semaglutide) stimulate insulin production and reduce appetite.
  • SGLT2 inhibitors (e.g. dapagliflozin) lower blood sugar levels by causing sugar to leak into the urine and be removed from the body.

Injectable treatments include:

  • Insulin- In many patients, particularly after several years of treatment, insulin production falls and is insufficient to meet the patient's needs such that patients with type 2 diabetes may need treatment with insulin injections, either alone or in combination with tablets.
  • GLP-1receptor agonists (e.g. semaglutide) stimulate insulin production and reduce appetite so can help with weight loss.

Are there any side-effects to the treatment?

Insulin treatment can cause weight gain and low blood sugar. In addition, there may be discomfort at the injection site. There are several types of tablets used to treat diabetes and they have different side-effects. The most common are diarrhoea (metformin), nausea (GLP-1 agonists), weight-gain (sulphonylureas and pioglitazone), low blood sugar (sulphonylureas) and genital thrush (SGLT2 inhibitors). However, not all patients will experience some or any of these side-effects and patients should discuss any concerns with their doctor.

What are the longer-term implications of diabetes mellitus?

Having diabetes requires life-long treatment and follow-up by health professionals. It is associated with an increased risk of strokes, heart attacks, nerve damage, eye and kidney problems, and poor blood circulation to the legs. Medical care aims to minimise these risks by controlling blood sugar levels, blood pressure, cholesterol, and screening for possible complications caused by the diabetes. 

Maintaining a healthy lifestyle with regular exercise and a healthy diet helps with glucose control and managing diabetes in the long term. With careful monitoring and appropriate treatment, diabetes patients can lead full and active lives. 

Women with diabetes who are planning to start a family should discuss this with their doctor as good glucose control is important both prior to conception and throughout pregnancy.

Last reviewed: May 2022