Diabetes is a chronic disease in which insulin production and/or tissue sensitivity to insulin is impaired. As a result, circulating blood sugar (glucose) levels rise (i.e. hyperglycemia, an excess of sugar in the bloodstream).
Normally, when we eat or drink, the carbohydrates in our diet are digested and broken down into small sugar molecules called glucose. Glucose is then absorbed into the bloodstream, so that it can supply energy to various tissues and organs throughout the body, especially the muscles, brain and liver cells. The amount of glucose produced from a particular food or drink depends on its glycemic index (see Glycemic Index below). Examples of carbohydrates include starchy foods such as potatoes, bread, pasta, oats, fruits and vegetables.
When glucose enters the bloodstream it triggers the release of a hormone called insulin, which is produced by specialised cells in the pancreas called beta cells. The pancreas is an organ situated in the upper abdomen. Insulin enters the circulation and lowers blood glucose levels by promoting its uptake from the circulation into various tissues. Glucose taken-up by tissues can either by metabolised to yield energy or stored as glycogen for use later (when dietary intake is low, eg overnight). As a result of insulin production after food intake (and insulin suppression when fasting), blood glucose levels are normally maintained within a fairly narrow range to provide a constant supply of fuel to tissues.
In diabetes, blood glucose levels can become dangerously elevated and can remain so for long periods of time. This arises either because insulin production from the pancreatic beta cells fails (or is insufficient) or because the sensitivity of tissues to the glucose-lowering effect of insulin is reduced, i.e most types of diabetes are due to a defect in insulin production and/or insulin action. With all forms of diabetes this can lead to a myriad of harmful effects such as damage to blood vessels in the heart, brain and lower limbs, damage to nerve cells, impaired eyesight, and reduced kidney function, The overall survival of patients with diabetes, on average, is less than their non-diabetic counterparts but with modern treatments and technologies to reduce blood sugar levels this excess risk of long-term health problems is greatly reduced. It is therefore important to have a clear understanding of your diabetes and to treat it promptly and effectively so that you can control your blood glucose levels and avoid the various complications.
There are three main types of diabetes
Type 1 diabetes
Type 1 diabetes occurs when the number and function of insulin-producing beta cells in the pancreas decreases. This is often because the cells are destroyed by our own immune system, but sometimes the cause is unclear. Type 1 diabetes mainly occurs in children or young adults, but not infrequently it arises in middle-to-old age and can be mistaken for type 2 diabetes. Type 1 diabetes is characterized by a fairly sudden reduction in insulin production (over days or weeks), which can trigger a life-threatening condition known as diabetic ketoacidosis, but in older adults type 1 diabetes may be less abrupt in onset. Weight loss, thirst and tiredness are typical symptoms.
Insulin replacement therapy is an essential treatment for people with type 1 diabetes. None of the other treatments used in type 2 diabetes is licensed for use in type 1 diabetes. Different types of insulin include rapid, short, intermediate or long-acting formulations, and these can be prescribed individually or in a pre-mixed combination. The mode of administration of insulin includes subcutaneous injection, using pen-like devices with needles, and insulin pumps, which can deliver insulin as a continuous subcutaneous infusion with supplementary “bolus” doses to cover mealtime surges in glucose. The amount and type of insulin prescribed and method of administration will be determined by your lifestyle, age, meal plan and general health.
Type 2 diabetes
Type 2 diabetes (previously referred to as maturity-onset diabetes) usually occurs when the tissues, eg muscle and liver, become less responsive to the glucose-lowering effects of insulin (a condition referred to as insulin resistance), and/or when there is insufficient insulin production from the beta cells to prevent rises in blood glucose following a meal (i.e impaired insulin secretion). Increasing age, obesity, a high-fat diet and a sedentary lifestyle all tend to cause insulin resistance. Initially, insulin resistance leads to a compensatory increase in insulin production by the beta cells but over time the beta cells gradually become exhausted and insulin production falls, worsening hyperglycaemia.
After ingesting a meal, gut-derived hormones known as incretins stimulate the beta cells to produce insulin but in type 2 diabetes patients this incretin effect is lost and post-meal glucose levels rise. When the liver becomes resistant to the insulin signal, glycogen is broken down and glucose released into the circulation. Therefore, increased output of glucose from the liver tends to worsen hyperglycaemia in patients with type 2 diabetes.
Type 2 diabetes tends to be slowly progressive. In the early stages, when beta cell function is maintained, it is often treated with lifestyle changes alone, such as nutrition and exercise, but over time patients with type 2 diabetes will usually need oral medications as well as lifestyle intervention. Injectible therapies, including insulin, may be needed once type 2 diabetes has been present for a few years. The first line therapy for type 2 diabetes is usually a drug called metformin, which lowers blood glucose by stopping the liver from producing excess glucose and by increasing muscle insulin sensitivity (1).
Gestational diabetes is a temporary state during pregnancy in which blood sugar levels rise due to insulin resistance and insufficient insulin production.
The hormonal increases during pregnancy and other pregnancy-related factors can lead to worsening of insulin resistance, and gestational diabetes occurs when the beta cells are unable to produce enough insulin to overcome the reduction in insulin sensitivity. Gestational diabetes is associated with higher health risks for both the mother and the foetus. It tends to occur more often in women who had some of the risk factors for type 2 diabetes prior to pregnancy, e.g a family history or they are overweight.
This type of diabetes occurs in 3-5% of all pregnancies and is mostly treated with nutrition and exercise, although some women will need short-term treatment with insulin. It normally disappears after the pregnancy is over, although having this type of diabetes can predispose you to developing type 2 diabetes in later life (outside of pregnancy).
Other types of diabetes
There are other less common types of diabetes which are often inherited and occur due to single gene defects. The family of disorders known as Maturity-onset diabetes of the young (MODY) is classed in this group. Diabetes can also occur due to some drug treatments, eg steroid therapy, and can occur when people are unwell for other reasons. It can also develop if patients have other forms of disease affecting the pancreas, e.g pancreatitis.
Incidence of diabetes
Around 3.5 million people have been diagnosed in the U.K with diabetes. As people with type 2 Diabetes are often unaware that they have this disease, it is estimated that there are over 4 million people living with diabetes in the U.K.
Approximately 5-10 % of all diabetes diagnoses are type 1. Type 2 diabetes accounts for at least 90 % of people diagnosed with diabetes (2,3). A third of this incidence is due to obesity and the rest is due to ageing and a change in the ethnic structure of the population (4). All forms of diabetes are influenced by family history and genetic susceptibility, as well as environmental factors such as diet and lifestyle.
Type 2 Diabetes used to be called adult-onset diabetes, because it was mostly diagnosed in adults. The modern western lifestyle has led to rising rates of childhood obesity, which is linked with the development of type 2 diabetes in young adults.
— May Meleigy
- Viollet B., Guigas B., Garcia N.S., Leclerc J., Foretz M., Andreelli F. Cellular and molecular mechanisms of metformin: an overview. Cellular and molecular mechanisms of metformin: an overview. Clin Sci (Lond). 2012 Mar 253-270. doi: 10.1042/CS20110386
- National clinical guideline for management in primary and secondary care (update). London: Royal College of Physicians, 2008.
- International Diabetes Federation. Types of diabetes. http://www.idf.org/types-diabetes
- Observatories (APHO) Diabetes Prevalence Model: estimates of total diabetes prevalence for England, 2010–2030. Diabetic Medicine 2011;28(5):575-825