Insulin is a hormone made by an organ located behind the stomach called the pancreas. There are specialised areas within the pancreas called islets of Langerhans (the term insulin comes from the Latin insula that means island). The islets of Langerhans are made up of different type of cells that make hormones, the commonest ones are the beta cells, which produce insulin.
Insulin is then released from the pancreas into the bloodstream so that it can reach different parts of the body. Insulin has many effects but mainly it controls how the body uses carbohydrates found in certain types of food. Carbohydrates are broken down by the human body to produce a type of sugar called glucose. Glucose is the main energy source used by cells. Insulin allows cells in the muscles, liver and fat (adipose tissue) to take up this glucose and use it as a source of energy so they can function properly. Without insulin, cells are unable to use glucose as fuel and they will start malfunctioning. Extra glucose that is not used by the cells will be converted and stored as fat so it can be used to provide energy when glucose levels are too low. In addition, insulin has several other metabolic effects (such as stopping the breakdown of protein and fat).
The main actions that insulin has are to allow glucose to enter cells to be used as energy and to maintain the amount of glucose found in the bloodstream within normal levels. The release of insulin is tightly regulated in healthy people in order to balance food intake and the metabolic needs of the body. This is a complex process and other hormones found in the gut and pancreas also contribute to this blood glucose regulation. When we eat food, glucose is absorbed from our gut into the bloodstream, raising blood glucose levels. This rise in blood glucose causes insulin to be released from the pancreas so glucose can move inside the cells and be used. As glucose moves inside the cells, the amount of glucose in the bloodstream returns to normal and insulin release slows down. Proteins in food and other hormones produced by the gut in response to food also stimulate insulin release. Hormones released in times of acute stress, such as adrenaline, stop the release of insulin, leading to higher blood glucose levels to help cope with the stressful event.
Insulin works in tandem with glucagon, another hormone produced by the pancreas. While insulin's role is to lower blood sugar levels if needed, glucagon's role is to raise blood sugar levels if they fall too low. Using this system, the body ensures that the blood glucose levels remain within set limits, which allows the body to function properly.
If a person accidentally injects more insulin than required, e.g. because they expend more energy or eat less food than they anticipated, cells will take in too much glucose from the blood. This leads to abnormally low blood glucose levels (called hypoglycaemia). The body reacts to hypoglycaemia by releasing stored glucose from the liver in an attempt to bring the levels back to normal. Low glucose levels in the blood can make a person feel ill.
The body mounts an initial 'fight back' response to hypoglycaemia through a specialised set of of nerves called the sympathetic nervous system. This causes palpitations, sweating, hunger, anxiety, tremor and pale complexion that usually warn the person about the low blood glucose level so this can be treated.
However, if the initial blood glucose level is too low or if it is not treated promptly and continues to drop, the brain will be affected too because it depends almost entirely on glucose as a source of energy to function properly. This can cause dizziness, confusion, fits and even coma in severe cases.
Some drugs used for people with type 2 diabetes, including sulphonylureas (e.g. gliclazide) and meglitinides (e.g. repaglinide), can also stimulate insulin production within the body and can also cause hypoglycaemia. The body responds in the same way as if excess insulin has been given by injection.
Furthermore, there is a rare tumour called an insulinoma that occurs with an incidence of 1-4 per million population. It is a tumour of the beta cells in the pancreas. Patients with this type of tumour present with symptoms of hypoglycaemia.
People with diabetes have problems either making insulin, how that insulin works or both. The main two types of diabetes are type 1 and type 2 diabetes, although there are other more uncommon types.
People with type 1 diabetes produce very little or no insulin at all. This condition is caused when the beta cells that make insulin have been destroyed by antibodies (these are usually substances released by the body to fight against infections), hence they are unable to produce insulin. With too little insulin, the body can no longer move glucose from the blood into the cells, causing high blood glucose levels. If the glucose level is high enough, excess glucose spills into the urine. This drags extra water into the urine causing more frequent urination and thirst. This leads to dehydration, which can cause confusion. In addition, with too little insulin, the cells cannot take in glucose for energy and other sources of energy (such as fat and muscle) are needed to provide this energy. This makes the body tired and can cause weight loss. If this continues, patients can become very ill. This is because the body attempts to make new energy from fat and causes acids to be produced as waste products. Ultimately, this can lead to coma and death if medical attention is not sought. People with type 1 diabetes will need to inject insulin in order to survive.
If insulin does not work properly on its receptor it may lead to type 2 diabetes. Type 2 diabetes can be caused by two main factors and its severity will depend on how advanced the problems are. Firstly, the patient’s beta cells may have problems manufacturing insulin, so although some insulin is produced, it is not enough for the body’s needs. This is ‘relative insulin deficiency’ rather than the ‘absolute insulin deficiency’ that is seen in type 1 diabetes. Secondly, the available insulin doesn’t work properly because the areas in the cell where insulin acts, called insulin receptors, become insensitive and stop responding to the insulin in the bloodstream. This is called insulin resistance and is very common in obesity and type 2 diabetes. These receptors appear to malfunction more in people who carry excessive amount of weight. Some people with type 2 diabetes might initially experience very few symptoms and the raised blood glucose is only picked up when a routine blood test is arranged for another reason; other people might experience symptoms similar to those seen in patients with type 1 diabetes (thirst, frequent urination, dehydration, hunger, fatigue and weight loss). Some patients with type 2 diabetes can control their symptoms by improving their diet and/or losing weight, some will need tablets, and others will need to inject insulin to improve blood glucose levels. See the article on diabetes mellitus for more information.
Insulin is produced in higher amounts when sugar passes through the gut compared to when glucose is injected into the veins (called the ‘incretin effect’). This is because when sugar enters the gut, messages are sent from the gut to the pancreas to produce more insulin. These messages are chemicals called peptides. The best known gut peptide controlling insulin is called GLP1 (Glucagon Like Peptide 1). GLP1 ‘speaks’ to the pancreas after glucose enters the gut, telling it to produce insulin at the right time. It is now known that in type 2 diabetes, GLP1 levels are too low, which is partly why blood sugar levels go too high after a meal (‘post-prandial hyperglycaemia’). There are now many medicines that increase GLP1 levels to control post-prandial hyperglycaemia which are therefore good treatments for type 2 diabetes.
Last reviewed: May 2021