Insulin is a hormone made by an organ located behind the stomach called the pancreas. Here, insulin is released into the bloodstream by specialised cells called beta cells found in areas of the pancreas called islets of langerhans (the term insulin comes from the Latin insula meaning island). Insulin can also be given as a medicine for patients with diabetes because they do not make enough of their own. It is usually given in the form of an injection.
Insulin is released from the pancreas into the bloodstream. It is a hormone essential for us to live and has many effects on the whole body, mainly in controlling how the body uses carbohydrate and fat found in food. Insulin allows cells in the muscles, liver and fat (adipose tissue) to take up sugar (glucose) that has been absorbed into the bloodstream from food. This provides energy to the cells. This glucose can also be converted into fat 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).
When we eat food, glucose is absorbed from our gut into the bloodstream. This rise in blood glucose causes insulin to be released from the pancreas. Proteins in food and other hormones produced by the gut in response to food also stimulate insulin release. However, once the blood glucose levels return to normal, insulin release slows down. In addition, hormones released in times of acute stress, such as adrenaline, stop the release of insulin, leading to higher blood glucose levels. The release of insulin is tightly regulated in healthy people in order to balance food intake and the metabolic needs of the body.
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.
People with type 1 diabetes mellitus are given drugs including insulin treatment to lower their high glucose levels. However, if a person accidentally injects too much insulin, cells will take in too much glucose from the blood. This leads to abnormally low blood glucose levels (hypoglycaemia). The body reacts to hypoglycaemia by releasing stored glucose from the liver in an attempt to bring the levels back to normal. However, persistently low glucose levels in the blood can make a person feel ill.
Unlike other cells in the body, nerve cells depend almost entirely on glucose as a source of energy. When the glucose level is too low, the majority of the symptoms result from these nerves not functioning properly. The brain is particularly affected by low glucose levels. Symptoms include dizziness, confusion and even coma in severe cases. In addition, the body mounts a 'fight-back' response through a specialised set of nerves called the sympathetic nervous system. This causes palpitations, sweating, hunger, anxiety, tremor and a pale complexion.
This is commonly seen in people with diabetes and is caused by different pathways in people with type 1 diabetics compared with people with type 2 diabetes.
In some people, the pancreas is unable to make enough insulin, for example, in a condition called type 1 diabetes. This condition is caused when the beta cells that produce insulin have been destroyed. 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. 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.
Type 2 diabetes can be caused by two factors. Firstly, the patient’s beta cells may have an impaired ability to manufacture insulin. This means that while some insulin is produced, it is not enough for the body’s needs. Secondly, the insulin receptors, which allow insulin to exert its effects on individual cells, become insensitive and stop responding to the insulin in the bloodstream. A combination of these factors leads to similar symptoms as seen in type 1 diabetes.
Last reviewed: Feb 2015