What is insulin and glucose function?

Insulin is a hormone that helps your body control the amount of sugar (glucose) in your blood. It is produced by the pancreas, a large gland that is located in the abdomen behind your stomach.

How does insulin work?

Insulin is produced in the islets of Langerhans (pancreatic islets), which are small isolated clumps of special cells in the pancreas. Insulin works alongside glucagon, another hormone produced by the pancreas, to manage the levels of glucose in your blood.

Both insulin and glucagon are secreted directly into your bloodstream, and work together to regulate your blood glucose levels. Insulin should stop your blood sugar from rising too high and glucagon should prevent it from becoming too low.

Insulin is produced by the beta cells of the pancreatic islets. Insulin is released when you have just eaten a meal and the level of glucose in your bloodstream is high. It works by stimulating the uptake of glucose into cells, lowering your blood sugar level. Your liver and muscles can take up glucose either for immediate energy or to be stored as glycogen until it’s needed.

Glucagon is produced by the alpha cells of the pancreatic islets. It is released when your blood sugar levels are low (for example overnight, or if you have been fasting or exercising). Glucagon stimulates cells in the liver and muscles to convert stored glycogen to glucose. The glucose is then released into the bloodstream, raising your blood sugar level.

What is insulin resistance?

Insulin resistance is when the liver and muscle cells stop responding properly to insulin.

The initial response of the pancreas is to make more insulin to help glucose enter cells, but the pancreas usually cannot keep making more and more insulin to overcome the insulin resistance. Eventually the insulin-producing cells of the pancreas start to burn themselves out and blood glucose levels start to rise (hyperglycaemia).

After some time, insulin resistance can cause type 2 diabetes, because there are no longer enough insulin-producing cells and also, the remaining ones are not as effective.

What is pre-diabetes?

Pre-diabetes is the condition when blood sugar levels are raised, but are not high enough for a diagnosis of type 2 diabetes to be made. Insulin resistance is the process by which pre-diabetes comes about.

People with pre-diabetes have so-called impaired fasting glucose (IFG) or impaired glucose tolerance (IGT).

• Impaired fasting glucose (IFG) is when your blood sugar levels are raised after fasting (not eating) but the levels are not high enough to diagnose diabetes. IFG is due to the liver releasing too much glucose into the blood when you are fasting overnight because it has become resistant to the effects of insulin.
• Impaired glucose tolerance (IGT) is when blood glucose levels are higher than normal (and also higher than in IFG) but still not high enough to make a diagnosis of diabetes. In IGT, blood glucose levels may be raised after meals or at other times. IGT happens when the body’s cells stop responding properly to insulin (insulin resistance) and/or there is not enough insulin to meet the body’s needs.

Your risk of developing pre-diabetes is increased if you are overweight or don’t get enough physical activity. Pre-diabetes risk factors are similar to those for type 2 diabetes.

Many people with pre-diabetes go on to develop type 2 diabetes. However, making lifestyle changes, such as eating a healthy balanced diet, losing weight and getting regular physical activity, can stop pre-diabetes from progressing to type 2 diabetes.

Insulin and diabetes

Diabetes mellitus is a condition where the blood contains too much glucose. The high blood glucose can be due to the pancreas not producing enough insulin, or to insulin resistance, where the body’s cells are not responding to the insulin being produced.

The most common types of diabetes include:

• type 1 diabetes, where the pancreas produces little or no insulin because the beta cells have been destroyed by the immune system;
• type 2 diabetes, where the body stops responding to insulin – at first, the pancreas reacts by producing extra insulin (hyperinsulinaemia), but eventually it is unable to make enough to meet the body’s needs; and
• gestational diabetes, where diabetes develops during pregnancy.

Achieving normal blood glucose levels

People with type 1 diabetes must take daily insulin to control their blood glucose levels. Insulin is usually given by injection (using a needle and syringe or insulin pen) or via an insulin pump.

People with type 2 diabetes can often control their blood sugar levels with a healthy diet, sufficient exercise and/or the use of tablets known as oral hypoglycaemic medicines, such as metformin. Eventually, however, many people with type 2 diabetes will also require insulin injections to keep their blood sugar levels steady.

People with diabetes need to monitor their blood sugar levels regularly. Your general practitioner (GP), endocrinologist (diabetes specialist) and diabetes educator will work with you to set goals for your optimum blood glucose levels before and after meals.

Blood sugar level targets, or goals, are usually set specifically for an individual. Factors that help work out your personalised target blood sugar levels include the type of diabetes you have and whether you are prone to episodes of low blood sugar (hypoglycaemia).

1. Diabetes: Detection and diagnosis (published Nov 2013; amended Feb 2015). In: eTG complete. Melbourne: Therapeutic Guidelines Limited; 2017 Jul. https://tgldcdp.tg.org.au/etgcomplete(accessed Aug 2017).2. Management of type 1 diabetes (published Nov 2013). In: eTG complete. Melbourne: Therapeutic Guidelines Limited; 2017 Jul. https://tgldcdp.tg.org.au/etgcomplete (accessed Aug 2017).3. Management of type 2 diabetes (published Nov 2013). In: eTG complete. Melbourne: Therapeutic Guidelines Limited; 2017 Jul. https://tgldcdp.tg.org.au/etgcomplete (accessed Aug 2017).

4. Diabetes Australia. Insulin (published June 2016). https://diabetesnsw.com.au/useful-tools/information-sheets/ (accessed Aug 2017).

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A person with diabetes being injected with insulin to regulate their blood sugar levels.

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).

How is insulin controlled?  

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.

What happens if I have too much insulin?

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.

What happens if I have too little insulin?

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.

What happens if insulin does not work properly?

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.

The link between the gut and the pancreas

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