Insulin’s role in the body 

Hormones are tiny chemical messengers that travel between cells. They can travel quickly through the body by hitching a ride in our blood. Hormones like insulin are perfect messengers for information that needs to reach large numbers of cells. They don’t aim for one particular place, they just carry their message across the whole body.

Insulin is made by an organ called the pancreas. When blood sugar is high, the pancreas releases billions of insulin molecules into the bloodstream. The tiny insulin molecules are carried along in the blood, spreading out into all our blood vessels. From the bloodstream they can reach all the cells of the body.

When cells receive insulin’s signal, this tells them that blood sugar is high. “Blood sugar” means the concentration of glucose (a type of sugar) in our blood. Blood glucose levels go up after meals, especially if we eat sugary foods.  

Glucose is really important to us because it is the main energy source for cells. We need it to power our bodies. But if we have too much glucose in our blood it can cause some nasty problems. So our bodies need to control glucose levels to make sure there is always enough - but never too much – glucose available.

Cells react to insulin’s signal by letting glucose come in through their cell membranes. This restocks their supplies and removes glucose from the blood. Some cells can also change glucose into a different form for storage. 

Where does insulin come from?

Insulin is made by an organ called the pancreas. The pancreas is a spongy organ tucked away behind the stomach. It’s roughly the size and shape of a bendy parsnip. Most of the cells in the pancreas make enzymes to help digestion. But there are also special areas that act as hormone factories. In these areas, different types of cells make different hormones. And some make insulin.

Insulin is a protein so it’s made – like all proteins - by joining amino acids together. The pancreas cells do this following instructions provided in their DNA. In this way, the pancreas makes huge numbers of insulin molecules, which wait inside the cells until the signal comes for them to be released.

The insulin stays trapped inside the pancreas cells until blood glucose levels rise. But this won’t happen until the person eats or drinks something. Carbohydrates in food are broken down into glucose, which is absorbed into the blood. And this flood of glucose from the gut increases blood glucose levels. This is what the pancreas has been waiting for. 

When the pancreas cells detect that glucose levels are high, they release (“secrete”) their store of insulin into the bloodstream. Vast numbers of insulin molecules are secreted in this way, spreading out into the blood, following different blood vessels, reaching every part of the body.

When the body’s cells receive insulin’s signal, they take glucose out of the blood. This reduces blood glucose levels back to normal levels. When the pancreas detects that blood glucose is no longer high, it will stop secreting insulin. This stops blood glucose levels getting too low.

Because the pancreas makes and releases hormones, it’s called an “endocrine gland”. The word “endocrine” means “about hormones”, and “gland” means “organ that secretes useful things”. 

What does insulin actually do?

Insulin, like all hormones, is a signalling molecule. Its only role is to carry a message. It doesn’t aim for any particular cells, or even know where it’s going. The insulin molecules are just carried along by the blood, like rubber ducks dropped in a river.

Insulin spreads throughout the whole body. This includes getting up close to cells. Most cells have a special insulin-receptor protein sticking out of their cell membrane. These receptors are shaped so they can grab hold of the insulin. The receptor and hormone fit together perfectly. This type of binding is called lock-and-key binding, because the shape of the ‘key’ (the hormone) has to fit the ‘lock’ (the receptor). In reality both parts wriggle around a bit during binding, but they end up with a perfect fit.

This is the moment that the signal from the pancreas is passed to the cell. The receptor protein then passes the signal inside the cell.

Insulin’s job is done. The message from the pancreas has arrived: “Blood sugar is high!”. This message, sent through insulin and its receptor, now sets off a complex signalling network inside the cell. Different cells will react in different ways to the signal.

How do cells react to insulin?

When cells receive insulin’s message, they know that there is glucose outside their cell membrane that they can use to refill their supplies. 

High blood sugar! Open the gates!

Firstly, the cell makes it easier for glucose to move through its cell membrane. Normally, it’s difficult for glucose to move into, or out of, a cell. This is good because it keeps the cell’s store of glucose safely inside, but also means that new glucose can’t get in.

When a cell detects insulin, it knows there is lots of glucose outside. In response, the cell puts channels in its membrane which allow glucose to move (diffuse) through. Because there is more glucose outside than inside, more glucose will enter than leave the cell.  
This provides the cell with new glucose to power its activities. It also reduces the amount of glucose in the blood. 

High blood sugar! Fill the stores!

Some cells can also store glucose for later use. This is a great way to get it out of the blood (where it can be dangerous) while still keeping it in the body.

The insulin signal tells these cells that they should refill their stores. As glucose is moved into storage, the levels inside the cell fall. But because glucose levels are high in the blood, more comes in through the channels in the cell membrane, refilling the cells own supply.

Liver cells store glucose by sticking it together to form huge, branched molecules called glycogen (a type of stored carbohydrate). When blood glucose is low, glycogen can be broken down back into glucose. This glucose is then released back into the blood to help bring levels back to normal.

Muscle cells can also store glycogen, although this is only for their own use.

Fat cells use spare glucose to make more fat, and the insulin signal also stops them breaking down the fat that they already have. Fat has a lot of uses in the body, including as a glucose store to be used when not enough glucose is coming in from meals.

Other things too!

Insulin has other effects too. Endocrinologists are still learning about everything it does. But helping manage blood glucose levels is its most important role.  

When things go wrong

Insulin is a key hormone in the body. When the sugar control system fails, it causes big problems. 

Signal failure: Diabetes mellitus

When the insulin signalling system fails, it causes a disease called diabetes mellitus.

If the pancreas can’t tell cells when to take glucose from the blood, cells become starved of glucose. And without glucose being removed from the blood, blood glucose levels get dangerously high.

There are two main ways this can happen: Some people can’t produce insulin because they have lost the cells that make it (this is Type I diabetes). In other people, their cells don’t react to insulin (this is type II diabetes).

It’s also possible for people’s cells to be a bit less reactive to insulin – this is known as insulin resistance. It’s not a total failure, but it still causes problems.

When blood glucose levels get too high, it can damage blood vessels and affect organs including the kidneys, eyes and brain. There is so much glucose in the blood that it gets into the urine, taking water with it. This means people with high blood glucose wee a lot. They then need to drink a lot to replace the lost water. It’s very easy for them to become dehydrated.

But even when blood glucose is high, most cells are unable to take in enough glucose without insulin. This means they become starved for fuel. They scavenge fuel where they can, breaking down fat and muscle, leading to people becoming tired and losing weight.

Too much insulin

Treatments for diabetes include injections of insulin. If someone accidentally injects too much insulin, their cells will take too much glucose out of the blood, leading to low blood glucose.

There is also a very, very rare cancer that can grow too many insulin-producing cells. Too many cells means that too much insulin is released, again leading to low blood glucose.

The body’s first reaction to not having enough glucose in the blood is to break down its glycogen stores in the liver. The glycogen is turned back into glucose and released into the blood. This is what your body does when you don’t eat for a while. But if there is too much insulin in the body, this may not be enough to bring levels back to normal.

People with dangerously low blood glucose usually feel very unwell. They go pale and tremble. They feel sweaty, hungry, and have anxiety and palpitations.

These symptoms can act as a useful warning for people to know they need to test their blood sugar, and eat something sugary to quickly raise their blood glucose. Some people also have medical alert dogs which can smell when they are unwell and warn them to take action.

If blood glucose remains too low for too long, cells will start running out of glucose for fuel. People may feel dizzy and confused, have fits and finally become unconscious. 

Insulin as a medicine

Insulin is used as a treatment for the disease diabetes mellitus. Its discovery and use were an incredible step forward, immediately saving lives.

At first, the insulin was taken from animals. Today, it is made in large quantities in bacteria and yeast. Most often, a gene for human insulin is put into the bacteria or yeast – this means they make exactly the same version of the hormone that we would normally have in our bodies.

There are also slightly different versions of insulin which have slightly different activity. Insulin is a protein made of chains of amino acids. By making small changes in the sequence of amino acids, it’s possible for example to make a version that is longer lasting.

We still don’t know everything about insulin and its activity in the body. But this is an active area of research. This brings the hope that we will make new discoveries that will lead to better treatments in future. 

Find out more about sugar control in the body  and diabetes mellitus here.