Sugar control in the body
It's really important that we always have enough glucose in our bodies, and that it gets to all the right places. But, we also need to make sure we don't have too much glucose, as that can cause its own problems. So, the body needs a way to control glucose levels. But this is not easy - maybe we just had a sugary drink, or just did lots of exercise!
Our sugar control system uses hormones to signal when levels are getting too high or too low. Their job is to let our cells know that they need to change what they’re doing, to re-balance our sugar levels. The most important hormones for sugar control are insulin and glucagon.
What is sugar?
Small and sweet
Sugar is sweet, and dissolves in water. There are different types of sugar – for example the sugar fructose is found in fruit, and the sugar lactose is found in milk. But the most important sugar for life is glucose.
Glucose is made by plants during photosynthesis. Energy from sunlight is captured in the glucose molecule’s bonds. Later, breaking these bonds can release the energy. In this way, glucose acts like a battery that stores the Sun’s energy for later use.
Our cells use glucose to power their activities. The level of glucose in our blood must be carefully controlled to make sure our cells receive enough to function properly.
To access glucose’s energy, cells use a process called cellular respiration. This process transfers energy from glucose into smaller molecules called ATP. One glucose molecule contains enough energy for 38 ATP molecules. The ATP provides tiny packets of easily-accessed energy which are then used to power all the cell’s activities.
Complex carbohydrates
Simple sugars like glucose can be stuck together to form larger structures. These structures aren’t always called sugars, but are still related to sugars. The word carbohydrate is used for the whole group of molecules, including both sugars and the larger structures they form. The largest structures are called complex carbohydrates.
Plants store their spare sugar as a complex carbohydrate called starch. Starch molecules are made from thousands of glucose molecules joined together into long chains. Starch doesn’t taste sweet, and is not soluble in water. It is a safe way of storing sugar inside cells.
There is usually a lot of starch in a plant’s storage organs, such as in potatoes and grain.. When we eat these foods, the long chains of starch are digested by enzymes to release the individual glucose molecules.
Our bodies can store sugar too, but not as starch. Our liver and muscle cells can make a different complex carbohydrate called glycogen. Like starch, glycogen is not sweet, not soluble, and a safe way to store glucose. It just has a different shape, with the glucose molecules joined together in a branched structure.
Sugar control HQ
The task of managing glucose levels is handled by the pancreas, an organ hidden behind the stomach. Every moment of every day, special cells in the pancreas check how much glucose there is in the blood. If glucose levels are high or low, a signal is sent out to alert other cells.
For its warning signals, the pancreas uses hormones. Hormones are small chemical messengers that travel in the blood – which makes them perfect for this role. Nerves can send signals more quickly, but they only reach certain places. In contrast, billions of hormones can be sent out into the bloodstream, spreading the message widely to reach every part of the body.
When a cell receives a hormone signal from the pancreas, its reaction depends on what type of cell it is, and the type of signal it receives. A cell might take in more glucose, put glucose into storage (in the liver and muscle), or take glucose out of storage to send it back into the blood.
Sometimes, steps in this system fail. This can lead to blood glucose levels getting too high or too low. This is what happens in the disease diabetes mellitus. Understanding how things can go wrong helps endocrinologists develop new treatments to help the body rebalance its glucose levels.
A tricky balance
The pancreas doesn’t have an easy job; our glucose levels are changing all the time.
Firstly, glucose is always being used up. Glucose is broken down inside cells to access its energy in a process called cellular respiration. Even while you sleep, your body is constantly using up glucose. Its energy is used to fuel movement of your heart and lungs and to power the constantly busy world inside every cell.
But, if you exercise, or think really hard, your body uses a lot more! Your pancreas keeps a constant look-out for drops in blood glucose so that it can quickly react if your energy demands increase.
The glucose our bodies use up must be replaced by what we eat. Glucose is made by plants, using energy from the Sun. When we eat a potato, rice, fruit or any other plant-based meal, the carbohydrates they contain are broken down into glucose and absorbed into our blood. The blood then carries the glucose to our cells.
Different foods have different effects on our blood glucose levels. And it’s not always the sweet-tasting foods that have the biggest effects. Potatoes don’t taste sugary, but they contain a lot of starch. Starch is made from long chains of glucose molecules, which are easily released and absorbed. A baked potato will flood the blood with glucose more quickly than an apple!
The pancreas needs to react fast when a meal raises blood glucose levels. It must prevent the levels getting too high, and quickly bring them back down to normal again.
The way in which our bodies control their blood glucose levels is just one example of homeostasis. Homeostasis is the process of maintaining a stable internal environment to keep our cells healthy.
Sugar storage
If the only way to raise our blood glucose levels were by eating, we’d need to snack all day long. It would be really difficult to maintain a healthy balance. Happily, this isn’t necessary, because our bodies have ways to store glucose outside the bloodstream. This stored glucose can be used to boost blood glucose levels when needed.
Our bodies store glucose as the complex carbohydrate glycogen. Glycogen contains thousands of glucose molecules linked together in huge, branched structures.
Glycogen is mostly made in the liver, where large amounts can be stored. When blood glucose is low, this glycogen is broken up to release glucose back into the bloodstream. Muscle cells can also store glycogen, but only enough for their own use.
Another way to store glucose is to use it to make fat molecules. But it takes a bit longer to turn fat back into glucose, so that’s a longer-term storage option.
The pancreas
The control centre of our sugar control system is a spongy organ shaped like a parsnip. It can be found neatly tucked away behind the stomach, beside the liver.
The Pancreas doesn’t only produce hormones. In fact, most of its cells make enzymes to help digestion in the gut. But because it also produces and secretes hormones into the blood, it can be called an endocrine gland.
Different cells in the pancreas make different hormones. The most important are insulin and glucagon. Insulin is released when blood glucose is high (usually after a meal), and glucagon is released when blood glucose is low.
Hormone messengers: insulin and glucagon
Insulin signals: “Blood sugar is high!”
Glucagon signals: “Blood sugar is low!”
When the pancreas detects high or low blood glucose, it releases billions of hormone molecules. These are carried in the blood, where they spread out to reach every part of the body. Cells in different parts of the body react in different ways to their message, together helping to bring glucose levels back to normal.
Insulin
When cells detect insulin, it tells them there must be lots of glucose outside their cell membrane. By making it easier for glucose to cross the membrane, they let it come into the cell where it can be used. As long as there are more glucose molecules outside than inside, more will diffuse into the cell than will diffuse out.
Insulin also tells liver and muscle cells to start making glycogen (the storage molecule made by sticking glucose molecules together). As they use up glucose by doing this, more glucose will come into the cells to replace it.
As glucose goes into cells, and is put into storage, the levels in the bloodstream fall.
You can find out more about insulin, what it is, how it is produced and released, and exactly how it works, in our Insulin article
Glucagon
Glucagon carries the signal “Blood sugar is low!”.
At this signal, liver cells slow down the rate at which they are using up glucose. They also start breaking down their glycogen stores to release glucose molecules back into the blood. This is what your body does if there is a long gap between meals.
Other cells react in different ways to help boost blood glucose levels, or at least to prevent them dropping too much further. This can include breaking down other molecules to provide glucose.
These reactions to glucagon help raise blood glucose levels.
When things go wrong
Sometimes, the body is unable to keep blood glucose at a healthy level. This might be because there is not enough food coming in, or because there is a problem with the glucose control system.
Diabetes mellitus is a common disease where the insulin signal doesn’t work. This can be either because the body doesn’t make insulin, or (more often) because cells don’t respond to insulin properly.
It’s important to treat diabetes mellitus because if blood glucose levels get too high this can cause big problems, including damage to blood vessels and dehydration. And without insulin’s signal, cells are also unable to take in enough glucose to fuel their activities. There are different treatments for different types of diabetes, including insulin injections.
Blood glucose levels can also get too low. This can happen if someone doesn’t eat, or a diabetes patient accidentally injects themselves with too much insulin. If this happens, cells are starved of energy and the body may damage itself in its attempt to solve the problem.
Find out more about diabetes mellitus in our article here.
By learning more about the body’s sugar control system, endocrinologists can develop new and better treatments for diabetes mellitus and other related diseases.
