Diabetes mellitus

Diabetes mellitus is a disease that affects how the body controls sugar. It is caused by a hormone called insulin not working properly. There are different ways this can happen, so there are different types of diabetes mellitus. The most common are Type 1 and Type 2, which are explained below. 

Without insulin’s signal, the cells that normally store spare sugar can’t take it in. Sugar builds up in the blood, causing problems including damage to nerves, eyes, and blood vessels. Sugar also escapes in the urine, taking water with it. This makes it easy for people living with diabetes to become dehydrated as they make many trips to the toilet. 

An ancient disease

Diabetes mellitus is nothing new - it has been around since ancient times. But it has become a lot more common in recent years. About one in every twenty people in the UK have diabetes mellitus, so it’s very likely that you know someone who is diabetic. If left untreated, it is a very dangerous disease. But by learning about insulin and how it works, endocrinologists have been able to develop effective treatments to prevent or delay the worst symptoms.

Ancient descriptions of diabetes can be recognised because of its most obvious symptom – needing the toilet a lot more than usual. The 1st-Century Greek who named the disease wrote: “The patients never stop making water and the flow is incessant as if from the opening of aqueducts.” (The extant works of Aretaeus, Welcome Collection).

“Diabetes” is now known to include different diseases. But the disease diabetes mellitus has another key symptom. A 7th-Century Indian surgeon wrote of ants being attracted to patients’ urine, which he described as sweet and sticky. This combination of symptoms – lots of sweet urine – is a clear description of diabetes mellitus.

For many years, doctors assumed diabetes mellitus was a kidney disease. After all, the kidney makes urine. But in the 18th Century, a Liverpool physician discovered that it wasn’t only the urine which had too much sugar – the blood did too. This important discovery set off new research that led to the first effective treatments.

We now know that diabetes mellitus is caused by problems with a signalling hormone called insulin. The person’s insulin is either missing, or not working properly. 

What’s the problem?

Sugar – and especially a sugar called glucose – is vital to life. Every living cell on Earth needs glucose as an energy source. So it’s really important that all the cells in our body get enough to power their activities. 
We get our sugar from the food we eat, mostly from complex carbohydrates like bread, pasta and potatoes. These carbohydrates are digested to provide glucose, which is absorbed from our gut into our bloodstream. The blood then delivers it to our cells. But because we get our glucose from food, it arrives into our bodies in surges. Every time we eat carbohydrates, we get a burst of new glucose coming into our blood.

This is a problem because having too much blood glucose is dangerous. So, our blood glucose level needs to be controlled, while making sure that our cells still have enough glucose to power their activities. This is done by moving the excess sugar into storage for later use.

In diabetes mellitus, this control system fails.

Where it goes wrong is with a hormone called insulin. Insulin is the chemical messenger our bodies use to tell cells when blood glucose is high. An organ called the pancreas monitors glucose levels, and when they are high it releases billions of insulin molecules into our blood.

Glucose can be safely stored by liver, muscle and fat cells. When these cells receive insulin’s signal, they take in glucose, both for their own use and to move it into storage. As they remove sugar from the blood, blood glucose falls back to healthy levels.  

But - if no insulin is made (Type 1 diabetes), or its signal doesn’t work (Type 2 diabetes), this doesn’t happen. Glucose is trapped in the blood at dangerously high levels. And liver and muscle cells can struggle to get enough glucose even just to power their normal activities. This is what happens in untreated diabetes mellitus.

You can find out more about glucose control and storage in our article Sugar Control in the Body.

Too much blood glucose

High blood glucose is bad for the body and makes people feel unwell. As well as needing the toilet a lot, they may feel weak, tired and thirsty. They may have blurred vision and start losing weight.

One big problem with high blood glucose is that it can cause damage to blood vessels. This damage builds up over time and increases the chances of heart disease or stroke. The damage can also reduce blood supply to the legs. Meanwhile, damage to the smallest blood vessels (called capillaries) can affect nerves, and organs including the eye and kidney.

Another major problem is that glucose escapes into the urine. Our kidneys don’t usually let glucose leave the body, but if there is too much then they are unable to stop it. This might sound like a good thing because it removes glucose from the blood, but it takes a lot of water with it. This is why large amounts of urine are produced. Going to the toilet so much is dangerous because the lost water must be made up by drinking more. It is easy for people with high blood glucose to become dangerously dehydrated.

High blood glucose affects the body in other ways too, some of which are still being discovered. This isn’t a surprise - glucose is a key molecule with many important roles throughout the body. 

Starving cells

It would be easy to imagine that high blood glucose should mean all the body’s cells would be flushed with glucose, with all the energy they needed and more. Unfortunately, this isn’t the case.

Glucose can’t pass through cell membranes by itself. This means cells have some control over how much is allowed in. To allow glucose through the membrane, cells use special protein channels. The more channels they put into their membrane, the more glucose can come in.

One of insulin’s roles is to signal to glucose-storing cells that they should put more glucose channels in their membranes. Glucose can pass either way through these channels, but insulin is only released when blood sugar is high. Because there is more glucose outside than inside at these times, more glucose will come in than go out. In this way the cells refill their supplies during times of high blood sugar. This is what happens with liver, muscle and fat cells. When they receive insulin’s signal, they let glucose come in.  

But … If there’s no insulin signal, then the cells won’t add channels. So the glucose can’t get in. This is a problem especially for liver and muscle cells, which need a lot of glucose to function. The blood may be sweet with excess glucose, but these cells will be starved of energy if they are unable to bring enough inside. This is a big danger in untreated Type 1 diabetes, where no insulin is made at all.

Without insulin, the body reacts as though it were starving. Cells start breaking down carbohydrate and fat supplies to release glucose and other energy sources. If this happens too quickly, it can also release dangerous molecules into the blood. The body will risk everything in its attempt to find fuel. Cells start dismantling themselves to find sources of glucose. 

Treatments

The more we learn about diabetes mellitus and our sugar-control systems, the more ways we will be able to help people who are living with this disease. People living with diabetes can use small, home-test kits to check their blood glucose levels. Medical alert dogs can also be trained to recognise when someone has high or low blood glucose levels. Being able to monitor blood glucose helps people work out how to manage their levels. And, most importantly, when they need to take action.

Insulin can be injected as a drug to raise its levels in the body. It is grown in genetically-modified bacteria and yeast for this purpose. Other drugs can increase insulin’s effectiveness, or work on other parts of the sugar control system to help reduce blood sugar.

Lifestyle changes also help. By learning to eat carefully, it’s possible to reduce how quickly blood glucose levels change after and between meals. This helps make it easier to keep the levels within safe limits by using drugs and other treatments.

For some people, changing lifestyle habits can also improve their insulin response. This can be done by changing diet, or becoming more active. Positive changes could be as simple as going for a walk, or taking the stairs instead of the lift.

Type 1 Diabetes

Type 1 diabetes is much less common than Type 2. It happens when there is no insulin at all. 
In people with Type 1 diabetes, their own body has attacked and killed the cells that normally make insulin (the islets of Langerhans in the pancreas). Nobody knows exactly why this happens. It’s not to do with diet or lifestyle. It’s a real medical mystery which a lot of researchers are busy trying to answer.

Type 1 diabetes often starts in childhood, although it can start at any time in life. It’s not a genetic disease – the children of parents with Type 1 diabetes don’t usually develop it themselves. They may be more at risk than other people, but this is also true of Type 2 diabetes.

The main treatment for Type 1 diabetes is to have injections of insulin. By injecting themselves, the person is doing the job of their missing pancreas cells. To keep their blood sugar level through the day, they need to work out how much to inject, and when to do so. They will usually have to inject themselves two to three times each day.

There are also insulin pumps which attach to the body and can release insulin as required.  

Eating carefully helps as it makes blood glucose changes slower and more predictable. And small blood-test kits are used to check levels to make sure everything is on track.

It’s important not to inject too much insulin, as low blood glucose comes with its own dangers. Low blood glucose symptoms include going pale, sweaty and shaky. People have anxiety and feel their heart beating strangely (palpitations). If blood glucose remains too low for too long, cells start running out of glucose for fuel. People may feel dizzy and confused, have fits and finally become unconscious.

In severe cases, it is possible to carry out a pancreas transplant to replace the lost cells. But transplant surgery is currently risky and difficult, so this is not a common treatment.

There is a lot still to learn about Type 1 diabetes, giving the hope for better treatments in future. Just understanding why it happens would be a huge step forward. 

Type 2 Diabetes and Insulin Resistance

Type 2 diabetes is by far the most common type of diabetes. About 90% of people with diabetes are Type 2. In Type 2 diabetes, the body stops responding properly to insulin. People living with Type 2 diabetes still have insulin, their cells just aren’t listening to its message.

Like with Type 1 diabetes, if your parents have it, then you are a bit more likely to get it too. But there are many other factors involved.  

People are a lot more likely to get Type 2 diabetes if they are overweight (especially around their belly) and not very active. It’s not impossible for slim and active people to get it, and not every overweight person will, but it just changes the probabilities. This helps explain why Type 2 diabetes has become so common in recent years – as more people become obese, it is not surprising that more people are developing the disease.

Type 2 diabetes can develop slowly over many years. This is good because it gives doctors a chance to spot it before it becomes a major problem. The main warning sign is that the body will start needing more and more insulin to get a reaction. This is called insulin resistance. The pancreas makes more insulin to make up for this, allowing people to control their blood sugar just enough that they don’t yet have diabetes mellitus. But at some point their pancreas won’t be able to keep up. Being able to identify insulin resistance early gives people the chance to change their lifestyles to reduce their chance of developing Type 2 diabetes.

Type 2 diabetes is treated with a variety of drugs and lifestyle changes. Changing diet can help avoid the big glucose highs that come after sugary meals, making it easier for the body to cope. There are drugs that increase insulin release, and others which can decrease insulin resistance (they make insulin work better). If this all isn’t enough, insulin can also be given. 

Research and the Future

There is a lot of current research into diabetes mellitus, with the very real hope that new and better treatments can be developed. We can also hope to find ways to reduce the number of people who develop Type 1 and Type 2 diabetes in the first place. The causes of Type 1 diabetes in particular are still a bit of a mystery – there is still a lot to learn.