Why is sugar bad for you?

coffee cup and sugar dispenserWhen I was a child the only bad thing I ever remember hearing about eating sugar was that it would give you bad teeth. Occasionally someone might say you would get fat if you ate too much, but not very often.

Now it seems that sugar is about the worst thing you can eat. We constantly hear about how toxic sugar is, and how it can cause all kinds of health problems, including diabetes, liver disease and cancer.

So what is it about sugar that is so bad for you? Let’s start with a look at some different types of sugar.

What types of sugar are there?

As you will know, sugar is a kind of carbohydrate, and carbohydrates are an important source of energy in your body. Green plants trap the sun’s energy when they make sugars, and the metabolism in your body cells releases it again for us to use.

There are two main types of carbohydrate: complex carbohydrates and simple carbohydrates. The major difference between the two is that complex carbohydrates need to be digested and simple carbohydrates don’t (or at least not very much).

Sugars (and carbohydrates generally) are often described by how many units there are in the molecule.

If there’s one unit it’s a monosaccharide. If it’s two units joined together it’s a disaccharide. If it’s lots of units joined together it’s a polysaccharide.

The monosaccharides (one unit) and disaccharides (two units) are what are usually described as simple sugars. Polysaccharides (lots of units) are the complex carbohydrates.

A lot of the dietary advice about carbohydrates (or carbs for short) says that we should eat more complex carbohydrates and less simple sugars.

Simple carbohydrates

You will have heard of some of the monosaccharides, or single sugars. Examples include glucose (which is called dextrose in medical circles for some reason) and fructose.

Glucose is found in glucose syrup (surprise, surprise) and fructose is found in fruit, so it is sometimes called fruit sugar.

Disaccharides consist of two single sugars joined together. The disaccharide we consume most often is Sucrose, which is one unit of Glucose joined to one unit of Fructose.

When you eat food containing these simple carbohydrates the sugars are already so small that they are absorbed through the walls of your intestines into your bloodstream very quickly. No need for them to be broken down by digestive enzymes.

It’s for this reason that food containing a lot of simple sugars causes your blood sugar to rise very quickly after eating it. This can lead to what is sometimes called a “sugar rush”.

Complex carbohydrates

The main type of complex carbohydrate that we eat is Starch. After plants have made sugar (by trapping the sun’s energy) they store it as starch. The sources of Starch in the food we eat are the places that plants store energy, including corn, wheat, rice, and potatoes.

Starch is made from a large number (millions and millions) of Glucose units joined together. Before the Glucose in the Starch can be absorbed into the bloodstream it needs to be released by digestive enzymes in the intestines.

This means that complex carbohydrates cause your blood Glucose to rise more gradually after eating them. Which means that your blood glucose is easier to control.

Control of blood sugar

Carbohydrates are an important source of energy for the body, and Glucose in the blood is the main way this source of energy is delivered to the body’s cells.

The level of Glucose in the blood needs to be carefully controlled within normal limits.

Low blood sugar symptoms

If your blood Glucose gets too low, this can be an immediate problem. Your brain needs a constant supply of glucose to supply the cells with the energy they need.

Everyone knows that your brain can be damaged if is it deprived of oxygen for two minutes. But what do you think the oxygen is for? It’s to help “burn’ the energy stored in glucose to keep the brain cells working.

If your blood glucose level is too low to deliver enough to your brain you can feel dizzy and pass out. If your blood sugar gets low enough you can have a seizure (similar to an epileptic seizure), go into a coma, or even die.

So why is sugar bad for you?

It seems that sugar is very important for providing an immediate source of energy for our body cells, so how can sugar be bad for you?

As usual with these things, it’s too much sugar that’s bad for you.

Well, way back in the past sugar, like many types of food, was quite hard to come by. We are designed for periods when we do have enough to eat, that might be followed by periods when we don’t have enough.

The problem nowadays is that there is so much food available, and the types of food that have become more available often contain a lot of sugar.

The other big problem with sugar is that in the past you had to eat a lot of rice, wheat, potatoes etc to get a lot of sugar. Now sugar is refined and purified by food companies, so in most developed countries you can eat a lot of simple sugar very easily. And that’s bad!

There are various types of refined simple sugars produced by food companies, and we consume them in different ways. As we’ll see, some are much worse than others.


Sucrose is the type of sugar you are most likely to come across in a packet of sugar. It’s sometimes called table sugar and people put it in their tea or coffee, and use it when baking.

As we saw earlier, sucrose is a disaccharide (two sugar units joined together), and is made from glucose joined to fructose. Digestive enzymes break the sucrose down and as the glucose is absorbed the level of glucose in the blood rises.

High blood sugar symptoms

Unlike low blood glucose, high blood Glucose tends to be a more long-term problem (although not always).

One of the main problems caused by high levels of blood glucose is that the sugar sticks to blood cells and blood vessel walls.

Normal levels of blood Glucose causes normal amounts of glucose to stick. So everything still functions normally.

High levels of blood blood Glucose causes high levels of Glucose to stick. This can lead to blood vessel damage, which increases your risk of heart disease, stroke, impaired blood circulation, nerve, eye and kidney damage.

Insulin resistance

Rising blood glucose causes the pancreas to release insulin, which causes the body’s organ cells to take up the glucose from the blood, causing the level to come back down. This is part of normal blood glucose control.

Eating (and drinking) too much sucrose can cause problems because your blood glucose is constantly being added to from the excess sugar you eat and drink, which causes high blood sugar (hyperglycaemia).

The pancreas releases insulin to lower this raised blood sugar. However, now there’s a problem.

Since your blood glucose is constantly being topped up from the sucrose you eat and drink, high levels of insulin are constantly being produced.

If you were to take a drug constantly your body would become tolerant, or resistant, to it and it wouldn’t have the same effect after a while.

A similar thing happens when blood levels of insulin are constantly high. Your body becomes tolerant to it to it, and doesn’t respond to it any more. This is called insulin resistance.

Insulin resistance means that your pancreas is producing plenty of insulin, but your body isn’t responding to it. This means that your blood glucose stays high, which is called hyperglycaemia. This can lead to type 2 diabetes mellitus.


Fructose is found naturally in fruits and vegetables, and for this reason it is often considered to be a “good” sugar. In the past you would have to eat a lot of fruit and vegetables to get very much fructose.

The problem now is that food companies purify and refine the fructose and use it to sweeten various kinds of food and sweet drinks. The main form of this purified fructose is high-fructose corn syrup, which is very cheap to produce and is widely used in food and drink production.

Fructose is actually sweeter than glucose or sucrose, making it appeal to children, which is a problem in itself. Sweet tasting foods and drinks are highly addictive and people can start to crave more and more of it.

We saw above that the some of the main problems with glucose and sucrose are that they can cause in increase in blood sugar levels, which can lead to all sorts of health problems like type 2 diabetes. Fructose can affect your health in other ways.

Fructose absorbed into the bloodstream can’t enter most types of cells, and it is mostly broken down in the liver. The process of breaking down the fructose results in the production of fat, which can build up in the liver and cause liver disease.

The fat produced from fructose can also lead to obesity, diabetes mellitus, and increase your blood cholesterol and your risk of heart disease.

Sugar and cancer

There has been a lot of speculation lately about sugar feeding cancer cells, making tumours grow faster. Some studies have actually found that sugar can cause cancer, and if you deprive cancer cells of sugar they will go back to normal.

The research on this seems to relate to some differences in the way cancer cells metabolise glucose, rather than any suggestion that eating more sugar can cause cancer.

There does seem to be a link between insulin resistance and cancer risk, and insulin resistance can be caused by eating too much sugar.

Eat less sugar

So, to sum up what’s been written here, we need to eat less sugar. That seems to be pretty clear.

To do this we need to be aware of all the hidden sugar in the food that we eat. Here’s a short video with some tips on sugar words to look out for to get you started.

The video is from a website aimed at improving children’s health, and you can see the the whole page here.

And, if you insist on eating sugar, don’t forget to brush your teeth.

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