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  • Joanna Injore

Does sugar feed cancer?

There are so many confusing messages about sugar and cancer. I am often asked; does sugar feed cancer? If I eat sugar will it cause the cancer to grow? Isn’t sugar bad for you?

In this blog I will explore the facts about sugar, where the link between sugar and cancer started and whether there is any truth in it.

What is sugar?

Before we examine the science around sugar and cancer let’s talk about sugar. What is sugar? What are the different types and what does that mean for our health.

Simple sugars are a type of carbohydrate which chemical name is monosaccharides. Mono- means single which means these sugars only contain one sugar unit and cannot be broken down further. In foods you will see that these are labelled as glucose, fructose and galactose.

When two sugar units join together they form a disaccharides. The following are examples that you may be familiar with:

Sucrose - made of glucose and fructose

Lactose – made of glucose and galactose

Maltose – made of two glucose units

When more sugar units join together to make longer sugar chains they are called polysaccharides or ‘complex carbohydrates’ which are found in starchy food such as bread, pasta and rice.

sugar sucrose glucose frutose lactose maltose
Most common types of sugar

When you read about sugars they are often described as ‘free sugars’ or non free sugars.

Free sugars

Free sugar is sugar added to food or drinks (usually in the form of sucrose and glucose) but also included in this is the sugar that is in honey, syrup and fruit juice (1). The reason they are called free sugars is because they are ‘free’ and not already contained in the cells of the food.

What about other sugars?

As you see above there are sugars in fruit, vegetables and milk. These sugars are still contained in food cells (so not as quickly broken down in the body) and usually come with extra nutrients like fibre, vitamins and minerals. These sugars are thought to be not as harmful as free sugars as they come with the benefits of extra nutrients.

So you see, sugar is in everything that we eat, but that is useful. Our bodies need glucose to convert it into energy, which is the fuel for all our body cells.

Sugar and cancer what's the connection?

The myth around sugar being harmful for cancer patients comes from early research looking at cancer cells in the lab. These test-tube studies found that cancer cells thrived when given glucose (2) called the ‘Warburg effect’ (after the scientist that discovered it). This fuelled the idea that reducing sugar would prevent cancer cells from growing if they were in a low/no glucose environment.

However, looking at cancer cells in a test tube is quite different to investigating cancer in the human bodies. Therefore, we cannot apply findings from test-tube studies to humans until further research is conducted. It is also important to remember that all human body cells need glucose to function and it is nearly impossible to restrict all glucose in your diet.

A lot of human studies in cancer patients have looked at ketogenic diets; which is a high-fat and low-carbohydrate diet. The idea behind this approach is that a ketogenic diet produces a lower blood glucose level, so there is less circulating glucose for cancer cells to use as their energy source. However, a large review (3) of studies in this area found limited evidence for ketogenic diets in cancer patients.

The ketogenic diets are also very restrictive which could cause weight loss or unpleasant side effects such as diarrhoea and constipation, vomiting or fatigue (3). Cancer treatment is very tough on the body and cause side effects that could affect your eating and appetite. If you then follow a restrictive diet which does not provide enough energy and nutrients your body needs, this increases your risk of weight and muscle loss which could affect your recovery and tolerance to treatment.

What about sugar and cancer prevention?

There is an indirect link with sugar and cancer but that is because diets high in sugar can lead to weight gain. Obesity and being overweight is linked to some cancers so it is a good idea to limit sugar from sweets, cakes, fizzy drinks but no need to cut out completely!



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