Many people seeking to follow a healthier diet reduce their consumption of carbohydrates, especially sugar, and sometimes follow a specific ‘low-carb’ diet programme such as the ketogenic diet. We’ll look at the evidence about the suitability of these diets for people diagnosed with cancer.

The general advice for people diagnosed with cancer is to follow a healthy, balanced diet. The NHS has produced its ‘Eatwell’ guide to show what this includes and in what proportions. You can find the Eatwell guide here.

Carbohydrates fall into two categories:

• Simple carbohydrates (also called free sugars) found in foods such as sweets and ice cream, which contain one or two carbohydrate molecules joined together.
• Complex carbohydrates such as starch (made of longer chains of molecules) found in foods such as bread, pasta and potatoes. Complex carbohydrates in turn can be divided into processed or refined grains that have had their fibre removed (such as white bread) and wholegrain carbohydrates (such as wholemeal pasta) and potatoes (especially when eaten with the skin on) that contain fibre and other nutrients.

You can find more information about carbohydrates here.

As you can see from the Eatwell guide, the NHS recommends that a third of our diets should consist of starchy foods such as potatoes and grains, and another third of fruit and vegetables, which all contain carbohydrates. As well as carbohydrates, which are our bodies’ main source of energy, these foods also contain other nutrients such as vitamins, minerals, fibre and amino acids (the building blocks of proteins). Therefore, reducing or eliminating carbohydrates altogether will also have significant effects on the rest of your diet and nutrition.

All of the carbohydrates that we eat (apart from insoluble fibre, which passes through our bodies and helps waste to move through the bowels) are broken down through the process of digestion into glucose molecules that our cells use to produce energy.

Free sugars are digested and reach the bloodstream as glucose more quickly, whereas complex carbohydrates raise blood glucose more gradually. Any excess glucose that the cells don’t need for energy is stored as a molecule called glycogen. If the body’s glycogen stores are full, any extra glucose is converted to fat. You can read more about carbohydrates and their role in a healthy balanced diet here.

With regard to sugar and cancer, it is known that cancer cells undergo a change in their cellular machinery which means that they produce their energy in a different way from normal cells. This means that they need a lot of glucose in order to continue growing and multiplying. This is why some people believe that cutting down or eliminating sugar from their diets will prevent cancer from growing.

However, as we mentioned above, all the carbohydrates that we digest ultimately enter the bloodstream as glucose and there isn’t a way to stop only cancer cells from accessing glucose while maintaining the essential supply to healthy cells. You can read about this in more detail here.

There are other reasons to reduce our intake of foods that are high in sugar and/or processed or refined carbohydrates. The Eatwell guide says that foods high in salt, fat and sugar such as crisps, sugary drinks and chocolate aren’t needed in our diet and recommends eating these foods less often and in small amounts as a treat.

This is because they have limited nutritional value and if we eat too much of them they can lead to conditions such as obesity, high blood pressure and type II diabetes. However, it doesn’t mean that all sugars or carbohydrates are bad for us. As we mentioned earlier, many foods containing carbohydrates such as vegetables and whole grains also contain other important nutrients that our bodies need to stay healthy.

There are several different ‘low-carb’ diet programmes, and one that we are sometimes asked about is the ‘ketogenic diet,’ which is high in fat, moderate in protein and very low in carbohydrates. The aim of the diet is to cause cells to ‘switch’ from using glucose as their energy source to ‘ketones’ produced in the liver. It is sometimes used to treat intractable epilepsy in children and there is a current clinical trial investigating whether it may be helpful for glioblastoma, a type of brain tumour.

The ketogenic diet is very different from the healthy diet recommended by the NHS. It can also be challenging to follow. Not only does it involve keeping consumption of carbohydrates very low, but also careful planning to make sure that you get enough of the other nutrients that you would otherwise obtain from carbohydrate-rich foods. This can be complex and time-consuming and may make it more difficult for you to go out to eat socially if menus don’t include suitable dishes. Many people experience side effects, especially when they first start the diet and their bodies aren’t used to it, sometimes called ‘keto ‘flu.’ You can read about the experiences of a paediatric dietitian who tried the diet here.

Research hasn’t established a conclusive link between any particular food or diet and the risk of ovarian cancer. If you’re considering any significant dietary change, it’s important to seek medical advice as to whether it’s safe for you.

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If you have any questions or need any further information please contact our support service team on 0800 008 7054 or email [email protected]

Reviewed by Rhia Saggu, Specialist Dietitian at Imperial College Healthcare NHS Trust

V.1. Last reviewed November 2022, due for review November 2025

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