Does sugar cause cancer? Unpacking the sugar causes cancer myth

Sugar causes cancer… or does it?

The theory that sugar in your diet increases the chances of cancer developing crops up regularly. Where does this idea come from, and what does the evidence tell us?

Hannah Cook, Registered Dietitian, answers our questions.

What's behind the idea that sugar feeds cancer, and is there an evidence base for this?

There's no evidence behind this! I can see why this myth began, though. Glucose (sugar) is a source of fuel for our bodies that is broken down rapidly (much quicker than more complex carbohydrates, protein, fibre or fat, for example). Therefore, because cancer cells grow rapidly and require lots of energy to continue growing, it is a common misconception that sugar would feed cancer cells. But the reality is that cancer cells will use any fuel source... they’re not fussy! So, if all that was available in the body was fat cells, the body is able to convert the fat into a usable source of energy, and the cancer cells would use this to keep growing. Equally, if there is not enough energy or calories consumed, the body will start breaking down its own muscle to keep its fuel source up. So, in essence, cancer cells will break down (metabolise) any fuel source it can to allow itself to keep growing... which is why seeking treatment and advice from an oncologist is essential. 

Is there any truth in the idea that PET scans show cancer because the cancer cells feed on sugar?

A great question and one that comes up really frequently! There is no evidence behind this. PET scans pick up highly metabolically active tissues within our bodies. Cancer cells have a faster cell turnover, and they grow quicker than other cells in our body - meaning that they are more metabolically active than most structures in our body. 

During a PET scan, a radioactive substance containing glucose (sugar) is used so that metabolically active structures can be seen on the scans. As the structures utilise the glucose, the cells then become visible on the scan. Glucose is used because it is the quickest fuel to be broken down (as described above). Really, it just saves you needing to be in the scan for too long. If another substance was used in the radioactive formula, it would have the same impact; you'd just be in the scan for longer!

What is the advice for people worried about their diet causing cancer?
The best advice is to enjoy a healthy, balanced diet where possible. Sweet treats (biscuits, chocolate, ice cream etc) should be included in a balanced diet, but should ideally only make up a small proportion of your daily diet. 

What is the general dietary advice for someone going through cancer treatment? Are there any foods to cut out when going through cancer treatment?

This is a really tricky question to answer directly, as it depends on the type of cancer treatment: chemotherapy/radiotherapy/immunotherapy/surgery, etc and the type of cancer the individual has. Generally, please do not cut any foods or food groups out unless advised by a healthcare professional. 

The general advice for those who are not losing weight is to continue a balanced diet, just as we'd recommend for the general population (plenty of fruit and veg, heart-healthy fats like avocado and olive oil and good sources of protein such as dairy products, beans/lentils/pulses, meat, poultry and fish). Those who may be struggling with weight loss or low body weight and wanting to try to stabilise their weight should also continue on the above diet, but may need to add greater proportions of higher protein and higher calorie foods. 

For some great information on eating well when you have cancer, I’d recommend this guide by the World Cancer Research Fund. Also have a look at your local NHS trust websites; most will have a section on nutrition.

What should people do if they want to know more about diet and cancer?
Please ask your consultant or GP for a referral to a dietitian if you'd like some personalised advice. You can also take a look on the British Dietetic Association for helpful resources or to find a dietitian locally to you: https://www.bda.uk.com/about-dietetics/what-is-dietitian.html

For some other examples of the facts behind diet myths, see this article by the British Dietetic Association.