Does sugar feed cancer? Let's untangle the question

By Meredy Birdi, HCPC-registered Specialist Cancer Dietitian and Nutritional Therapist

Many people I meet after a cancer diagnosis have started looking at whether there are things they should change about the way they eat. Sugar is often one of the things they have read or heard about.

Some people cut down on sweets, cakes, biscuits and sugary drinks. I've also had many conversations with people who have stopped eating fruit, bread, potatoes or other carbohydrate foods because they're worried that the sugar in these foods might feed cancer or make it worse.

I completely understand how we have got here.

Cancer cells use glucose. PET scans use a glucose-like tracer. Insulin is involved in cell growth and signalling. There is research looking at metabolic health, inflammation and cancer. Ketogenic diets and other metabolic approaches are also being studied.

There's real science behind these questions. The difficulty is that several different questions have become tangled together. Add headlines and social media into the mix, alongside that feeling of wanting to do everything you possibly can after a cancer diagnosis, and foods we might once have eaten without thinking twice about can suddenly become something to analyse and overanalyse.

Is a banana too high in sugar? Is it okay to have this piece of cake? Should I avoid bread altogether?

People have told me that every mouthful can start to feel as though it's either helping or harming them. Food can become a series of pass-or-fail decisions.

I don't want nutrition to become another source of worry or pressure.

When we think about sugar and cancer, we need to separate out two distinct questions.

1. If I eat this piece of cake, dessert or biscuit, am I ‘feeding’ cancer and making it grow faster?

2. Can the way I repeatedly eat and live influence my metabolic health in ways that may be relevant to cancer?

They are both important questions, and they are not the same question. Let's start with the first.

Does eating sugar make cancer grow?

We do not have evidence in people that eating a sugar-containing food makes cancer cells grow faster.

Major cancer organisations are clear on this. The US National Cancer Institute says that no studies have shown that eating sugar will make cancer worse. World Cancer Research Fund, Macmillan Cancer Support, and Mayo Clinic similarly explain that eating sugar has not been shown to make cancer grow faster.

This doesn’t mean that our overall diet doesn’t matter. It absolutely does, and I’ll come back to that later. But we can stop asking one individual food to carry the whole weight of cancer nutrition.

If cancer cells use glucose, can we starve them of it?

Glucose isn't used only by cancer cells. It is an important source of energy throughout the body, including for healthy cells and tissues that rely heavily on it.

Our bodies regulate the amount of glucose circulating in the blood very carefully. Maintaining a supply of glucose is so important that when we don't get enough from food, the body can make it from other sources.

If we stop eating sugar, or substantially reduce carbohydrate, glucose doesn't disappear from the bloodstream.

We cannot starve cancer of glucose through diet alone.

There are many different types and subtypes of cancer, and their metabolism is not all the same. Cancer cells can use nutrients and metabolic pathways in varying ways, and can adapt to changes in their environment.

This is an active area of research, and I'll talk later about what this might mean for metabolic dietary approaches.

What do we mean when we say “sugar”?

When someone tells me they have “cut out sugar”, that can mean a number of things.

They may mean that they have stopped sweets, cakes, biscuits and sugary drinks. They may also have stopped eating fruit, bread, potatoes, pasta or other carbohydrate foods.

Sugar is one type of carbohydrate. Fruit and vegetables contain naturally occurring sugars, while milk and yoghurt contain a sugar called lactose. Free sugars include sugars added to foods and drinks, as well as those naturally present in honey and syrups. The sugars in fruit juice and smoothies are also classed as free sugars because processing releases them from the structure of the fruit.

Starch is another type of carbohydrate. It is found in foods such as bread, potatoes, rice, pasta, oats, beans and lentils, and is broken down into glucose during digestion.

These foods are not the same nutritionally.

A banana and a sugary drink such as cola or lemonade both contain sugar. A banana brings fibre, vitamins, minerals, and naturally occurring plant compounds. A sugary drink typically provides a large amount of free sugar without fibre and with very little nutritional value alongside it.

This is also where the overall balance of our diet comes in. If foods and drinks high in free sugars make up a large part of what we regularly eat, they can leave less room for foods that bring fibre, protein, vitamins, minerals and beneficial plant compounds.

Removing all foods that contain sugar or carbohydrate can mean missing out on foods, and nutritional benefits, that we have very good reasons to include.

If cancer cells light up on a PET scan, doesn't that show they feed on glucose?

This is a common concern, and I can understand why. A PET scan gives us a very visible demonstration that many cancer cells use glucose.

For the most common type of PET scan, a small amount of a radioactive form of glucose called FDG is injected into the bloodstream. Cancer cells are often more active than normal cells and take up more of the tracer, which helps them show up on the scan. Macmillan Cancer Support explains this too: more active cells use more glucose, which is why they take up more of the tracer.

Taking up more glucose is not the same as growing faster because more glucose is available.

A PET scan is showing us something about the activity of those cells and the way they use energy. It isn't telling us what happens to cancer growth when we eat a sugar-containing food. Mayo Clinic also makes this point.

Does that mean I don't need to think about sugar at all?

This brings us back to the second question we started with: can the way we repeatedly eat and live influence our metabolic health in ways that may be relevant to cancer?

This is where we need to zoom out from the individual food.

There is a growing body of research looking at cancer and metabolic health, including insulin resistance and persistently high insulin levels, blood glucose regulation, body composition and chronic systemic inflammation.

A metabolic picture develops over time. It isn't created by one food, one nutrient, one meal or one day. And it isn't shaped by food alone.

A metabolic state is not a food.

A piece of cake is not hyperinsulinaemia. A banana is not insulin resistance. Eating sugar is not synonymous with chronic systemic inflammation.

Insulin is an essential hormone that helps regulate blood glucose and also has roles in cell growth and signalling. A rise in insulin after we eat is part of normal physiology. Research in relation to cancer is looking at insulin and glucose metabolism in a number of ways, including what happens when normal metabolic regulation becomes disrupted. This includes insulin resistance, persistently high insulin levels and impaired blood glucose regulation over time.

A normal and natural response to eating is not the same as persistently high insulin levels.

Chronic systemic inflammation is also relevant to cancer biology, and our usual pattern of eating can play a part in levels of inflammation in the body. Again, this is about our diet as a whole rather than an individual food.

Our metabolic health is influenced by many interacting factors. Our diet is part of that, alongside physical activity, muscle and body composition, sleep and other aspects of our health.

Muscle is metabolically active tissue and plays an important role in glucose metabolism and insulin sensitivity, which is one of the reasons maintaining and using our muscles is valuable for metabolic health too.

We can get hung up on an individual biscuit or piece of cake, while losing sight of how we're eating overall, moving our bodies, nourishing ourselves and living over time.

Sugar can be part of the picture. It isn't the whole picture.

What does this look like in practice?

In terms of nutrition and metabolic health, I'm much more interested in somebody's usual pattern of eating than in occasional foods.

We are what we repeatedly eat.

Our usual dietary pattern is where things like fibre, plant diversity, protein, fat quality and the amount and type of carbohydrate we eat start to come together.

If your treatment, symptoms and nutritional needs allow, I would lean into whole fruit, plenty of colourful vegetables, wholegrain and higher-fibre carbohydrates, beans and lentils, nuts and seeds.

These foods bring so much to our diet: different types of fibre, vitamins, minerals and a huge variety of beneficial plant compounds, including polyphenols. They help nourish our gut microbiome and provide nutrients that support immune function. There are always ways we can shape this around what you can comfortably and safely manage.

I also think about what we eat alongside our carbohydrates. Including protein, fibre and beneficial fats can help create more balanced meals and snacks, rather than focusing on a carbohydrate food in isolation.

Protein is one of my ‘North Stars’. It's one of the foundations I come back to with my clients, particularly during and after cancer treatment, when maintaining muscle mass is an important priority.

Where do free sugars fit?

The advice here isn't different because somebody has cancer. I wouldn't recommend making sugary drinks, sweets, cakes and biscuits the foundation of anybody's usual diet when there are so many more nutritionally valuable foods to eat regularly.

Food is not an Excel spreadsheet of nutrients that we need to tick off perfectly. Eating brings pleasure, connection, culture, celebration and normality too. These are a meaningful part of our lives. We don't want to suck the joy out of eating and drinking.

My conversations about cancer nutrition are not led by what we can remove. I focus much more on what we can add because of the benefits it brings. As we build in more of the foods we want to eat regularly, a natural order starts to develop.

There is so much more we can do with nutrition than tell somebody not to eat sugar.

What if I'm losing weight or struggling to eat?

This is where nutrition has a hierarchy.

If you're losing weight and muscle, struggling with your appetite or finding it difficult to eat because of treatment or symptoms, we need to be flexible and adapt. Getting enough energy and protein to nourish you and help protect your muscle mass, strength and resilience becomes especially important. 

This absolutely doesn't mean we need to abandon a nourishing diet. We can make the food you are able to eat work harder for you, perhaps by adding olive oil, avocado, nuts or smooth nut butters, enriching foods with yoghurt or cheese, or choosing nourishing drinks.

What works will depend on your symptoms and what you can comfortably manage.

There may also be days when eating is really difficult. Maybe a crumpet with jam is one of the few things that feels possible.

I would reframe that as wisdom, not failure.

You're responding to what is possible that day, finding something you can manage and getting some nutrition on board.

There may be opportunities to add extra nourishment where you can, but we don't need to turn every mouthful into another task.

The right immediate nutritional priority for somebody losing weight and muscle may be different from the priority for somebody who is eating well, weight stable and looking at their longer-term metabolic health.

What about ketogenic diets and other metabolic dietary approaches?

You might be wondering: if we can't starve cancer of glucose by removing sugar and carbohydrate from our diet, why are researchers studying ketogenic diets and other metabolic dietary approaches?

Cancer metabolism is an active and evolving area of research. It involves much more than glucose, including the different ways cancer cells obtain and use nutrients, respond to signals and adapt to their surroundings. Different cancers can have different metabolic characteristics, and researchers are exploring whether some of these might create vulnerabilities that could potentially be targeted, including through nutrition.

This is where it's important to separate evidence about potential mechanisms from evidence of results. Research has identified potential mechanisms through which diet and nutrition could influence cancer biology. But identifying a potential mechanism doesn't tell us whether a dietary strategy based on it will translate into better clinical outcomes in people.

If a particular approach is shown to be useful, we will need to understand which specific cancers, subtypes or treatments it is relevant to. What would it mean in terms of foods and amounts? Would it need to be done alongside something else? How long would you do it for? Would anything need to be measured or monitored?

We also need to consider the potential downsides of a more restrictive dietary approach, particularly at a time when eating and meeting nutritional needs can be difficult. Restricting certain foods may mean missing out on some of the wider benefits they bring, beyond their individual nutrients. If an approach makes it harder to meet nutritional needs or contributes to loss of muscle mass, that's clinically significant.

Maintaining muscle mass and staying physically active are important parts of cancer care from day one. They can help support strength and physical function, treatment tolerance and recovery.

I'm keeping a close eye on this area of research to see where the science takes us.

Putting sugar and cancer into perspective

If you've come to this article worried about sugar and cancer, I hope you can step back from feeling that every mouthful needs to be analysed. It doesn't work like that.

The way we repeatedly eat influences our nutritional and metabolic health over time. There is so much we can positively do with nutrition to nourish ourselves, support our muscle and strength, and adapt as our needs change.

When we look at the bigger picture, sugar starts to take its proper place within it. It doesn't need to carry all of our attention, or all of our fear.

If worries about sugar or other aspects of your diet are making it difficult to know what to eat, I can help. Together, we can look at your individual needs, address your concerns and make sure you're getting the nutrition and support that's right for you.

Find out about my personalised nutrition support programmes.

This blog is for general information only and is not a substitute for personalised medical or dietetic advice.