Cancer and Nutrition: 8 Diet Myths Explained by the Evidence
Updated: Oct 14
Written by Rosy Allan, BSc (Hons), ANutr
Registered Associate Nutritionist | Level 3 Personal Trainer | Level 4 Cancer Exercise Rehabilitation Specialist
A cancer diagnosis can change the way you think about food.
Suddenly, everyday decisions about what to eat can feel much more significant. You may find yourself questioning whether sugar is harmful, whether certain foods could improve your recovery or whether you should be taking supplements.
And there's no shortage of advice.
Friends, family, social media and online health websites may all offer suggestions, often with the best intentions. Unfortunately, not all that information is supported by scientific evidence.
Having experienced cancer treatment myself, I understand the desire to do everything possible to protect your health.
But nutrition advice should provide clarity and support, not create unnecessary guilt or anxiety.
Let's explore eight common cancer-related diet myths, what the research actually tells us and how to apply that knowledge in everyday life.
Myth 1: Sugar feeds cancer, so you should avoid it completely
This is probably one of the most persistent misconceptions surrounding cancer and nutrition.
The idea comes from the fact that cancer cells use glucose for energy, sometimes at a higher rate than many healthy cells. But there's a crucial detail missing from this explanation. Healthy cells also require glucose.
When we eat carbohydrates, whether from bread, fruit, rice, potatoes or sugary foods, the body digests and processes them to provide energy.
The body also regulates blood glucose and can produce glucose when dietary carbohydrate intake is low. This means eliminating sugar from your diet does not selectively deprive cancer cells of energy.
Cancer Research UK explains that there is no evidence that following a sugar-free diet improves cancer survival or prevents cancer from developing (Cancer Research UK, 2023).
However, that doesn't mean the amount of sugar we consume is irrelevant to health.
Regularly consuming large amounts of foods and drinks high in free sugars can contribute to excess energy intake and weight gain. Excess body fat is an established risk factor for several cancers (World Cancer Research Fund, n.d.-a).
What can you do?
You don't need to eliminate fruit, carbohydrates or the occasional dessert because of a cancer diagnosis.
Instead, focus on overall dietary quality, including nutritious carbohydrate sources such as oats, potatoes, wholegrains, beans and fruit.
If you're undergoing treatment and struggling to eat enough, maintaining an adequate energy intake may be particularly important. Unnecessary food restrictions can make this more difficult (Macmillan Cancer Support, n.d.-a).
Myth 2: Superfoods can prevent or cure cancer
Blueberries, turmeric, green tea, kale and numerous other foods are regularly promoted for their supposed cancer-fighting properties.
Many of these foods contain valuable nutrients or naturally occurring compounds that researchers are interested in studying.
For example, fruit and vegetables can provide fibre, vitamins, minerals and phytochemicals.
However, research demonstrating an effect in isolated cells or laboratory animals does not automatically mean eating that food will produce the same effect in humans.
No individual food has been demonstrated to cure cancer.
Cancer Research UK advises that no single food or ingredient can prevent cancer, although eating a healthy, balanced diet can help reduce the risk of developing certain cancers (Cancer Research UK, n.d.-a).
What can you do?
Rather than purchasing expensive powders, juices or products marketed as superfoods, aim for variety.
Include different vegetables, fruits, wholegrains and sources of protein in your usual meals.
For example, frozen berries can be just as convenient as fresh ones, and ordinary vegetables such as carrots, cabbage and broccoli can all contribute valuable nutrients.
You don't need exotic ingredients to eat well.
Myth 3: Organic foods are always better for cancer prevention
Some people choose organic foods because of environmental concerns, farming practices or personal preferences.
However, organic food is sometimes marketed as though it offers additional protection against cancer.
The evidence is less straightforward.
Research comparing organic and conventionally produced food has produced mixed findings.
The World Cancer Research Fund notes that observational studies have reported different associations between organic food consumption and cancer risk. However, these studies cannot reliably establish that choosing organic food itself caused any differences in cancer incidence (World Cancer Research Fund, n.d.-b).
Other factors, including broader dietary habits and socioeconomic circumstances, may influence the results.
There is currently insufficient evidence to conclude that organic foods provide greater protection against cancer than conventionally grown foods.
What can you do?
Don't let the price of organic produce discourage you from eating fruit and vegetables.
Fresh, frozen and tinned vegetables can all contribute towards a balanced diet. Choose what is affordable, accessible and practical for you.
Wash fresh produce appropriately, and follow any additional food-safety advice given by your oncology team if your immune function is significantly reduced (Macmillan Cancer Support, n.d.-b).
A healthy diet shouldn't depend on having a large food budget.
Myth 4: You must completely avoid red meat after a cancer diagnosis
This is an area where it's particularly important to distinguish between exaggerated advice and genuine scientific evidence.
Research has established a relationship between red and processed meat consumption and colorectal cancer.
The World Cancer Research Fund concludes that there is convincing evidence that processed meat causes colorectal cancer and that red meat is probably a cause (World Cancer Research Fund, n.d.-c).
Processed meat includes products such as bacon, ham and sausages that have been preserved through processes including curing, salting or smoking.
Red meat includes beef, lamb and pork.
However, these findings do not mean that eating one serving of red meat will inevitably cause cancer.
Cancer risk is influenced by numerous factors, and the evidence primarily concerns dietary patterns and long-term exposure.
What do the recommendations say?
For cancer prevention, the World Cancer Research Fund recommends:
Limiting red meat to approximately 350–500g cooked weight per week, or around three portions.
Eating very little, if any, processed meat (World Cancer Research Fund, n.d.-c).
Red meat can provide useful nutrients, including protein, iron, zinc and vitamin B12.
Whether it should be included in someone's diet depends on their nutritional needs and preferences.
What can you do?
If you regularly eat red meat, consider alternating it with other protein sources.
For example, you could replace some of the beef mince in a spaghetti bolognese with lentils, or prepare meals using chicken, fish, tofu or beans.
You don't necessarily need to eliminate red meat entirely to improve your dietary pattern.
During cancer treatment, people with poor appetite, unintentional weight loss or other nutritional problems may require different priorities from those used in general cancer prevention guidance. These should be individualised with the clinical team (Macmillan Cancer Support, n.d.-a).
Myth 5: Dairy products cause cancer
Milk, yoghurt and cheese are sometimes portrayed as foods that should be avoided after a cancer diagnosis.
Concerns often centre on hormones naturally present in milk or the suggestion that dairy products promote tumour growth.
However, the available evidence does not support a blanket recommendation to eliminate dairy.
Cancer Research UK reports that dairy consumption is associated with a reduced risk of colorectal cancer, while there is no consistent, reliable evidence that dairy consumption increases overall cancer risk (Cancer Research UK, n.d.-c).
Research into some individual cancers, particularly prostate cancer, is more complex, and further high-quality studies are needed.
Dairy products can also contribute protein, calcium and other important nutrients.
What can you do?
If you enjoy dairy and tolerate it well, there's generally no reason to eliminate it solely because of cancer-related claims.
Plain yoghurt, milk and cheese can contribute to a balanced diet.
If you prefer plant-based alternatives, consider unsweetened products fortified with calcium and other relevant nutrients.
Of course, some people may need to avoid or modify dairy intake because of allergies, lactose intolerance, digestive symptoms or individual medical advice.
The important point is that unnecessary exclusion isn't the same as better nutrition.
Myth 6: An alkaline diet can prevent or treat cancer
The alkaline diet is based on the idea that certain foods make the body more acidic, potentially creating an environment in which cancer can develop.
Followers are often encouraged to avoid supposedly acidic foods and consume more alkaline foods, particularly vegetables and fruit.
However, the human body carefully regulates blood pH through physiological systems involving the lungs and kidneys.
Although diet can influence urine pH, this does not mean eating particular foods meaningfully changes blood pH.
Cancer Research UK explains that there is no good evidence that alkaline diets prevent or cure cancer (Cancer Research UK, n.d.-a).
What can you do?
Eating plenty of vegetables and fruit is beneficial, but not because they make your body alkaline.
Their benefits come from their nutritional contribution to your overall diet.
You don't need to purchase alkaline water, avoid ordinary foods because of their acidity or follow restrictive pH-based meal plans.
Myth 7: Fasting can starve cancer cells and improve treatment outcomes
Intermittent fasting has attracted interest from researchers investigating metabolism and cancer treatment.
Some laboratory and early clinical studies have explored whether fasting or fasting-mimicking diets might influence treatment responses or side effects.
However, the evidence is not sufficient to recommend fasting as a cancer treatment.
Importantly, fasting has not been established as a way to starve cancer cells or cure cancer.
Cancer Research UK explains that fasting can be unsuitable for people living with cancer, particularly those experiencing low body weight, unintentional weight loss or other health complications (Cancer Research UK, n.d.-b).
Macmillan also advises that the suitability and safety of fasting depend on the individual's circumstances and should be discussed with the treating healthcare team (Macmillan Cancer Support, 2025).
This is especially important because cancer and its treatment can increase nutritional vulnerability.
What can you do?
If you're undergoing treatment, avoid introducing a restrictive fasting routine without discussing it with your oncology team.
If you're experiencing nausea, poor appetite or weight loss, regular meals or smaller, more frequent eating opportunities may be more appropriate.
Some people may be able to follow particular eating schedules safely, but that does not mean fasting is beneficial as a cancer therapy.
For anyone at risk of malnutrition, nutritional assessment by an oncology dietitian should take priority.
Myth 8: Supplements can prevent cancer or stop it returning
It's understandable that someone facing cancer might want to explore every possible way of protecting their health.
The supplement industry offers countless products promising antioxidant protection, immune support or improvements in recovery.
But more isn't always better.
Macmillan Cancer Support explains that there is no good evidence that dietary supplements generally prevent cancer, treat cancer or stop it returning (Macmillan Cancer Support, n.d.-a).
Some high-dose supplements may also interfere with cancer treatments or increase the risk of harm.
For example, there is concern that high-dose antioxidant supplements could interfere with the intended effects of certain treatments, although the risks depend on the supplement and clinical circumstances.
This is different from obtaining antioxidants naturally through ordinary foods.
Does that mean nobody with cancer should take supplements?
No.
There are circumstances where supplements are medically appropriate.
For example, someone with a diagnosed deficiency may need a prescribed vitamin or mineral supplement.
Others may require nutritional supplements because they cannot meet their energy or protein requirements through normal food intake.
Some treatments also create additional bone-health considerations.
The key distinction is between correcting a nutritional need and taking high-dose supplements in the hope of treating cancer.
What can you do?
Before taking a new supplement, particularly during active cancer treatment, check with your oncologist, specialist nurse, oncology pharmacist or dietitian.
Bring a list of everything you're taking, including herbal products, protein powders, multivitamins and over-the-counter supplements.
This makes it easier for your healthcare team to identify potential interactions or unnecessary duplication.
So what should you actually eat during cancer treatment?
After exploring these myths, you might reasonably wonder what dietary advice you should follow.
The answer depends on your diagnosis, treatment, nutritional status and symptoms.
For someone who is eating well, maintaining their weight and experiencing few dietary difficulties, a varied and balanced diet may be appropriate.
However, someone experiencing significant weight loss, nausea, difficulty swallowing, digestive problems or malnutrition may need a very different approach.
Macmillan Cancer Support emphasises that there is no single diet suitable for everybody with cancer, and that dietary modifications may be necessary according to treatment effects and individual needs (Macmillan Cancer Support, n.d.-a).
A practical starting point
If you're currently receiving treatment, consider these priorities:
1. Eat enough to meet your needs. Unintentional weight loss and poor appetite deserve attention. Maintaining adequate nutritional intake may be more important than trying to lose body fat.
2. Include protein regularly. Foods such as eggs, fish, dairy products, meat, tofu and legumes can contribute to protein intake, which is important for maintaining body tissues.
3. Make eating manageable. Ready meals, frozen vegetables, microwave rice and other convenient foods can be valuable when fatigue makes cooking difficult.
4. Adapt food choices around symptoms. Taste changes, mouth soreness, nausea and bowel problems may require temporary changes in what and how you eat.
5. Ask for specialist advice when needed. If you're struggling to meet your nutritional needs, request referral to an oncology dietitian.
You don't need to follow every wellness trend or prepare every meal from scratch.
Sometimes the most helpful nutrition strategy is simply finding foods you can tolerate, enjoy and eat consistently.
A personal perspective
When you've experienced cancer yourself, you understand how easily dietary advice can
become another source of pressure.
People often want to help, but being told to avoid sugar, buy expensive supplements or follow a particular diet can leave you questioning everything you eat.
I believe nutrition advice should make life easier, not more complicated.
As a Registered Associate Nutritionist, my role is to explain the available evidence, support appropriate everyday dietary choices and recognise when specialist clinical nutrition input is required.
There is considerable value in helping people understand what they don't need to worry about.
Being able to enjoy food without believing every ingredient is influencing your cancer is important too.
Final thoughts
Nutrition has an important role in overall health, cancer prevention and supporting people during and after treatment.
However, the relationship between diet and cancer is complex.
Research does not support claims that eliminating sugar, following an alkaline diet, fasting or taking high-dose supplements can cure cancer.
There is stronger evidence supporting broader dietary patterns, including regular consumption of wholegrains, vegetables, fruit and beans, alongside limiting red and processed meat for cancer prevention (World Cancer Research Fund, n.d.-a).
During treatment, nutritional advice must also account for appetite, symptoms, body weight and the individual's clinical needs.
Good nutrition isn't about finding one perfect food or eliminating everything you've been told to fear. It's about making informed decisions that support your health and circumstances.
At ROSY, I provide evidence-based nutrition education and specialist cancer exercise rehabilitation, including online support throughout the UK and in-person rehabilitation in Bristol.
For complex nutritional problems during cancer treatment, I work within my professional scope and recommend appropriate clinical dietetic support.
Disclaimer: This article is intended for general educational purposes and does not replace personalised medical or oncology dietetic advice.
Dietary requirements during cancer treatment vary considerably. Anyone experiencing significant weight loss, malnutrition, swallowing difficulties, persistent digestive symptoms or other treatment-related nutritional problems should seek advice from their oncology team and an appropriately qualified dietitian.
Do not stop or alter prescribed cancer treatment, introduce restrictive diets or begin new supplements without discussing them with your treating healthcare professionals.
References
Cancer Research UK (2023) 'Sugar and cancer – what you need to know'. Available at: https://news.cancerresearchuk.org/2023/08/16/sugar-and-cancer-what-you-need-to-know/
Cancer Research UK (n.d.-a) Food myths. Available at: https://www.cancerresearchuk.org/about-cancer/causes-of-cancer/cancer-myths-questions/food-myths
Cancer Research UK (n.d.-b) Alternative cancer diets. Available at: https://www.cancerresearchuk.org/about-cancer/treatment/complementary-alternative-therapies/alternative-cancer-diets
Cancer Research UK (n.d.-c) Milk, dairy and cancer risk. Available at: https://www.cancerresearchuk.org/about-cancer/causes-of-cancer/diet-and-cancer/dairy-and-cancer-risk
Macmillan Cancer Support (2025) Healthy eating and cancer. Available at: https://www.macmillan.org.uk/cancer-information-and-support/impacts-of-cancer/healthy-eating-and-cancer
Macmillan Cancer Support (n.d.-a) Diet and food supplements. Available at: https://www.macmillan.org.uk/cancer-information-and-support/treatment/coping-with-treatment/complementary-therapies/diet-and-food-supplements
Macmillan Cancer Support (n.d.-b) Food safety when your immunity is low. Available at: https://www.macmillan.org.uk/cancer-information-and-support/impacts-of-cancer/healthy-eating-and-cancer/food-safety-when-your-immunity-is-low
World Cancer Research Fund (n.d.-a) Cancer Prevention Recommendations. Available at: https://www.wcrf.org/preventing-cancer/cancer-prevention/our-cancer-prevention-recommendations/
World Cancer Research Fund (n.d.-b) Myths and controversies about what causes cancer. Available at: https://www.wcrf.org/preventing-cancer/topics/myths-and-controversies-about-what-causes-cancer/
World Cancer Research Fund (n.d.-c) Limit consumption of red and processed meat. Available at: https://www.wcrf.org/research-policy/evidence-for-our-recommendations/limit-red-processed-meat/
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