Diet and Cancer: what should we eat?

There’s so much information about diet and cancer. Unfortunately much of the “advice” is not factually correct or supported by robust scientific evidence.

A plant-based diet is the way to go!

According to the evidence, a plant-based diet containing minimal amounts of added sugar and processed foods is best when it comes to cancer prevention. The same applies for when living with cancer. It might not sound glamorous compared to some headlines promoting restrictive diets or cleanses, but it really does make scientific sense!

Some incredible science

A landmark study by the American Gut Project in 2018 found that those who had 30 or more plants per week had a more diverse range of gut bacteria vs those who had 10 or less plants per week. Robust evidence shows that greater gut microbe diversity, the better off we will be, with systems in our body functioning like they should. 

What do our microbes do for us?

When you have a varied, plant-based diet, you’re feeding the trillions of microbes living in our large intestine (aka the gut microbiome). These beneficial bacteria enable us to digest our food efficiently as well as absorb vitamins and minerals effectively. This will contribute to improved energy and performance. They also play a role in hormone regulation and neurotransmitter production, which are the chemicals that control our mood and contribute to looking after our mental health. 

Something else found in our plants are prebiotics. These not to be mistaken for probiotics (which are the live bacteria found within supplements or fortified foods). Prebiotics are fibres found within our plants (which feed the microbes) which have a scientifically proven benefit. For example improved blood sugar control, appetite regulation, and immunity. 

How does this relate to cancer?

In terms of cancer prevention, these clever microbes have a protective role in the development of some cancers. They produce beneficial compounds known as short-chain fatty acids which have been shown to reduce risk of colon cancer. Also, within many of our plant foods are chemicals called polyphenols which have been associated with cancer protection. 

Interestingly, more recent studies found a diverse gut microbiome may affect how a person responds to cancer treatment, in terms of side effects as well as treatment success.

Have I sold you on eating more plants yet?!

What does a plant-based diet look like?

As a very rough guide, each day aim for:

  • 2-3 pieces of fruit (spread the intake out across the day)
  • 5-7 types of vegetables/salad (e.g., 3 and lunch and 4 at dinner)
  • 3 types of wholegrains (wholewheat pasta, quinoa, buckwheat, oats) 
  • 1-2 portions of nuts (or nut butters) and seeds
  • 1-2 types of legumes (beans, chickpeas and lentils)
  • Herbs & spices 

Does this mean a vegan diet?

A plant-based diet does not necessarily mean a vegan diet. But if you want to go vegan that’s ok too. There’s some key nutrients that need to be considered when removing all animal products from our diet. You may want to chat with me to make sure you’re getting everything you need if you’re vegan.

Meat, fish, and other animal products can definitely be included in your diet if you enjoy them. The global evidence base says it’s safe to do so. What we’re learning from the research is that it’s the balance of protein to fibre that’s important. When our microbes ferment upon protein within our gut, they produce compounds called uremic toxins. These cannot be eliminated from the body and build up in the bloodstream where they organs including the kidneys and heart. This has been associated with increased mortality in patients with chronic kidney disease. Studies suggest that dietary fibre may counteract the production of those harmful compounds. This essentially means we should be aiming for more plants (fibre) and less animal foods in our diet. 

If you are eating meat, ideally avoid processed meats such as ham and sausages which contain known carcinogenic (cancer-forming) properties. It’s one of the few things I tend to say to avoid – the evidence is pretty strong for this recommendation.  

Remember: we’re aiming for variety to feed as many species of microbes in our gut. Without variety, the microbes may not get the fuel they need and die off which can impact our digestion and overall health. 

What else should we be including in our diet?

> Protein

Include a protein source with each meal to promote healthy muscle growth and repair. This is especially important for someone who is going through cancer treatment where preserving muscle mass is crucial.

  • Aim to include oily fish 2 times per week
  • Limit red meat intake to 1-2 times per week (avoiding those processed meats)

If you don’t eat meat or fish include some of the following protein sources:

  • eggs
  • cheese
  • Greek yoghurt

Plant-based or vegan sources include:

  • beans
  • lentils
  • chickpeas
  • tofu
  • nuts

> Fats

Fats are important as they help absorb some of our vitamins. They also make up the surrounding layer of all the cells in our body.

Contrary to popular belief, we shouldn’t be shying away from fats. In fact, we should be aiming to include plenty of healthy sources such as: nuts, seeds, oily fish, extra virgin olive oil, avocado, and olives.

Unless we’re trying to gain weight, ideally we want to limit consumption of animal fats like butter, visible fat on meat, and large portions of cheese.

For cooking, use a extra virgin olive oil, rapeseed oil, butter, or coconut oil. Something to bear in mind is that if your oil is smoking then its chemical structure changes and can be more harmful when consumed. Save those nut, seed or other oil types as a garnish rather than cooking with them as they tend to have a much lower smoking point. 

> Fluids

Aim to drink at least 2 litres of fluid per day ensuring to sip fluid during mealtimes to help that fibre go through your digestive tract.

Where possible, limit fizzy drinks, squashes and cordials (we want to limit high sugar or lots of artificial sweeteners) and instead try flavouring water with things such as lemon, orange, mint leaves.


It’s never too late to introduce more plants into your diet. Remember that our microbes like variety, so aim for plenty of different colours, textures and flavours across your week. It will nourish your microbes, who will in turn look after your overall health.


McDonald D, Hyde E, Debelius JW, et al. American Gut: an Open Platform for Citizen Science Microbiome Research. mSystems. 2018;3(3):e00031-18. Published 2018 May 15. https://doi.org/10.1128/mSystems.00031-18

Rossi M, Johnson DW, Xu H, et al. Dietary protein-fibre ratio associates with circulating levels of indoxyl sulfate and p-cresyl sulfate in chronic kidney disease patients. Nutr Metab Cardiovasc Dis. 2015;25(9):860-865. https://doi.org/10.1016/j.numecd.2015.03.015

Yu, ZK., Xie, RL., You, R. et al. The role of the bacterial microbiome in the treatment of cancer. BMC Cancer 21, 934 (2021). https://doi.org/10.1186/s12885-021-08664-0

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