The good, the bad and the unicorns: diet and cancer

basics Oct 04, 2018

For today’s post, I’ve decided to try something a little different. I’ve recorded an audio version of this blog post (I guess this is technically a podcast then, right?! And there was me saying I’d probably never ever do a podcast…hmmmm).

I wanted to trial this as I think it might make your life a little easier. With the best will in the world, we don’t have time to read lengthy blog posts, and I couldn’t cut this one down any more.

So, ever the scientist, I’ll experiment with recording an audio version for a few weeks or months and I’ll see what the response is like. If it proves popular, then I guess I’ll have to start podcasting in the new year…!

You can access the audio here – download it to your phone and listen on the go:


Something I’m always being asked about is diet and cancer. It seems that hardly a day goes by without there being some bold claim in the media about this food causing cancer, or that superfood preventing cancer, but what should we believe? 

Until now, I’ve avoided writing about this because I felt “unqualified”…yeah, I know, I’ve got a PhD in the subject, but still! Somehow it feels like it’s beyond my expertise because I’m not a registered dietician or a trained nutritional therapist.

So, I decided that what I could do is lay out the cold hard facts – the evidence, as I see it, on what foods are linked to reducing cancer risk, what foods may cause cancer, and what foods there just isn’t enough evidence to say either way.

The good, the bad and the unicorns

Let’s start with the good. What do we know protects us from cancer?

Cancer-protective food #1: Fresh fruits and vegetables

Probably the most important facet of a healthy diet is an abundance of fresh fruits and vegetables (with the emphasis on fresh!). Research suggests that eating fruit and veg can reduce your risk of cancers of the mouth, upper throat, larynx and lung [1-4].

Why? In part, because they contain lots of different nutrients that hinder tumor growth, such as carotenoids, vitamin C and numerous phytochemicals. But they also have ‘anti-cancer’ properties because they are a rich source of natural fiber, which brings me nicely onto my next point.

Cancer-protective food #2: High fiber diet

A high fiber diet can reduce the risk of bowel cancer, and in fact almost a third of all bowel cancer cases are thought to be linked to a low fiber diet. Eating just 10g of fiber a day reduces bowel cancer risk by 10%, whilst 90g of fiber a day reduces it by 20%.

But how does fiber reduce cancer risk? Well, it does so because it helps push waste through the digestive tract, therefore reducing the contact time between the bowel and potentially harmful carcinogens in fecal waste. It also restores the balance of natural bacterial flora found in the bowel, making it harder for tumors to develop in the first place [56].

The bad

So, what about the bad? What do we know that causes cancer or increases cancer risk?

Cancer-causing food #1: Red and processed meats

When I think about foods that cause cancer, my mind immediately jumps to red meat and bowel cancer. In truth, it’s the excessive consumption of red or processed meats (so, think cured bacon, ham, beef burgers, etc.) that causes cancer. The International Agency for Research on Cancer (IARC) classifies processed meat as a cause of cancer, and red meat as a probable cause [7].

Eating 100g of red meat every day can increase an individual’s bowel cancer risk by up to 17%, and eating 50g of processed meat has a similar effect (18% increased risk) [5].

How does it cause cancer? Well, there are a few different chemicals in red and processed meats that play their part: the first is the red pigment protein called haem, which gives blood and red meat its characteristic red color. Haem stimulates our gut bacteria to produce known carcinogenic compounds that can cause DNA damage in our cells. So, I guess this would make vampires more at risk of developing cancer too, right?!

Some of the chemicals used to preserve processed meats include nitrites and nitrates, which can be converted into volatile carcinogenic compounds both during the curing process and inside the body. This might explain why there’s a higher cancer risk with eating processed meats rather than just red meat.

But why bowel cancer, specifically? Well, it’s all to do with contact (and contact time). We eat and digest our food fairly rapidly, but the waste (the poo!) can take a while to work its way through the gut and through the bowel. As it moves through, oozing carcinogens and stimulating gut bacteria to produce yet more carcinogens, our undigested waste unleashes an assault on the cells lining the bowel and colon. And this carcinogenic assault results in DNA damage, and ultimately, tumor growth.

Cancer-causing food #2: Salt-preserved foods

The next cancer-causing food on my ‘hit list’ is the long-time enemy of our hearts: salt. Salt-preserved foods can increase our risk of stomach cancers by damaging the lining of the stomach, causing inflammation, which promotes tumor growth. But also, by making the cells lining the stomach more sensitive to carcinogens, such as nitrates, which of course are produced by the gut bacteria. This may also help the ‘bad gut bacteria’ called Helicobacter pylori, which is known to cause stomach ulcers and gastric cancer, to colonize the site. And so these two factors work together to increase stomach cancer risk [8].

The link between the total amount of salt we consume and cancer risk is not so cut and dried, but we can’t rule out a link altogether. In any case, what we do know is that too much salt can cause increased blood pressure [9], leading to increased risk of heart disease and stroke [10], and so the recommendation is simple: cut your salt intake to less than 6g a day [11].

Cancer-causing food #3: Burnt or charred foods

The last big player on my list of cancer-causing foods is burnt or charred foods [12]. Yes, chargrilled chicken and summer barbecues may be doing you more harm than good. This is linked to the effects of cooking meat at high temperatures, which results in the formation of chemical structures called heterocyclic aromatic amines and polycyclic aromatic hydrocarbons. These compounds are definitely carcinogenic in animals, however the evidence in humans is less clear, but there’s enough evidence to support the recommendation to eliminate them from your diet.

So, what about burnt toast? I remember hearing an urban legend many years ago that burnt toast could give you cancer. Is it true? Well, the jury’s still out on this one. Cooking carbohydrate-rich foods like bread and potatoes at high temperatures until it burns (e.g. by frying or grilling or toasting) results in the formation of acrylamide, but the quantities typically consumed in cooked food are so low it’s unlikely to cause cancer (see Unicorn #1 below). In fact, in the most recent study [13] of the available data at that time, the conclusion was that “dietary acrylamide is not related to the risk of most common cancers”. 

Cancer-causing food (ok, drink!) #4: Alcohol

Excessive alcohol consumption is linked to seven different cancers: liver, breast, bowel, upper throat, mouth, esophageal and larynx [14]. But how exactly does alcohol cause cancer?

When we drink alcohol, it is broken down (or metabolized) in our liver by enzymes called alcohol dehydrogenases (ADH) to form acetaldehydes, and these acetalaldehydes are then converted to acetate by another set of enzymes called aldehyde dehydrogenases (ALDH). So it’s a two-step process. However, if we drink too much, the body can’t process it quickly enough and so there is a build-up of acetaldehyde, which is actually toxic to our cells and causes DNA damage, which results in mutations that give rise to cancer. 

Some people carry mutations in the genes encoding these ALDH enzymes, which means they can’t breakdown acetaldehyde to acetate and this makes them much more susceptible to alcohol-related cancers. This genetic predisposition is particularly common in Asian populations, which may explain the increased rates of esophageal cancer in China.

Something else that alcohol does is it enhances the ability of the cells lining the mouth and throat to absorb carcinogens in cigarette smoke, and so social smokers – those who only smoke when they’re on a night out – may actually have a higher risk of developing cancers of the mouth and throat.

The unicorns

So, we’ve looked at the good and the bad. Now let’s take a look at the unicorns: the things we may wish were real (who knows, maybe they are?!) but remain stubbornly elusive…I’m talking, of course, about those dietary factors for which there is no clear evidence either way. Perhaps unsurprisingly, this is the longest list of the three… 

Unicorn #1: Acrylamide

You may have heard the controversies around acrylamide [15] – the naturally occurring chemical that is formed when many foods, particularly starchy foods, are cooked at high temperatures for long periods of time (think baking, frying, toasting and roasting). It’s also found in biscuits, coffee, bread and fried potato products, like crisps, chips and fries. 

Now, the International Agency for Research on Cancer (IARC), which I mentioned before, defines acrylamide as a “probable human carcinogen”.  But interestingly, although studies in animals shows a link between acrylamide interacting with cellular DNA and cancer, the evidence in humans is sketchy at best.

However, if we stick to the other guidelines for following a healthy balanced diet that’s full of fresh fruit and vegetables, with minimal consumption of calorie dense foods such as crisps, chips, fries and biscuits – which of course are major sources of acrylamide – then we will naturally reduce our exposure anyway.

Unicorn #2: Artificial sweeteners

What about artificial sweeteners like aspartame and saccharin? They’ve had a bad rap over the years, but do they actually increase our risk of developing cancer? Well, once again, it appears that their reputation as potential cancer-causing nasties came from animal studies and actually subsequent studies in humans have provided strong evidence that these artificial sweeteners do not increase cancer risk [16]. In fact, people who eat a lot of sweetened foods actually consume well below the daily recommended level anyway. Which I have to admit is a bit of a relief because I do tend to drink a lot of diet soda (particularly Irn Bru – I'm such a Scottish stereotype!).

Unicorn #3: Green tea

Green tea has been touted as an anti-cancer ‘elixir’ for many years, but results from large cohort human studies have shown there is no strong evidence to support the idea that green tea can reduce cancer risk [17].

But where did this idea originally come from? Well, green tea contains lots of catechins – a natural plant metabolite with antioxidant properties. Studies in the lab have found that catechins could block cancer cell growth and prevent carcinogen activation, but once again, studies in humans have not been able to give any clear evidence that green tea can reduce cancer risk. 

Unicorn #4: Milk and dairy products

Although milk and dairy are good sources of dietary calcium and protein, we are the only species of animal that drinks the milk of another species, and there is some conflicting evidence around whether they increase or decrease cancer risk [18]. On the one hand, some studies have shown that the casein protein found in milk can cause cancer, yet other studies demonstrate that dairy products could, in fact, reduce the risk of bowel cancer. On balance though, there is no strong evidence to support any links between dairy products and cancer risk, and further larger cohort studies are really needed to resolve this uncertainty.

Unicorn #5: Pesticides

Perhaps one of the most controversial topics is the use of pesticides in food farming. High doses of some pesticides can cause cancer in animals, BUT…and it’s a big BUT! The levels found in OUR foods are so tightly regulated to make sure they are well below these doses that it is unlikely they pose a threat to our health [18]. Whilst some fruits and vegetables contain very small trace amounts of pesticides on their surface, there is no evidence to suggest that these small amounts actually increase the risk of cancer in people who eat them. Interestingly, and perhaps controversially, eating organic food, which usually doesn’t use pesticides, doesn’t actually appear to affect cancer risk either. All this is not to say, “don’t eat organic” – there are many health benefits to doing so; but more than anything I want to say to you and your patients, “don’t panic if you can’t afford to eat organic because evidence suggests it won’t affect your cancer risk.”

Unicorn #6: Soya products

Soya products made from soybeans, like tofu and soy milk, are rich in particular plant-based estrogens called isoflavones, which although similar in structure to human estrogen, have much milder effects. Some studies in the lab suggest that isoflavones mimic estrogen in our bodies and could therefore reduce the risk of some hormone-related cancers. But once again, the evidence for this in humans is lacking [18].

In some other studies, diets rich in soy are linked with reduced prostate cancer risk, but there isn’t any strong evidence linking soy with a reduced risk of any other type of cancer.

And if you want to go a bit deeper, many of these studies were conducted in Asian countries where diets are typically higher in soy than in Western countries, so for a start, it might not be relevant to Western incidences of hormone-related cancers, and then it also doesn’t take into account any confounding genetic or other environmental factors.

Unicorn #7: Superfoods

We’ve all read the news articles that claim “superfoods” can reduce your cancer risk but can they really? 

Well technically yes, but strictly speaking no, not exactly. Let me explain…

Extracts from superfoods may have beneficial health effects, but only in massive quantities [18].

Some of the superfoods commonly championed in anti-cancer nutrition include blueberries, broccoli, raspberries, green tea, goji berries, oily fish, garlic, beetroot, and even chocolate! There is no official definition of a “superfood” but the term is used to describe foods with apparent health benefits. Really, the term is more of a marketing tool than anything else!

Regardless, many superfoods contain natural compounds that have been tested in the lab and shown to have positive health effects, but the difficulty is that many of these studies use purified ingredients from a particular food in concentrations that are not found in its natural state. Add to that the fact that a chemical isolated from a superfood may actually behave very differently on its own in a test tube than when it is eaten as part of a whole food in a balanced diet (or even an unbalanced diet, for that matter!).

So, whilst there’s no direct evidence for superfoods reducing cancer risk, what is clear is that our diets are incredibly important for our health, but the concern is that we place too much emphasis on any one individual food and that this itself could lead to unhealthy eating habits. 

So, just as you can’t outrun a bad diet, no one food, even superfoods, can make up for an unhealthy diet full of processed products and refined sugars. Rather than focusing on superfoods, dieticians and nutritionists tend to talk about “super diets” with an emphasis on a healthy balanced diet rich in fruit and veg and unprocessed wholegrains.

Unicorn #8: Tomatoes

The tomato: is it a fruit? Is it a vegetable? Who knows? Who cares?! But it does contain a chemical called lycopene – an antioxidant which can mop up the dangerous free radicals that can cause DNA damage in our cells that might lead to cancer [18]. What’s really nice is that processing tomatoes doesn’t actually affect lycopene – it’s found in all forms of tomatoes and tomato products, including fresh, tinned, paste, juice and even ketchup! Good news for those of us with children who insist on drowning every dinner in liters of ketchup, eh?! My daughter does that, for sure!

Human studies have shown that lycopene may reduce prostate cancer risk, but the evidence is a little sparse so we can’t fully rely on it. However, these versatile little fruit/veg chameleons are an excellent source of vitamins A, C and E, and of course count towards your five a day.

Unicorn #9: Vitamin and mineral supplements

And last but not least, what about taking vitamin and mineral supplements? Well, first up let’s be clear about something – taking vitamin supplements cannot replace a healthy balanced diet that’s rich in fruit and veg. All I’m saying is you can’t get away with having a cigarette and a Centrum for breakfast!

It is thought that the vitamins and minerals found in fruit and veg interact with other chemicals within the same plant to produce their positive health benefits – so on their own, they’re perhaps less beneficial.

Several clinical trials have looked at the effects of vitamin supplements on cancer risk, and rather worryingly, some studies have found that very high doses could actually increase the risk of cancer [18]. There’s an excellent Cochrane Collaboration review that looks at the results of 78 clinical trials of vitamin supplements, where they found that instead of prolonging a person’s life, they actually had either neutral or harmful effects [19].

In the end, by far the best way to get your full range of vitamins and minerals is to eat a healthy, balanced diet with a wide variety of fruit and veg – so, eat a rainbow! (and not of the Skittles variety…). Vitamin supplements should be taken in addition to a healthy diet (really, the clue is in the name – supplement!). 

The bottom line

Clearly, diet is incredibly important in both preventing cancer in the first place, and in limiting its growth once developed. I am not a qualified nutritional therapist, and would strongly urge anyone reading this to seek professional advice from someone who is qualified. As with anything in science, ask yourself, “Is the evidence convincing?” and “Could there be something else at play here?”.

If you’d like to get your hands on a free infographic summarizing these key players in the anti-cancer diet, you can grab your copy here

And remember, just because you eat red meat, or maybe you drink excessively, or perhaps you don’t eat enough fruit and veg, this does NOT mean you are destined to get cancer. There are PLENTY of examples of super-fit, super-healthy people who unfortunately do get cancer, and there are also plenty of examples of people who are not super-fit, who are overweight, who are sedentary, who don’t get cancer...admittedly they’re more likely to have other health problems, but my point is this: it’s not always as straightforward as we’d like to think. 

If you do find yourself with a cancer diagnosis, please, please, please don’t blame yourself. You didn’t do anything to deserve it. You didn’t cause it. The majority of cancer cases are caused by an underlying genetic predisposition and/or a problem with the normal system of anti-cancer immunity. 

But know this: we can do a lot to help ourselves and reduce our cancer risk going forward.

October is Breast Cancer Awareness month, and I would love it if you could share this post on social media and tag me (@essentialcancereducation on Facebook and Instagram, @mhairimorris on Twitter). The more we can spread the word about making the best possible choices in terms of our own health, the more we can hope to reduce cancer incidence in the future. Thanks so much – I really do appreciate you :-)

And if you liked this post, you might also like the short introductory course on Cancer Essentials: The Fundamental Principles and Hallmarks of Cancer. Designed exclusively for health professionals working with those affected by cancer, this training will give you a strong foundation in the subject, thoroughly grounded in scientific evidence, and will help you to develop a deeper understanding of how cancer thrives. Because if we truly understand HOW cancer thrives, then we can begin to understand WHY our lifestyle choices have such power to affect secondary cancer risk. And you can then CONFIDENTLY pass this knowledge onto your patients and clients, helping them to embrace positive lifestyle changes.

Find out more here.

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[1] Vieira et al., 2016

[2] Wang et al., 2015

[3] Liu et al., 2013

[4] Maasland et al., 2015

[5] World Cancer Research Fund/American Institute for Cancer Research. Continuous Update Project Report: Food, Nutrition, Physical Activity, and the Prevention of Colorectal Cancer. 2011; 50(2): 167-178

[6] Aune et al., 2011

[7] Bouvard et al., 2015

[8] World Cancer Research Fund/American Institute for Cancer Research. Diet, Nutrition, Physical Activity and Stomach Cancer, 2016.

[9] Scientific Advisory Committee on Nutrition. Salt and health, 2003.

[10] Lewington et al., 2002

[11] Sadler et al., 2011

[12] National Cancer Institute (NCI): Dietary risk factors

[13] Pelucchi et al., 2015

[14] Cancer Research UK (CRUK): How alcohol causes cancer

[15] National Cancer Institute (NCI): Acrylamide and cancer risk

[16] National Cancer Institute (NCI): Artificial sweeteners and cancer

[17] National Cancer Institute (NCI): Tea and cancer prevention

[18] Cancer Research UK (CRUK): Food controversies

[19] Bjelakovic et al., 2012

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