By Keir Watson
The Five-a-Day message, originally concocted by the US National Cancer Institute, is now so well embedded in the public psyche that the word “Health” has become practically synonymous with “Fruit and Veg” — just consider how often you have seen an apple used as part of a health promotion image.
But are fruit and vegetables the health saviours they seem to be?
Incredibly, the science is not as clear cut as their squeaky-clean health image would have you believe. Not only that, but there is considerable evidence that plant-based diets have the potential to cause ill health for some individuals.
In this article we put the five-a-day claim under the microscope.
- Fruit, Veg and Cancer Protection
- Fruit, Veg and Heart Disease
- Fruit, Veg and All-Cause Mortality
- Where did “5-a-Day” come from?
- Fruit and veg as a source of antioxidants
- Historical caution around fruit and veg
- Other adverse conditions associated with plant-based diets
- Plants as pesticide factories
- Hormesis to the rescue
Read time: 14 minutes (2750 words)
Fruit, Veg and Cancer Protection
Evidence for the anti-cancer properties of plant-foods comes mainly from population studies where it is frequently found that those eating higher levels of fruit and vegetables have a lower incidence of certain cancers. [e.g.Turati, 2015]
Such studies, though, are often inconsistent or under-whelming. For example, in the European Prospective Investigation into Cancer and Nutrition (EPIC) study — which followed half a million Europeans for seven years, and was designed precisely to identify the links between cancer and food intake — the associations between fruit, vegetable, and fibre consumption and the risk of many different cancers found only limited evidence for a protective effect. Whilst high consumption did appear to be associated with a reduced risk of colorectal cancer, and total fibre intake with liver cancer, there was no link with cancers of the stomach, biliary tract, pancreas, cervix, endometrium, prostate, kidney, bladder, or lymphoma.
Even the small apparent protective effects, it appears, might be illusory: Researchers point out that high intake of fruits and vegetables was also associated with other lifestyle variables, such as lower levels of smoking and drinking, and higher levels of physical activity. They conclude that “although the analysis was adjusted for these factors, we cannot rule out the possibility of residual confounding.”
Tellingly, after years of recommending people eat more fruit and vegetables, the National Cancer Institute (NCI) programme has discontinued emphasising cancer reduction as the reason.
“It is widely believed that cancer can be prevented by high intake of fruits and vegetables. However, inconsistent results from many studies have not been able to conclusively establish (this).” Boffetta et al., 2010
So if it is not about cancer prevention, what else might make fruit and vegetables healthy choices?
Fruit, Veg and Heart Disease
One of the maxims of plant-based diets is that animal fats contribute to heart disease whilst plant-based diets reduce the risk. This has been said so often that most people take it as gospel, quite unaware that recent research challenges this view.
Take for example a recent study — one of the most comprehensive of its kind — which looked at cardiovascular disease mortality and risk markers across 42 European countries. They found clear evidence that meat and animal products were protective, whereas, refined carbohydrates (including starchy vegetables and grains) were associated with an increased cardiovascular risk. Fruit and vegetable consumption showed little correlation at all. (See my summary of this study)
These results do not appear to be limited to Europe, with similar findings coming from the PURE study (Fig 1). The lead author, cardiologist Dr Salim Yusuf, summarised some of the key findings at a conference in 2017 [See video here], which included the following:
- Greater fat, including high-fat dairy intake, was found to be protective
- Carbohydrates were harmful
- Saturated fat from meat was neutral
- Fruit was beneficial, but no additional benefit over 2 portions per day
- Legumes were beneficial
- Eggs, fish and vegetables were neutral
Dr Yusuf emphasised that vegetables, in particular, showed little effect either way, yet in the context of poorer countries represented expensive luxuries. He suggests that any surplus income would be better spent on meat and dairy products rather than fruit or vegetables.
Fruit, Veg and All-Cause Mortality
Of course, fruit and veg may have benefits other than cancer or heart disease, in which case the statistic we want to examine is all-cause mortality. If a plant-based diet is healthier than a standard Western diet we would expect this to show up in life expectancy of vegetarians when compared to meat eaters.
But again, such studies have failed to identify any such benefits. In 2015 a studycomparing UK vegetarians and meat eatersfound no significant difference in mortality. The same null-result was confirmed independently two years later in a similar Australian study.
(caption) A UK study in 2015 compared different dietary groups to regular meat eaters and found no difference in all-cause mortality. Higher bars indicate greater incidence of premature death. The grey bars indicate confidence intervals, showing that apparent differences in outcome are statistically insignificant.
Where did “5-a-Day” come from?
Much of the original impetus behind the 5-a-Day campaign came from the fact that fruit and veg are a rich source of micronutrients, while being generally low in fat and calories. With the goal to reduce the nations consumption of fat already established, fruit and veg seemed the obvious next step. Add to this animal studies demonstrating cancer-fighting properties of isolated fruit and veg compounds and you can see how 5-a-day seemed like a no-brainer.
You might assume that before launching the campaign on an unsuspecting public, trials were conducted showing that five portions per day were superior to four, three, two, one or even none at all. But you would be wrong. The choice of “5” was little more than a marketing gimmick — a catchy little number that seemed achievable and made the bitter-pill of governmental nannying a little easier to swallow.
Out of the public gaze, however, nutritional researchers were quite clear that evidence was distinctly lacking.
“Fruit and vegetables have long been considered a panacea against major chronic diseases, including cancer. However, there is no convincing epidemiological, clinical or experimental evidence supporting fruit and vegetable’s chemopreventive ability.”
Fruit and veg as a source of antioxidants
Fruit and vegetables famously are good sources of antioxidants: Vitamin C, E and beta-carotenes are familiar examples but less well known are the vast array of polyphenols packed into plant foods, many of which show extremely strong anti-oxidant activity in the lab.
Over the last two decades, however, the health benefits of dietary antioxidants have been called into question. This is partly because trials of supplements have failed to produce the expected benefits, and in some cases, have actually produced harm. [e.g. see here]. Then as an understanding of the body’s in-built antioxidant system grew, it became clear that these effects were far more complex than initially believed..
By way of example, one recent study found that feeding rats a fixed blend of 5 fruit and veg for just ten days led to an unexpected increase in oxidative stress as well as reduced resistance to cancer: the very reverse of what 5-a-day is supposed to achieve.
The authors (Bonamassa et al) conclude that dietary advice should be changed to emphasise “daily diversification” of fruit and vegetables to prevent these adverse effects — a subtlety not captured in the 5-a-day hyperbole. They also suggest that their work may explain why vegetarian and vegan diets are not as healthy as expected, and why colorectal cancer is higher in vegetarians than in meat eaters (Key et al, 2009). They also point to the infamous CARET trial in which high dose supplemental beta-carotene given to reduce the incidence of lung cancer among smokers and asbestos workers had to be halted early because of the higher rates of cancer. (See my essay on Vitamin A from Animal Sources)
Historical caution around fruit and veg
We sometimes forget that foods we take for granted and eat regularly today were once looked upon with suspicion. The Victorians, for instance, grew egg-plants (aubergines) as ornamentals because they believed them to be poisonous, yet when we learn of this most of us tend to assume that they were simply over-cautious. But perhaps they were on to something that we have forgotten. There are certainly people who cannot eat any Solanaceousvegetables at all, including potatoes, tomatoes, aubergines, peppers and goji berries. Such nightshade allergy can lead to breathing problems, a rash, or eczema shortly after consuming these foods.
I heard recently that the Italians used to peel tomatoes and peppers, and remove the seeds before consumption, and originally ate them cooked, not raw. These traditional food processing techniques are interesting, as it is now known that in general the anti-nutrients in fruits, including lectins, saponins and alkaloids, tend to be concentrated in the skins and seeds rather than in the flesh. This even applies to the poisonous red berries of the yew tree, although trying to eat just the flesh of those tiny fruits is a fool’s errand — don’t try it, please.
Is it possible that the Italians, who were regularly eating these nightshade foods, were protected from the negative effects because of their fastidious preparations? Are we as careful today? And if not, might this be contributing to the modern disease burden?
The potential for fruits and vegetables to cause health problems extends beyond the nightshade family, yet most people will only realise that a food is harming them once they develop overt problems. What the effects of long term sub-acute toxicity might be is anyone’s guess.
Other adverse conditions associated with plant-based diets
|Brassicas (cabbage, cauliflowers, broccoli, Brussel’s sprouts); Millet||Goitrogens||Iodine deficiency; Thyroid dysfunction; Goiters [ref]|
|Peanuts, rhubarb, spinach, beets, chocolate and sweet potatoes||Oxalates||Kidney stones [ref]; acute poisoning in extremis|
|Whole grains, soy protein||Phytate, polyphenols||Iron and zinc deficiencies [ref]|
|Cassava (tapioca); apple pips; bitter almonds; apricot kernels; undercooked lima beans||Cyanogenic glycosides||Cyanide poisoning (lethal or sub-lethal) [ref] [ref] [ref]|
|Vegan diets||low in zinc, calcium, vit. D, B6, B12, DHA/EPA||Higher risks of deficiencies; long term adverse effects on offspring [ref]|
|Rice (and to a lesser extent|
Apple & pear juice)
|Arsenic||Chronic toxicity; [ref]|
|Star Fruit (Averrhoa carambola)||Neurotoxins|
|Toxic, especially for people with chronic kidney disease (CKD) [ref]|
|Leafy vegetables, salads, berries, melon||Major sources of food borne pathogens||Hospitalisations and deaths from Salmonella(ref), Listeriaand E coli (ref).|
Plants as pesticide factories
To understand plants — and hence the potential issue with eating them — we need to recognise that unlike animals they cannot run away from predators and they do not have an adaptive immune system to deal with pathogens. Instead, they produce mechanical barriers (thorns, bark etc) and, importantly, a plethora of chemical deterrents.
With regards the latter, plants are nature’s chemical weapons factories, engaged in an evolutionary arms race against insects, bacteria, fungi and herbivores. It is estimated that plants have evolved at least 200,000 such defensive compounds which represent their own insecticides, fungicides and digestive inhibitors. Despite constituting up to one third the dry weight of plants, these compounds serve no direct role in their growth or reproduction, so are classed as secondary metabolites.
When ingested, these compounds act on many biochemical pathways, and include nerve agents, digestive inhibitors, emetics (causing vomiting), irritants and toxins with potential targets all over the body. Mammals, and particularly herbivores, have evolved detoxification mechanisms to help them cope, along with the good sense not to eat the plants most poisonous to them. Frequently such plants are bitter tasting — providing an early warning system that they contain poisons.
Some plant compounds are strongly poisonous to humans, which is why many wild plants and fungi are classed as inedible, toxic or deadly. This is also why many beans require cooking before consumption, as cooking destroys these toxins and renders them safe to eat. On the other hand, in controlled doses, some of these compounds have found medicinal uses as modern pharmaceuticals or traditional herbal medicines whose use extends back hundreds of thousands of years, even to our Neanderthal cousins.
The ability to act both as toxins and medicines is due to the way plant compounds interfere with biochemical pathways. For example, willow bark is a source of the painkiller aspirin. It works by blocking cyclooxygenases leading to pain reduction, but too much can cause temporary tinitis. Foxglove (Digitalis) reduces sodium-potassium ATPase which increases cardiac output. It was once used successfully in congestive heart failure and arrhythmia but was discontinued in its herbal form because the therapeutic dose was too close to the toxic dose. The isolated glycoside digoxin is still used in cardiology. Deadly nightshade (Belladonna) is a source of atropine which is still used to dilate the pupil aiding eye examinations.
Plant compounds, then, can have both positive and negative effects; as Paracellsus said: “Tis the dose that maketh the poison”.
When we eat fruit and vegetables the bulk of the “micronutrients” we ingest are actually these natural toxins. 99% of these plant compounds have no direct nutritional value and are broken down and eliminated within hours of ingestion. Because they have no specific metabolic function — no receptors, carriers or target sites they are able to reach all parts of the body and so potentially affect any and all organs.
What exactly all these plant compounds do once inside the body is a matter of ongoing enquiry and not one likely to be completed any time soon. Consider aspirin: one of the most studied plant-derived compounds; its multiple modes of action have taken decades to tease out, yet many mysteries still remain to this day. Reflect then, on the unquantifiable biochemical complexity that takes place when you eat something as apparently innocent as an apple or lettuce leaf.
Once you realise that eating plant foods is a kind of self-medication, in which you expose yourself to thousands of potential toxins and allergens it is unsurprising that they have the potential to do harm as well as good. Wheat is a classic example: its ability to induce autoimmune diseases such as coeliac, gluten ataxia and type 1 diabetes, can be traced to its natural insecticidal properties. It appears that at least part of wheat’s damaging effect on the gut is due to an ancient conserved response to infection that is shared between us and certain insects that might otherwise eat it (See my article: The Chemical Warfare on Your Plate for a riveting account).
Hormesis to the rescue
Although we might be in the dark about some of the specific effects of plant secondary metabolites there is an important generalised physiological response that has come to light in the last twenty years or so: hormesis. When our cells experience mild stress — from exercise, UV exposure, fasting or plant nutrients — they often respond by beefing up their defensive mechanisms. This can include up-regulating detoxification pathways, cellular repair mechanisms and anti-cancer defences. It now appears that the majority of benefits of plants foods stem from our body’s own response to a sub-acute dose of plant poisons.
Studies of olive oil, for example, have found that it is the phytochemicals in extra-virgin olive oil (the green hazy colouration) that provide the majority of the observed health benefits, rather than the mono-unsaturated oil that was first thought to explain the health benefits of the Mediterranean diet.
“It is suggested here that the Mediterranean diet can be conceptualized as a form of chronic hormetic stress”
As already noted, many plant toxins are bitter, and it was once thought that bitter taste receptors on the tongue were an adaptation to help us avoid poisonous plants. However, humans display a distinct penchantfor bitter foods suggesting that we actively seek out low level doses of plant toxins. The discovery that we have bitter taste buds, not only on our tongue, but all over the body, including inside organs such as the brain and testes, opens a fascinating insight into evolution and diet which I have explored in two essays:
- The Bitter Truth is Sweeter Than We Thought
- How Can Bitter Foods Be Good For Us When They Taste So Bad? — Resolving The Paradox
Plant foods certainly produce effects in the body, and sometimes healthful responses. Coffee, tea, olives and dark chocolate are good examples of plant-based foods that have an increasing amount of evidence on their side as I discovered when researching an article in 2016 (Five Ancestral Foods with Proven Health Benefits). But these results are only true at the population level: When it comes down to the individual there will always be some who are harmed not helped even by the latest “superfood”. In one recent case, reported by the BBC, a man required a liver transplant following a three month course of green-tea supplements which were identified as the likely cause.
The bottom line is that the majority of benefits that come from fruit and vegetables are derived from plant secondary metabolites: aka toxins. While the body has the ability to react positively to these toxins via hormesis, the point at which benefits tip over into harms is not at all well established and unarguably varies from person to person and with different foods or combinations of foods in each individual.
The toxic potential of fruit and veg may explain why some people who try a plant-free (carnivore) diet experience dramatic improvements in their health. You can read hundreds of such testimonials on the website Meat Heals where you can search by medical condition.
At Rosemary Cottage Clinic many patients are found to have immune reactions to quite unexpected plant-foods, such as vanilla, peppermint, wine, asparagus, potato, corn, sesame, teff and tapioca to name just a few. For most people, the probability of identifying which plant foods in their diets might be contributing to their health problems is vanishingly small: especially as in many cases the effects of ingestion may not show up until hours or days afterwards and may persist for days or even weeks after ingestion.
When health breaks down, it seems perfectly possible that plant foods could be far more irritating to the system than meat. Elimination diets which remove all plant-based foods from the diet can provide a safe and effective starting point for those who want to establish whether plant-foods are contributing to their health problems. In many cases, eating just meat, eggs, fish and dairy (if tolerated) for a few weeks will reveal whether plants were the problem. If health improves on such a carnivore-diet, then vegetables can be gradually reintroduced carefully watching for reactions. I am currently in the middle of a two-month trial of the carnivore diet myself, to see whether I have any health issues with plant-foods. I’ll let you know the results in another post.
To be clear, I am not saying fruit and veg are necessarily bad, or that you should not eat them — I have grown and eaten fruit and vegetables myself for years and find they add variety and palatability to meals. But I’m now deeply sceptical about the uncritical, one-sided health claims that have become attached to them, and I’m more than a little jaded by health images covered in a rainbow of fruit and vegetables. I think it’s time to take the five-a-day dogma down from the pedestal we’ve placed it on.