The eat-your-fruits-and-vegetables 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 vegetables” - simply 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.

Much of the original impetus behind the eat more fruit and vegetables campaign (the amount will depend in the country) came from the idea that fruits and vegetables are a rich source of micronutrients, while being generally low in fat and calories. With the goal to reduce the consumption of fat already established, adding in more fruit and vegetables appears to be the obvious next step.

There are a few animal studies demonstrating cancer-fighting properties of isolated plant compounds making adding more plant matter to the diet an even more obvious choice

Out of the public gaze, however, nutritional researchers have been quite clear that evidence is 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 chemo preventive ability.”

What About 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 laboratory models.

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. 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.

Further studies have concluded that dietary advice should be changed to emphasise “daily diversification” of fruit and vegetables to prevent these adverse effects. They also suggest that this 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.

Historical Caution With Plant Foods

You may have forgotten that foods you 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 today you might assume that they were simply over-cautious. Perhaps, however, they were on to something that current society may have forgotten. There are certainly people who cannot eat any Solanaceous (nightshade) vegetables at all, including potatoes, tomatoes, aubergines, peppers and goji berries. A nightshade allergy or sensitivity can lead to breathing problems, a rash, or eczema shortly after consuming these foods.

Until recently, 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.

Is it possible that the Italians, who were regularly eating these nightshade foods, were protected from 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) and Millet contain goitrogens which have been linked to Iodine deficiency, thyroid dysfunction and goiters

• Peanuts, rhubarb, spinach, beets, chocolate and sweet potatoes contain oxalates associated with kidney stones and acute poisoning in extremis

• Whole grains and soy protein contain phytate, polyphenols resulting in iron and zinc deficiencies

• Cassava (tapioca); apple pips; bitter almonds; apricot kernels and undercooked lima beans contain cyanogenic glycosides causing cyanide poisoning (lethal or sub-lethal)

• Vegan diets are low in zinc, calcium, Vit. D, B6, B12, DHA/EPA meaning those following a vegan diet have higher risks of deficiencies and long term adverse effects on offspring

• Rice (and to a lesser extent Apple & pear juice contain arsenic resulting in chronic toxicity

• Star fruit (Averrhoa carambola) is filled with neurotoxin, nephrotoxins and oxalates which aretoxic, especially for people with chronic kidney disease (ckd)

• Leafy vegetables, salads, berries and melon are major sources of food borne pathogens culm inating in hospitalisations and deaths from salmonella, listeriaand E coli.

Plants As Pesticides

To understand plants and the potential issue with eating them it is important to recognise that unlike animals they cannot run away from predators and do not have an adaptive immune system to deal with pathogens. Instead, they produce mechanical barriers (thorns, bark and the like) and, importantly, a plethora of chemical deterrents.

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.

These compounds include nerve agents, digestive inhibitors, emetics (causing vomiting), irritants and toxins with potential targets all over the body and act on many biochemical pathways. Mammals, and particularly herbivores, have evolved detoxification mechanisms 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. Many beans and legumes require cooking before consumption, as cooking destroys these toxins and renders them safe to eat. 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.

The ability to act both as toxins and medicines is due to the way plant compounds interfere with biochemical pathways.

Some examples include:

·       Willow bark is a source of the painkiller aspirin. It works by blocking cyclooxygenases leading to pain reduction. Too much, however, can lead to temporary tinnitus.

·       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 – the poison is in the dose!

When you eat fruits and vegetables the bulk of the “micronutrients” 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 and thus no receptors, carriers or target sites they are able to reach all parts of the body and 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 almost self-medication, in which you expose yourself to thousands of potential toxins and allergens it is not a stretch to understand 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 you and certain insects that might otherwise eat it.

Hormesis

Although you 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 recently: hormesis. When your 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 your 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.

As already noted, many plant toxins are bitter, and it was once thought that bitter taste receptors on the tongue were an adaptation to avoid poisonous plants. However, humans display a distinct penchant for bitter foods suggesting that they might actively seek out low level doses of plant toxins. The discovery of bitter taste buds, not only on your tongue, but all over the body, including inside organs such as the brain and testes, opens a fascinating insight into evolution and diet.

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, 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”.

Should You Eat Fruits & Vegetables?

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.