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AFTER decades of celebrating the benefits of fruit and veg, some corners of the foodie media are voicing a radical new claim: fresh produce is not only unnecessary for human health, it is actually quite toxic. With countless devotees, this school of thought argues that most plants have evolved elaborate toxins to defend themselves from predation, which makes animal-based foods the safer and healthier choice.

One statistic circulating on social media is that 98.8 per cent of plants are toxic whereas 99.9 per cent of animals are edible. As strange as it may seem, this narrative 鈥� at least, when it comes to plants 鈥� has a bit of truth to it, although that doesn鈥檛 mean you should stop eating plenty of fruit and vegetables. So let鈥檚 unpack the botany behind this claim.

Unlike animals, plants can鈥檛 run away or hide from threats, so they use a different strategy: chemical weapons. They pack their cells with pungent, bitter-tasting compounds, caustic irritants and deadly nerve toxins to inflict damage on predators, humans included. This isn鈥檛 just exotic plants in far-off jungles either 鈥� in fact, a whole range of ordinary edible crops belong not only to the same genus, but even the same species as some proper toxic ones.

Potatoes, courgettes and pumpkins are filled with such substances in the wild, and it was only after millennia of brilliant breeding work by the indigenous peoples of the Americas that they were rendered edible. Every now and then, some plants revert to their wild ways, meaning home growers can be poisoned by the cucurbitacins found in improperly grown courgettes, for example. You could add everything from carrots and rhubarb to kidney beans and nutmeg to the 鈥渢oxic crops鈥� list too, with literally hundreds of everyday examples. So how prevalent is this toxicity?

Well, when you start digging, things turn out to be quite complex. There are 300,000 to 400,000 plant species on Earth, of which roughly 50,000 to 80,000 are edible . The huge range of estimates is because there isn鈥檛 a clear-cut way of defining what counts as edible.

鈥淐ompounds made by broccoli as a toxic defence against insects are also what give it many of its nutritious effects鈥�

There is, after all, a difference between technically edible (think of the pine bark and mosses once ground into bread during times of famine) and popularly eaten (the wheat, rice and corn that make up 60 per cent of the plant-based calories we consume worldwide). Plus, with thousands of plants being newly recorded by science every year, we don鈥檛 even know how many plant species are out there, hence the varying estimates. Tricky stuff.

However the numbers are crunched, though, I have yet to find any evidence behind the claim that 98.8 per cent of plants are toxic. I suppose the argument is that many of the plants we already eat use such defences, and so 鈥渆dible鈥� and 鈥渢oxic鈥� may not be mutually exclusive terms. So how do we define what is and isn鈥檛 edible?

Despite us often talking about toxicity as if it is a strict either/or concept, the lines are actually more than a little blurry. The reality is that it is entirely dose-dependent. Water and even oxygen can be deadly when consumed to excess, despite being essential for life. The same is true for many of the substances plants produce to tackle bacteria, fungi and insects, which can actually have the opposite effect in our bodies.

The sulphur compounds produced by broccoli as a toxic defence against insect attacks are the exact same ones that seem to help give the vegetable many of its nutritious effects in humans, for example. The beta-carotene that makes an excess of carrot juice a bad idea is also converted into vitamin A by the body, which is essential to our survival in the correct amounts.

Even determining the dose at which these compounds become problematic is tricky. For many plants, the only sure-fire way to do this would be to run large clinical trials that intentionally poison hundreds of people to establish the exact safe dose, so we instead rely on quite arbitrary estimates based on limited evidence from accidental poisonings and animal studies. As a result, what is legally considered an unsafe level of bitter almonds or cherry extract in some countries is perfectly permissible in others.

This huge level of complexity means that the toxic and edible labels are largely cultural constructs in many cases. As such, creating an extreme narrative based on these terms is very easy to do and still technically correct, though very misleading.

Fortunately, we have decades of consensus from scientific studies, consistently showing that people who consume higher levels of fruit and vegetables tend to also enjoy far better health outcomes, which would be rather a surprise if they were really so toxic.