How would I go if I tried to convince you that most food contains poison? You might nod and agree but I am not talking about processed junk laced with manmade chemicals, but rather this is all about the natural plants that make up the majority of our diet. Toxins are essential to the success of plants and they play an inescapable ecological role. They range from the spectacularly deadly like curare and oleander, to the mildly annoying and everything in between.
Not all foods contain toxins. Those that do not often rely on other kinds of defences. Most animal based foods do not contain toxins since they can rely on behaviour to protect themselves. Some plants rely on physical barriers, like a thick shell on a nut, or spines on a juicy stem, meaning the plant can function without additional layers of chemical protection. Ripe fruit also often contain relatively little chemical defence unless specialised to be eaten only by a limited range of animals. Our hominin ancestors relied on many of these food sources before the adoption of fire for cooking. This key technology allowed many toxic plants to be rendered harmless and greatly expanded the range of plant species that could be consumed. Leaching and fermentation are also useful techniques to defuse chemical defences.
Humans, with remarkably weak mouths and digestive systems and highly demanding brains need to eat relatively concentrated foods. When a plant accumulates resources (starch/oil/protein/minerals) in a concentrated organ like a tuber or seed it represents a tempting target for other organisms to take for themselves. The plant needs to invest additional resources in preventing that from happening. Plants use a wide variety of active agents to do the job, with a few prominent examples from domesticated food crops listed below.
Calcium oxalate crystals- Found in taro and rhubarb leaves as needle like crystals that pierce soft tissues causing intense irritation.
Cyanide as glycosides- Toxin that prevents blood carrying oxygen found linked to sugars in cassava, barley, sorghum, flax and bitter almonds.
Alkaloids- nitrogen containing toxins in many plants with a wide range of effects but best known in potato and its relatives.
Lectins- Sugar binding proteins found in many plants but richest in plant foods, toxic examples include raw kidney beans and other legumes that cause clumping of red blood cells.
Phytate- A stored form of phosphorus that is concentrated in all seeds that prevents the uptake of minerals during digestion.
Enzyme inhibitors- Proteins that bind to your digestive enzymes and prevent them from breaking down food, found in all grains and legumes and sweet potato.
Saponins- Soap like molecules that inhibit feeding and damage membranes, high levels present in unwashed quinoa.
Glucosinolates- sulfur containing chemicals that prevent iodine uptake, found in Brassicas like kale.
Tannins- Astringent chemicals that bind to protein making it indigestible, found in high levels in berries, legume seeds, acorns, unripe persimmon.
This is a cursory list and the complete range of plant defence chemicals is quite extraordinary. This diversity exists because each toxin is of limited use ecologically. If too many plants use a particular defence strategy the reward for an animal species that breaks through is too great. This principle applies to both natural and artificial chemicals used on excessive scales. Instead a functional ecosystem forms a patch work of plants with different defences. An animal species must then settle on a limited number of plant species to specialise in eating. The most toxic plants often end up with the most specialised predators, for example monarch butterflies on milkweed, crow butterflies on oleander and cabbage white butterflies on our garden Brassicas. The various beetles that specialise on toxic Solanums and Cucurbits also fit this pattern. Indigestible grains often get infested by highly specialised beetles. Toxins often work by limiting the amount of a particular plant a herbivore can safely eat, limiting the amount of damage to an acceptable level. Humans sometimes suffer effects even in cooked food if our diets are lacking in diversity, for example peas and lentils contain non-protein amino acids that can cause toxicity at higher doses. Oxalate overload is also becoming more common, especially in people rapidly shifting to “healthier” diets, causing many effects on top of painful kidney stones.
Plants also play an optimisation game with their defences since these chemicals require energy and nutrients to create. A plant that produces a high level of chemical defences will grow and reproduce more slowly than one that invests less, so plants tend to produce the lowest level of defence necessary to achieve satisfactory rates of reproduction. Many plant defence chemicals are targeted to the most significant threats- microbes and insects- with their effect from vertebrates being a side effect. For example the hot chemical capsaicin found in chillis is meant to prevent fungi growing on the seeds with the effect on human taste buds being a random side effect. Phytoestrogens can poison livestock that eat excess clover but its primary role is also as an antifungal.
Wild plants often contain higher levels of defence chemicals than their domesticated counterparts. For example wild relatives of potatoes contain much higher levels of toxic alkaloids, requiring them to be processed and detoxified in the bright ultraviolet light of the high Andes. As strains emerged with lower alkaloid levels they were preferentially grown since they required less intensive preparation. Cassava is a widely grown starchy staple tuber that recently received the attention of western plant breeders who produced low cyanide varieties. These “superior” strains were mostly rejected by subsistence farmers since they were too vulnerable to pests. Exchange of crop plants from one culture to another often happens without the accompanying cultural practices that can manage the plant toxins appropriately. A great example of this is the adoption of soybean in western agriculture, without the Asian appreciation of the potential health impacts of unfermented soy foods. Prolonged dietary intake of plant defence chemicals can lead to gradual adaptation among the human populations consuming them. For example populations that consume many oxalate rich foods are known to acquire gut bacteria that help break down and detoxify the oxalate.
One plant that has recently appeared on my radar is the locally grown black bean tree, or moreton bay chestnut (Castanospermum australe). It is a large rainforest tree in the bean family that produce large starchy seeds about the size of a matchbox. Recent genetic analysis has shown the southern populations were probably spread by human activity since they were an important staple crop of the local Aborigines. Like any large concentrated nutrient prize they are protected by toxins. They contain a potent alkaloid called castanospermine that inactivates your enzymes that break down starch and glycogen, causing you to starve internally even if plenty of carbohydrate energy is present. They also contain bitter saponins that make the raw seeds unpalatable. Through a combination of roasting, cutting into thin slices, soaking in running water and cooking as a mash the seeds can be made edible. I am hopeful this long process can be simplified by adding fermentation steps in the future but I will need to proceed with caution.
The human capacity to detoxify a wide range of high value plant foods is one of our most important tricks. It allowed us to be the first truly generalist herbivore as hunter-gatherers that could access all of the most concentrated plant foods in an ecosystem, then allowed us to cultivate crops that were toxic to all but the most specialised insect pests. No other species has struck such an ecological jackpot, but it has also allowed us to break free from the checks and balances of ecology. We even adapted beyond the limits of living biomass to start extracting energy first from firewood and eventually from burning a wide range of fossil fuels, and even started to burn uranium atoms themselves left over from the debris of exploding stars. Each of those energy sources had their own toxins to contend with, from carcinogenic wood smoke to radioactive waste. Every prize comes with a price.
Zero Input Agriculture 
Another informative article. Thank you for writing.
“Wild plants often contain higher levels of defence chemicals than their domesticated counterparts.”
This probably explains why cultivated hallucinogenic plants like Peyote have lower alkaloid contents than those found in the wild. This is one example were domestication or cultivation brings on the opposite of the desired effect.
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I saw an even more interesting example. Echinacea became a popular herbal medicine a while ago, so farmers began growing it all over the place. When they tested the level of the supposed active compound in the crops they found a weird pattern. The farmers who grew Echinacea inside its original natural range had high levels of the medicinal compounds. Those farmers who were growing outside the natural range of the wild species had very low levels and were pretty much useless. Something about the combination of soil/climate/microbiome inside its natural range allowed it to make those chemicals which were presumably for its own defences.
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