The plant poisons that shape our daily lives
An exploration of nature’s toxins reveals complex relationships between humans and the plant chemicals we use as foods, medicines and mind-altering drugs
Seemingly harmless chrysanthemums make a toxin used in pesticides and louse shampoos.Credit: VCG/VCG via Getty
Most Delicious Poison: The Story of Nature’s Toxins — From Spices to Vices Noah Whiteman Little, Brown Spark (2023)
One beautiful summer’s day about 30 years ago, my father had a mini-stroke. He was diagnosed with a heart arrhythmia and was prescribed the blood-thinning drug warfarin. But after a few years of relative stability, tests showed that his body was no longer metabolizing the warfarin properly. Searching for an explanation, doctors eventually realized that my father had started drinking grapefruit juice with his breakfast. The fruit contains chemicals called furanocoumarins, which stop warfarin being metabolized in the liver.
As my father’s experience shows, plants that we might think of as benign can — depending on our circumstances — be anything but. In fact, many are bioactive. In Most Delicious Poison, biologist Noah Whiteman delves into why plants’ myriad natural toxins arose, how animals have adapted to them and how humans have attempted, for better or worse, to harness them for our benefit, without fully understanding the effects that these poisons have on our brains and bodies.
Whiteman explores these ideas by looking at plant chemicals that people use for medicine, food and pleasure, including toxins such as ethanol (from plant sugars) that can end up as addictive substances.
Having lost his own father to alcohol-use disorder, the author seeks to understand the biochemistry and genetics of alcohol addiction, focusing on the possible role of proteins in the brain called GABAA receptors. When activated by molecules of the neurotransmitter GABA, these receptors have a calming effect. As the author finds, it is not only naturally occurring GABA molecules that trigger these receptors — ethanol seems to do so too, as do some commonly used sedatives. When considering how this interaction might have affected the people in his life who relied on alcohol, Whiteman writes: “Their GABAA receptors fired away as the alcohol did its thing, dampening their worries, numbing their pain, and transforming them into different people.”
GABAA receptors are also activated — at least in some species — by the chemical α-pinene, which is found in the oil and resin produced by balsam firs and their relatives. Fir trees, Whiteman explains, probably evolved the ability to make the chemical to trap insect predators and stop them from eating their leaves. Whether α-pinene interacts with GABAA receptors in people has yet to be proved, but the author proposes that this might explain some of the benefits of walking in the deep woods. For many, the scent of balsam seems to slow time, making the world fall away for a moment.
Why do plants make chemicals that target the nervous system? As Whiteman notes, it probably gives them an evolutionary edge. Plants are easy targets for herbivores, and a chemical defence can be crucial for survival.
Evolutionary quirks
Elsewhere in the book, Whiteman has more colourful stories illustrating the many uses that humans have for plant chemicals. The daisy family alone, for instance, makes alkaloids, flavonoids and terpenoids, and humans have co-opted, copied and synthesized these chemicals for use as anti-inflammatory medicines, pesticides, antimalarial drugs and more.
Grapefruit juice contains a chemical that prevents drugs such as warfarin from being metabolized by the liver.Credit: Wellford Tiller/Shutterstock
One daisy, the chrysanthemum, serves as an example of how evolutionary quirks can dictate the ways in which we use plant toxins. If you worry about ticks when out walking, you might rely on an insecticide called permethrin, which is a synthetic equivalent of chrysanthemum chemicals called pyrethrins. Both the natural toxin and the synthetic version interact with proteins in nerve cells, causing uncontrollable firing. These chemicals are relatively harmless to humans, but lethal to insects, thanks to a single genetic difference that makes insect nerve cells 100 times more responsive to them than are human ones. Pyrethrin is used in louse shampoos, and permethrin in insect-repelling clothing and flea collars for dogs. But cats must steer clear — another genetic difference means that they do not produce the enzyme that allows humans and dogs to detoxify the chemical.
Even spices might have evolved as toxins. Mustard oil, for example, is highly poisonous both to herbivorous insects and to the plants that make it, which include watercress, rocket and wasabi. To prevent the toxin from damaging healthy leaves, the plants make inactive precursors called protoxins. Like “bombs with unlit fuses”, the protoxins are stowed away in one type of cell, and the enzymes that activate them are confined in another. When a leaf is chewed — by a grazing insect or a person eating salad — the cells break apart, the protoxins and enzymes come into contact and, kaboom, the protoxin is converted into poison. The insect might die or leave its meal uneaten, but humans ingest such small amounts of the toxin relative to our size that it is not harmful to us. Instead of being poisoned, we can enjoy the spicy bloom of mustard flavour in our mouths.
The stories Whiteman chooses are often complex, because each class of chemical exists in a web of related poisons. In places it is easy to get lost in the chemistry, but the author deftly navigates readers through nature’s chemical mazes. In doing so, he reveals that plant toxins have helped shape who we are today. They expand our minds, interact with important enzymes and receptors in our bodies, spice up our foods and medicate us.
The author’s passion for his subject matter comes through on almost every page of Most Delicious Poison, and the book’s illustrations — collages of key plants, chemical structures, target species and human use — provide succinct visual summaries. Aficionados of chemical form, people interested in botanical pharmacology and toxicology, and those who are simply curious about the origins of their drugs and spices will find much to enjoy in this fascinating compendium.
Nature 622, 689-690 (2023)
doi: https://doi.org/10.1038/d41586-023-03303-z
This story originally appeared on: Nature - Author:Emily Monosson
