When asked to name a poisonous plant, you probably don’t think of potatoes or apples. However, everyday vegetables and fruits contain compounds that can have harmful effects. In some rare cases, they have even been fatal to humans. Meanwhile, plant compounds that are harmless to humans can be dangerous to pets and livestock.
Did you know? The term theobromine is derived from a Greek word meaning “food of the gods.”
Many plants produce chemical compounds in order to ensure their survival. They use the compounds to protect themselves against immediate predators such as birds, insects, and fungi. The chemicals produced by most edible plants generally do not affect larger animals, such as dogs, cats, and humans. However, they still contain chemical compounds that can be toxic in larger doses.
One everyday example is the potato, which is a member of the nightshade family of plants. Potatoes contain a toxic substance called solanine, a member of a group of naturally-occuring chemical compounds called alkaloids. Solanine can cause vomiting, diarrhoea, and headaches. At particularly high levels, it can even cause hallucinations, paralysis, and death.
Normally, potato tubers contain very little solanine. However, when they are exposed to light, solanine levels increase enough to cause harmful effects to humans. Reports of potato poisoning occur once in a while, and they can all be attributed to improper storage. Prominent cases include the recent death of a Russian family, whose members succumbed to gas build-up from rotten potatoes, and a 1983 incident where 60 Alberta students suffered from vomiting and diarrhoea minutes after eating a baked potato school lunch.
The incidence of potato poisoning is thankfully low because the production of solanine, which gives potatoes a bitter taste, usually occurs at about the same rate as the production of chlorophyll, which gives potatoes a greenish tinge. And who wants to eat green potatoes? The US National Institute of Health recommends that potatoes that are green below the skin not be eaten.
Did you know? A modified form of amygdalin, called laetrile, was used to treat cancer in the 1950s. It was later found to be ineffective.
Other members of the nightshade family also produce varying levels of alkaloids. For example, the leaves and stems of tomatoes and eggplants are slightly lower in alkaloids than potatoes. Other members of the nightshade family, such as deadly nightshade (belladonna) and henbane, are highly toxic to humans. Yet cattle and goats seem to be able to eat all parts of belladonna without suffering any negative effects. A very low dosage (0.15-0.3 mg of alkaloid per day) of belladonna extract has been used as homeopathic cure for stomach spasms.
Cyanoglycosides are another class of potentially dangerous substances found in edible plants, and which your body converts into the highly-toxic free hydrogen cyanide. Hydrogen cyanide, consisting of a carbon atom triple-bonded to a nitrogen atom, prevents the creation of ATP (adenosine triphosphate) molecules that your body needs to produce energy. As a result, exposure to significant amounts of hydrogen cyanide can result in death. Amygdalin is the name of the cyanogenic glycoside present in apples, bitter almonds, peaches and apricots. It is found mainly in skins and seeds, which can contain enough amygdalin to cause severe health issues and even death. One woman in New Zealand had to be hospitalized after she ate 30 apricot kernels!
Some edible plants are perfectly harmless to humans, but toxic for cats and dogs. For example, onion and garlic contain sulfoxides that decrease the lifespan of red blood cell lifespan in both cats and dogs, resulting in anaemia. Even low doses of theobromine, found in chocolate and coffee, can cause severe health issues, such as heart failure, in canines and felines.
Perhaps the Roman philosopher Lucretius said it best: “What is food to one man is bitter poison to others.”
Horrific Tales of Potatoes That Caused Mass Sickness and Even Death (K. Anabelle Smith, Smithsonian Magazine) Potato plant poisoning - green tubers and sprouts (MedlinePlus, US National Institute of Health) Cyanogenic Glycosides – Information Sheet (New Zealand Food Safety Authority) Foods That Are Hazardous to Dogs (American Society for the Prevention of Cruelty to Animals) Foods That Are Hazardous to Cats (American Society for the Prevention of Cruelty to Animals) Rotten Potato Gas Kills Four in Russian Garage – Investigators (RIA Novosti) Atropa Belladonna (European Agency for the Evaluation of Medicinal Products)
Vetter J. 2000. Plant cyanogenic glycosides. Toxicon. 38:11-36.