What do corn, potatoes, beans, and cashews have in common? They can all be poisonous under the right circumstances. Some of the world’s most important food crops contain toxic compounds that require them to be cooked or combined with other foods to make them safe. Some, like the grass pea, have earned a world-wide reputation for turning a famine into an even more tragic catastrophe.
Also called chickling vetch, this pea has been a dietary staple in the Mediterranean, Africa, India, and parts of Asia for centuries. Like most legumes, it is an excellent source of protein, but it has one serious drawback: it contains a neurotoxin called beta-N-oxalyl-diaminopropionic acid, or beta-ODAP. The first symptom of beta-ODAP poisoning, or lathyrism, is a weakening of the legs. Eventually, the toxin kills nerve cells and victims become paralyzed from the waist down. Without treatment, they will die.
How has this pea remained such a popular ingredient in flours, porridges, and stews? If they are soaked for a long time in water or fermented in breads or pancakes, they pose little risk. Grass peas are one of the few food crops that can survive a serious drought. People are then left with little else to eat — and not enough water to soak the peas.
Hippocrates warned that people who “ate peas continuously became impotent in the legs.” Today one of the great tragedies of famines in places like Ethiopia and Afghanistan is that the high-protein pea is typically reserved for men to give them strength so that they can feed their families. Instead, it has the opposite effect, reducing them to crawling on their knees (and as one report noted, “Wheelchairs aren’t an option for most lathyrism sufferers, as they tend to live in dirt-floor huts”). Even if the drought receded and they stopped eating the peas, they might still be disabled for life.
Francisco Goya depicted the ravages of lathyrism in his circa 1810 aquatint print called "Gracias a la Almorta," or “Thanks to the Grass Pea.” He was portraying a grueling outbreak that occurred during Spain’s war for independence against Napoleon’s army.
The grass pea resembles a sweet pea. It is a climbing vine with fine tendrils and blue, pink, purple, or white flowers. It is often used as a fodder crop for cattle, and still shows up in the cuisine of many countries around the world.
Native people in the Americas knew how to prepare this local crop safely. Traditional recipes called for adding slaked lime or calcium hydroxide, a naturally occurring mineral, to corn. (The basic recipe for tortillas still includes the addition of lime.) Without it, the niacin in corn cannot be absorbed. This is not a problem unless corn is eaten by itself and makes up most of a person’s diet. When that happens — as it did with early settlers who did not understand the risks — the result is severe niacin deficiency called pellagra.
As early as 1735, when corn was imported from the New World, impoverished people in Spain and other European countries showed symptoms of pellagra. Those symptoms came to be known as the four D’s: dermatitis, dementia, diarrhea, and death. In fact, a pair of researchers writing for a British medical journal suggested that the ghastly symptoms of pellagra could have inspired European myths of vampirism in Bram Stoker’s Dracula: pale skin that erupted in blisters when exposed to the sun, sleepless nights brought on by dementia, an inability to eat normal food because of digestive problems, and a morbid appearance just before death.
During the first half of the twentieth century, pellagra sickened three million Americans and killed one hundred thousand. The disease was not entirely understood until the 1930s. Today corn is considered to be a perfectly safe and healthy part of the diet as long as it is eaten in combination with other foods.
Rheum x hybridum
The leaves of this Asian plant contain high levels of oxalic acid, which can cause weakness, difficulty breathing, gastrointestinal problems, and even coma and death in rare circumstances. In 1917 the Times of London reported on the death of a minister who died after eating a dish made from rhubarb leaves. The unfortunate cook admitted that she had used a recipe that she found in the newspaper titled “War Time Tip from the National Training Schools of Cookery.” In fact, there was a war on, and food was scarce, but recipes like this one added yet another threat to both soldiers and civilians.
Red Kidney Bean
Perfectly safe and healthy, except if eaten raw or undercooked. The harmful compound in kidney beans is called phytohaemagglutinin, and it can bring on severe nausea, vomiting, and diarrhea. People usually recover quickly, but it takes only four or five raw beans to bring on these extreme symptoms. The incomplete cooking of raw beans in a slow cooker is a common source of red kidney bean poisoning.
There’s a reason why grocery stores don’t sell raw cashew nuts. Cashews are part of the same botanical family as poison ivy, poison oak, and poison sumac. The cashew tree produces the same irritating oil, urushiol. The nut itself is perfectly safe to consume, but if it comes into contact with any part of the shell during harvest, it will give the person who eats it a nasty rash. For that reason, cashews are steamed open, making them partially cooked even if they appear to be raw. In 1982, a Little League team in Pennsylvania sold bags of cashew nuts that were imported from Mozambique. Half of the people who ate them developed rashes on their arms, groin, armpits, or buttocks because some of the bags of nuts contained pieces of cashew shells, which would have had the same effect as mixing poison ivy leaves with the nuts.
An important food crop in Latin America, Asia, and parts of Africa, the root is cooked in much the same way that potatoes are. Starchy flour derived from the root is used to make tapioca pudding and bread. There’s just one problem: cassava contains a substance called linamarin that converts to cyanide in the body. The cyanide can be eliminated through careful preparation that involves soaking, drying, or baking the root, but this process is imperfect and can take several days. In times of drought, cassava roots may produce higher levels of the toxin, and people in famine-stricken areas may eat more of the root and take less care with preparation.
Cassava poisoning can be deadly. Even at lower levels it can cause a chronic condition known in Africa as konzo. Symptoms include weakness, tremors, lack of coordination, vision problems, and partial paralysis.
The ackee fruit plays an essential role in Jamaican cuisine. Only the aril (the flesh surrounding the seeds) is safe to eat, and the fruit must be harvested at a precise point of ripeness or it may be toxic. Ackee poisoning, or Jamaican vomiting sickness, can be fatal if untreated.
This fruit, popular in jams, cakes, and pies, is much more dangerous when consumed raw. In 1983 a group of people attending a retreat in central California had to be flown by helicopter to a hospital after drinking fresh elderberry juice. Most parts of the plant, including the uncooked fruit, may contain varying levels of cyanide. Generally, people experience severe nausea and recover.
Amy Stewart is the award-winning author of six books on the perils and pleasures of the natural world, including four New York Times bestsellers, The Drunken Botanist, Wicked Bugs, Wicked Plants, and Flower Confidential. She lives in Eureka, California, with her husband Scott Brown. They own an antiquarian bookstore called Eureka Books and tend a flock of unruly hens in their backyard.