Apricots, bamboo shoots, cassava, corn, wild cherry, elderberries, flaxseed, and lima beans all share a surprising trait: they’re all sources of cyanide.
These plants, along with almost 2,000 more, contain phytochemicals called “cyanogenic glycosides.” Cyanogenic glycosides have a chemical structure that contains one carbon with a cyanide group linked to a sugar (“glyco” means sugar). During digestion, the cyanide group is released and forms hydrocyanic acid (HCN). HCN is toxic to humans in small amounts (0.5 to 3.5 mg per kg), but if this amount is eaten over time, it’s not necessarily dangerous. Healthy humans can safely eat some cyanogenic plants if their diet includes adequate complete protein, which provides sulfur amino acids that help detoxify the HCN.
Bamboo shoots contain the highest amount of cyanogenic glycosides, with up to 8,000 mg per kg. Pandas can eat bamboo because they’ve adapted to a diet high in HCN, but people shouldn’t eat bamboo shoots raw or in great quantities. Cassava root contains a smaller amount of cyanogenic glycosides than bamboo (240 to 890 mg per kg), but is responsible for the majority of cyanide poisonings because it’s eaten as a staple food in tropical countries.
The cyanide-food connection
Among the twenty-four leading food plants in the world, sixteen are cyanogenic. Why are so many food plants cyanogenic? Scientists speculate that cyanogenic glycosides deter animals from feasting too heavily on the plant. The surviving cyanogenic plants were then more available to prehistoric humans—provided that people could process the plants to release HCN before consumption. Humans developed grinding, rinsing, and cooking methods to do this, and because animals don’t do any such processing, many tasty but cyanogenic plants were eventually adopted into agriculture.
Later in human history, we began to breed varieties with lower cyanide contents. For example, wild lima beans are much higher in cyanogenic glycosides than the supermarket limas we eat today. Older cookbooks recommend that limas always be boiled well in an open pot—for one reason, to break down cyanogenic glycosides that were still present at higher levels in older varieties. Boiling removes the HCN, but leaves the glycoside sugar, making the beans sweeter as well as safer.
Herbs that contain cyanogenic glycosides include bitter almond, elderberry, eucalyptus, flaxseed, and wild cherry. And there’s a relationship between these compounds and the plants’ healing properties.
For example, wild cherry is a traditional American remedy for coughs, while bitter almond is the traditional Chinese treatment. These two plants are both in the genus Prunus, along with sweet almond, peach, plum, other cherries, and apricot. Prunus species contain the cyanogenic glycosides amygdalin and prunasin, which form HCN when digested. HCN tranquilizes our respiratory “cough center,” so amygdalin and prunasin may be responsible for wild cherry and bitter almond’s abilities to aid hacking coughs.
Traditional herbal preparations of wild cherry and bitter almond probably contained cyanogenic glycosides or they wouldn’t have been considered helpful for so long. However, today’s commercial almond extracts (those used for cooking) and cherry cough syrups are unlikely to produce any HCN because grinding and high temperatures used during processing release the compound.
Elderberry and eucalyptus also are traditional remedies for coughs, colds, and flu, but their healing properties are probably not from cyanogenic glycosides alone—other phytochemicals provide more benefits. Note that elderberry bark, leaves, and unripe berries can produce toxic levels of HCN, so if you like to collect and use wild elderberry, make a tea from the flowers or cooked ripe berries.
Flaxmeal (what’s left after the oil is pressed out) is used as bulk animal feed, with no major toxicity reported from the cyanogenic glycosides contained in the whole plant. The oil pressing and grinding processes liberate some or all of the HCN. Flaxseed meal and whole flaxseed in smaller quantities of 25 to 75 g per day are considered safe and beneficial for humans. If you have concerns, cooking or baking flaxseed meal, which is already ground, will certainly release any remaining HCN.
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Bhatty, R. S., P. Cherdkiatyumchai. “Compositional analysis of laboratory-prepared and commercial samples of linseed meal and hull isolated from flax.” Journal of the American Oil Chemists’ Society 1990, 67:79–84.
Cunnane, S. C., et al. “High a a-linoleic flaxseed (Linum usitaissimum): Some nutritional properties.” British Journal of Nutrition 1993, 69:443–453.
Jones, D. A. “Why are so many food plants cyanogenic?” Phytochemistry 1998, 47:155–162.
C. Leigh Broadhurst holds a doctorate in geochemistry and is a nutrition consultant in Clovery, Maryland.
James Duke is a member of the Herbs for Health Editorial Advisory Board. His most recent book is The Green Pharmacy (Rodale, 1997).