The plant kingdom has developed an arsenal of poisons to deter predation. Among these are pyrrolizidine alkaloids. Some 3 percent of all flowering plants produce these chemicals, including such herbal-garden favorites as borage and comfrey. When bees pollinate such plants, the little buzzers risk seeding the honey they produce with worrisome concentrations of the poisons, a new study charges.
In 1988, the World Health Organization designated pyrrolizidine alkaloids as a serious health threat. Even persistent, low-dose exposure to the poisons can cause cirrhosis of the liver or other diseases, notes John A. Edgar, a consulting organic chemist and toxicologist in Melbourne, Australia. The alkaloids have killed animals eating tainted feed or forage and struck people who unwittingly consumed contaminated grain or certain herbal drugs, he notes.
German-government regulations issued in 1992 limit the alkaloids’ concentrations in herbal medicines intended for oral use to 1 microgram per daily dose. The regulations also prohibit pregnant women and children from using such drugs and restrict their use by other people to no more than 6 weeks annually. Germany set tolerable doses of the alkaloids 90 percent lower still for products that might be consumed regularly. Edgar says the European Union is now considering adoption of similar limits.
The irony, he notes, is that the few chemical analyses of honeys from bees that visited alkaloid-producing plants reveal the poisons at concentrations that can greatly exceed their German limits in herbal drugs.
In the May 8 Journal of Agricultural and Food Chemistry, Edgar and his colleagues from Germany and the United States review dozens of studies underpinning their concerns about possible pyrrolizidine-alkaloid poisonings. Some of the studies reported concentrations of the alkaloids at up to 3.9 mg/g in honey. That’s a hefty amount considering that some adults report consuming 90 g of honey daily over long periods, and even infants may down 30 g of the sweetener per day.
People typically encounter traces of dozens of potentially deadly agents in foods, Edgar observes, such as aflatoxins in peanuts and fumonisins in corn flakes. He says: “I just want regulators to take pyrrolizidine alkaloids as seriously” as other toxins–by monitoring foods and working to minimize the alkaloids’ concentrations in our diets.