Chocolate as Poison – Wired Science

Chocolate as Poison – Wired Science.

 

In honor of Valentine’s Day, I have once again updated this – one of my favorite posts – on the poisonous chemistry of chocolate.

The Latin name for the cacao tree – the tropical plant source of all things chocolate – consists of two words packed with candy-loving scientific exuberance.  Theobroma cacao. It derives from the Greek words for god (theo) and food (brosi), roughly translating to  “food of the gods”.

Well, sure you say. Obviously. This is chocolate, after all.  Almost goes without saying. Which is why I won’t. Actually, I’m mostly trying to explain why the  most potent chemical compound  in chocolate – a plant alkaloid, slightly bitter in taste, surprisingly poisonous in some species – is called theobromine.

And while chocolate, as a whole, has a wonderfully seductive  chemistry, this poison-obsessed blog will remain, well, obsessed. Today’s obsession is inspired by  the fact that  every Valentine’s season, in addition to stories about love and lace, newspapers run cautionary candy tales. The website PetMD sends out reminders about its Chocolate Toxicity Meter. And this year, the Pet Poisoning Hotline was inspired to include a warning rhyme in its Valentine’s Day tips, which goes “Roses are Red/Violets are Blue/Chocolate can be toxic/And lilies are too.”

Except for the lilies, of course, we’re talking about theobromine.

So theobromine is an alkaloid, meaning it’s part of the everyday chemistry of the plant world.  Plant alkaloids are nitrogen-based, typically with with flourishes of carbon, hydrogen and occasionally other atoms such as oxygen.  The recipe (or as chemists like to say, formula) for theobromine is seven carbon atoms, eight of hydrogen, four of nitrogen and two of oxygen.

And while this may sound like a recipe for the routine, alkaloids are anything but.  The first plant alkaloid isolated (in 1804) was morphine from the flowering poppy.  Other notable examples include cocaine (1860),  nicotine (1828), caffeine (1820), strychnine (1818) and a host of pharmaceuticals including the anticancer drug Vincristine; the blood pressure medication, reserpine; and the antimalarial compound, quinine.

By this standard, theobromine discovered in cacao beans in 1841, might sound to you

like a basic wuss of the alkaloid family. It’s mostly known as a mild stimulant in humans; it contributes (along with caffeine and a few other compounds) to that famed lift that people get from eating chocolate.

There is some evidence that if people get carried away with chocolate consumption, of course, theobromine will make them a  little twitchy. According to the National Hazardous Substances Database: “It has been stated that “in large doses” theobromine may cause nausea and anorexia and that daily intake of 50-100 g cocoa (0.8-1.5 g theobromine) by humans has been associated with sweating, trembling and severe headache.”  Occasionally, people (mostly the elderly) have needed hospital treatment for a theobromine reaction.

But if one looks at LD50 values, it’s obvious that the alkaloid is far more threatening to other species. LD50 is an oral toxicity measurement; it refers to the dose that will kill 50 percent of a given population and is usually calculated in milligrams of poison per kilograms of body weight.  The theobromine LD50 is about 1000 mg/kg in humans. But for cats it’s 200 mg/kg and for dogs it’s 300 mg/kg – in other words, dangerous at a far lower dose.

This varies, of course, by animal size and shape and breed. A few years ago, in fact, National Geographic published a fascinating interactive chart so that pet owners could search out the individual risk.   The chart focuses on dogs because they are more likely than cats to eat something sweet. And it notes that theobromine is more concentrated in dark chocolates making them more dangerous than milk or “white” chocolate. The dark chocolate effects are so acute for canines, that the alkaloid has been tested with some success as a means of controlling coyote populations. (Interestingly,  rats and mice are much less affected; their theobromine LD50 is much more like that found in humans.) Dogs are so sensitive though that, as the Pet Poison Hotline, notes, they’ve been poisoned by everything from cocoa mulch to cocoa powder.

The different toxicities have to do with the way different species metabolize the alkaloid; humans process it much more efficiently than canines. And in small amounts, theobromine’s effects can make it medically useful.  But even here, it shows complexity. It increases heart rate and at the same time it dilates blood vessels, acting to bring down blood pressure. It can also open up airways and is under study as a cough medication.  It stimulates urine production and is considered a diuretic. It interacts with the central nervous system, although not as effectively as caffeine.

At toxic levels – in a characteristic dog death, for instance – all of this adds up acute nausea, convulsions, internal bleeding and often lethal over-stimulation of the heart. WebMD tells the story of a couple who fed their poodle a full pound of Valentine’s Day chocolate; the dog was on anti-seizure medications for a full five days afterward. Another column, written by a vet, suggests rather hopefully that an evening walk is far more romantic and less likely to feature pet vomit (which she describes in revoltingly foamy detail).

We had that same foamy experience in our household in December, actually, when our dog discovered our son’s holiday stash. We all survived but the humans in the house are a lot more careful about where they leave their chocolate. And this Valentine’s Day, we’re sticking to champagne. Sure, ethanol is also a poison in its own right. But that’s a different story.

Homepage photo: kev-shine/Flickr

Deborah Blum is a Pulitzer-Prize winning science writer and the author of five books, most recently the best-seller, The Poisoner’s Handbook: Murder and the Birth of Forensic Medicine in Jazz Age New York. She writes for a range of publications including Time, Scientific American, Slate, The Wall Street Journal, The Los Angeles Times (and even the literary journal, Tin House). She is currently working on a sixth book about poisonous food.