The Curious (Toxic) 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. In the last few days alone, there have been headlines ranging from Chocolate Poisoning and More to  Pets and the Peril of Chocolate.

And that’s entirely due to, yes, 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

Theobromine, 3-D model

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.)

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. “See a vet immediately” is the message of one cautionary post, titled Toxic Chocolate. 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.


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22 Responses to The Curious (Toxic) Chemistry of Chocolate

  1. Moreno says:

    Fantastic reading! A sweet death indeed.

    BTW, the link to the Nat Geo chart is broken.

  2. Deborah Blum says:

    Thanks! And thanks for the tip – the link is now working.

  3. Tim Reynolds says:

    Hmm interesting. I note that the LD50 OF Bisphenol A is over 2000 mg/kg for lab animals. I guess a ban on the manufacture and consumption of chocolate must be a priority for legislation now?! Happy Valetines y’all.

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  7. Roger says:

    Lowers blood pressure? I’m going to keep some in my desk.

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  9. Robert Ovalle says:

    Fantastic article, well articulated and witty. However, I have a few items to point out. For the lay person, it would be helpful to elaborate the idea of enantiomers/isomers since not all 7 carbon molecules with 8 Hydrogens, 2 nitrogens, etc result in Theobromine. The uninformed reader may be misdirected by the lack of clarification.

    There are issues with formatting. The line “By this standard, theobromine discovered in cacao beans in 1841, might sound to you

    like a basic wuss of the alkaloid” is broken by bad formatting.

    Paragraph 8 would best be supported by a an example. You could state something along the lines of “A typical 200 lb man would need to consume X amount of chocolate to be in the LD50 range”, etc. Also, a sentenced can be introduced at paragraph 8, or the previous one, making the connection between the anxiety and rapid heart beat to those being the initial signs of toxicity of theobromine.

    The claim that dogs are more likely to eat something sweet than cats, in my opinion, requires a source. Can you name a study that cites this, or is it merely conjecture?

    You start off your thesis saying you will attempt to explain why this alkaloid in chocolate is called theobromine or “food of the gods”, but at the end o the paper you divert to another topic, a family pet emergency, but fail to “bring it home” and state why it is called “food of the gods”. You had a good direction towards a brilliant conclusion when you stated the benefits of chocolate, how it lowers blood pressure, and all of those good things. Later on you diverged into a different direction.

    Also, the few sentences about how it’s more “romantic” to walk your dog… that left me confused as to how that pertains to the topic, or how that even fits in the article itself. It seems extraneous and unrelated. Perhaps it needs to be developed more so that the reader can see the connection.

    And although you might think I’m being a bastard for ripping apart your paper in the middle of Thanksgiving holiday, I think your original article could be better if a few ideas were developed more. It’s such a good topic and such a unique one at that, that it merits further development and details. I really liked and enjoyed your take on the chemistry of theobromine and chocolate.

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