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I was wondering why dogs shouldn't eat chocolate. Can't dogs just excrete the indigestible component in their droppings?

It's common knowledge that dogs shouldn't eat chocolate. What I don't know is why chocolate would kill them, from a specifically biological perspective.

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    $\begingroup$ Welcome to Bio.SE! I upvoted this question because it generated an interesting answer to me. Generally, for future posts do consider doing a bit more research and explaining what you discovered before posting. This community isn't a "homework service". $\endgroup$
    – James
    Jun 12, 2017 at 13:25
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    $\begingroup$ Please note that chocolate is toxic also to humans, the difference is that we can tolerate much higher doses. There are also some positive effects of cocoa ingestion so I am not asking anyone to stop eating cocoa-based products. Just adding some context. $\endgroup$
    – okolnost
    Jun 12, 2017 at 16:38
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    $\begingroup$ Dogs can eat chocolate just like you can eat nightshade: once. Maybe update the question to say "shouldn't"? $\endgroup$
    – zzzzBov
    Jun 12, 2017 at 20:04
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    $\begingroup$ @zzzzBov can have (at least) two meaning - being physically capable of and being permitted to do something. E.g. "you cannot drive more then 50 mph in this zone". $\endgroup$
    – Maciej Piechotka
    Jun 12, 2017 at 22:17
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    $\begingroup$ "Why shouldn't humans eat cyanide? Can't humans just excrete the indigestible component in their droppings?" $\endgroup$
    – geometrian
    Jun 14, 2017 at 2:25

3 Answers 3

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The reason is simple: Chocolate contains cocoa which contains Theobromine. The darker the chocolate is (meaning the more cocoa it contains) the more theobromine it contains. This is a bitter alkaloid which is toxic to dogs (and also cats), but can be tolerated by humans.

The reason for this is the much slower metabolization of theobromine in the animals (there are reports for poisonings of dogs, cats, birds, rabbits and even bear cubs) so that the toxic effect can happen. Depending on the size of the dog, something between 50 and 400g of milk chocolate can be fatal. As mentioned by @anongoodnurse the cocoa content in milk chocolate is the lowest and much higher the darker the chocolate gets.

The poisoning comes from the Theobromine itself, which has different mechanisms of action:

First it is an unselective antagonist of the adenosine receptor, which is a subclass of G-protein coupled receptors on the cell surface which usually bind adenosine as a ligand. This influences cellular signalling.

Then it is a competitive nonselective phosphodiesterase inhibitor, which prevents the breakdown of cyclic AMP in the cell. cAMP is an important second messenger in the cell playing an important role in the mediation of signals from the outside of the cells via receptors to a reaction of a cell to changing conditions. The levels of cAMP are tightly controlled and the half-life of the molecule is generally short. Elevated levels lead to an activation of the protein kinase A , an inhibition TNF-alpha and leukotriene synthesis and reduces inflammation and innate immunity. For references see here.

The LD50 for theobromine is very different among species (table from here), with LD50 as the lethal dose killing 50% of the individuals and TDlo the lowest published toxic dose:

enter image description here

The LD50 also differs between different breeds of dogs, so there are online calculators available to make an estimation, if there is a problem or not. You can find them for example here and here. The selective toxicity makes it even an interesting poison for pest control of coyotes, see reference 4 for some details.

References:

  1. Chocolate - Veterinary Manual
  2. Chocolate intoxication
  3. The Poisonous Chemistry of Chocolate
  4. Evaluation of cocoa- and coffee-derived methylxanthines as toxicants for the control of pest coyotes.
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    $\begingroup$ Very interesting, thanks! I always assumed that dogs couldn't/shouldn't eat chocolate because of their lactose intolerance, similarly to why you might regret giving your cat milk if it hasn't been drinking milk its whole life. $\endgroup$
    – James
    Jun 12, 2017 at 13:22
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    $\begingroup$ Milk chocolate is rarely a cause of poisoning in dogs because it's the lowest in cocoa powder. $\endgroup$
    – anongoodnurse
    Jun 12, 2017 at 15:26
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    $\begingroup$ From these articles, I gather that you can, however, give your dog white chocolate because it contains barely any theobromine. Or is that a gross misunderstanding of the article? $\endgroup$
    – Nzall
    Jun 12, 2017 at 20:46
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    $\begingroup$ I may or may not have been calculating my chocolate LD50 for my bodyweight when I saw that table. You know, just to be safe! $\endgroup$
    – Cort Ammon
    Jun 13, 2017 at 17:23
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    $\begingroup$ So humans can literally die from eating too much chocolate!? A casual search suggests dark chocolate contains about 10% theobromine, so eating 0.7Kg (1.5 lb) of chocolate will kill the average (70kg) person. That means the name given to the popular death by chocolate recipe maybe wasn't given in jest. $\endgroup$
    – Bohemian
    Jun 13, 2017 at 17:29
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The toxic ingredient in chocolate is in the mythylxanine class, a substance called theobromine. It is much like theophylline; overdoses of theophylline used to be very common before the advent of inhalers for the treatment of asthma. (Chocolate also has some caffeine in it, which may exacerbate the effects of theobromine.) As @Chris stated, it is only slowly metabolized in dogs.

Generally, toxicity depends on the size, age, the breed of the dog, any medical problems the dog may already have, as well as what type of chocolate was consumed. The larger the dog (given no other predisposing factors), the more theobromine they can handle, and older dogs tend to have more problems than younger ones.

Here is the dose of theobromine that can cause dangerous symptoms, and death:

20 mg/kg (or 20 mg/2.2 pounds, or about 10 mg/pound) may cause agitation, hyperactivity and gastrointestinal signs such as drooling, vomiting, and diarrhea.

At >40 mg/kg, cardiac signs can be seen, and include a racing heart rate, high blood pressure, or even heart arrhythmias.

At >60 mg/kg, neurologic signs can be seen, including tremors, twitching, and even seizures.

Fatalities have been seen at around 200 mg/kg (~ 100 mg/lb), or when complications occur.

Here are the approximate amounts of theobromine per ounce of chocolate:

Cocoa powder: up to 800 mg/oz
Baker’s chocolate (unsweetened): up to 450 mg/oz
Dark chocolate: 150 mg/oz
Milk chocolate: 50 mg/oz

So, theoretically a 40 pound dog (medium-sized dog? My Border Collies each weigh 42 pounds, and are considered medium-sized) will not die if it eats a Giant Hershey's Symphony Bar (a very yummy 6.8 oz / 204g) because that's less than 10 mg/pound. I've never put this to the test, however, and would not recommend it.

Info at https://vcahospitals.com/know-your-pet/chocolate-poisoning-in-dogs

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    $\begingroup$ Would you be willing to specify theobromine content in some other, more internationally useful, unit as well? Say, mg/g or mg/kg, or mg/litre, or whatever is convenient. $\endgroup$
    – user
    Jun 12, 2017 at 21:17
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    $\begingroup$ I have answered the toxicity part in mg/kg, which is an international unit of measurement. There are 28.5 grams per ounce in milk chocolate, slightly more in baker's chocolate and cocoa powder. I have no idea why. $\endgroup$
    – anongoodnurse
    Jun 13, 2017 at 13:36
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    $\begingroup$ @MichaelKjörling there is a Firefox extension that will convert stuff on the fly. It can be set to show conversions on a menu, or annotate inline, or various other options. Never fret communicating with your Americqn friends again! Esp. useful for cooking sites and such where traditional measurements reign supreme. $\endgroup$
    – JDługosz
    Jun 13, 2017 at 16:05
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    $\begingroup$ In this case, I think his list is perfect with the mismatched units! We have figures for toxicity in mg already, and chocolate candy is sold and sized in oz. So this allows ready values to be found, looking up and converting in one step. $\endgroup$
    – JDługosz
    Jun 13, 2017 at 16:07
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    $\begingroup$ @JDługosz "chocolate candy is sold and sized in oz" In some countries, perhaps, but not where I'm at. I looked all over the wrapper for a candy bar I had at home, and nowhere does it say oz anything. And of course, I think anongoodnurse's comments that the number of grams per oz differs based on the product being measured is proof enough that such information helps international readers! $\endgroup$
    – user
    Jun 13, 2017 at 19:11
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I had a chocolate loving dog. She was 12 lbs. The vet said she could have it, provided she wasn't given too much. So his rule of thumb was not to feed her more than I would allow the average 2 year old in 24 hours. Which is two pips ( or two pieces from Hershey's bar). She lived to 16 yo. So yes, chocolate is a hazard to dogs, but like all things done in moderation it should be fine. Check with your vet.

Chocolate contains substances known as methylxanthines (specifically caffeine and theobromine), which dogs are far more sensitive to than people. Different types of chocolate contain varying amounts of methylxanthines. In general, though, the darker and more bitter the chocolate the greater the danger....For instance, 8 ounces (a ½ pound) of milk chocolate may sicken a 50-pound dog, whereas a dog of the same size can be poisoned by as little as 1 ounce of Baker's chocolate!

Chocolate Toxicity Meter for Dogs

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    $\begingroup$ Could you support your answer with any credible online reference? Almost all the time answers on Bio Stack Exchange are supported with reference(s) so that the readers can verify the claim put forward by the answerer. And accordingly upvote or downvote. One can also comment on (others') posts just to share and not to answer, but it will require them 50 reputation points which one has to earn by writing Qs and As (which are eventually upvoted as a sign of appreciation {I think}). $\endgroup$
    – Tyto alba
    Jun 12, 2017 at 16:32
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    $\begingroup$ Obviously, depending on the dosage you can feed dogs some chocolate without outright killing them. They might have some digestion problems, vomit, or look just fine even though cocoa is toxic for them. But why on Earth would you do that? What are the benefits? $\endgroup$
    – okolnost
    Jun 12, 2017 at 16:43
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    $\begingroup$ @anongoodnurse Similar principle, quite different magnitudes. Also, human deciding for himself about costs and benefits seems to me as different from poisoning a dog on purpose. Are you actually supporting this or is this more of an intellectual exercise (not a bad one, in fact). $\endgroup$
    – okolnost
    Jun 12, 2017 at 17:23
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    $\begingroup$ @okolnost - It depends on how you look at it. I want my dogs to be happy, so they get treats and all kinds of exercise/interactions. A little milk chocolate on occasion doesn't hurt them any more than running after tennis balls. A little bit of milk chocolate on occasion won't harm the vast majority of dogs. But there are those purists that won't indulge a dog, but will indulge themselves in grilled meats, alcohol, etc. $\endgroup$
    – anongoodnurse
    Jun 12, 2017 at 17:31
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    $\begingroup$ @anongoodnurse Understood. I would probably try to find a different and healthier treat for my dog (if I had one), as I do with my treats. But I can see your point and I when I think about it I would probably let you take care of my dog (again, if I had one). :-) $\endgroup$
    – okolnost
    Jun 12, 2017 at 17:38

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