Ignoring high sugar and fat content of chocolate, it contains a chemical called theobromine that dogs cannot digest. In dogs this causes tachycardia, increased blood pressure and kidney problems that can be fatal if untreated.
Back in the late 1990s, I was an avid watcher of the BBC’s Animal Hospital programme and almost every week the vets would see a dog that had been brought in having raided its owner’s stash of chocolate. The problem that dogs have with chocolate is a serious one and it revolves around how they digest a specific ingredient called theobromine, a bitter-tasting alkaloid found in chocolate, coffee and tea.
Theobromine is part of a family of naturally occurring chemicals called methylxanthines that also include caffeine and the theophylline used in asthma medication. Methylxanthines stimulate the heart, causing tachycardia, relax certain muscle groups and act as a diuretic. Humans have little problem metabolising (breaking down) theobromine and excreting it, but in dogs the process is much slower. Theobromine is absorbed by the dog’s intestines after which it’s carried through the hepatic portal vein to the liver. Some is successfully excreted by the liver, but some makes it into the inferior vena cava; from here to the heart and lungs and into the main circulatory system. The situation is compounded by the fact that some of the by-products excreted by the liver into the bile ducts can be converted back into methylxanthines and reabsorbed by the small intestine, triggering a feedback loop.
Ultimately, this means that dogs cannot effectively breakdown and excrete theobromine and it hangs around in their systems for much longer than in humans. Indeed, most veterinary manuals suggest that dogs take about two and a half times longer to break down theobromine than humans do, although I’ve seen estimates suggesting some breeds may take up to five times longer. During this time, the theobromine is exciting the dog’s central nervous system, while also stimulating the heart and increasing blood pressure. Indeed, signs of theobromine poisoning in animals usually include excitement (i.e. nervousness and trembling), vomiting and/or diarrhoea, excessive thirst, muscle spasms and possibly seizures. Coma and eventually death can result from alteration of normal heart rhythm, although such extreme outcomes are rare. If caught in time, theobromine poisoning can be treated with IV fluids, emetics (vomit-inducers), activated charcoal, anti-seizure medications and cardiac medications.
On their website, the American Society for the Prevention of Cruelty to Animals note that, in dogs, theobromine toxicity typically starts at about 150 mg per kilogram body weight, but this varies according to the breed and poisoning has been documented at levels as low as 20 mg per kg. Milk chocolate contains theobromine at anywhere up to five grams per kilogram, the average being about 2 g/kg – dark chocolate can have twice as much. As a broad rule of thumb, the darker the chocolate the higher the theobromine content.
Ultimately, it can be very difficult to guess how much chocolate is likely to be too much. It will vary from dog to dog based on the individual’s susceptibility, size, breed, the purity of the chocolate given, what else the dog has eaten and so forth. Furthermore, it should also be remembered that it’s not just domestic dogs that are at risk; most mammals are susceptible to theobromine poisoning, with cats, rats and mice among those shown to suffer potentially fatal responses to even low doses. There are reports from the wild of foxes, badgers and even bears dying after having consumed too much chocolate. When you consider that chocolate also tends to be high in sugar and fat, the safest option is to avoid giving chocolate to any non-human animals.