It was a sorry end. Cut down in his prime, the cunning thief lay on the slab, his cold body offering pathologist Brett Gartrell no outward sign of how he had met his maker. Once Gartrell had wielded his scalpel, however, the cause became clear: a belly stuffed with sticky brown gunk. Diagnosis? Death by chocolate. Divine – yes. Delicious – absolutely. But deadly? For some it certainly is. The corpse on Gartrell’s slab belonged not to a human but to a kea, an endangered New Zealand parrot. Like many animals, keas are acutely sensitive to chemicals in chocolate that are harmless to humans in all but huge doses. Scientists are now studying these chemicals, along with other substances in cocoa, hoping to exploit their toxic effects to control pests or microbes. If you’re reading this after scoffing your fifteenth chocolate Santa, don’t panic: we humans have been safely enjoying the beans of the cacao plant, Theobroma cacao, for millennia. Theobroma is Greek for “food of the gods”, reflecting the Mayan belief that cocoa had divine origins. Every April, they sacrificed a dog with cacao-coloured markings in honour of Ek Chuah, the god of cacao.
Knife-wielding priests aside, chocolate is still bad news for many animals. Cocoa beans are naturally rich in caffeine and its chemical relatives theobromine and theophylline, collectively called methylxanthines. To humans these are little more than benign stimulants, but to a number of animals they are highly toxic. Just 240 grams of unsweetened dark chocolate contains enough methylxanthines to kill a 40-kilogram dog, about the size of a German shepherd. It was methylxanthines that did for the kea too. Gartrell, a wildlife pathologist at Massey University in Palmerston North, New Zealand, is wearily familiar with keas’ propensity to poison themselves. Besides being arguably the world’s smartest birds, keas are extraordinarily inquisitive foragers, using their beaks to rip open tents and backpacks, open garbage bins and even pry pieces off cars in their quest for food. “They’ll try anything that is vaguely edible, which is part of the reason they get into trouble,” says Gartrell.
The dead kea was found outside a hotel kitchen in the holiday resort of Mount Cook Village in the Southern Alps. It had eaten more than 20 grams of dark chocolate, presumably pilfered from the kitchen garbage (New Zealand Veterinary Journal, vol 55, p 149). “He’d really pigged out,” says Gartrell. The ill-fated kea was by no means alone in its folly. Veterinary journals are peppered with stories of dogs, cats, parrots, foxes, badgers and other animals dropping dead after finding chocolate or being fed it by well-meaning humans. “The dead kea had eaten more than 20 grams of dark chocolate, pilfered from the kitchen garbage” The reason humans don’t turn up their toes after bingeing on chocolate is largely down to the speed at which our bodies metabolise theobromine, the most abundant methylxanthine in chocolate. Rats metabolise it much more slowly than humans, and dogs are slower still. There are no reliable figures for theobromine toxicity in humans, but based on caffeine toxicity an average adult would have to gorge on around 50 kilograms of milk chocolate in a single sitting to get anywhere near a lethal dose.
The observation that methylxanthines are highly toxic to animals, with dogs being especially vulnerable, prompted John Johnston, a chemist at the US Department of Agriculture in Fort Collins, Colorado, to investigate chocolate as a more selective way of controlling coyotes (Journal of Agricultural and Food Chemistry, vol 53, p 4069). Coyotes are a serious pest in the US, killing $44 million worth of livestock each year, damaging property and attacking people and pets. Measures such as fences are often ineffective. Sometimes culling them is the only option but unfortunately the poisons now used, such as sodium cyanide, are toxic to humans and most other animals too. “If we can come up with something that is more selective, it offers an advantage,” says Johnston. “It’s a more responsible approach.” Methylxanthines looked as though they might fit the bill. After testing the toxicity of several different types of chocolate, Johnson came up with a mixture of theobromine and caffeine that killed coyotes quickly and with minimal distress. The mixture can be hidden in bait and is currently undergoing field tests.
Methylxanthines are also shaping up as a way to dispatch other pests. Earl Campbell of the US Pacific Basin Agricultural Research Center in Hilo, Hawaii, discovered that caffeine sprays could kill two species of noisy and ecologically damaging Caribbean tree frogs that have plagued the island since they were accidentally introduced in the 1980s. Campbell noticed that the spray also killed slugs. His colleague Robert Hollingsworth then found that caffeine spray made snails kick the bucket too (Nature, vol 417, p 915). Hollingsworth is now developing caffeine as an alternative to conventional pesticides, such as those used in slug pellets. “There’s a huge amount of interest in using botanical extracts,” he says. “People are more comfortable with things that are natural.” The methylxanthines are just a start. “Cocoa is a real gold mine of different components,” says Herwig Bernaert, research manager at Barry Callebaut, a chocolate manufacturer in Zurich, Switzerland. Cocoa contains more than 700 compounds and there is a great deal of research on which of these can affect people or other creatures. Some of these compounds, such as the flavonoids, have commanded a lot of attention for their apparent health benefits, but researchers are also keen to exploit chocolate’s more sinister side.
Some studies suggest cocoa extracts can prevent Helicobacter pylori, the bacterium that causes stomach ulcers, from setting up shop in the lining of the gut. Others suggest that the extracts block the growth of disease-causing strains of the gut bacterium E. coli. Compounds isolated from chocolate could even be used to prevent tooth decay. Phil Marsh, a microbiologist at the Leeds Dental Institute in the UK, reported last year that a cocoa extract rich in polyphenols discouraged mouth bacteria from sticking to teeth and forming plaque. It also reduced the tooth-rotting powers of the bacterium Streptococcus mutans by reducing its ability to produce acid (European Journal of Oral Sciences, vol 114, p 343). But if you think that means it’s time to ditch the toothpaste and reach for that jumbo jar of chocolate spread instead, forget it. There’s far too little of these polyphenols in chocolate to outweigh the damage that will be done by all the sugar it contains. “There are many other compounds in a chocolate bar that would overwhelm any benefit of the polyphenol,” Marsh warns. As manufacturers fall over themselves to advertise chocolate’s health-boosting potential, there’s a wry satisfaction to be had in knowing that its sinister side is being put to good use too. While this may be good news for us, just make sure your furry or feathered friends can’t get their sticky paws or claws on it.
January 8, 2008
Original web page at New Scientist