Sugar and spice and everything nice hold no interest for a cat. Our feline friends are interested in only one food: meat. This predilection is not solely the result of an inner killer just waiting to catch a bird or torture a mouse. It also occurs because cats lack the ability to taste sweetness.
The cause has been traced to a gene. The tongues of most mammals hold taste receptors—proteins on the cellular surface that bind to an incoming substance, activating the cell’s internal workings and leading to a signal being sent to the brain. Humans enjoy at least five kinds of taste buds: sour, bitter, salty, umami (or meatiness) and sweet (as well as possibly fat). The sweet receptor is actually made up of two coupled proteins generated by two separate genes: Tas1r2 and Tas1r3.
All cats, however—including lions, tigers and British longhairs, oh my!—lack a chunk of DNA that occurs in the Tas1r2 gene of other mammals. As a result, the depleted feline gene (more properly called a pseudogene) does not code for the proper protein and thereby prevents cats from tasting sweets. “They don’t taste sweet the way we do,” says neurobiologist Joe Brand, former associate director and now member emeritus of the Monell Chemical Senses Center in Philadelphia.
Brand and his colleague Xia Li discovered the pseudogene about 12 years ago, when they followed up on decades of anecdotal evidence of cats’ being sweetness-impaired, such as cats showing no preference for sweetened over regular water.
Of course, there are also plenty of anecdotal accounts pointing in the other direction: cats that eat ice cream, relish cotton candy or chase marshmallows. “Maybe some cats can use their [Tas1r3 receptor] to taste high concentrations of sugar,” Brand says. “Or perhaps they are cuing on some other compound that we cannot appreciate.”
Scientists do know, however, that cats can taste things we cannot, such as adenosine triphosphate (ATP), the compound that supplies the energy in every living cell. “There isn’t a lot hanging around in meat, but it’s a signal for meat,” Brand says. Plenty of other animals have a different array of receptors, Li says, from chickens that also lack the sweet gene to catfish that can detect amino acids in water at nanomolar concentrations. “Their receptor is more sensitive than the background concentration,” Brand notes. “The catfish that detects the rotting food first is the one that survives.”
Cats are not alone. Brand and his colleagues have since found that all felines (some 36 species) and several other species of strictly meat-eating mammals all lack a sweet receptor. Cats may also lack other components of the ability to enjoy (and digest) sugars, such as glucokinase in their liver—a key enzyme that controls the metabolism of carbohydrates and prevents glucose from flooding the animal’s bloodstream.
Despite the cat’s inability to handle sugar, most major pet food manufacturers use rice or other grains in their meals. “This may be why cats are getting diabetes,” Brand offers. “Cat food today has up to 20 percent carbohydrates. Cats are not used to that—they can’t handle it.” What these fearsome predators of suburbia cannot taste may be hurting them. But it also means that most cat lovers don’t have to worry about Kitty snatching their unattended dessert.