Scattered on mammalian tongues are specialized taste bud receptors that recognize specific tastes, including, of course, sweet. Two different proteins hook together to make each sweet receptor. According to Brand, Xia Li (lead author of the study), and their coauthors, cats do not produce one of the proteins, because the gene that codes for it is a pseudogene and is not functional. It is this lack of a functional sweet receptor that explains cats' indifference to sweet stimuli.
Cats have no way to recognize carbohydrates, and therefore eat an "Atkins-like" diet of meat and fat. These findings lead to more questions, Brand says. "The overarching evolutionary questions are: when and why did cats lose the ability to taste sweet things? Which came first, the carnivorous behavior or the inability to taste sweets?"
Brand adds, "I say jokingly, no wonder cats are cranky--not only do they have to hunt for their food, but they also can't enjoy a sweet dessert!"
Citation: Li X, Li W, Wang H, Cao J, Maehashi K, et al. (2005) Pseudogenization of a sweet-receptor gene accounts for cats' indifference toward sugar. PLoS Genet 1(1): e3.