Consider this 2008 experiment, led by a team of scientists at Duke University. The researchers began by testing a mouse strain that, thanks to a genetic mutation, can't taste sweetness. As a result, these mice showed no immediate preference for sugar water.
But then the scientists discovered something odd, which is that the mutant mice gradually came to prefer the sugar water, even though they couldn't taste the sweetness. How could this be? How could an animal prefer what it couldn't even perceive?
The first clue came from a control experiment with sucralose, an artificial sweetener used in diet sodas. When the mutant mice were given sucralose water, they never developed a preference for it, which suggested that they were drawn to the calories of sugar rather than to its taste.
Sure enough, subsequent experiments demonstrated that "post-ingestive" factors—the ability of the gut to detect calories—were driving this preference. The mutant mice couldn't enjoy the taste of sugar, but they learned within a few hours to enjoy the taste of energy.
The detection of calories in the stomach and intestines also led to measurable changes in the brain. After the mice drank sugar water, there was an increased release of dopamine, a neurotransmitter associated with the processing of rewards. (Previously, it was assumed that only the taste of a food could trigger the release of dopamine.) Interestingly, the mutant mice only showed this spike of neurotransmitter when drinking real sugar water—the artificial sweetener gave them no chemical satisfaction.
--Jonah Lehrer, WSJ, on redundant calorie-seeking systems. HT: Franklin Shaddy