The neuroscience of junk food:
While the McGriddle is a tasty creation, the most pleasurable thing about the sandwich isn't the pancake or the bacon: it's the calories. According to a recent paper in Neuron, the brain also receives rewarding input from metabolic processes that have nothing to do with the tongue. When you eat at McDonald's, a big part of the pleasure comes from the fact that the food is sustenance, fuel, energy. Even mediocre food is a little rewarding.
The scientists at Duke came up with a clever paradigm for isolating this more indirect rewarding pathway: they studied mice without a functional TRPM5 channel, which is essential for detecting sweetness. As a result, these mutant mice showed no immediate preference for sugar water.
But here comes the cool part of the experiment. The scientists then allowed the mice to spend some time with the sugar water and normal water. After a few hours, it became clear that the mutant mice greatly preferred the sugar water, even though they couldn't taste the sugar. (A control experiment with sucralose, an artificial sweetener, demonstrated that the rats were responding to the caloric intake, not the sweet taste.)
Finally, the scientists measured dopamine levels (via in vivo microdialysis) in the nucleus accumbens (a brain area that processes rewards) in the mutant mice and normal mice.* While normal mice exhibited an increase in dopamine in response to both fake sugar and real sugar - the reward was the sweet taste - the mutant mice only demonstrated a dopaminergic spike when consuming genuine sugar water. What they enjoyed were the calories.
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