The brain's ability to sniff out calories in the form of sugar depends upon sugar's drug-like effect on the dopamine-rich reward center known as the nucleus accumbens, according to a study published in the March 27 issue of Neuron . This tiny structure in the mid-brain is also the locus of reward activity for all addictive drugs.
In the study, Ivan de Araujo and colleagues at Duke University and the Universidade do Porto in Portugal demonstrated that lab mice lacking the ability to taste sweet foods still preferred sugary water to regular water. The genetically altered mice, lacking functional taste receptor cells for bitter and sweet, consistently chose to consume sugar water--even though they could not sense the sugar. (The lab animals were also prevented from smelling or sensing textural differences in the offerings.)
"Our findings suggest that calorie-rich nutrients can directly influence brain reward circuits that control food intake independently of palatability or functional taste transduction," the researchers wrote.
The findings offer new clues to obesity, and also bolster the contention that simple carbohydrate foods--because of their effect on reward pathways in the brain--can be addictive for certain people. As Tamas Horvath of Yale University's School of Medicine told Science News (sub. required): "This is a very exciting new element in how you get addicted to food. It doesn't even matter how it tastes."
In the same article, written by Amy Maxmen, study author de Araujo said: "The animal's reward processing systems were sensitive to changes in metabolism, not just flavor. This is a new system."
The "sweet-blind" animals did not go for the low-cal alternative, when they were offered water mixed with sucralose, otherwise known as Splenda. Low-cal sweeteners did not result in a similar dopamine boost along the reward pathways of the brain.
The brain's ability to "sense" calories may help explain why diet foods are often ignored in favor of sweets. As Ewan Callaway of New Scientist put it, "Anyone who has devoured a tub of ice cream in one sitting knows that delicious foods can override our body's pleas of 'enough.'" We have increased levels of dopamine in the nucleus accumbens to thank for that.
As De Araujo explained toNew Scientist, "even when you do not stimulate the sensory pathways in the mouth you still have this reward signal in the brain."
In a preview of the article in the same issue of Neuron , Zane Andrews and Tamas Horvath speculate that "high-fructose corn syrup is an ubiquitous sweetener in American society.... It may be that fructose produces stronger activation of the reward system and that removing high-fructose corn syrup as a sweetener will curb some desire for these products."