27 March 2008

Brain Has Sixth Sense For Calories

by Kate Melville

The latest issue of the journal Neuron carries news of a "sixth sense" that scientists say can detect calories in food without the involvement of our regular taste sensors.

In the experiments, the researchers genetically altered mice to remove their sweet taste receptor cells, making them "sweet-blind." The research team then performed behavioral tests in which they compared normal and sweet-blind mice in their preference for sugar solutions (sucrose) and those containing the non-caloric sweetener sucralose. The researchers found that the sweet-blind mice showed a preference for calorie-containing sugar water that did not depend on their ability to taste.

In analyzing the brains of the sweet-blind mice, the researchers showed that the animals' reward circuitry was switched on by caloric intake, independent of the animals' ability to taste. Those analyses showed that levels of the brain chemical dopamine, known to be central to activating the reward circuitry, increased with caloric intake. Also, electrophysiological studies showed that neurons in the food-reward region, called the nucleus accumbens, were activated by caloric intake, independent of taste.

Interestingly, the researchers found that a preference for sucrose over sucralose developed only after ten minutes of a one-hour feeding session and that neurons in the reward region also responded with the same delay.

"We showed that dopamine-ventral striatum reward systems, previously associated with the detection and assignment of reward value to palatable compounds, respond to the caloric value of sucrose in the absence of taste receptor signaling," the researchers noted. "Thus, these brain pathways do not exclusively encode the sensory-related hedonic impact of foods, but might also perform previously unidentified functions that include the detection of gastrointestinal and metabolic signals."

The finding that the brain's reward system is switched on by this "sixth sense" machinery could have implications for understanding the causes of obesity. For example, the findings suggest why high-fructose corn syrup, widely used as a sweetener in foods, might contribute to obesity. 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.

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Source: Cell Press