No such thing as sweet tooth: Study

Your fondness for ice-creams, cakes and chocolates may have nothing to do with taste buds, say researchers who claim that the human brain 'senses' foods that are high in calories and 'rewards' people by releasing hormones that make them feel happier.

Such an appeal for sugary foods may imply that the thing called 'sweet tooth' does not exist.

Understanding why high-calorie food triggers such a response in the brain could hold the secret to 'turning off' the pleasurable feelings associated with eating sweet food.

Ivan E de Araujo of Duke University's medical centre, who led the study, said that researchers had identified a "calorie-sensing" system in the brain.

The questions raised by that discovery were 'extremely important' to understanding the development of human obesity, he added.

The research team looked at mice that had been genetically altered to make them unable to taste the sweetness in food.

Next, the researchers performed behavioural tests in which they compared normal and sweet-blind mice in their preference for sugar solutions and those containing the non-caloric sweetener sucralose.

In those tests, the sweet-blind mice showed a preference for calorie-containing sugar water that did not depend on their ability to taste, but on the calorie content.

In analysing 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.

Significantly, the researchers found that a preference for sucrose over sucralose developed only after 10 minutes of a one-hour feeding session and that neurons in the reward region also responded with the same delay.

"In summary, 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," said the researchers.

"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," they added.

The scientific questions raised by the discovery of the brain's calorie-sensing system "are extremely important to understanding the pathogenesis and sociology of human obesity," said the researchers.

The study is published in the journal Neuron, published by Cell Press.