Brain's 'sixth sense' for calories discovered

The brain can sense the calories in food, independent of the taste mechanism, researchers have found in studies with mice. Their 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.

Ivan de Araujo and colleagues published their findings in the March 27, 2008, issue of the journal Neuron, published by Cell Press.

In their experiments, the researchers genetically altered mice to make them “sweet-blind,” lacking a key component of taste receptor cells that enabled them to detect the sweet taste.

The researchers next performed behavioral tests in which they compared normal and sweet-blind mice in their preference for sugar solutions and those containing the noncaloric 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 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.

Significantly, 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.

“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,” concluded 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 wrote.

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,” wrote Zane Andrews and Tamas Horvath in a preview of the article in the same issue of Neuron.

“For example, high-fructose corn syrup is a ubiquitous sweetener in American society, and evidence suggests that fructose is not as effective as sucrose in terminating a meal. 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. Regardless, the present study alone will further galvanize the scientific community to understand how higher cognitive centers in the brain control food intake and body weight regulation,” wrote Andrews and Horvath.

Source: Cell Press

Related stories:

Emotional 'bummer' of cocaine addiction mimicked in animals
Cocaine addicts often suffer a downward emotional spiral that is a key to their craving and chronic relapse. While researchers have developed animal models of the reward of cocaine, they have not been able to model this emotional impact, until now.
Genes may make some people more motivated to eat, perhaps overeat
Science has found one likely contributor to the way that some folks eat to live and others live to eat. Researchers at the University at Buffalo, The State University of New York, have found that people with genetically lower dopamine, a neurotransmitter that helps make behaviors and substances more rewarding, find food to be more reinforcing than people without that genotype. In short, they are more motivated to eat and they eat more.
Level-headed: Economics experiment finds taste for equality
The rich don't get richer -- at least not in laboratory games. According to a new study of behavioral economics, published in the April 12, 2007 issue of Nature, people will spend their own money to make the rich less rich and the poor less poor. They do so without any hope of personal gain, acting, it seems, out of a taste for equality and sense of fair play.
Genetic studies endow mice with new color vision
Although mice, like most mammals, typically view the world with a limited color palette – similar to what some people with red-green color blindness see – scientists have now transformed their vision by introducing a single human gene into a mouse chromosome. The human gene codes for a light sensor that mice do not normally possess, and its insertion allowed the mice to distinguish colors as never before.
Researchers find 'switch' for brain's pleasure pathway
Amid reports that a drug used to treat Parkinson's disease has caused some patients to become addicted to gambling and sex, University of Pittsburgh researchers have published a study that sheds light on what may have gone wrong.
Your sweet tooth may really be in your brain's 'pleasure hotspot'
What makes those holiday candies and Christmas cookies look so tempting? University of Michigan researchers have discovered a "pleasure spot" in the brains of rats, helping neuroscientists understand where and how pleasure is generated in humans.
Protein on 'speed' linked to ADHD
A genetic change in the dopamine transporter – one of the brain's dopamine-handling proteins – makes it behave as if amphetamine is present and "run backward," Vanderbilt University Medical Center investigators report this week in The Journal of Neuroscience.
Does Gene Variant Make Women More Prone to Alcoholism?
A particular gene variant might make women more susceptible to alcoholism. At least, a study carried out by the Universities of Bonn and Sweden’s Karolinska Institute makes this a plausible conclusion.

News discussion:

Medicine & Health news