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Of Obesity, Sweets and Numb Tongues

Corpulence leads to an increased craving for sweets, and it may be because our taste receptors need an extra jolt.

More than 60 percent of adult Americans are obese, and ever-increasing levels of fat and sugar are sneaking into processed foods. Could blunted taste buds be partly to blame?

Neuroscientists at Pennsylvania State University have discovered that obesity gradually numbs the tongues of rats, depriving them of taste sensations for sweet foods and spurring them to eat more and sweeter meals. While previous studies had suggested corpulence leads to an increased craving for sweet foods, little had been known about why fatter and leaner people have different levels of taste and desire for sweets.

“When you have a reduced sensitivity to palatable foods, you tend to consume it in higher amounts,” said Andras Hajnal, associate professor of neural and behavioral sciences at Penn State College of Medicine, in a press release. “It is a vicious circle.” The findings appeared recently in the Journal of Neurophysiology.

To investigate these differences, Hajnal and his colleagues studied the taste responses of two strains of rats. One was a lean, healthy variety. The other group of rats began with normal body weight but tended to chronically overeat because of a missing satiety signal, which tells the brain when the body has been fed to capacity. These rats developed obesity and diabetes and displayed a heightened preference for sweets, stopping at nothing to obtain saccharine-flavored prizes for completing their learning tasks.

“When you have excess body weight, the brain is supposed to tell you not to eat more, or not (to) choose high(ly) caloric meals,” Hajnal said. “But this control apparently fails and thus the obesity epidemic is rising, and we want to find out how the sense of taste drives up food intake.”

Accordingly, the researchers implanted electrodes in the rodents’ brains to record the firing of nerve cells when the rats’ tongues were exposed to various tastes — ranging from salt to citric acid to water and including six different concentrations of sucrose. While the response to salty foods was the same between the two groups of rats, the obese rodents had about 50 percent fewer neurons firing when their tongues tasted sucrose, suggesting that obese rats are overall less sensitive to the flavor.

However, when the obese rats were fed a stronger concentration of sucrose, their nerve cells fired more vigorously than did those of the lean rats. “These findings tell us that there is a difference in activation of neurons between lean and obese rats when they are exposed (to) varying concentrations of sucrose,” Hajnal noted. “If you sense sweetness less, you may be inclined to eat sweeter foods.”

So what does this mean for humans? In obese people, an increase in the weight-height ratio is normally joined by an increase in dopamine, a neurotransmitter alighted with the brain’s ability to sense and deliver pleasure. “In these obese rats, like in humans, the dopamine system is suppressed and it is very possible that the obese rats are seeking a hedonistic experience or reward by eating larger meals and when they have a chance they also eat more sweets,” Hajnal said.

He added that the enhanced taste of processed foods stimulates people’s taste and food reward neurons on a chronic basis, making them less sensitive over time. “Instead of eating less, we seek out higher palatability,” Hajnal explained. “We simply start putting an extra spoonful of sugar in our coffee.”