Washington, Nov 27 : Have you ever wondered why fat people have an endless urge for sweets, cakes and chocolates? Well, now a new study in mice has shown that obesity gradually numbs the taste sensation to sweet foods, which drives up the consumption of larger and ever-sweeter meals.
The new study, conducted by researchers at Penn State College of Medicine, could reveal a critical link between taste and body weight, and reveal how flab hooks the brain on sugary food.
"When you have a reduced sensitivity to palatable foods, you tend to consume it in higher amounts. It is a vicious circle," said Andras Hajnal, associate professor of neural and behavioral sciences at Penn State College of Medicine.
For the study, Hajnal and his colleague examined the taste responses of two strains -- OLETF and LETO rats. Compared to the lean and healthy LETO rats, the taste responses in OLETF rats mirror those in obese humans.
These rats have normal body weight at first, but they tend to chronically overeat due to a missing satiety signal, become obese and develop diabetes. The obese rats also show an increased preference for sweet foods and also are willing to work harder to obtain sweet solutions as a reward for their learning.
"When you have excess body weight, the brain is supposed to tell you not to eat more, or not choose high caloric meals. 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," said Hajnal.
The researchers implanted electrodes in the rodents' brains to record the firing of nerve cells when the rats' tongues were exposed to various tastes -- salt, citric acid, plain water and six different concentrations of sucrose.
The researchers specifically looked at differences in processing taste in the pontine parabrachial nucleus (PBN), a part of the brain that uses nerve cells to relay information from the surface of the tongue to the brain.
"We found that compared to the LETO rats, the OLETF rats had about 50 percent fewer neurons firing when their tongues were exposed to sucrose, suggesting that obese rats are overall less sensitive to sucrose," said Hajnal.
The response to salt was the same for both strains. However, when the obese rats were fed a stronger concentration of sucrose, their nerve cells fired more vigorously than in the lean rats. In other words, obese rats have a weaker response to weak concentrations and a stronger response to strong concentrations.
"These findings tell us that there is a difference in activation of neurons between lean and obese rats when they are exposed varying concentrations of sucrose. If you sense sweetness less, you may be inclined to eat sweeter foods," said Hajnal.
The study has appeared in a recent issue of the Journal of Neurophysiology.