High on Calories

Obese people are not able to regulate high-calorie food intake because of changes in their brain.
If your overweight children binge eat, blame it on their flawed brain circuitry. Scientists have now found that despite the desire to cut their food intake, obese individuals will not be able resist junk food, which is very high in calories. That’s because their persistent eating behaviour has precipitated changes in the brain similar to that found in heavy smokers and drug addicts. The study appeared yesterday in the journal Nature Neuroscience.

After spending years studying brain changes associated with drug abuse and smoking, Paul Kenny — a neurobiologist at the Scripps Research Institute in Florida, the US — recently turned his attention to obesity. He and his graduate student, Pal Johnson, wanted to understand the strong yet not-so-easy-to-fathom link between obesity and depleted levels of dopamine or D2, a brain chemical associated with feelings of pleasure.

Scientists in the past had observed that obese individuals have reduced levels of dopamine, but weren’t sure if it was triggered by obesity. It was also known that food intake was associated with dopamine release and the degree of pleasure from eating correlates with the amount of chemical released. Evidence has shown that in comparison to lean people, obese individuals have fewer dopamine receptors in the brain. And people with fewer dopamine receptors need to take in more of a rewarding substance — such as food or drugs — to get an effect that others get with a lesser amount. But the underlying mechanisms are poorly understood.

“What we have achieved is proven in our experiments with laboratory rats that obesity can elicit these brain changes,” Kenny told KnowHow over the telephone.

For this, the Florida scientists embarked upon a series of meticulously planned experiments. In the first, they offered rats — which were grouped into three categories — different menus. While the first group had access solely to less-appetising but healthier chow, the other two were offered a choice of high-calorie food such as bacon, sausage, cake and chocolate in addition to chow, but for varying durations. Some rats had access to the rich fare for just one hour, while the other group could gorge on it most part of the day. The animals were fed this way for 40 days. All of them were wired to record even the slightest change in their D2 levels.

The rats in the first set — which were fed only chow — maintained their weight, while those belonging to the second set — which had restricted access to rich food — exhibited insignificant increase in their body weight. On the contrary, the third group — which had unlimited access to calorie-rich food — gained weight rapidly. These animals were found to be gobbling up twice as much as the other two groups. As the days wore on, their dopamine levels plummeted, requiring them to consume higher quantities of high-fat food to get the feeling of satiation. This is quite like the case of a smoker who has to puff away more cigarettes to get the same high that he or she earlier got with one. Or a drug addict who has to continuously increase the dose for getting a kick.

A second set of experiments with the rats showed that this blunting of the reward sensitivity does not return for a good two weeks even after the high-calorie food was withdrawn.

A true addict, whether rat or human, will compulsively consume the addictive substance even when it is clearly detrimental to health. In the third experiment the scientists tested this hypothesis. To do this, the team trained the rats to expect painful foot shocks upon seeing a light signal. Although the normal rats stopped eating even the most delicious junk food when the light came on, the obese ones used to a high-calorie diet simply ignored the cue and kept feeding.

Anoop Misra, head of internal medicine at Fortis Hospitals in New Delhi, says this explains why obese people find it difficult to modulate their junk food eating habits.

“The US scientists’ experiment has substantiated many assumptions associated with obesity and dopamine levels in the brain fairly well,” says Nihal Thomas, an endocrinologist at the Christian Medical College, Vellore. “The exercise and methodology followed are exquisite.” The findings may help develop drugs that may target dopamine receptors to treat obesity, he observes.

Source : The Telegraph (Kolkata, India)


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