Two people, one slender and one obese, may dip into a cup of frozen yogurt in much the same way, but their bodies’ reactions to the treat may actually be quite different.
According to new research from Yale on obesity in rats, some who ate high-fat, high-calorie foods became obese — while others did not — based in part on how their brains are wired.
The findings, reported online the week of Aug. 2 in the journal Proceedings of the National Academy of Sciences, are a “double-edged sword,” said experimenter Barry Levin, a professor at the Department of Neurology and Neurosciences at New Jersey Medical School. While the biological basis for obesity may reduce some of the stigma associated with it, psychologically it can serve as a crutch for people struggling to lose weight, he said.
Still, the researchers are not trying to discourage anybody from shedding extra pounds, said Yale School of Medicine comparative medicine professor Tamas Horvath, the lead author for the study.
“There’s a biological underpinning for the difficulty [in losing weight],” he said. “And once you understand that, you can work accordingly.”
The researchers bred rats so their susceptiblity to diet-induced obesity would be known. The team then took rats from two different populations — one vulnerable to diet-induced obesity and one resistant to it — and monitored how their brains responded to high-fat, high-calorie foods. What the scientists found was that the brain talked to the body differently in obese individuals, said Levin, who is also a staff neurologist of the VA Medical Center in East Orange, N.J.
The rats prone to obesity took weeks to figure out they were eating too much when given the fatty, high-caloric diet, Levin said. The obesity-resistant rats, on the other hand, figured out they were eating too many calories within two to three days and adjusted their intake accordingly.
The reason behind this adjustment rests in nerve cells located in the hypothalamus, a part of the brain that controls thirst, hunger and other homeostatic systems, Horvath said. In the rats prone to obesity, those neurons are “more lazy” and do not signal early enough to tell the rats to stop eating, he said.
Evolutionarily, that laziness makes sense, Levin said.
“Obesity is not something that humans have had to contend with through the eons. We were designed basically to be very efficient and take in calories when they were available and hold on to them when they weren’t,” he said.
The obesity-resistant rats may not have been susceptible to the host of problems, including diabetes, that can accompany obesity, but they would not have done very well in nature because they would not put on enough fat to get through the winter, Levin said.
“Obesity-prone animals ignore a lot of signals that tell them they should stop eating,” he said. “That’s a pretty good way of promoting survival.”
But ignoring these signals to stop also means that the rats begin to experience inflammation in the brain, Horvath said. Such inflammation can permanently change the brain’s architecture and make it harder to go back to a normal body weight, he added.
Indeed, when researchers tried to put obese rats on diets, the rats decreased energy expenditure and were constantly hungry, researchers said. When those same rats were allowed to eat as much as they wanted, they became obese once again.
This mirrors what is observed in humans, the researchers said. Ninety percent of people who are obese and try to diet away their fat go back to the same body weight or higher in two years, Levin said.
But genetics is only one of the factors that influences the development of obesity, the researchers reminded. Even obesity-resistant rats will become obese if given a highly tasty diet, such as chocolate Ensure, a nutritional drink, Levin said. And environmental factors, changes in the way genes are expressed and time spent in the womb may all play a role in developing obesity, researchers said.
Horvath said he and his team plan to publish the results of a study on the maternal influence on the developing brain in the next few months.