Shedding extra pounds and keeping them off can be hard, and a new study suggests one reason why: A high-fat diet, followed for even a short time, injures the brain.

Researchers looked at the brains of rodents that were bred to become obese and found that when placed on a high-fat diet, the animals developed injuries to the hypothalamus, an area of the brain that controls your urge to eat and sends signals to stop eating when you're full.

The researchers found signs of similar damage in the same brain area in obese people.

"Within 24 hours of switching rodents to a high-fat diet, we found injury in the hypothalamus area," said co-author Dr. Michael Schwartz, an endocrinologist at the University of Washington School of Medicine.

"We don't know what causes the injury and we don't know for certain that it's a cause of obesity, but that part of the brain does control body weight," Schwartz said.

The findings were published Dec. 27 in the Journal of Clinical Investigation.

Obesity may play a role

About a third of Americans are obese, according to the Centers for Disease Control and Prevention.

Obesity causes inflammation in the tissues and organs. This isn't the same type of inflammation you get during an allergic reaction. Instead it's a low level of inflammation that persists in the body.

Schwartz and colleagues speculated that obesity might also be linked with inflammation in the hypothalamus, "which may prevent it from responding to hormones like insulin that regulate our body weight," said co-author Dr. Joshua Thaler, also an endocrinologist at the University of Washington.

Researchers compared rats and mice that ate a high-fat diet with those that ate a regular diet over a four-week period. Within the first week, they found gliosis — an overgrowth of cells that is a sign that the brain has tried to heal itself from injury.

They also found that though the brain's repair effort was effective, inflammation and gliosis persisted as long as the animals remained on a high-fat diet.

Moreover, the brain images of 34 healthy people, who ranged from lean to obese, revealed a link between body weight and gliosis similar to what was found in rodents. "There seemed to be more gliosis in people who were obese than those who were lean," Thaler said.

Because the hypothalamus is involved in our urge to eat, the findings imply that obesity, or the eating habits that lead to obesity, "caused damage to brain areas responsible for keeping our body weight stable," Thaler said.

Whether the damage to the hypothalamus is permanent remains unclear.

Preventing obesity is the best bet

The study was mostly done in rodents, cautioned Dr. Stephen Hammes, chief endocrinologist at the University of Rochester Medical Center, who was not involved with the work.

"We don't know if humans respond the same way as rodents, but this study is still intriguing," Hammes said.

He also noted that the study does not show whether the "hypothalamus caused obesity, or if the obesity caused the changes in the hypothalamus." The findings show only a correlation.

For now, Hammes said he believes the best way to control obesity is to prevent it.

"Some people have a genetic predisposition to obesity, but they still have to eat too much and exercise too little to be obese," he said. "You can overcome this predisposition by staying active and eating healthy."

Pass it on: Eating a high-fat diet may change the brain in ways that makes it harder to lose weight.

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