Paul MacLean raises a lot of fat rats. MacLean is a professor at the University of Colorado's School of Medicine where he studies the metabolisms of rats to learn about the metabolisms of people.

He says one of the biggest problems doctors face in treating fat patients, is getting them to keep weight off after they've lost it. MacLean's fat rats may hold a clue as to why that's so.  

The experiment called for rats to eat like people

MacLean says the best way to learn about gaining weight is to get rats to act as much like people as possible.  

"We give them too much fat, and we put them on an energy restricted low-fat diet just like humans go through," MacLean said.

"Once we have a weight reduced rat, we model the holidays and allow them to go off of their diets and we look at various aspects of their metabolism," he added.

After allowing the rats to gorge on food and regain weight, MacLean divided them into two groups. One group remained sedentary. The other exercised daily.  

After weight loss, exercise suppresses appetite

What MacLean found was that when he exercised the animals by giving them a daily bout of treadmill exercise, similar to what a lot of people do, it changed their metabolism.

"It lowered their hunger that they were experiencing on a daily basis," MacLean said. "And it reduced the amount of weight gain early on, as they relapsed to obesity and ultimately lowered the body weight. So it changed their biology," he said.

MacLean added that the exercising rats didn't stay thinner because they were burning calories every day. That might have played a part in their weight control, he said, but the exercise program actually changed the biological drive to eat, and suppressed it.

"We were changing how they regulated body weight," he explained. MacLean says that may mean exercise can help people stay on their diet and resist the temptation to, "succumb to those biological urges of hunger pains that they feel on a daily basis after they've lost weight."

Of course, MacLean pointed out, people are different from rats; humans don't just eat because they're hungry. They eat to socialize, or when they see delicious looking food or when others pressure them to just try a little bite. But he said, people could react just like rats in that when they exercise, they might keep that weight off.

MacLean's research is published in the American Journal of Physiology ? Regulatory, Integrative and Comparative Physiology.