Researchers think it may be key to treating obesity. More than 60 percent of people in the U.S. are considered overweight.
"It's abnormal, it's unusual to be lean in this country," said Dr. Samel Klein, director of the Center for Human Nutrition.
For some, obesity will lead to Type 2 diabetes, heart disease and high blood pressure. But for 25 percent of obese people, there are no adverse effects. Now researchers are trying to learn why.
The goal of the study is to find out why obesity causes metabolic diseases, such as diabetes and heart disease, in some people but not in others. The findings will help identify the factors that are protecting some obese people from those adverse effects.
"We really are paying people to gain weight. There's no question about it," said Klein.
As part of the "overfeeding" study, nurse Dawn Freeman is getting $3,500 plus food expenses to eat an extra 1,000 calories a day.
Klein says using fast food is a cheap and easy way to track calories. Freeman gained 20 pounds in two months.
"I couldn't climb stairs after two to three weeks. I was tired. I couldn't breathe," said Freeman.
Klein said if researchers can understand that link better, then they can develop better therapies to break that link.
After gaining 5 percent of her body weight, Freeman was put on a six-month weight-loss program. five months into it, she dropped the pounds and gained a taste for something else.
"I consciously put vegetables on my plate now," she said.
Klein says preliminary study results show some people really are resistant to the adverse effects of weight gain. The study is funded by the National Institutes of Health and is still enrolling. Volunteers must already be obese.