However, there are some people who are gorging on food and intentionally packing on the pounds in name of science.
French fries, chocolate cake, steak -- imagine eating it all, gaining lots of weight, but not suffering any of the health consequences. For some obese people -- that's a reality.
Now researchers believe finding out why could be key in treating obesity, and they are putting their money where our mouths are.
"It's a worse epidemic than the bubonic plaque, than HIV infection," said Dr. Samuel Klein of Washington University School of Medicine.
More than 60 percent of people in the United States are considered fat.
"It's abnormal, it's unusual to be lean in this country," said Klein.
For some, obesity will lead to type two 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 -- using a variety of fast food.
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.
People like O-R nurse Dawn Freeman. As part of the "overfeeding" study, she's getting $3,500 plus food expenses to eat an extra 1,000 calories a day.
"McDonald's, I finally settled on their Angus burger," said Freeman.
Klein says using fast food is a cheap and easy way to track calories. Dawn 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.
"If we can understand that link better, we can develop better therapies to break that link," said Klein.
After gaining 5 percent of her body weight, Dawn 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," said Freeman.
Back to her normal weight and back to normal eating. Preliminary study results suggest that some people really are resistant to the effects of obesity.