Big Fat Experiment: Participants Paid to Eat Fast Food

Researchers are trying to find out why weight gain can make some people develop metabolic diseases.
3:00 | 10/02/12

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Transcript for Big Fat Experiment: Participants Paid to Eat Fast Food
and for those who salivate at the very sight of a drive-thru window, it sounds like a dream gig or scam. Get paid thousands of dollars simply to chow down on greasy burgers or taco combos, once a day, every day for three months. But this was a real dp perexperiment, a jufrpg food binge in the name of science y some people get diabetes or hypertension when they gain and some don't. Here is john donvan. Two sausage butteryburr tishgs -- a number three. He goes to the drive-thru. Again and again. And again. Thank you very much. And why? For science. Dave giocolo is a guinea pig of Back up a few months, put dave in his car and here was the heaad he heard. This study includes a short term high coalorie diet and then diet program. Reporter: Offering to pay people to add fat to their own bodies, adding an extra meal each day, 1,000 calories worth of fast food and a lot of people came forward. Like dave. A salesman. Once I got into work I called right away and I tried to e-mail them. Reporter: For what? Research. You can see this is x-ray slice through the abdomen where you can see the thick rim of fat around the outside. Reporter: The doctor wanted to find out why only some people who gain weight also get diabetes and hypertension and others do not something he could not research by feeding food pellets to lab animals. What you learn in rodents does not always translate to people. What you learn in flies and worms won't translate to people. Reporter: Fast food is a perfect food pellet replacement, good for when you want to measure exactly what people are eating. These five restaurants were chosen for the study. They have very regulated food content, we know the cal rories and composition of each restaurant. It's an easy and tasteful way to get people extra calories. Reporter: Also a cash incentive, up to $3000 depending how long it took to gain weight. It took a month to get approved. Reporter: Dawn is a nurse and when we met her she had actually completed the program. Weight gain from 170 pounds in her first week -- I had shakes and burgers and fries -- Reporter: Did you like it? Tacos? To 186 aeight weeks later. Report Reporter: She was paid for the effort, including to earn the last 50 losing all of the weight again, which she did buy dieting and a lot of walk exercise like this the hospital guides them through the weight loss. So before dave even started dawn knew that gaining weight fast with a doctor's permission even only sounds easy. It even seemed easy 'first meal. A big mac -- Reporter: Large fries? It was great. Really good. Next night went to taco bell and it was wonderful. Reporter: And again, this is after you have already eaten dinner? This is after I've already eaten dinner. Reporter: But doctor klein could almost predict what she would see next. This is not present for them, not easy to stuff your fast every day for a long period of time. Reporter: For two weeks it was all right and then I started really feeling awful. I couldn't hardly breathe anymore. Vrt. Reporter: She so is glad it's over and dave, his starting weight 249.9 with a goal of adding 15 or so pounds for the study and he was loving the thought of that. Reporter:HIS IS LIKE Paradise? That's why it sounded so GOOD, I EIGHT at McDonald's, burger king. Reporter: How do you see the next several months? Should be pretty easy. Reporter: This from a guy who start every morning by a breakfast burrito. He then knew the numbers by heart. 707 calories. Reporter: Putting down quarter pounders and caught up with him around week four. Bleh. How do you feel about week five? My knees are aching, my ankles are aching, it's harder to move. Reporter: He tmetabolism is a mysterious thing. The weight went on slowly. One week he lost a pound. They told him to up the quantities. Go up to this, to this. Reporter: Around week 11 he stepped away from his burger and showed me. Can you feel the weight on your body? Where have you put it on? Here. Here. Reporter: Bigger in there? Stomach is bigger. Reporter: You feel it? Belt had to come out. Had to buy different pants. Reporter: You just want this to be over? I'm done with it now. Reporter: Except you're not it now. Reporter: A few days ago dave fin frishd 249.9 to 268. Just over 18 pounds which he now has to lose, helped perhaps by the fact, he says, he's lost his appetite for fast food for a while. I'm john donvan for "nightline" in st. Louis.

This transcript has been automatically generated and may not be 100% accurate.

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