"And they like it more. They like that they're always able to look forward to the next day when they can eat whatever they want. They are able to feel normal sometimes."

“So, do people do this for their entire life?” I asked. “Will they eventually lose all of the weight and weigh nothing?”

“As an obesity researcher, I’ve seen that obviously you can’t just go on a diet for a while and lose weight, then go back to what you were doing. You’re going to gain that weight back. You need to find something you can adhere to.” That’s where her “maintenance regimen” comes in. The NIH study will transition to a point where instead of eating 500 calories on the fasting day, people eat 1,000. Her early data says that works for keeping weight steady.

The results seem clear, but their cause is not. "The reason they lose weight is that something keeps people from really binging on that feed day,” Varady says. “Something changes in the body on the fasting days. We're not sure what. It may be hormone changes, or that the stomach shrinks."

Measuring the size of a stomach is invasive—it involves putting a balloon in a person's stomach while they are undergoing an MRI—but she says that's something she will eventually do.

She says the first 10 days are "pretty difficult," but after that, people seem to get used to it.

There's also an idea, proposed by many fasting adherents, that the practice helps us become familiar and comfortable with the sensation of hunger. One voice for that idea is John Berardi, an Ontario-based nutritionist with a PhD in exercise physiology and nutrient biochemistry. He has been an adviser to Apple, Equinox, and Nike, and Livestrong.com named him among “20 of the Smartest Fitness Trainers You Might Not Know.”

Berardi is also a self-described “professional dieter”—which is to say, he doesn’t just talk smart, he talks from experience. He claims to have personally tried “nearly every diet or nutritional protocol that’s around, to test its efficacy.”

“As a competitive, masters-level track athlete and life-long fitness enthusiast”— in 1995 Berardi won a national junior bodybuilding title—“I wanted to test a new way to drop fat and get extremely lean, while staying strong and powerful,” Berardi wrote in his eminently readable e-book, Experiments With Intermittent Fasting. Berardi details over 80 pages his experiences with six different fasting protocols over the course of six months.

"I kept meticulous notes on everything," he writes, "from scale weight, body-fat percentage, and blood/hormonal markers, to lifestyle markers like energy levels, cognitive thought, and pain-in-the-ass factors."

He came away with the idea that even a few days spent messing around with this type of dieting can be beneficial long-term. Berardi himself ended up losing 20 pounds, decreasing his body fat from 10 to 4 percent (he was doing very well already, and by the end he was a spectacle of leanness). He came out of that trial period with “intermittent fasting strategies that [he] could follow indefinitely with no problem,” he writes, “in a way that was easier and less time-consuming than ‘traditional’ dieting.”