Bestselling Book Spurs New Diet Phenomenon

"The Fast Diet" requires people to consume little to no food two days a week.
3:22 | 02/28/13

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Transcript for Bestselling Book Spurs New Diet Phenomenon
Reporter: Maybe it's the stiff upper lip. But it seems much of england is obsessed with fasting. Not every day. Just two days a week. I'll have the almond croissant. Reporter: The other days, you can eat what you want. And it's called the fast diet. I step on the scale each week and the pounds are dropping off. Reporter: Terra says she lost 36 pounds over 7 months. My appetite is reduced. When I try to overeat, I find it quite challenging. Reporter: The two fasting days allow just 25% of a normal adult's daily intake. A measly 500 calories for women. Roughly 600 for a man. That's a lot of food. That's breakfast on a fasting day. It can be eaten in one meal or spread out the day. The british fasting craze is the brainchild of dr. Michael mosley. You weren't motivated to get in a little black dress. A doctor by training, hisbbc documentary "eat, fast and live longer," began with a wake-up call from his doctor. A shock. My fasting glucose level was diabetic. Reporter: He met with american researchers, getting astonishing scientific results from calorie restriction. Decreased cancer risk. Increased life expectancy. Improved brain function. It was tested in rats. But it has come to humans. Reporter: Mosley came up with a human diet plan to see if he could mimic the labs. You're like patient zero. Like patient zero. Reporter: He lost nearly 20 pounds in 2 months, which clearly shows on the outside. But on the inside, he says his makeover was far more dramatic. My body fat went down from 28% to 20%. And my blood glucose went down from diabetic to normal. My cholesterol went down from needing medication to normal. Reporter: But some say it's impossible to know if the diet is safe without more human testing. It's not safe for somebody under 20. Or somebody who is growing. Or kids. And it may not be safe for all adults, as well. Reporter: But for a throng of brits, dr. Mosley's example is good enough to give it a go. You can see why it would be controversial. But the two fast days are not starvation days. I want to show you what 600 calories look like. Two soft boiled eggs. For a woman, remove an egg and a plum. That's 500 calories. You skip lunch. And the author says, we often mistake boredom or thirst for fatigue for hunger. Get up and have some tea. For dinner, you have a steak, looks plentiful. 500 calories. Two cups of salad. And you only do this two days a week. The other five days, you can have whatever you want. Basically, what it's doing is changing the body's chemistry. And he's finding that he's healthier at the end of it, as well as losing weight. That's the point. To find something that works for your body. I can do that. I'd try it. I'm in. It's a dietary "deals &

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

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