Controversial 'Fast Diet' Instructs 2 Days of Fasting

British doctor who created the feast and famine weight loss plan says it can help you live longer.
6:58 | 02/28/13

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Transcript for Controversial 'Fast Diet' Instructs 2 Days of Fasting
It is the hottest new trend in modern day weight loss with principles as old as humanity itself as long before hunting and gathering involved drive through windows. Our ancient ancestors -- as much as they could whenever they could and then starve until the next feast. The same pattern has now become the basis for new -- taken -- lost world by storm. Come mining fasting with feasting in the pursuit of not only a smaller size -- -- longer life. Here's ABC's -- check. When it comes to hot British imports there's one direction song what makes you. How -- the American fascination with the royal couple and the pregnancy. But the next big British invasion hitting our shores this week. Just might be the fast diet -- already feel like the diet eat all the rage in England because it's not just about losing weight to its creator sees it as a radical new approach to eating away to a longer life. You playing discovered the fountain of youth relatively something really really well for -- -- they get up. Assistant understated -- The British fasting craze is the brainchild of this man doctor Michael -- he embarked on a quest for the scientific -- -- -- youth movement. He discovered that you can get radically healthier by doing something that sounds truly unhealthy. Fasting just two days a week the other five days you can eat whatever -- -- just. Be clear you're not telling people to -- them -- let -- -- in. The -- regime that I'm suggesting the -- when you ever do that through these two that they have to eat pizza and drink -- and eighteenth and have Christmas and invest them. Which requires no will -- Safe happy and uncle son and they. She escaped. -- Laughlin lost 36 pounds over seven months and says her dress size isn't the only thing that's changed. My normal dang now my appetite is this the case -- even when -- future. Eight I sometimes find it quite challenging. On the two fasting days you're not actually starving yourself but close. It's just 25% of the normal adults daily intake for measly 500 calories for women. -- 600 for a man. But for fifteen and lots of fruits and vegetables are highly recommended for feedback is a -- report from within. A doctor by training Michael Moseley BBC documentary airing on PBS the spring eat fast and live longer. Begins with a wake up call from his doctor. He'd already lost his dad to diabetes. -- discovered that my eye popping group has -- I would that the diabetic but don't the that you need to stop medication that. As you wage is structures deteriorate. And you'll balance -- -- It was clear doctor Mosley could stand to start aging a little more gracefully six point five ninths that the Vegas. Researchers across the United States have been finding astonishing results from severe calorie restriction. Decreased cancer risk increase life expectancy even improved brain function the election was that done in -- but. -- -- -- The -- humans by mimicking feast or famine conditions researchers at Johns Hopkins and US -- Discovered that the body and the brain responded fascinating way this who. -- -- They asked what they found in -- is. When -- deprived -- -- that brings us produce a protein called brain grew your country affected and what this does is it makes you feel that it. And would also appears to do -- make you -- Doctor mostly tracked down a human trial of alternate day fasting led by doctor -- -- at the University of Illinois. They met for a burger uneasy stick to it really the calorie goals on the best days. You can literally eat whatever you like the -- she compared the two fasting groups one eating high fat foods on their feet day the other low fat. Surprisingly. In terms of cardiovascular disease risk it didn't matter if you're eating -- Instead of gorging and making up for lost calories people on the study -- just 10% more on feed -- Overall women cut their risk of breast cancer by 40%. So doctor mostly created the fast diet my fasting on two random days of the week. -- it is really important get a decent amount of them protein even on the -- -- day right. Because your body doesn't store -- and then he created low calorie nutrient dense recipes they get a lot of bulk -- find her. The 50 -- -- -- peaceful. It will profile regular. Here -- accordance with important. But there aren't nutrition experts concerned that it's too big a leap to go from one case study to a -- a -- best seller. With a crop of similar books also making -- It's it's anecdotal on his -- and his experience that's an opinion. I like to make recommendations are based on good solid science them we're not there yet. Keep a new worries about nutritional deficiencies overtime and a prospect of people taking fasting to the extreme your criticism are extremely valid. And yet. Hasn't academic and as someone who understands the impact of these longevity studies done to find an intriguing -- absolutely intriguing lots of things are intriguing that I wouldn't necessarily recommend until I know a lot more. For doctor mostly after eight weeks the moment of true. Hey that is spent -- state. That is a 173. Point eight pounds which means we'll have lost well over a stone. My belief that when we did it before -- was 27%. And it's put it twenty. -- -- Over two months doctor mostly wound up losing nearly twenty pounds which clearly shows on the outside. But on the inside he says is make over was far more dramatic. My body fat went down from 28% 20%. And my -- good -- went down from diabetic to normal. But -- -- banned from -- medication to normal do you really weren't motivated by not get into the little black dress you learn -- -- Well ahead visceral fat back in got -- -- if you get rid of that then -- to significantly reduce your risk of diabetes and heart disease. -- -- approach to the test for now doctor Mosley seems to be the poster child for turning back time in my Internet. So we asked him to take the balance age test again. That's good that's good. Still not graceful but a heck of a lot healthier that was literally like sixteen you don't really have. -- yes yes use your body has become much younger for Nightline I'm juju Chang in New York.

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

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