Dina Spector writes:

The FastDiet, also known as the 5:2 diet, is based on something called intermittent fasting. This involves eating normally for five days a week and "fasting" for two. On those two fasting days, you eat about a quarter of your typical calorie intake — 500 calories for women and 600 calories for men. The rest of the time you can eat whatever you want. 

The book, only recently released in the U.S., was written by Dr. Michael Mosley and Mimi Spencer, both of the U.K. They claim this diet will make you lose weight faster than other standard diets — Mosley says he lost 20 pounds within a few weeks of starting the diet.

Although the extreme eating regime has been met with criticism in the health community, Mosley says the evidence to support intermittent fasting is there. The entire first chapter is even devoted to the "science of fasting."

Dina Spector explains the science:

When you don't eat, your body must turn somewhere else besides food for energy and begins to feed on the glucose in your blood.

Once all of the glucose in your blood is eaten up, your body turns to stored glucose — which is kept in complex carbohydrate molecules called glycogen.

And finally, when all of that stored glucose is used up, your body taps into fat stores for energy and glucose. 

But, continuing to do this for long stretches of time is not sustainable. In fact, your body might go into starvation mode, where your metabolism actually slows down in response to a decrease in calorie intake.

The 5:2 diet, based on intermittent fasting, is different from calorie restriction or starving.

Intermittent fasting is not the same as prolonged calorie restriction, or starving. That would be when our body doesn't take in enough nutrients to maintain a normal, healthy lifestyle.  

Intermittent fasting involves "eating fewer calories, but only some of the time," Mosley writes. 

Mosley admits that prolonged fasting is not sustainable and will ultimately result in muscle loss.

With the FastDiet, you never go for longer than 24 hours without eating. 

Another way fasting works is by reducing the amount of IGF-1 your body produces, according to Valter Longo at the University of Southern California. Less IGF-1, an insulin-like growth hormone, has been shown to reduce your risk of many age-related diseases, like cancer.