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, released last year 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 claims to have 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 devoted to the “science of fasting.” And, some fitness experts have come out in favor of the diet. A 2007 review also found that alternate-day fasting may moderate disease-risk.
Here we break down how and why the 5:2 diet works, according to its authors.
First, this is what happens to your body when you fast.
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.