Is going super low-calorie two days a week the trick to fighting flab? The author of a new book on intermittent fasting explains how it works, and Health provides recipes that fit the plan.

Getty Images A buzzy British eating plan called the Fast Diet grabbed our attention when it jumped the pond to the United States this spring. The diet—which involves cutting your calories two days a week—was created by Dr. Michael Mosley, MBBS, a science journalist, and built on research suggesting that intermittent fasting (IF) can not only help people shed weight but also reduce their risk of heart disease and diabetes. While the concept of fasting for health benefits and weight control has been around for centuries (even Hippocrates recommended it!), it has been gaining momentum in recent months thanks to Dr. Mosley's book The FastDiet (as well as other expert-penned IF offerings, including The 2-Day Diet). Curious? Here is an exclusive excerpt of the book—along with our own version of a 500-calorie day.

Why I created this plan, by Dr. Michael Mosley, MBBS

About 15 months ago when I was 55, I went for a medical checkup and had a nasty shock. I discovered that although I looked quite slim, I was actually a TOFI (thin on the outside, fat on the inside). Internal fat, also called visceral fat, is the most dangerous sort of fat because it wraps itself around your internal organs and puts you at increased risk of heart disease and diabetes. Blood tests showed I was a borderline diabetic and had a cholesterol score that was way too high. Obviously I was going to have to do something about this.

What convinced me to try intermittent fasting? Well, a large number of studies with animals and humans have shown that it's good for health and weight loss. One of the best-researched forms of intermittent fasting is alternate-day fasting, in which you cut calories every other day. I, however, found ADF too difficult to do on a regular basis.

Instead I decided to cut my calories just two days a week. I started calling it the 5:2 Fast Diet because for five days a week I was eating normally, and for two days a week I limited myself to 600 calories a day.

After three months, I had lost 19 pounds (down from 187 to 168), and my body fat had dropped from 28 to 21 percent. I lost 3 inches around my waist and stopped snoring, which delighted my wife and quite possibly the neighbors. Even better, my diabetes and heart disease risks, as indicated by blood tests, improved in spectacular fashion. My wife, Clare, who is also a doctor, was impressed. She regularly sees overweight patients who have blood chemistry like mine had been, and she said none of the advice she gives them has had anything like the same effect.

The basics

The Fast Diet is different from other programs because you diet for just two days a week. On fast days you eat healthily, but around a quarter of what would be normal for someone of your gender (500 calories for women or 600 for men).

There are various ways to handle fast days; some people prefer to eat all their calories in one sitting, others split them between breakfast and dinner, and others have three small meals over the course of the day. You have to see what works for you. [Note from the editors of Health: We recommend dividing the calories into mini meals, as shown at right.]

How to choose the right food

Because you eat less than normal on a fast day, it's important that you choose foods that will keep you satisfied longer.

Aim for fare that's high in protein, rich in fiber and has a low glycemic index, since high-GI foods (like bread, potatoes and rice) are likely to make your blood sugar spike, then crash. The Fast Diet doesn't recommend boycotting carbs entirely or living permanently on a high-protein diet. But the combination of proteins and low-GI foods on a fast day are helpful weapons in keeping hunger at bay.

On your fast days, go for healthy options that are low in saturated fat, like steamed white fish, skinless chicken, low-fat dairy, shrimp, tuna, tofu and other plant-based proteins. Nuts, seeds and legumes (beans, peas and lentils) have lots of fiber to fill you up (nuts may be high-calorie, but they're generally low-GI and satiating). Eggs, meanwhile, are low in saturated fat and full of nutritional value. They won't adversely affect your cholesterol levels, and they score a mere 90 calories each, so an egg-based breakfast on a fast day makes perfect sense; poach or boil them to save on calories. Two eggs plus a 1¾-ounce serving of smoked salmon clocks in at a sensible 250 calories.

When to fast, when to feast

Monday is an obvious choice for a fast day, particularly if it follows a social weekend. For that reason you might avoid Saturdays and Sundays, when family lunches and brunches, dinner dates and parties make calorie-cutting a chore. Thursday would then make a sensible second fasting day. But be flexible; don't force yourself to fast when it feels wrong. If you're stressed or tired on a day that you have designated a fast, try again another day. But do aim to establish a pattern. That way your fasts will become familiar. Be kind to yourself, but be tough, too.

Reshaping your body—and appetite

The first thing you can expect is to lose weight, thanks to the simple law of thermodynamics (weight loss occurs when energy in is less than energy out). If you cut to 500 calories two days a week and don't compensate by eating more on other days, you should lose 1 to 2 pounds a week. This won't be all fat: Some of it will be water and some digested food in your body. You can expect, however, to lose around 10 pounds of fat over a 10-week period.

You should find that over the weeks your BMI, body fat and waist measurement will drop. Your cholesterol count and blood glucose levels will also improve. The changes will start to show up in the mirror as your body becomes leaner and lighter.

Hopefully your food preferences will adapt, so that even on non-fast days you will begin to choose healthy foods by default. You will also learn to recognize the sensation of being pleasantly full—satiated, not stuffed. The upshot? No more food hangovers, improved digestion, more bounce.

If you continue to fast and feast with awareness, all kinds of other changes should occur. You may discover that you've been suffering from portion distortion. Muffins will start to look vast. You might go from a Venti to a Grande to wanting only half a cup, no sugar, no cream. If you are like me, then one day soon, you'll arrive at a place where you say no to the cheesecake because you don't want it, not because you are denying yourself. This is the baseline power of intermittent fasting: It encourages you to recheck your diet. And that's your long-haul ticket to health.