"The Fast Diet" is a bestselling book that outlines a weight-loss plan that's raising some eyebrows. In one week, the plan says, participants can eat what they want for five days, but must fast for two non-consecutive days.

Dr. Michael Mosley, the book's author, defines fasting as 500 calories a day for women and 600 for men. Adults can consume these calories, which amount to 25 percent of a normal adult's daily food intake, in one mid-day meal or spread across the course of the day. But protein and lots of fruits and vegetables are highly recommended.

The other five days supposedly require little willpower. Backers of this diet claim fast dieters can eat whatever they want, including cheeseburgers and croissants, in a guilt-free way.

MORE: Controversial 'Fast Diet' Uses Feast and Famine

Mosley appeared on "Good Morning America" today to discuss the diet.

The British man said he never set out to create the next diet craze. In an interview with ABC News' "Nightline," Mosley said he was inspired to create the diet after a wakeup call from his own doctor.

WATCH: More About the Fast Diet

"I had a bit of a nasty shock because I discovered that my fasting glucose levels were those of a diabetic," Mosley, who lost his father to diabetes, told ABC News. "And my cholesterol levels [were] about twice [what] they should be."

Mosley - who will April 3 appear in the PBS special "Eat, Fast and Live Longer with Michael Mosley" - said he lost nearly 20 pounds in two months on his Fast Diet and his overall health improved.

"My body fat went down from 28 percent to 20 percent, and my blood glucose went down from diabetic to normal," he said. "My cholesterol went down from needing medication to normal."

He also said he lost the fat in his gut, significantly reducing his risk of diabetes and heart disease.

Recipes for Restricted-Calorie Meals

Breakfast (190 calories)

Oatmeal made with 1.4 ounces steel-cut oats (160 calories) and water

Scant (slightly less than 1/2 cup) fresh blueberries (30 calories)

Dinner (306 calories)

Chicken Stir-Fry (281 calories)

Cut a 5-ounce chicken breast fillet (148 calories) into strips. Stir-fry in a nonstick skillet in 1 teaspoon olive oil (27 calories) with 1 teaspoon finely chopped ginger (2 calories), 1 tablespoon chopped cilantro (3 calories), 1 clove garlic, crushed (3 calories), 2 teaspoons soy sauce (3 calories), and the juice of 1/2 lemon (1 calorie), until the chicken is lightly browned. Add water if it sticks.

Add 1/2 cup trimmed snow peas (12 calories), 1 1/2 cups shredded cabbage (26 calories), and two large carrots, peeled and cut into thin strips (56 calories). Stir-fry for 5 to 10 minutes more, until the chicken is cooked through. Add water if necessary.

1 tangerine (25 calories)

Daily Total: 496

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Breakfast (178 calories)

4 ounces smoked salmon (132 calories)

1 plain Ryvita cracker (35 calories)

1 ½ teaspoons low-fat whipped cream cheese (11 calories)

Dinner (322 calories)

Thai Salad (322 calories)

Soak 1.8 ounces rice vermicelli noodles (194 calories) in water, according to package instructions. Combine 2 tablespoons Thai fish sauce (20 calories), the juice of 1 lime (1 calorie), 1 teaspoon sugar (16 calories), 2 scallions (white and green parts), trimmed and thinly sliced (5 calories), and 1 very small red chile, finely chopped (1 calorie) in a bowl. Mix well. Add 10 very small cooked and peeled shrimp (30 calories) and 2 large carrots, peeled and grated (55 calories). Drain the noodles and add. Toss well.

Daily Total: 500 calories

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Breakfast (223 calories)

1 small apple, sliced (47 calories)

1 small mango, peeled and pitted (86 calories)

1 small boiled egg (90 calories)

Dinner (267 calories)

Tuna, Bean, and Garlic Salad (267 calories)

Combine 1 1/2 cups canned cannellini beans, drained and rinsed (108 calories), one 5-ounce can solid white tuna in spring water, drained (119 calories), 2 ounces grape tomatoes (16 calories), and 1 loosely packed cup baby spinach (8 calories) in a salad bowl. In a small bowl, combine 1 clove garlic, crushed (3 calories), the juice and grated zest of lemon (1 calorie), ½ teaspoon olive oil (12 calories), and a splash of white wine vinegar. Drizzle over the salad and toss to mix well.

Daily Total: 490 calories

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Breakfast (140 calories)

1 small boiled egg (90 calories)

3 ultra-thin slices 97 percent fat-free ham (25 calories)

1 tangerine (25 calories)

Dinner (358 calories)

Vegetable Pizza (358 calories)

Preheat the oven or a toaster oven to 400°F. Top one 8-inch whole-wheat tortilla (144 calories) with 1 tablespoon tomato puree (5 calories) and 2 ounces fresh mozzarella cheese, diced (159 calories). Scatter with about 6 ounces chopped lightly steamed vegetables (50 calories); mushrooms, red pepper, zucchini, red onion, eggplant, spinach are all OK. Sprinkle with Italian herb seasoning. Bake for 5 to 10 minutes, until the cheese has melted.

Daily Total: 498 calories

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Breakfast (331 calories)

2 small soft-boiled eggs (180 calories)

5 lightly steamed asparagus spears (33 calories), to dip

1 slice whole-grain toast (78 calories)

2 small plums (40 calories)

Dinner (260 calories)

Thai Steak Salad (260 calories)

Grill a 5-ounce sirloin steak (188 calories) until cooked to your preferred doneness. Set aside to cool to room temperature. Slice the steak very thin across the grain. In a bowl, combine 2 cups shredded romaine lettuce (16 calories) and 1 cup shredded savoy cabbage (24 calories). In a separate bowl, combine the juice of 1 lime (2 calories), 1 teaspoon sugar (16 calories), 1 clove garlic, crushed (3 calories), 1 very small red chile, seeded and finely chopped (1 calorie), and 1 tablespoon Thai fish sauce (10 calories). Pour over the salad and toss to combine.

Place the salad on a plate and arrange the steak slices on top.

Daily Total: 591 calories