Some people fast to "rest" the digestive tract, while others do so as part of a religious tradition. The last time you fasted may have been before a medical test, such as a colonoscopy.

But as a weight-loss technique, fasting has always been controversial. Its detractors claim that it shifts the body into a starvation mode that makes unwanted pounds even harder to drop.

What's gaining favor: More and more scientists are now studying fasting as a method for losing extra pounds and fighting disease. But does it work? As one of the few scientists worldwide who has studied fasting in humans, I consider it to be the most effective-and healthful-method for most people to lose weight.* How it works...

The simple formula

With intermittent fasting, you eat a reduced number of calories every other day. Scientifically, this is called alternate-day modified fasting.

The principle is simple: Most people find it easier to stay on a diet in which they can eat whatever they want half of the time. In the eight clinical studies I have conducted involving about 600 people (including an ongoing three-year study funded by the National Institutes of Health), intermittent fasters typically have lost 1.5 to 3 pounds per week, depending on how much weight they had to lose.

People lose weight by eating just 500 calories one day ("fast day") and all they want and anything they want the next day ("feast day")-alternating fast days with feast days until their weight-loss goal is reached. Goal weight is maintained by increasing calories on fast days to 1,000 three days a week and enjoying feast days the rest of the time.

Why it works

Key points about using this method to lose weight...

Why 500 calories? Animal studies showed that consuming 25% of the normal calorie in take on fast days produced the best results in preventing and reversing disease.

Translating this finding to people, I calculated 25% of daily recommended calories, which resulted in a general recommendation of 500 calories on fast day using foods with optimal nutrients.

Those 500 calories are consumed with one 400-calorie meal and a 100-calorie snack, since people tend to overeat if calories are broken up throughout the day. Lunch or dinner works best for the meal-if you eat your 400-calorie meal for breakfast, you'll be too hungry later in the day.

Example of a lunchtime meal: A turkey and avocado sandwich (two slices of turkey, one slice of Swiss cheese and one-quarter of an avocado on one slice of multigrain bread) and fruit (such as one-half cup of strawberries) for dessert.

Before or after your meal, you can have a snack such as a smoothie.

Tasty option: In a blender, mix one cup of unsweetened chocolate almond milk with one-half cup of unsweetened frozen cherries and one cup of ice.

Hunger disappears. After two weeks of alternate-day modified fasting, hunger on fast day disappears for most people. During those two weeks, ease your fast-day hunger by drinking eight to 10 eight-ounce glasses of water and other no-cal beverages such as coffee and tea and chewing sugar-free gum. Some people reported mild constipation, weakness and irritability, which subsided after two weeks.

You won't overeat on feast day. My studies show that people almost never overeat on feast day-on average, they consume 110% of their normal caloric intake. Over the two-day fast/feast cycle, that's an average of 67.5% of normal caloric intake-a perfect formula for safe, steady weight loss but without the non-stop deprivation of every-day dieting.

Add exercise-and lose twice as much weight. Every-other-day fasters can exercise on fast day without feeling weak or lightheaded. Exercising before the fast-day meal is best because you'll feel hungry afterward and can eat.

Good news: People who go on an intermittent fast and exercise (45 minutes of brisk aerobic exercise, three times a week) lose twice as much weight, on average, as people who only fast. You can exercise on both fast and feast days.

You won't lose muscle. Five out of six conventional dieters who lose weight gain it all back. That's probably because the typical dieter loses 75% fat and 25% muscle-and never regains that calorie-burning muscle mass after the diet is over.

But people who lose weight using alternate day modified fasting lose only about 1% muscle-a unique and remarkable result. And my one-year maintenance studies show that these alternate-day fasters maintain their weight. Longer-term studies are also needed.

As a Disease-Fighter...

People who have followed alternate-day modified fasting not only lose weight but also improve their overall health. In weight-loss studies of 600 people that lasted up to one year, average reductions in risk occurred for...

Heart disease. Total cholesterol dropped 21% .. . and LDL "bad" cholesterol dropped 20 points. Triglycerides fell from 125 mg/dL (considered "normal") to 88 mg/dL (defined as "optimal").

Type 2 diabetes. Glucose (blood sugar) levels dropped by up to 10% after eight weeks on the diet.

Animal studies have shown that intermittent fasting may help prevent...

Cancer. The diet may also slow the growth of existing malignancies.

Cognitive decline. Intermittent fasting helped protect the brains of mice genetically programmed to develop Alzheimer's... stopped the early development of nervous system problems in mice programmed to develop Parkinson's... and helped animals recover from stroke.

*Check with your doctor before trying this diet-especially if you have diabetes. Fasting is not recommended for pregnant women.

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