Intermittent Fasting


Intermittent fasting (IF) is a name for a group of calorie restriction (CR) diets in which the dieter alternates between periods of fasting (usually understood to mean consumption of water or no-calorie beverages only) and nonfasting. IF is based on time periods rather than on meal plans, food lists, recipes, calorie counting, or similar features of most weight loss diets. One common form of IF appears to be a two-day cycle in which 24 hours of fasting is followed by 24 hours of feeding. This pattern is known as alternate-day fasting (ADF), every-other-day fasting (EOD), or every-other-day feeding (EODF). Other IF diets may divide each day into a period of fasting and a period of feeding, such as 20 hours of fasting and 4 fours of feeding or 19 hours of fasting and 5 hours of feeding. Another variation is to consume a very limited number of calories (usually 15% to 20% of normal intake) on fasting days rather than taking in no calories at all.


Intermittent fasting as a dietary regimen appears to have originated with laboratory experiments on animals (mice in most cases) in the 1940s, in which researchers discovered that calorie restriction (CR) in the form of intermittent fasting appeared to extend the animals' life spans. Calorie restriction without malnutrition has been shown to extend the median and maximum life spans in such different species as yeast, fish, and dogs, as well as mice, but its effects on humans are not yet fully understood because of the length of the human lifespan in comparison to that is of other animals.


IF seems to be practiced primarily by male bodybuilders and athletes in developed countries, who may combine it with food cycling regimens of various types. Food cycling refers to the practice followed by some weight trainers of reversing the proportions of fats and carbohydrates in the diet according to the phase of the training schedule—usually high carbohydrate/low fat on training days and low carbohydrate/high fat on rest days.


There is no single IF diet, but rather several different regimens. Some of the better-known IF regimens include 2 Meal, LeanGains, Fast-5, Eat Stop Eat, and the Warrior Diet.

2 Meal (IF Life)

The 2 Meal version of intermittent fasting has been popularized by Michael O'Donnell, a personal trainer and fitness coach who began to write about his approach in 2007 and refers to himself as “2 Meal Mike.” The 2 Meal system, also called IF Life, is the easiest version of IF to tailor to an individual's preferences; O'Donnell describes it as “a simple way to eat less overall for weight loss.” The “2 Meal” in the program's title refers to limiting one's eating to two meals a day. O'Donnell states that he rarely eats breakfast, has a late lunch, and then eats one other meal per day, usually in the early evening. Rather than specify an ideal length of time for fasting/feeding, O'Donnell notes that his own daily feeding window can vary from 6 to 10 hours in length. Although he advises beginners to start with set times for meals so that they do not have too many variables to adjust, he emphasizes that “less is more” and that “many ways can work” for people to benefit from the 2 Meal approach.

O'Donnell's claims for his version of IF are modest. He notes that no one diet plan works for everyone, and that such factors as insulin resistance, general level of activity, choice of foods and total calorie consumption, amount of rest and sleep, and the presence of any metabolic disorders can affect the rate of weight loss and success in weight maintenance. His overall advice is to avoid making intermittent fasting unduly complicated—the focus should be on enjoying life rather than worrying about the diet.

O'Donnell has a website called IF Life, last updated in 2017. In addition to pages explaining his theory of weight loss, he offers the most recent (fifth) edition of his 100-page book, The 2 Meal Solution. The book is a PDF download rather than a hardcover or paperback. He includes a disclaimer: “2 Meal Solution is meant for healthy individuals and may not be suitable for everyone. All people are advised to talk with their physician before attempting. Use at your own risk. Results can vary by person and are not guaranteed.”


Leangains is an IF program devised and popularized by Martin Berkhan, a Swedish personal trainer and magazine writer who maintains a blog about his dietary recommendations at . Berkhan, who holds an undergraduate degree in public health sciences and education, maintains that he became interested in IF when he found that the sixmeals-a-day regimen often recommended for athletes did not work for him either physically or psychologically. In particular, he noticed that his life had started to revolve around food: “The constant meal preparing, the obsessiveness about eating the perfect meals at the right time, and the way I sometimes made excuses not to participate in social gatherings in order to meet my calorie and macronutrient goals for the day.” He experimented with intermittent fasting, noted the range of IF patterns reported in the literature, and settled on one that worked for him and his bodybuilding clients.

Berkhan's version of IF is based on a 16/8 daily pattern of fasting/eating rather than an alternate-day pattern. (For women, Berkhan recommends shortening the fasting phase to 14 hours). He and most of his clients eat three meals during the eight-hour feeding window: a pre-workout meal and two post-workout meals. Unlike O'Donnell, Berkhan has strong convictions about the proper balance of nutrients in the preand post-workout meals, stating that the pre-workout meal should be light (about 500 calories), with equal amounts of carbohydrates and proteins, as well as “some fat for taste.” A typical pre-workout meal for Berkhan might include 5 ounces of lean meat, a potato or other vegetable, and a large apple. The post-workout meals should account for 80% of the day's calorie intake and be high in carbohydrates, moderate in protein, and low in fat. Berkhan states that he usually eats one post-workout meal immediately after the workout and the second meal about an hour before bedtime. Like most proponents of intermittent fasting, Berkhan notes that limiting one's total calorie consumption is still necessary to lose weight, and that IF will not work if people regard the eating window as an excuse to binge.

Fast-5/Appetite Correction

The Fast-5 approach to intermittent fasting was devised in the early 2000s by a physician named Bert Herring and his wife, also a physician. The Herrings published a 52-page book on their version of IF in 2005, followed by a 158-page paperback released in 2015 and titled AC: The Power of Appetite Correction. “AC” appears to be the Herrings' current term for the original Fast-5 diet, based on their theory that intermittent fasting is a form of appetite correction that essentially works by resetting the dieter's feelings of satiety.

The Fast-5 program resembles O'Donnell's 2 Meal approach in that it is relatively flexible. It is based on a 19/5 daily pattern of fasting/eating, with all eating to be done within the five-hour window. During that five-hour period, the dieter is to eat as much as they want, as long as they are truly hungry. No liquids containing calories are to be consumed during the 19-hour fasting period, although the dieter may drink as much water or other calorie-free beverages as desired. The Herrings emphasize that any period of five consecutive hours is fine to use as the eating window, so that users can identify a time frame that works for them.

The Herrings outline two ways to start the Fast-5 program: a “cold turkey” approach, in which the dieter simply waits to eat until the chosen five-hour window, or a gradual adjustment approach, in which the timing of the eating window is pushed back by half an hour or an hour every day or every few days until the person reaches the desired time setting for the window. With regard to choice of foods, the Herrings recommend a variety of fruits and vegetables containing fiber; a variety of protein sources that include fish, eggs, and meat; and nuts or sunflower seeds—in short, “a balance of carbohydrate, fats, and protein.”

The Herrings advise that people may not notice weight loss until they have used the Fast-5 approach for three or four weeks, and that they may find they are losing inches from the waistline before their scale registers a loss in weight. The Herrings refer to the initial three weeks as the adjustment period, and maintain that dieters using the Fast-5 approach should begin to lose about a pound per week after a month or longer on the program. The updated AC version of the Fast-5 diet recommends several “tweaks” if the dieter does not see results after a month or so on the diet:

Eat Stop Eat

Eat Stop Eat is a version of intermittent fasting popularized by Brad Pilon, a Canadian with a degree in nutrition who blogs at . Pilon is the author of two e-books, Eat Stop Eat (in two versions, one for men and one for women) and The Zen of Nutrition. Pilon's approach to IF consists of one or two 24-hour fasts per week, the day(s) chosen by the user. Pilon recommends that people choose days when they are not too busy. Exercise should be done when energy levels are highest, which is usually toward the beginning of the fast. The fast usually begins on the evening before the fast day and ends 24 hours later on the following evening. During the fast, the dieter may drink any fluid that does not contain calories: coffee, unsweetened tea, water, club soda, diet soft drinks, and the like.

The most distinctive feature of Pilon's approach to IF, most likely derived from his academic background, is his philosophical approach; that is, analyzing how one thinks about food. “We ask ourselves, either consciously or subconsciously, ‘What is the right thing to do now—Eat, or not eat?’…This is the basic most fundamental philosophy behind intermittent fasting. If you consider Intermittent Fasting to be the ability to practice patience when it comes to the act of eating—a conscious polite restraint when it comes to food intake, then the philosophy is simply—we do not have to eat all the time, therefore we are free to choose when we eat.”

Warrior diet

The Warrior diet also differs from the IF plans described above in its rules about cooking and eating. Like Berkhan, Hofmekler believes in food cycling; dieters should alternate between high-fat and high-carbohydrate days in order to maximize the body's fat burning during exercise. Hofmekler advises people to avoid storing foods in plastic containers or purchasing foods wrapped or contained in plastic. He believes that only bottled water should be used for drinking and cooking and that supermarket foods contain estrogenic compounds. Hofmekler's preoccupation with estrogens in the environment has no parallel in other IF regimens. Last, Hofmekler has a store at through which he markets a number of dietary supplements intended to help the body burn fat, detoxify, rid itself of estrogenic compounds, and maintain a normal hormonal balance. These products are quite expensive, which again sets the Warrior diet apart from other IF regimens.


The function of intermittent fasting is to help otherwise healthy adults lose weight at a slow but steady rate while choosing foods that appeal to them and selecting an overall fasting/feeding pattern that works for them as individuals. Berkhan also maintains that IF can be used by bodybuilders to increase lean muscle mass and/or decrease the proportion of body fat, and the Herrings maintain that IF can retrain the dieter's appetite to desire less food.


In addition to a healthful rate of weight loss, proponents of IF maintain that it offers several other benefits:

Calorie restriction (CR)—
The limitation of daily calorie intake below a person or animal's previous unrestricted level, most often by 10%–25%. Some researchers believe that CR could extend the human life span, as it appears to have that effect in some other species.
Insulin resistance—
A state or condition in which a person's body tissues have a lowered level of response to insulin, a hormone secreted by the pancreas that helps to regulate the level of glucose (sugar) in the body.
The quality or state of feeling comfortably full. It is sometimes used as a criterion for evaluating people's satisfaction with diets or diet products.
The term used in intermittent fasting to refer to the time period when eating is allowed. It is also called the eating window.
Yo-yo dieting—
A recurring pattern of losing and then gaining weight. It is also called weight cycling.


Although Martin Berkhan claims that he has clients with diabetes who have successfully used his approach to IF, most IF proponents state that their programs are intended for otherwise healthy adults who need to lose weight; they are not intended for children or adolescents who are still growing, pregnant or lactating women, or anyone with a chronic disorder, including diabetes, cancer, or heart disease.

Following an IF regimen can be difficult. People with a history of eating disorders, stress, or anxiety disorders may find that the restrictions of IF can worsen these conditions. The diets can be hard to follow for an extended period of time, especially for people who live with others who do not practice IF. Some researchers are concerned that IF promotes weight cycling, or yo-yo dieting, due to the extremes of eating nothing and eating anything.


Repeated fasting can result in nutrient deficiencies. People, particularly women, may also feel fatigued during the fasting periods. Others report feeling muscle weakness or chronic headaches. Anyone interested in IF should first consult their physician to help avoid any complications.

Research and general acceptance

Relatively little scientific research had been done on IF in humans prior to 2012, possibly because of the various fasting/feeding patterns that are identified as intermittent fasting. In addition, many of the writers in the field are personal trainers or bodybuilders rather than research scientists. Since 2012, however, physicians studying metabolic disorders, particularly type 2 diabetes, as well as those concerned with weight management in general have been looking into IF diets as possible answers to the high rates of overweight and obesity in developed countries. As of early 2018, however, most researchers maintained that the long-term effects of IF diets in humans are not yet known. The Academy of Nutrition and Dietetics had published only one article on IF as of 2018, which took a skeptical position: “For the everyday active person, intermittent fasting may present some challenges and may not be the best approach for managing your weight.”


Some physicians point out that adherence to a diet is the most important factor in weight loss and maintenance, and that “regular professional contact and supportive behavioral change programs” are critical to success. Others have noted that IF diets are essentially based on calorie restriction and that they work only “because of overall decreased caloric intake.” One group of researchers in California, however, thinks that IF regimens are promising because they reduce nighttime eating and prolong nightly fasting; and thus “may result in sustained improvements in human health.”

Of the 24 clinical trials of IF registered with the National Institutes of Health in 2018, only four were studies of the effects of IF on weight loss; the others focused on the effects of IF on drug uptake, high blood pressure, insulin resistance, behavioral modification, cancer, and aging. No results had been posted for any completed clinical trials of IF.

See also Calorie restriction ; Malnutrition ; Weight cycling .



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Academy of Nutrition and Dietetics, 120 S. Riverside Plaza, Ste. 2190, Chicago, IL, 60606-6995, (312) 899-0040, (800) 877-1600,, .

Center for Food Safety and Applied Nutrition, Food and Drug Administration, 5001 Campus Dr., HFS-009, College Park, MD, 20740-3835, (888) 723-3366, .

International Food Information Council Foundation, 1100 Connecticut Ave. NW, Ste. 430, Washington, DC, 20036, (202) 296-6540,, .

Rebecca J. Frey, PhD

  This information is not a tool for self-diagnosis or a substitute for professional care.