Intermittent fasting (IF) is a term used to describe a type of calorie-restricted diet in which the dieter alternates between periods of fasting (usually defined as drinking only water or no-calorie beverages) and non-fasting. IF is based on time periods rather than meal plans, food lists, recipes, calorie counting, or other weight-loss diet features. One common type of IF appears to be a two-day cycle consisting of 24 hours of fasting followed by 24 hours of feeding. This pattern is also referred to 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 fasting period and a feeding period, such as 20 hours fasting/4 hours feeding or 19 hours fasting/5 hours feeding. Another variation is to consume a very limited number of calories (usually 15% to 20% of normal intake) on “fasting” days rather than none at all.


Intermittent fasting as a dietary regimen appears to have originated in the 1940s with laboratory experiments on animals (mostly mice), in which researchers discovered that calorie restriction in the form of intermittent fasting appeared to extend the animals’ lives. Calorie restriction without malnutrition has been shown to increase the median and maximum life spans in a variety of species, including fish, dogs, and mice, but its effects in humans are not fully understood due to the length of the human lifespan in comparison to other animals.

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


Intermittent fasting serves the purpose of assisting otherwise healthy adults in losing weight at a slow but steady rate while selecting foods that appeal to them and an overall fasting/feeding pattern that works for them as individuals. It is also suggested that bodybuilders use IF to increase lean muscle mass and/or decrease body fat proportion.


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

  • Calorie restriction caused by IF has been shown to reduce inflammation, lower blood pressure and glucose levels, lower triglyceride and total cholesterol levels, and reduce body fat while sparing lean muscle tissue.
  • The flexibility of IF frees people from the emotional burdens of calorie counting, obsessive meal planning and preparation, and feelings of deprivation associated with “forbidden” foods. During the feeding window, most proponents of IF encourage people to have something they consider a pleasure or a “treat.”
  • IF also enables people to change the timing of their feeding window for eating out, business trips, social events, and other similar occasions, alleviating concerns about social isolation.
  • Because they are not constantly preoccupied with food or food preparation, many users report increased mental energy and ability to concentrate.
  • Other users report a decrease in interest in sweets and processed foods, as well as an increase in enjoyment of natural flavors in foods.
  • People with food allergies who need to avoid certain foods that are commonly included in conventional weight loss plans can easily accommodate IF.
  • Because users do not have to spend money on exotic foods or cooking equipment, dietary supplements (with the exception of the Warrior diet), diet books, membership in a diet program, or other similar expenses, IF is easy on the food budget. They usually discover that they save money on food because they eat less.

Intermittent Fasting Diets

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)

Michael O’Donnell, a personal trainer and fitness coach who began writing about his approach in 2007 and refers to himself as “2Meal Mike,” popularized the 2 Meal version of intermittent fasting. The 2 Meal system, also known as IF Life, is the simplest version of IF to customize to an individual’s tastes. According to O’Donnell, it is “a simple way to eat less overall for weight loss.”

The term “2 Meal” refers to limiting one’s eating to two meals per day. According to O’Donnell, he rarely eats breakfast, has a late lunch, and then eats one other meal per day, usually in the early evening. Rather than specifying an ideal fasting/feeding time, O’Donnell notes that his own daily feeding “window” can range from 6 to 10 hours in length. While he recommends that beginners start with set meal times so that they don’t 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 makes modest claims for his version of IF. He emphasizes that no single diet plan works for everyone, and that insulin resistance, general level of activity, food selection and total calorie consumption, amount of rest and sleep, and the presence of any metabolic disorders can all affect the rate of weight loss and weight maintenance success. His general advice is to avoid overcomplicating intermittent fasting—the emphasis should be on enjoying life rather than worrying about the diet.


Martin Berkhan, a Swedish personal trainer and magazine writer who maintains a blog about his dietary recommendations, created and popularized the LeanGains IF program. Berkhan, who has a bachelor’s degree in public health sciences and education, claims that he became interested in IF after discovering that the six-meal-a-day regimen commonly recommended for athletes didn’t work for him, either physically or psychologically; in particular, he noticed that his life had begun to revolve around food: “The constant meal preparation, the obsessiveness about eating the perfect meal at the right time, and the way I sometimes made excuses not to participate in social gatherings in order to meet my daily calorie and macronutrient goals.” He experimented with intermittent fasting, taking note of the various IF patterns reported in the literature, and eventually settling on one that worked for him and his bodybuilding clients.

Berkhan’s version of intermittent fasting is based on a 16/8 daily fasting/eating pattern rather than an alternate-day pattern. During the eight-hour feeding window, he and the majority of his clients eat three meals: one pre-workout meal and two post-workout meals. Unlike O’Donnell, Berkhan is adamant about the proper nutrient balance in pre- and post-workout meals, stating that the pre-workout meal should be light (around 500 calories), with equal amounts of carbohydrates and proteins, as well as “some fat for taste.”

Berkhan’s typical pre-workout meal might consist of 5 ounces of lean meat, a potato or other vegetable, and a large apple. Post-workout meals should account for 80% of total calorie intake for the day and be high in carbohydrates, moderate in protein, and low in fat. Berkhan claims that he typically consumes one post-workout meal immediately following the workout and a second meal about an hour before bedtime. Berkhan, like most proponents of intermittent fasting, notes that limiting total calorie consumption is still required to lose weight, and that IF will not work if people use the eating window as an excuse to binge.


Bert Herring, a physician, and his wife, also a physician, developed the Fast-5 approach to intermittent fasting in the early 2000s. The Herrings published a 52-page book on their version of IF, which can be found at http://www.fast-5.com.

In that it is relatively flexible, the Fast-5 program is similar to O’Donnell’s 2 Meal approach. It is based on a 19/5 daily fasting/eating pattern, with all eating taking place within the five-hour window. The dieter is allowed to eat as much as they want during that five-hour period, as long as they are truly hungry. During the 19-hour fasting period, no calorie-containing liquids may be consumed, though the dieter may drink as much water or other calorie-free beverages as desired. The Herrings emphasize that any period of five consecutive hours can be used as the eating window, allowing users to find a timeframe that works for them.

The Herrings outline two approaches to starting the Fast-5 program: a “cold turkey” approach in which the dieter simply waits until the chosen five-hour window to eat, 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. In terms of food, the Herrings recommend a variety of fiber-rich fruits and vegetables; a variety of protein sources such as fish, eggs, and meat; and nuts or sunflower seeds—in short, “a balance of carbohydrates, fats, and protein.”

The Herrings caution that people may not notice weight loss until they have used the Fast-5 approach for three or four weeks, and that they may notice they are losing inches from their waistline before their scale registers a weight loss. The Herrings refer to the first three weeks as the adjustment period, and they claim that dieters following the Fast-5 approach should start losing about a pound per week after a month or more on the program.

Eat Stop Eat

Brad Pilon, the author of two e-books, Eat Stop Eat (in two versions, one for men and one for women) and The Zen of Nutrition, popularized this version of intermittent fasting. Pilon’s method of IF consists of one or two 24-hour fasts per week, on the day(s) of the user’s choosing. Pilon advises people to pick days when they are not overly busy. Exercise should be done when energy levels are at their peak, which is usually near the start of the fast. The fast usually starts the evening before the fast day and lasts 24 hours, ending the next evening. During the fast, the dieter may consume any non-calorie liquid, such as coffee, unsweetened tea, water, club soda, diet soft drinks, and so on.

Warrior Diet

Because it is based on a daily cycle of overeating and undereating, the Warrior diet is commonly lumped in with other IF regimens, although it differs from intermittent fasting in a number of ways. The Warrior diet is a whole workout, fitness, and nutrition program designed by Ori Hofmekler, a former member of the Israeli Defense Forces (IDF). It is not primarily a weight-loss program.

The Warrior diet’s undereating phase isn’t truly a fast; for the 10 to 18 hours a day that make up the undereating period, the dieter is allowed to eat light snacks of raw fruits or vegetables or a light protein source like yogurt. Furthermore, according to Hofmekler, people do not need to calculate calories during the binge eating stage; they can eat as much as they want. Most IF proponents, on the other hand, argue that the eating window in their various programs is not an excuse to binge, and that calories do matter if the user wants to lose weight. The Warrior diet is supplemented by a tough and tough training program consisting of 45-minute whole-body workouts.

In addition, the Warrior diet differs from the IF diets discussed above in terms of cooking and eating requirements. Hofmekler, like Berkhan, believes in dietary cycling, in which dieters alternate high-fat and high-carbohydrate days to enhance fat burning during exercise. People should avoid storing food in plastic containers or purchasing items that are wrapped or encased in plastic, according to Hofmekler. He believes that drinking and cooking should only be done with bottled water, and that store foods contain “estrogenic chemicals.” Other IF regimens don’t include Hofmekler’s obsession with estrogens in the environment.


Although Martin Berkhan claims that his approach to IF has helped clients with diabetes, most IF proponents state that their programs are for otherwise healthy adults who need to lose weight; they are not for children or adolescents who are still growing, pregnant or lactating women, or anyone with a chronic disorder, such as diabetes.

There are numerous challenges to sticking to an IF diet. The constraints of IF may aggravate eating problems, stress, or anxiety disorders in people who have a history of them. The diets can be difficult to stick to for a long time, especially if you live with those who do not observe IF. Because of the extremes between eating nothing and eating everything, some experts believe that IF promotes weight cycling or yo-yo dieting.


Fasting on a regular basis might lead to nutrition deficiency. During fasting periods, people may feel tired. Anyone considering IF should speak with their doctor first to avoid any issues.

Research and General Acceptance

Because there are several alternative fasting/feeding patterns that describe themselves as intermittent fasting, there has been relatively little scientific research on IF in humans. Rather from being research scientists, many of the writers in this field are personal trainers or bodybuilders. As of summer 2012, the Academy of Nutrition and Dietetics had published no reviews of IF, either of the practice in general or of individual IF regimens. Only one of the nine clinical trials of IF registered with the National Institutes of Health in 2012 was a weight loss research; the other seven were all studies of IF’s effects on medication absorption and metabolism. The ninth study, which is currently recruiting participants, aimed to examine alternative meal frequency patterns, specifically the effects of intermittent fasting to a three- or six-meal-per-day eating pattern.

In comparison to studies using mice models, there are only a few published studies on IF in humans. According to one study, IF has no effect on the metabolism of various nutrients in healthy subjects when compared to control people on a normal diet. In 2011, a British study found that intermittent fasting is at least as efficient as a daily calorie-restricted diet for weight loss and insulin sensitivity in overweight women.

The favorable effects recorded by adults utilizing an IF feeding regimen may be attributed in part to the way IF resets the body’s circadian rhythm, according to a team of Israeli researchers in 2010. The majority of registered dietitians believe that additional research on intermittent fasting in humans is needed to acquire a better knowledge of its impacts on the human body or the aging process, as well as the metabolic pathways or other explanations for its favorable effects.

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