Rebecca J Frey & Teresa G Odle. The Gale Encyclopedia of Medicine. Editor: Jacqueline L Longe, 6th Edition, Volume 5, Gale, 2020.
Definition
Intermittent fasting (IF) is a name for a group of calorie restriction (CR) diets in which the dieter alternates between periods of fasting (usually consuming water or no-calorie beverages only) and non-fasting. 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 are followed by 24 hours of feeding. This pattern is alternately 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/4 fours of feeding or 19 hours of fasting/5 hours of feeding. Another variation is to consume a very limited number of calories (usually 15-20% of normal intake) on fasting days rather than taking in no calories at all.
Description
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 in humans are not yet fully understood because of the length of the human life span in comparison to the life spans of other animals.
IF seems to be practiced primarily by 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. The most common methods are fasting on alternate days, for whole days with a specific frequency per week, or during a specific time frame. Some of the better-known IF regimens include 2 Meal, LeanGains, Fast-5, Eat Stop Eat, 5:2 Fast Diet, 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 six to 10 hours in length. While 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.
Lean Gains
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. Berkhan, who holds an undergraduate degree in public health sciences and education, maintains that he became interested in IF when he found that the six-meals-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. 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 pre- and post-workout meals, stating that the preworkout 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 oz. (142 g) 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 postworkout 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
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 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, so 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.
Eat Stop Eat
Eat Stop Eat is a version of intermittent fasting popularized by Brad Pilon, who 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 will usually be 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.
Warrior diet
The Warrior diet is sometimes loosely grouped together with other IF regimens because it is based on a daily cycle of overeating and undereating, but it differs from intermittent fasting in a number of important respects. Designed by Ori Hofmekler (1952-), a former member of the Israeli Defense Force (IDF), the Warrior diet is a total workout, fitness, and nutrition program; it is not primarily a weight-loss program. Second, the undereating phase of the Warrior diet is not really a fast; the dieter is allowed to consume light snacks of raw fruits or vegetables or a light protein food like yogurt for the 10-18 hours a day that constitute the undereating period. In addition, Hofmekler maintains that people do not need to count calories during the overeating period; they can basically eat as much food as they wish. This advice is quite different from that of most IF proponents, who state that the eating window in their various plans is not an excuse to binge, and that calories do count if the user wishes to lose weight. Third, the Warrior diet is accompanied by an intense and rigorous exercise program based on whole-body workouts that may last as long as 45 minutes.
The Warrior diet also differs from other IF plans 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 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—$170 for a 30-day “protein support kit” as of 2012—which again sets the Warrior diet apart from other IF regimens.
Purpose
The purpose of IF is to help otherwise healthy adults lose weight at a slow but steady rate while choosing foods that appeal to them and 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.
In addition to a healthful rate of weight loss, proponents of IF maintain that it offers several additional benefits:
- The calorie restriction that occurs with IF has been shown to reduce inflammation, lower blood pressure and blood glucose levels, lower triglyceride and total cholesterol levels, and reduce body fat while sparing lean muscle tissue.
- The flexibility of IF relieves people of the emotional burdens of calorie counting, obsessions with meal planning and preparation, and feelings of deprivation related to “forbidden” foods. Most proponents of IF encourage people to have something they consider a pleasure or a “treat” during the feeding window, which can lead to better adherence of the plan.
- IF also allows people to adjust the timing of their feeding window for eating out, business trips, social events, and similar occasions, thus easing concerns about social isolation.
- Many users report increased mental energy and ability to concentrate because they are not constantly preoccupied with food or food preparation.
- Other users report less interest in sweets or processed foods and heightened enjoyment of natural flavors in foods.
- IF can easily accommodate people with food allergies who may need to avoid certain foods often included in conventional weight loss plans.
- IF is easy on the food budget 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 similar expenses. They also usually find that they save money on food because they are eating less.
Risks
Although 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.
There are many difficulties to following an IF regimen. 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 between eating nothing and eating anything.
Repeated fasting can result in nutrient deficiencies. People may feel fatigued during the fasting periods. Anyone interested in IF should first consult with their physician to help avoid any complications.
Research and General Acceptance
Most research on the health effects of fasting have been carried out in animals, with calorie restriction being shown to increase lifespan and improve tolerance to various metabolic stresses in the body. Until recently, relatively little scientific research had been done on IF in humans, possibly because there are a number of different fasting/feeding patterns that identify themselves as intermittent fasting and because many of the writers in the field were personal trainers or bodybuilders rather than research scientists. However, due to the popularity of this approach, several recent studies have investigated its effectiveness in weight loss. A systematic review of 40 studies looking at their effects concluded that the evidence does suggest that they are effective in promoting weight loss, with a typical loss of 7-10 pounds over 10 weeks. However, methodological differences between these studies made comparisons of findings difficult. A randomised controlled trial following obese individuals for a year did not find IT to be more effective than daily calorie restriction.
Other suggested benefits of this approach include decreased triglycerides and LDL-cholesterol, reduce blood pressure and reductions in insulin resistance. The fact that IF diets may improve insulin sensitivity is an attractive option for people with diabetes. This has been supported by some research demonstrating decreases in glucose and insulin secretion in patients with type 2 diabetes, although there is a need to study longer term effects.
The Academy of Nutrition and Dietetics states in March 2014 that although animal studies and early human studies show positive effects, additional studies in humans are needed. Fasting for at least 16 hours is generally easier for elite athletes to maintain, yet the organization’s review also stated that competitive athletes should work with a coach, athletic trainer, and a registered dietitian or nutritionist when controlling weight and body composition. Further, people who are not elite athletes may not be best served by IF. Problems such as dizziness, fatigue, or nausea can accompany long periods of fasting. And people who are not in the same shape as competitive athletes may not be able to perform regular physical workouts while restricting food intake.
Most registered dietitians maintain that much more research needs to be done on IF in humans to gain a clearer understanding of its effects on the human body or the aging process, and the metabolic pathways or other reasons for those beneficial effects.