Intermittent fasting (IF) is a way of eating that involves periods of fasting alternating with eating windows. Different methods of IF vary in fasting/eating periods and restrictions on what you are allowed to eat and drink.
IF is often used for weight loss and preventing or managing some chronic diseases. Fasting is also used for non-diet reasons, such as religious beliefs and traditions.
This article reviews reasons for following an intermittent fasting eating pattern, different methods of IF, and the health benefits and safety concerns of IF.
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Why Do People Start Intermittent Fasting?
People choose to start IF for various reasons. Aside from fasting for religious beliefs or cultural reasons, the most common health-related reason for fasting is for weight loss.
A randomized controlled trial involving people with prediabetes (the stage before type 2 diabetes) who were overweight or had obesity divided them into one of two intermittent fasting methods or a control group.
At the end of the trial, both fasting groups had greater reductions in body weight, body mass index (BMI), and waist circumference compared to the control group.
BMI is a dated, flawed measure. It does not take into account factors such as body composition, ethnicity, sex, race, and age. Even though it is a biased measure, BMI is still widely used in the medical community because it’s an inexpensive and quick way to analyze a person’s potential health status and outcomes.
A systematic review of 40 studies on IF found that it helped people lose weight, with a usual weight loss of 7 to 11 pounds over 10 weeks.
Other reasons may include helping prevent or manage some chronic medical conditions, enhancing physical performance, or helping improve thinking and memory.
Intermittent Fasting vs. Fasting Before Bloodwork
Intermittent fasting is different from fasting for blood work. Some blood work tests require you to fast for at least 8 hours or overnight before having your blood drawn. This is done so the test can be as accurate as possible. Fasting for blood work is only done when instructed by a healthcare provider before certain labs are drawn.
There are several different methods of IF, but all involve periods of fasting or modified fasting. Fasting windows may range from 10 to 36 hours, depending on the method. In some types of IF, you cannot eat any food in a fasting period, while others are modified fasts, allowing for around 500 to 600 calories of food and/or beverages.
Continue reading to learn about seven different methods of intermittent fasting.
1. Time-Restricted (16:8 or 14:10) Intermittent Fasting
Time-restricted fasting is one of the most commonly used methods of IF, especially for beginners. This method follows a 16:8 or 14:10 schedule, in which you fast for 16 or 14 hours a day and have an 8- or 10-hour eating window, respectively.
- Schedule: There are no restrictions on when you need to have your fasting hours begin or end. However, it is important to be consistent with what you choose. Most people have the majority of their fasting hours overnight. An example might be to fast from 6 p.m. to 10 a.m. for a 16:8 schedule or 7 p.m. to 9 a.m. for a 14:8 schedule.
- Rules: Finish your meal before your eating window ends. Water and other no-calorie beverages, such as herbal tea and black coffee, are allowed during the fasting window.
2. Overnight (12:12) Intermittent Fasting
A shortened version of time-restricted fasting is to fast overnight for 12 hours. This method of IF might be a good option for beginners, as most of the fasting window is while you’re sleeping.
- Schedule: Choose a 12-hour overnight fasting period—for example, 8 p.m. to 8 a.m.
- Rules: Finish your meal before your eating window ends in the evening. Water and other no-calorie beverages, such as herbal tea and black coffee, are allowed during the fasting window.
3. Alternate Day Intermittent Fasting
Sometimes referred to as the 4:3 IF method, alternate day fasting involves fasting every other day. Being a more extreme method, this variation of IF may not be appropriate for beginners or people with certain medical conditions.
- Schedule: Fast for a full day, followed by a day of eating regularly. Continue alternating between fasting and eating days. Some variations include fasting past 24 hours and up to 36 hours, then eating for 12 hours.
- Rules: This method has a couple of variations. One is to fast (go without food completely) on your fasting day. Another is to eat minimally, or around 500 calories on your fasting day. Water and other no-calorie beverages, such as unsweetened tea and black coffee, are allowed on the fasting day.
4. 5:2 Intermittent Fasting
The 5:2 diet, also known as the Fast Diet, involves eating regularly five days a week and following a modified fast two days. This method is a little more flexible, as you are allowed to choose which days of the week are your modified fasting days.
- Schedule: During a seven-day week, choose two days to fast. It’s best to break up fasting days so as not to be consecutive. The remaining five days are regular eating days.
- Rules: On your fasting days, you may eat minimally, around 500 to 600 calories per day. On eating days, eat a normal day's worth of calories—but don’t overeat to compensate for the fasting days. Water and other no-calorie beverages, such as unsweetened tea and black coffee, are allowed on the fasting days.
5. Warrior Diet (20:4) Intermittent Fasting
A more drastic variation of IF, the warrior diet involves eating minimally (usually raw fruits and vegetables) during the day, followed by eating one large meal at night within a four-hour period.
- Schedule: Twenty hours of eating very little is followed by a four-hour window in which you are allowed to eat the remaining calories for the day.
- Rules:During the 20-hour modified fast, you may consume some raw fruits and vegetables, small amounts of protein, and beverages such as milk, water, coffee, and unsweetened tea. During the four-hour eating window, you may eat however much and whatever you want, but are encouraged to eat nutrient-dense foods.
6. One Meal a Day (OMAD) Intermittent Fasting
One of the most extreme methods of IF, the OMAD diet. is a 23-hour fast followed by a 1-hour eating window in which you eat the entire daily calories. This fasting method may not be appropriate for beginners or people with certain medical conditions.
- Schedule: Fasting for 23 hours, followed by 1 hour of eating a very large meal. The1-hour eating window should be within the same 4-hour time frame each day.
- Rules: You are encouraged to eat as many calories as you normally would during a day in your 1-hour eating window. There are no restrictions on the types or amount of food you can eat. During the 23-hour fasting period, no- or very-low-calorie beverages are allowed, such as water, unsweetened tea, and black coffee.
7. Eat Stop Eat Intermittent Fasting
The Eat Stop Eat method of IF involves fasting for 24 hours one or two nonconsecutive days a week. This method is similar to the 5:2 fasting diet. However, the 5:2 method allows for 500 to 600 calories on fasting days, while the Eat Stop Eat method does not.
- Schedule: Eat normally for at least one day, then stop eating (fast) for a full 24 hours (i.e., 5 p.m. until 5 p.m. the next day). Resume eating normally again for one or more days. You may choose one or two days of fasting during the week, spaced with at least one normal day of eating in between.
- Rules: No food is allowed during the fasting period. However, water, unsweetened tea, and coffee are allowed. There are no restrictions on what or how much you eat on non-fasting days.
Besides Weight Loss, Does Intermittent Fasting Have Other Benefits?
Some studies have shown promising results for health benefits of IF beyond weight loss. The same randomized controlled trial mentioned previously that showed IF beneficial for weight loss also found the two intermittent fasting groups to have significant decreases in both blood glucose (sugar) and triglyceride (a type of fat) levels.
A different randomized controlled trial looked at the effects of a 5:2 fasting diet in people with metabolic syndrome (a group of symptoms occurring together that increase the risk of type 2 diabetes, stroke, and cardiovascular disease),
Over eight weeks, they found that the fasting group improved certain cardiometabolic risk factors, including reducing fat mass, oxidative stress (an imbalance between free radicals and antioxidants in your body), and inflammation.
In addition, IF significantly altered the gut microbiota (the collection of bacteria and other organisms in the gastrointestinal tract), improving overall gut health and carbohydrate metabolism.
Some studies have shown beneficial effects of IF on physical activity performance, including studies done on elite cyclists, generally healthy active women and men. However, one small study showed a negative impact on sprinting performance in males.
Some studies concluded that IF may improve memory and brain health. Other research has shown that IF may result in improved metabolism.
Additional, larger clinical trials and research is needed to determine whether IF is beneficial in other ways and in the long term.
What Is Metabolism?
Metabolism is a term that refers to all of the chemical processes or changes that occur in the body at the cellular level. These changes make energy and the materials organisms and cells need in order to grow, reproduce, and maintain life.
In relation to diet, metabolism is the process of digesting or breaking down the food you eat into a simpler form the body is able to use. Metabolism is also involved when your body breaks down muscle or stored fat to be used as energy during periods of strenuous exercise or fasting.
Safety of Intermittent Fasting: Who Shouldn’t Try It
While IF may be beneficial for someone trying to lose weight, it isn’t for everyone. Always talk with a healthcare provider before starting a new diet or eating pattern, especially if you have a known medical condition.
IF may be unsafe for people with diabetes who take insulin or other glucose-lowering medications and people who are on medication that needs to be taken with food. Individuals with a history of disordered eating, children under 18, and people who are pregnant or breastfeeding should not follow IF diets.
Possible Risks in Men vs. Women
A review of human trials found that fasting may reduce levels of androgens (reproductive and growth hormones), which can negatively impact metabolic health and libido (sexual desire) in males.
Some studies suggest that women may be more sensitive to fasting, most likely relating to fluctuating hormones. One small study concluded that IF may increase stress in women,
More research is needed to determine the different effects of IF on men versus women.
Water Fasting: What You Need to Know
Summary
Intermittent fasting (IF) involves periods of fasting alternating with periods of eating. There are several methods of IF with variations in the amount of time spent fasting versus eating. Fasting for health is done for different reasons, including weight loss, disease prevention or management, and to help improve other aspects of health, such as physical performance, metabolism, and brain health.
People with diabetes who take insulin or other glucose-lowering medications and those who are on medication that needs to be taken with food should not follow IF eating patterns. Others who should also avoid IF include individuals with a history of disordered eating, children under 18, and pregnant or breastfeeding people. Talk with a healthcare provider if you are considering trying IF.
