Intermittent fasting is based on the idea that you can lose weight if you limit the window of time where you're allowed to eat. This makes it unique from many other diets.
"Intermittent fasting focuses on when a person should eat food, not what type of food he or she eats," says Divya Selvakumar, a registered dietitian, author, and founder of Divine Diets, LLC.
Eating for only a limited window means that you fast for the rest of the time, but are allowed to consume calorie-free beverages like coffee. The purpose of the fast is to kickstart a metabolic state called ketosis. While in ketosis, your body burns fat for fuel instead of sugar.
According to the limited, short-term studies, intermittent fasting does work to help people lose weight in a matter of weeks. However, there are many different approaches to intermittent fasting, so here's a guide for what to know to help you decide which fasting method may be right for you.
The 16:8 approach is possibly the most common method of intermittent fasting. It's sometimes called time-restricted fasting, and there are several slightly different variations.
In the 16:8 version, you fast for 16 hours and limit your eating to an eight-hour window of time during the day.
Most people skip breakfast as part of the 16-hour window, says Selvakumar. So, you might eat between the window from 12 pm to 8 pm, for example.
However, some may choose to skip dinner, instead. For this, you might limit your eating window to between 9 am and 5 pm each day.
According to registered dietitian Natalie Allen, a clinical assistant professor at Missouri State University, most people choose some version of the 16:8, because it's easier to follow than some other versions.
Research suggests that this type of intermittent fasting can help you lose fat and perhaps even improve your cholesterol levels, among other health benefits.
As the name implies, alternate-day intermittent fasting is when you fast or severely restrict your caloric intake every other day.
Allen says that the alternate-day fasting approach is much less popular. Plus, it may be the most challenging type of intermittent fasting to attempt. As a 2017 article in the Annual Review of Nutrition puts it, "[T]his fasting regimen may not be practical because it leads to intense hunger on fasting days."
Another 2017 study in JAMA Internal Medicine found that most of the people who dropped out of an intermittent fasting research study in an attempt to lose weight were the participants who were attempting to do alternate day fasting. Also, it didn't produce greater weight loss or weight maintenance.
"Generally, we don't recommend it," Allen says. "It's hard to not eat for a whole day. I would worry about your blood sugars, your insulin levels, your energy levels, your ability to think. And are you going to be hangry?"
For 5:2 intermittent fasting, you eat normally for five days of the week and drastically reduce your caloric intake for two non-consecutive days. Women restrict themselves to 500 calories, and men to 600 calories on fasting days.
Some people prefer the 5:2 approach. But a 2018 study in The American Journal of Clinical Nutrition found that while this version was effective on the weight loss front, it was not more effective than a continuous calorie restriction approach in which people tried to reduce the number of calories they consumed each day.
The OMAD (one meal a day) diet is where you limit your eating window to just one hour each day and fast for the remaining 23 hours. This is an extreme type of intermittent fasting, and it can be an effective weight-loss strategy for some people.
It can even help to lower some of the risk factors for cardiovascular disease. However, you have to be able to last the entire 23 hours between meals and resist the temptation to overeat during that window, as intense hunger is a common side effect of this method.
Your own tolerance for hunger pangs may guide you in choosing which version of intermittent fasting is best for you. While intermittent fasting is generally safe for most healthy adults, it's not meant for everyone.
"People who are diabetic, pregnant, nursing, athletes, or children: none of those people should be doing intermittent fasting," says Allen. People with eating disorders also should avoid intermittent fasting and restrictive diets, in general.
For the average healthy adult, Allen gives the following advice: "If you want to try it, pick the hours that you want to eat and that you want to fast. Make healthy choices during those times and see if it works for you."
In fact, what you eat when intermittent fasting may even help prevent hunger pains.