Intermittent fasting is an eating pattern that alternates periods of eating with periods of either no food or very little food.
The idea of intermittent fasting isn’t to intentionally restrict calories — although it often leads to consuming fewer calories — but to lower inflammation, improve cell repair processes, and flip the metabolic switch from glucose as its main fuel source to fatty acid-derived ketones (1).
There are many variations of intermittent fasting, each varying in the duration and timing of the hours in which you fast. Some of the most popular ones include (1):
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The 16/8 Method: This method involves fasting every day for 16 hours and eating during the remaining hours.
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The 5:2 Diet: With this method, you eat normally five days of the week, and then eat 0 to 25% of your daily calorie needs (0 to 500 calories for the average person) for the other two days of the week.
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Whole-Day Fasts: Also known as eat-stop-eat, you don’t eat anything for 24 hours once or twice per week.
Generally, no food is allowed during the fasting windows, except with certain variations like the 5:2 diet, but you can drink beverages that don’t contain calories or contain negligible amounts such as water or unsweetened tea or coffee (coffee still provides around two calories per eight-ounce cup).
Depending on the reason for intermittent fasting, some people drink coffee containing medium-chain triglyceride (MCT) oil, ghee, coconut oil, or butter. These items technically break a fast, but keep ketones as the body’s fuel source instead of glucose.