20:4 fasting involves fasting for 20 hours per day and eating during a four-hour window. It is a type of intermittent fasting that is categorized as time-restricted eating (TRE), where the focus is on when you can and cannot eat, rather than what you can and cannot eat.
20:4 fasting was popularized by the book The Warrior Diet by Ori Hofmekler. The book is based on Hofmekler’s experiences of staying fit while in the Israeli special forces by practicing the lifestyle habits of ancient warriors, who typically ate little to nothing during the day, exercised vigorously, and then feasted at night.
While the strict version of this fast requires you to limit yourself to water and other calorie-free beverages during the fasting window, modified versions allow you to eat small portions of foods such as fruits, vegetables, dairy, and hard-boiled eggs during the fasting window to keep you going.
When you fast for 20 hours a day, your body goes into a metabolic state known as ketosis1, where it starts to tap into stored fat for energy. This fasting state can help promote weight loss2, in addition to offering other cardiometabolic benefits3, such as reduced blood glucose, blood pressure, and cholesterol, says Madiha Saeed, M.D., an integrative family physician and author of The Holistic Rx.
“Longer fasts also enable autophagy, which helps your body detox and repair itself,” Saeed adds.
The 20:4 fasting protocol doesn’t restrict any food groups and doesn’t require you to count your macros. Modified versions also allow you to choose the timings of your fasting window.
However, some people may find it hard to fast for 20 hours at a time. Intermittent fasting beginners may find it easier to start with a less restrictive fasting regime instead.