20:4 Fasting: How To Decide If It's Right For You + Easier Alternatives

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 timing of your fasting window. 

While there are many research studies documenting the benefits of intermittent fasting and time-restricted feeding, few pertain specifically to 20:4 fasting.

A comprehensive review covering 46 studies concluded that intermittent fasting can help improve several cardiometabolic markers4 such as weight, body mass index (BMI), blood pressure, cholesterol, and triglyceride levels.

A review published in the ​​Journal of the Academy of Nutrition and Dietetics notes that fasting for 20-hour intervals can help lower blood glucose levels6. This happens because fasting reduces the amount of glucose synthesized by the liver and helps the body use glucose more efficiently3. 

A 2019 study found that time-restricted eating promotes autophagy7, which is a natural housekeeping process by which your body repairs, recycles, or disposes of damaged cells. 

Autophagy generally kicks in after 17 hours of fasting, says Saeed. "It's a hyperactive healing state where your body detoxes itself and repairs mitochondrial damage."

In this state, your body accelerates the process by which it repairs damaged DNA molecules and produces antioxidants to fight off harmful free radicals, Steve Hendricks, author of The Oldest Cure in the World: Adventures in the Art and Science of Fasting, previously told mindbodygreen.

Yes, 20:4 fasting can help you lose weight, if that's your goal. A small randomized controlled trial with 11 overweight adult participants found that fasting for 20 hours per day for eight weeks helped participants lose approximately 3% of their body weight2.

Going without food for a period of time is a powerful way to boost metabolism, Chris Palmer, M.D., a Harvard psychiatrist, previously told mindbodygreen. In fact, research shows us that intermittent fasting can be as effective for weight loss9 as traditional diets that require you to limit your daily calorie intake.

However, it's important to keep in mind that fasting is not a miracle solution. Whether you lose weight, and how much weight you lose, will depend on what you eat on fasting and non-fasting days. Eating a lot of high-calorie junk food during your fasting window or on non-fasting days may even cause you to gain weight.

While the original Warrior Diet recommends eating your primary meal at night, other interpretations of the 20:4 fast give you the freedom to choose your feeding window. Depending on your lifestyle and personal preferences, you could choose to schedule it in the morning, afternoon, or evening.

For instance, if you wake up early and work out in the morning, you may choose to schedule your eating window to begin a little before your workout so you can fuel up before you exercise and refuel for the rest of the day. However, not being able to eat in the evening may put a damper on your social life.

On the other hand, if you're not a breakfast person and prefer to prolong your fasted state for as long as possible, you can schedule your feeding window in the afternoon or evening.

Try different meal combinations and timing options to see what works best for you. Within the four-hour feeding window, you can choose to eat one big meal or a couple of smaller meals with some snacks.

8 a.m.: Apple slices with nut butter

8:30 a.m.: Workout

9:30 a.m.: Avocado toast topped with a fried egg

11 a.m.: Blueberry smoothie

2 p.m.: Burrito bowl

3 p.m.: Yogurt with berries

4 p.m.: Kale chips

6 p.m.: Grilled salmon with salad

4 p.m.: Lemon chicken with broccoli

5 p.m.: Fruit salad

6 p.m.: Mixed nuts

One major downside of a 20:4 fasting regimen is the risk of nutritional deficiencies. The less you eat, the fewer essential vitamins, minerals, and protein you get into your system. "It can be difficult to get adequate nutrition in such a small feeding window," says Saeed.

This fasting regimen may work for some people, but it's not safe for everyone. Children, pregnant or breastfeeding people, people with eating disorders, and people who are underweight should avoid this fasting protocol, Saeed states.

Saeed recommends that women and people who menstruate undertake this fasting regimen cautiously, since it can place considerable stress on the body and may affect fertility. It also may be too much for women who are postmenopausal.

Some of the other potential side effects of fasting13 include fatigue, hunger, irritability, low mood, difficulty concentrating, and obsessive food-related thoughts.

Because of its restrictive nature, this fasting protocol may not be ideal for the long term. Shorter feeding windows are generally linked to poorer adherence rates3.

How long you can practice this fast can vary from person to person, says Saeed. "Some people thrive on this eating schedule, but others suffer. So, listen to your body." If you ever get dehydrated, feel faint, or experience blood sugar instability, you should stop the fast.

20:4 fasting is an advanced intermittent fasting protocol that requires you to fast or severely restrict your calories for 20 hours in a day. Emerging research shows us that staying in a fasted state for several hours may promote weight loss and offer other health benefits. This form of fasting may not be safe for everyone, so it's best to check with your health care provider whether it's appropriate for you.