How to start 20/4 intermittent fasting?

October 17, 2025 •

2 min read

According to a 2020 study, practicing 20 hours of fasting daily over eight weeks led to an average of 3% body weight loss in participants. This guide will show you how to start 20/4 intermittent fasting, a time-restricted eating method also known as the Warrior Diet, by preparing your body and mind for the advanced schedule.

Quick Summary

This article explains the advanced 20/4 intermittent fasting method, its benefits, and potential side effects. It provides a phased approach for beginners, tips for choosing an eating window, and advice on proper hydration and nutrition. It also compares 20/4 to other fasting protocols and offers strategies for sustainable long-term success.

Key Points

Start gradually: Do not begin with 20/4 immediately; ease in with a shorter, more manageable schedule like 16:8 or 18:6 to allow your body to adapt.
Prioritize nutrition during eating window: Focus on consuming nutrient-dense foods including lean protein, healthy fats, fiber, and vegetables during your four-hour window.
Stay hydrated: Drink plenty of water, black coffee, and herbal tea during your 20-hour fasting period to manage hunger and prevent dehydration.
Choose the right eating window: Align your 4-hour eating window with your daily schedule and natural hunger patterns for better long-term adherence.
Listen to your body: Pay close attention to how you feel. If you experience negative side effects like extreme fatigue or irritability, consider adjusting your schedule or reverting to a less intense method.
Consult a professional: Before starting any restrictive diet, especially for individuals with health conditions, it's essential to consult a doctor or registered dietitian.

The 20/4 intermittent fasting method, often referred to as the Warrior Diet, involves fasting for 20 consecutive hours and consuming all of your daily food intake within a compressed four-hour eating window. This advanced protocol requires significant adaptation and is best suited for those already comfortable with shorter fasting windows, not first-timers. A phased approach can help you transition smoothly and avoid fatigue or burnout.

Phase 1: Mastering Shorter Fasts

Before attempting a 20-hour fast, build consistency with less restrictive schedules to help your body adapt to burning fat for energy.

Start with 12:12: Fast for 12 hours overnight for two to four weeks. This is a gentle introduction, often aligning with typical sleep and meal times.
Progress to 16:8: Extend the fasting window to 16 hours, commonly achieved by skipping breakfast. This is a popular intermediate step.
Advance to 18:6: Further compress the eating window to six hours after successfully maintaining 16:8 for several weeks.

Phase 2: Implementing the 20/4 Schedule

Once adapted to shorter fasts, you can move to the 20/4 protocol, which requires careful planning.

Choosing Your Eating Window

Select a 4-hour eating window that fits your lifestyle:

10 a.m. to 2 p.m.: Suitable for early risers who prefer earlier meals.
2 p.m. to 6 p.m.: A common option aligning with standard workdays.
4 p.m. to 8 p.m.: Ideal for those who prefer evening meals and social eating.

Nutrition During the 4-Hour Window

Focus on nutrient-dense, whole foods to get all necessary nutrients within the short window.

Protein: Include lean meats, fish, eggs, and legumes to preserve muscle mass.
Healthy Fats: Add avocados, nuts, seeds, and olive oil for sustained energy.
Fiber-Rich Carbs: Choose complex carbs like sweet potatoes and quinoa for satiety.
Vegetables: Eat a variety of vegetables for vitamins and antioxidants.

Staying Hydrated During the 20-Hour Fast

Stay well-hydrated with calorie-free drinks.

Water: Drink plenty throughout the day to prevent dehydration and manage hunger.
Black Coffee & Tea: Can help suppress appetite and support fat oxidation; consume unsweetened.
Herbal Tea: Offers variety and potential digestive benefits.

Comparison of 20/4 vs. 16/8 Fasting


Feature	20/4 Intermittent Fasting	16/8 Intermittent Fasting
Fasting Window	20 hours	16 hours
Eating Window	4 hours	8 hours
Difficulty Level	Advanced; longer adaptation	Beginner-friendly; easier
Metabolic Benefits	More pronounced fat burning, deeper ketosis	Improved insulin sensitivity
Flexibility	Less flexible; can impact social life	More flexible and adaptable
Nutritional Intake	Requires careful planning in 4 hours	Easier to get nutrients in 8 hours
Weight Loss Potential	Potentially faster due to restricted window	Sustainable weight loss, potentially slower

Conclusion

Starting 20/4 intermittent fasting requires a phased approach, building up from shorter fasts. Strategic, nutrient-dense eating during the four-hour window and consistent hydration are essential. While beneficial for experienced fasters, it's not for everyone and needs careful attention to your body's signals. Always consult a healthcare professional before starting. With proper preparation, 20/4 can be a part of your health journey.

Optional Resources

For additional support on choosing eating windows and managing hunger, explore resources like the Zero Longevity blog.

Frequently Asked Questions

The 20/4 intermittent fasting method involves fasting for 20 consecutive hours each day and consuming all daily calories within a condensed 4-hour eating window.

No, 20/4 is an advanced intermittent fasting protocol that is not recommended for beginners. It's best to start with an easier schedule, such as 16/8 or 18/6, to allow your body to adapt first.

During the fasting window, you can drink calorie-free beverages such as water, black coffee, and unsweetened tea. Staying well-hydrated is key to managing hunger and supporting your body.

The best eating window depends on your lifestyle. Some people prefer a midday window (e.g., 12 p.m. to 4 p.m.) while others opt for an evening window (e.g., 4 p.m. to 8 p.m.) to accommodate social events and family dinners. Experiment to find what works for you.

While longer fasts increase muscle loss risk, it can be minimized by consuming adequate protein during your eating window and incorporating resistance training into your exercise routine.

The 20/4 method provides a 4-hour eating window, allowing for potentially two smaller meals, whereas OMAD restricts eating to a single one-hour period.

Initial changes, such as appetite adaptation and some water weight loss, can be noticeable within the first one to two weeks. More significant fat loss and body composition changes may become visible after four weeks or more.