What is the protocol for a one day fast?

November 16, 2025 •

4 min read

According to Johns Hopkins Medicine, research shows that intermittent fasting can boost memory and heart health. A one day fast, typically 24 hours, follows a simple protocol that can offer these and other potential health benefits when done correctly. This guide will detail the steps for a safe and effective 24-hour fast, from preparation to reintroduction of food.

Quick Summary

This guide outlines the complete protocol for a 24-hour fast, covering preparation, hydration, what to expect during the fast, and the proper way to break it. It addresses common mistakes and includes safety considerations for different individuals.

Key Points

Prepare Effectively: Eat a nutrient-dense meal high in protein, fiber, and healthy fats before you begin your fast to sustain your energy levels.
Hydrate Constantly: Drink plenty of water, black coffee, or unsweetened tea throughout the 24 hours to prevent dehydration, headaches, and fatigue.
Plan Your Fasting Window: An 'Eat-Stop-Eat' schedule, such as fasting from dinner to dinner, is an effective way to manage the 24-hour period.
Listen to Your Body: Pay attention to how you feel; if you experience severe dizziness, confusion, or extreme weakness, break the fast and seek medical advice if necessary.
Break the Fast Gently: Reintroduce food slowly with small, easily digestible portions of broth, smoothies, or cooked vegetables to avoid digestive upset.
Know Who Shouldn't Fast: Certain individuals, including those with eating disorders, Type 1 diabetes, pregnant or breastfeeding women, and those under 18, should not attempt a fast.
Consider a Modified Fast: For beginners, a less extreme protocol like the 5:2 diet, where you restrict calories for two days, might be a more suitable starting point.

Preparing for Your One Day Fast

Proper preparation is crucial for a successful and safe 24-hour fast. Rushing into it unprepared can lead to unnecessary discomfort and potential side effects like headaches and fatigue.

Mindful Eating Before the Fast

Your last meal before beginning the fast is essential. To sustain yourself, focus on nutrient-dense, satisfying foods that provide lasting energy.

Include Protein: Lean proteins like chicken, fish, or legumes help promote satiety.
Add Fiber: High-fiber foods, such as vegetables and whole grains, help you feel fuller for longer.
Incorporate Healthy Fats: Sources like avocado and nuts can provide sustained energy.
Hydrate Well: Drink plenty of water leading up to the fast to start fully hydrated.

Timing Your Fast

One of the simplest ways to execute a 24-hour fast is the “Eat-Stop-Eat” method, which involves fasting from one meal to the same meal the next day. For example, you might have dinner at 6 p.m. and not eat again until dinner at 6 p.m. the following day. Aligning a significant portion of your fast with your sleep can make it feel much more manageable.

During the 24-Hour Fast

Staying hydrated and managing energy levels are the primary goals during the fasting window. While no food is consumed, certain calorie-free beverages are permitted and encouraged.

Acceptable Fasting Beverages

Water: Drinking plenty of plain, mineral, or sparkling water is vital to prevent dehydration, which is a common cause of headaches and fatigue during a fast.
Black Coffee: Unsweetened, black coffee can be consumed and may help suppress appetite for some, though excessive amounts can cause jitters.
Unsweetened Tea: Herbal and black teas without milk or sugar are also acceptable options that can aid in staying hydrated.
Electrolytes: For some, especially those new to fasting, adding a pinch of sea salt to water or using a low-carb electrolyte supplement can help prevent mineral loss and associated symptoms like dizziness.

What to Expect Mentally and Physically

Between 8 and 12 hours into the fast, your body will transition from burning glucose to burning stored fat for energy in a process called metabolic switching. While this is a desired effect, it can also lead to side effects. You might experience initial hunger pangs, irritability, or decreased concentration, but these often subside as your body adjusts. Keeping busy with light activities and staying well-hydrated are key strategies to overcome these feelings.

Breaking the Fast Safely

The most critical part of the one day fast protocol is how you reintroduce food. Breaking a fast with a large, heavy meal can overwhelm your digestive system, causing discomfort and bloating.

Easing Back into Eating

Start with small, easily digestible portions to gently reawaken your digestive system.

Broth: A low-sodium vegetable or bone broth is gentle on the stomach and helps replenish electrolytes.
Smoothies: A simple smoothie with a small amount of fruit and liquid is easy to digest and provides quick nutrients.
Fermented Foods: A small amount of strained yogurt can reintroduce healthy gut bacteria.
Cooked Vegetables: Steamed or cooked, non-starchy vegetables are easier to handle than raw ones.

Foods to Avoid When Breaking a Fast

Wait at least 2-3 hours after your initial light meal before eating a larger portion. Avoiding certain food types immediately after a fast can prevent digestive upset.

High-Fiber Foods: While healthy, high-fiber foods like legumes can be difficult to digest right away.
Heavy, Fatty Meals: Large amounts of rich, fatty foods can cause stomach upset.
Added Sugars: Processed foods and drinks high in sugar can cause a sharp spike and crash in blood sugar levels.

Comparison of One Day Fasting Approaches


Feature	Water-Only Fast (Eat-Stop-Eat)	Modified Fasting (5:2 Diet on Fast Days)
Calorie Intake	0 calories	Restricted to 500-600 calories
Flexibility	Less flexible, requires complete calorie restriction	More flexible, allows for small meals or snacks
Ease for Beginners	Potentially more challenging due to complete restriction	Often easier for beginners to manage
Primary Beverage	Water, black coffee, unsweetened tea	Same as water-only, with limited exceptions
Mental Focus	May experience stronger initial hunger but potential for sharpened mental clarity as body adapts	Lesser hunger pangs due to some calorie intake, possibly more consistent energy
Long-Term Adherence	May be difficult to sustain for some people long-term	Often considered more sustainable for long-term practice

Safety Considerations and Who Should Not Fast

While generally safe for many, a 24-hour fast is not for everyone. Before starting any fasting regimen, it's crucial to consult with a healthcare provider, especially if you have underlying medical conditions. Individuals who should generally avoid or modify fasting include:

Individuals under 18: Still in a critical period of growth and development.
Pregnant or breastfeeding women: Have increased caloric and nutrient needs.
People with a history of eating disorders: Fasting can trigger unhealthy patterns.
Diabetics (especially Type 1): Can be dangerous due to medication and blood sugar management.
Individuals on medications that must be taken with food: Can interfere with absorption or cause side effects.

Conclusion

A one day fast, when executed with a clear protocol, can be a manageable and potentially beneficial health practice. Success hinges on careful preparation, which includes eating a nutrient-rich final meal, and a gentle transition back to eating. Staying hydrated with calorie-free beverages throughout the fast is non-negotiable. By choosing a fasting method that aligns with your experience level and heeding crucial safety warnings, you can navigate the process effectively. Remember to always prioritize your body's signals and seek professional medical advice before undertaking a new dietary regimen like fasting. For some, a modified fast might be a more sustainable and less challenging option, while others may prefer the simplicity of a complete 24-hour fast. In any case, listening to your body and planning ahead are the cornerstones of a successful fast.

Visit InsideTracker for more in-depth articles on fasting and metabolic health.

Frequently Asked Questions

Start with a nutritious, balanced meal containing plenty of protein, fiber, and healthy fats. Many people find it easiest to fast from dinner to dinner, as it incorporates sleep into the fasting window.

You can drink zero-calorie beverages such as water, black coffee, and unsweetened tea. Staying hydrated is critical, and some find adding a pinch of salt for electrolytes helpful.

Many people feel hunger pangs, especially at the beginning, but these feelings often subside as your body shifts from burning glucose to fat for energy. Keeping busy and drinking plenty of water can help manage hunger.

Break your fast with a small, easily digestible meal like a vegetable broth, a simple smoothie, or some cooked vegetables. Avoid large, heavy, or sugary meals, as they can cause digestive upset.

No, fasting is not safe for everyone. It is not recommended for children, pregnant or breastfeeding women, individuals with a history of eating disorders, or those with certain medical conditions like Type 1 diabetes.

It is generally safe to do a 24-hour fast once or twice per week, but it is important to eat normally and nutritiously on your non-fasting days. Excessive fasting is not recommended.

During the first eight hours, your body uses stored glucose for energy. After that, it transitions to burning stored fat for fuel, a process known as metabolic switching.