Can a 24 hour fast be beneficial? An in-depth look at the science and safety

October 10, 2025 •

4 min read

According to scientific studies, after just 12-24 hours without food, the human body can deplete its glycogen stores and begin to burn fat for energy. This fundamental metabolic shift is a key reason why many ask: Can a 24 hour fast be beneficial?

Quick Summary

An occasional 24-hour fast can promote metabolic shifts to fat burning, aid weight loss, and trigger cellular cleanup (autophagy). However, it requires careful preparation and refeeding to avoid risks like dehydration and fatigue.

Key Points

Metabolic Switch to Fat Burning: During a 24-hour fast, your body switches from using glucose for energy to burning stored fat, a process known as ketosis.
Cellular Renewal through Autophagy: Fasting for 24 hours can trigger autophagy, a cellular process that recycles damaged and dysfunctional cell components.
Improved Insulin Sensitivity: Regular 24-hour fasting has been linked to lower fasting insulin and glucose levels, which can improve insulin sensitivity.
Risks and Side Effects: Potential side effects include fatigue, headaches, and irritability. Individuals with certain health conditions, like diabetes or eating disorders, should avoid it.
Proper Preparation and Re-feeding are Key: To minimize risks, stay hydrated during the fast and break it gently with small, easily digestible, nutrient-dense meals.
Not a Universal Solution: While potentially beneficial, a 24-hour fast is not necessarily more effective for weight loss than consistent calorie restriction and may be harder to sustain.

What Happens to Your Body During a 24-Hour Fast

For a healthy individual, a 24-hour fast can initiate a series of metabolic adaptations. This process is far from 'starvation mode' in the short term, as the body is highly efficient at using its stored energy reserves.

The Metabolic Shift: From Glucose to Ketones

During the first 8-12 hours of a fast, your body primarily uses glucose from your last meal for energy. After that, it turns to stored glucose (glycogen) from the liver. Once liver glycogen is depleted, typically between 18 and 24 hours, the body enters a state known as ketosis. In this state, it begins to break down stored fat for fuel, creating molecules called ketones that can be used by the brain and muscles. This shift from burning carbohydrates to burning fat is one of the main goals for those undertaking a 24-hour fast for weight management.

Cellular Recycling: The Autophagy Process

One of the most frequently discussed benefits of a 24-hour fast is its effect on autophagy. Autophagy is a natural, regulated mechanism of the cell that disassembles unnecessary or dysfunctional components. It essentially acts as a cellular 'housekeeping' service. Studies suggest that autophagy ramps up significantly after about 16-18 hours of fasting and may peak around the 24-hour mark. This process is linked to cellular renewal and may offer protective benefits against chronic diseases.

Potential Benefits and Considerations

While research into intermittent fasting is ongoing, and benefits are not universally proven across all studies, several potential advantages have been identified.

Possible Metabolic and Health Benefits

Weight Loss and Fat Reduction: By creating a significant calorie deficit and shifting the body to burn fat for energy, a regular 24-hour fast can lead to weight and fat mass reduction over time.
Improved Insulin Sensitivity: Fasting can help lower fasting glucose and insulin levels, which improves the body's sensitivity to insulin. This is a key benefit for individuals at risk of developing type 2 diabetes.
Enhanced Cardiovascular Health: Some studies indicate that regular fasting may help reduce blood pressure and improve cholesterol profiles, which are risk factors for heart disease.
Reduced Inflammation: Short-term fasting may help decrease inflammation in the body, which is a factor in many chronic diseases.
Increased Human Growth Hormone (HGH): Fasting has been shown to increase HGH production, a hormone important for metabolism, weight loss, and muscle growth.

Safety, Risks, and How to Fast Responsibly

It is crucial to approach a 24-hour fast with caution. It is not suitable for everyone, and the risks should be carefully considered, especially without medical supervision.

Who Should Avoid a 24-Hour Fast?

Certain individuals should not attempt a 24-hour fast without first consulting a healthcare professional:

Pregnant or breastfeeding women
Children and adolescents under 18
Individuals with a history of eating disorders
People with diabetes (especially type 1) or other conditions affecting blood sugar regulation
Those with certain medical conditions like kidney or heart disease
Individuals who are underweight

Potential Side Effects

Even for healthy individuals, a 24-hour fast can cause temporary side effects:

Extreme hunger and irritability
Headaches and dizziness, often due to dehydration or low blood sugar
Fatigue and low energy
Constipation or other digestive changes
Bad breath, a common side effect of ketosis

How to Prepare and Break a 24-Hour Fast

Preparation and re-feeding are as important as the fast itself. To do it correctly:

Hydrate: Drink plenty of water throughout the fast to prevent dehydration and reduce hunger pangs. Unsweetened black coffee or tea is also acceptable.
Eat a Balanced Pre-Fast Meal: Before starting, consume a meal rich in fiber and protein to promote satiety and help manage cravings during the fast.
Break the Fast Slowly: Re-feeding properly is critical. Start with a light, nutrient-dense meal to avoid overwhelming your digestive system. Think small portions of easy-to-digest foods like bone broth, smoothies, or fermented foods before moving back to a regular, healthy diet.
Avoid Binging: Resist the temptation to overindulge in processed, sugary foods after the fast, as this can negate the benefits and cause digestive distress.
Listen to Your Body: If you feel unwell, dizzy, or experience heart palpitations, stop the fast and re-feed carefully. A fast should not cause severe discomfort.

Comparison of Fasting Methods

While a 24-hour fast (often called the 'Eat-Stop-Eat' method) is one option, it is one of the more intense forms of intermittent fasting. Below is a comparison with other popular methods.


Feature	24-Hour Fast (Eat-Stop-Eat)	16:8 Method	5:2 Diet
Frequency	Once or twice a week	Daily	Two non-consecutive days per week
Fast Duration	24 hours	16 hours	Entire day with calorie restriction (500-600 calories)
Eating Window	All calories in one or two meals on non-fast days	8-hour window each day	Regular eating for 5 days a week
Difficulty	High; requires mental discipline	Beginner-friendly; often skips one meal	Moderate; requires tracking calories on fast days
Potential for Autophagy	Strong potential due to longer fasts	Initiates autophagy, but less profound	Potential due to calorie restriction, but may be less effective than full fasting
Sustainability	May be challenging to maintain long-term	Highly sustainable and easy to integrate	Good for those who prefer structured weeks

Conclusion

So, can a 24 hour fast be beneficial? For a healthy individual, integrating a 24-hour fast into a healthy lifestyle once or twice a week can offer metabolic benefits like increased fat burning and improved insulin sensitivity. The activation of autophagy, the body's cellular recycling process, is another significant potential advantage. However, it is a more challenging form of intermittent fasting and carries risks like dehydration, fatigue, and hunger, especially for beginners. It is not appropriate for everyone, and certain populations should avoid it without medical guidance. Those considering it should prioritize proper hydration, eat balanced meals outside the fasting window, and break the fast gently to maximize benefits and minimize side effects. As with any significant dietary change, a discussion with a healthcare provider is the safest first step. NIH Physiology, Fasting

Frequently Asked Questions

During a 24-hour fast, you can drink calorie-free beverages such as water, black coffee, and unsweetened tea to stay hydrated and help manage hunger.

Yes, a 24-hour fast can help with weight loss by creating a calorie deficit and encouraging the body to burn stored fat for energy. However, it is not necessarily more effective than other calorie restriction methods.

People who are pregnant, breastfeeding, under 18, underweight, or have certain health conditions like diabetes, heart disease, or a history of eating disorders should avoid 24-hour fasting without medical supervision.

To break a 24-hour fast, start with small portions of easily digestible, nutrient-dense foods like bone broth, cooked vegetables, or fermented foods. Avoid overeating or consuming heavy, processed foods immediately.

Most experts suggest doing a 24-hour fast no more than once or twice per week. Frequent, prolonged fasting can lead to nutrient deficiencies and other side effects.

A 24-hour fast involves abstaining from food for a full 24 hours, typically once or twice a week. The 16:8 method is a daily fasting regimen with a 16-hour fast and an 8-hour eating window.

For a healthy person, a single 24-hour fast is unlikely to cause significant muscle loss. The body prioritizes fat burning, and growth hormone spikes during fasting help conserve lean muscle mass.