Is it good to fast for 24 hours?

October 17, 2025 •

4 min read

According to a 2022 review of 43 studies, intermittent fasting, which can include a 24-hour fast, was found to produce more significant reductions in body weight, waist circumference, and fat mass than a non-intervention diet. While this suggests potential benefits, determining if it is good to fast for 24 hours requires a deeper look into the metabolic changes, risks, and best practices involved.

Quick Summary

A 24-hour fast can offer metabolic improvements like weight loss, enhanced insulin sensitivity, and cellular repair through autophagy. It is generally considered safe for most healthy adults but poses risks for certain individuals and requires proper preparation and hydration.

Key Points

Metabolic Shift: During a 24-hour fast, your body uses up stored glycogen and shifts to burning stored fat for energy, a state known as ketosis.
Autophagy Activation: Fasting for 24 hours can trigger autophagy, a cellular process of recycling and removing damaged components to promote cellular renewal.
Health Benefits: Potential advantages include weight loss, improved insulin sensitivity, better cardiovascular health markers, and reduced inflammation.
Common Side Effects: Initial side effects like hunger, fatigue, headaches, and irritability are common but often diminish as the body adapts.
Who Should Avoid Fasting: Individuals with a history of eating disorders, type 1 diabetes, or those who are pregnant, breastfeeding, or underage should not attempt a 24-hour fast without medical supervision.
Safe Practice is Key: To fast safely, stay hydrated, avoid intense exercise on fasting days, and break the fast slowly with easily digestible, nutrient-dense foods.
Listen to Your Body: If you experience concerning symptoms like severe dizziness or weakness, it is critical to end the fast immediately.

What Happens to Your Body During a 24-Hour Fast?

When you start a 24-hour fast, your body goes through several key metabolic shifts. The initial hours are spent using up your body's primary energy source: glucose from recent food and stored glycogen.

Hours 1–8: Your body is still digesting your last meal and using the glucose from that food for energy. Glycogen stores, which are reserves of glucose in your liver and muscles, are also used.
Hours 8–24: With glycogen depleted, your body switches to using stored fat for energy, a process known as ketosis. This metabolic state is where many of the proposed benefits of fasting originate.
Stimulating Autophagy: Fasting for 24 hours can trigger autophagy, a cellular repair process where your body recycles old or damaged cell parts. This 'self-eating' process helps clean out cellular debris and create new, healthier cells, which is linked to longevity and disease prevention.

Potential Health Benefits of a 24-Hour Fast

For many healthy individuals, incorporating a 24-hour fast can offer several health advantages when done safely and periodically.

Weight Loss and Fat Reduction: By forcing your body to use fat for fuel, a 24-hour fast can help with weight management and reduction of body fat. However, its effectiveness for weight loss is comparable to regular caloric restriction and requires consistency.
Improved Insulin Sensitivity: Fasting can lead to lower blood pressure, improved cholesterol profiles, and better regulation of blood sugar and insulin levels, potentially reducing the risk of type 2 diabetes and heart disease.
Cardiovascular Health: Regular 24-hour fasting may lower levels of trimethylamine N-oxide (TMAO), a compound associated with an elevated risk of coronary artery disease.
Reduced Inflammation: A short-term fast can decrease markers of inflammation, which is a key factor in many chronic diseases.

Risks and Side Effects to Consider

While generally safe for healthy adults, a 24-hour fast is not suitable for everyone and comes with potential side effects.

Hunger, Headaches, and Fatigue: Especially for beginners, hunger pangs, headaches, irritability, and low energy are common. These side effects often subside as the body adapts over time.
Dehydration: The body releases water and salt during the initial stages of fasting. It is crucial to stay well-hydrated with water and calorie-free drinks during the fasting period to prevent dehydration.
Risk of Overeating: Some people may feel a strong urge to binge eat after a fast, undoing the potential calorie-reduction benefits. Mindful re-feeding is essential.
Medical Contraindications: Certain individuals should avoid fasting or do so only under medical supervision. This includes people with diabetes, a history of eating disorders, pregnant or breastfeeding women, and those on specific medications that must be taken with food.

Comparison Table: 24-Hour Fast vs. Time-Restricted Eating


Feature	24-Hour Fast (e.g., Eat-Stop-Eat)	Time-Restricted Eating (e.g., 16:8)
Frequency	1–2 times per week	Every day
Fasting Window	A full 24-hour period of no solid food	A daily 16-hour fasting period with an 8-hour eating window
Intensity	More challenging initially, potentially causing greater fatigue	Generally easier to sustain and less intense side effects
Energy Source	Primarily relies on fat stores for fuel after glycogen depletion	More balanced energy use, but can also lead to ketosis
Beginner Suitability	May be too intense for beginners; best for experienced fasters	Often recommended for beginners to ease into fasting
Nutrient Intake	Requires careful attention on non-fasting days to ensure nutrient needs are met	Easier to meet daily nutrient requirements within the eating window

How to Safely Complete a 24-Hour Fast

If you decide to try a 24-hour fast, following a few simple steps can maximize its effectiveness and safety.

Before the Fast

Eat Nutrient-Dense Meals: On the day before your fast, focus on consuming meals rich in protein, healthy fats, and fiber to help you feel satiated longer and provide a good nutritional foundation.
Choose a Low-Stress Day: Pick a day when you have minimal physical and social commitments. The fast can be physically and mentally draining, so rest is important.

During the Fast

Stay Hydrated: Drink plenty of water throughout the 24 hours. Calorie-free beverages like black coffee or unsweetened tea are also acceptable. Consider adding electrolytes to your water to counter any salt loss.
Listen to Your Body: Pay attention to how you feel. If you experience severe dizziness, confusion, or extreme weakness, it's a clear signal to end the fast early and eat something.

Breaking the Fast

Ease Back into Eating: Do not break your fast with a large, heavy meal. Start with a small, easily digestible meal to avoid shocking your digestive system.
Choose Whole Foods: Options like bone broth, cooked vegetables, soft fruits, or eggs are excellent for breaking a fast. Avoid simple carbs and sugary foods.

Conclusion

For many healthy individuals, incorporating a 24-hour fast into their routine can offer significant health benefits, such as weight management, improved insulin sensitivity, and enhanced cellular repair through autophagy. However, it is not a suitable practice for everyone, especially those with pre-existing medical conditions, and requires a cautious and well-prepared approach. By understanding the potential benefits and risks, listening to your body, and consulting with a healthcare professional, you can determine if and how to safely implement a 24-hour fast. Ultimately, the success of any fasting regimen relies on consistent, healthy eating patterns outside of the fasting window and prioritizing overall well-being. A balanced diet and regular exercise remain proven fundamentals for maintaining a healthy lifestyle, regardless of fasting practices. For more detailed information on metabolic changes during fasting, consider reading resources from authoritative sources like the National Institutes of Health (NIH). NIH News in Health provides valuable insights into the science behind fasting.

Frequently Asked Questions

For most healthy individuals, a weekly 24-hour fast is generally considered safe. However, it's essential to stay hydrated, listen to your body, and consult with a doctor, especially if you have underlying health conditions.

When you fast for 24 hours, your body primarily uses stored fat for energy after its glucose reserves are depleted, not muscle. Muscle is typically not broken down for fuel unless the fast is prolonged for several days or weeks.

During a 24-hour fast, you can drink plenty of water, black coffee, or plain, unsweetened tea to stay hydrated without breaking your fast. Some people also use electrolyte drinks or add a pinch of Himalayan salt to their water.

To break a 24-hour fast, you should start with small, easily digestible foods like bone broth, cooked vegetables, or soft fruits. Avoid large, heavy meals or foods high in sugar, as this can cause discomfort.

It can take two to four weeks for your body to fully adjust to a regular intermittent fasting routine. Side effects like hunger and irritability are common in the beginning but often lessen over time as your body becomes accustomed to the schedule.

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

A 24-hour fast is a type of intermittent fasting known as the 'Eat-Stop-Eat' method. Intermittent fasting is a broader term that includes various eating patterns, such as the 16:8 method or alternate-day fasting.