Is Fasting One Day a Week Harmful? Understanding the Risks and Benefits

December 6, 2025 •

4 min read

Studies on intermittent fasting suggest it can lead to short-term benefits like weight loss and reduced inflammation, which often prompts questions about its safety and long-term effects. A weekly 24-hour fast is a common approach, but it's essential to understand both its advantages and potential downsides before starting.

Quick Summary

A weekly 24-hour fast can offer healthy adults potential benefits such as weight management and metabolic improvements, yet carries risks like fatigue, headaches, and increased hunger. The practice is not suitable for everyone and requires careful consideration of personal health and lifestyle.

Key Points

Generally Safe for Healthy Adults: A weekly 24-hour fast is typically not harmful for most healthy individuals when done correctly.
Metabolic Shift: During a 24-hour fast, your body transitions from burning glucose to burning stored fat, which can aid in weight loss.
Who Should Avoid: Pregnant or breastfeeding women, individuals with diabetes or a history of eating disorders, children, and those on specific medications should not fast without medical supervision.
Potential Side Effects: Common temporary side effects include headaches, fatigue, dizziness, and irritability, particularly during the initial adjustment period.
Hydration is Key: Staying well-hydrated with water and calorie-free beverages is crucial to manage side effects and support bodily functions during the fast.
Ease In and Out: Prepare for the fast by eating nutritious meals beforehand and break it gently with small, light portions to avoid digestive distress.

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

When you begin a 24-hour fast, your body goes through several metabolic changes. For the first several hours after your last meal, your body continues to use glucose from your last meal for energy. As this stored glucose, known as glycogen, is depleted, typically around the 18 to 24-hour mark, your body shifts its primary fuel source. This metabolic transition is where the most significant effects of fasting occur.

The Shift to Fat Burning

After exhausting its glucose reserves, your body enters a state called gluconeogenesis, where it begins to create its own glucose, primarily from stored fat. This process is the key mechanism behind the fat loss benefits often associated with fasting. During this time, your body uses its stored energy reserves more efficiently. While this is a normal process, it's a stark contrast to a regular eating pattern where your body has a constant supply of new glucose to use as fuel.

Potential Benefits of Weekly Fasting

For healthy individuals, regular 24-hour fasts can offer several notable benefits. The consistent, short-term calorie restriction can lead to a manageable calorie deficit over time, supporting weight loss efforts without the daily counting associated with traditional diets. Some research suggests fasting can also contribute to improved metabolic health, including better insulin sensitivity and regulation of blood sugar levels. Furthermore, studies indicate potential benefits for cardiovascular health by helping to manage blood pressure and cholesterol levels, and reducing markers of inflammation.

Common Side Effects of Weekly Fasting

While generally safe for healthy people, weekly fasting isn't without its immediate side effects. These are most often experienced during the adjustment period as your body gets used to the new schedule. The most common side effects include:

Hunger pangs and cravings
Fatigue and low energy
Headaches
Irritability or mood swings
Difficulty concentrating
Nausea
Dizziness or lightheadedness
Constipation or other digestive issues

Most of these temporary effects typically subside as your body becomes more accustomed to the fasting routine. Adequate hydration is crucial to mitigate many of these symptoms, especially headaches and dizziness.

Potential Health Risks and Who Should Avoid Fasting

While a single 24-hour fast per week is generally safe for most healthy adults, it is not suitable for everyone. Certain individuals face a higher risk of complications and should avoid this practice unless medically supervised. These high-risk groups include:

Individuals with a history of eating disorders: Fasting can trigger or exacerbate unhealthy and disordered eating behaviors.
Those with diabetes: Especially type 1 diabetes, as fasting can disrupt blood sugar control and increase the risk of hypoglycemia (low blood sugar).
Pregnant or breastfeeding women: Nutritional needs are significantly higher during these periods and restriction can harm both mother and child.
Children and adolescents: They are still growing and require consistent nutrients and calories for proper development.
Anyone on medications that require food intake: Some drugs can cause severe side effects if taken on an empty stomach.
Individuals with certain chronic conditions: Those with a history of heart arrhythmias or who are at risk for bone loss should consult a doctor first.

It is also important to note that overly aggressive or unsupervised fasting can lead to severe malnutrition and dehydration. The key to any fasting regimen is a balanced, mindful approach.

Safely Practicing a One-Day Fast

To ensure your one-day fast is as safe and effective as possible, follow these guidelines:

Prepare your body: The day before your fast, consume nutrient-rich, fiber-filled meals to help you feel full longer. Drink plenty of water and get sufficient sleep.
Stay hydrated: Drink plenty of water throughout the fasting period. Black coffee, green tea, and herbal teas without sugar or milk are also typically allowed and can help curb hunger.
Keep busy: Boredom can lead to thoughts of eating. Engage in light, engrossing activities to distract yourself during the fast.
Break your fast mindfully: When the 24 hours are up, ease back into eating with a light, nutritious meal. Avoid large, heavy meals, which can overwhelm your digestive system. Opt for foods like fruits, vegetables, and soups.
Listen to your body: If you feel unwell, weak, or sick at any point, stop the fast. Your health should always be the priority.

Comparison of Weekly Fasting: Benefits vs. Risks


Aspect	Potential Benefits	Potential Risks
Metabolism	Improved insulin sensitivity, enhanced fat burning.	Temporary metabolic slowdown with excessive or long-term fasting.
Weight	Calorie deficit aids in weight loss, fat mass reduction.	Potential for overeating or binge eating on non-fasting days.
Cardiovascular Health	Reduced blood pressure and cholesterol levels.	Increased risk of heart arrhythmias or other complications for certain individuals.
Inflammation	Lowered markers of systemic inflammation.	Can cause stress on the body if done improperly or by those with underlying conditions.
Mental State	Increased mental alertness and focus after adaptation.	Irritability, mood swings, and difficulty concentrating, especially initially.
Overall Health	Cellular repair process (autophagy) may be triggered.	Risk of nutrient deficiencies if non-fasting nutrition is poor.

For more detailed information on intermittent fasting approaches, including the 5:2 diet which also involves weekly fasting, consult trusted health resources like the Cleveland Clinic on Intermittent Fasting Explained.

Conclusion

For most healthy adults, fasting one day a week is not inherently harmful and can offer several health benefits, particularly related to weight management and metabolic health. However, it requires a careful, mindful approach and is not a one-size-fits-all solution. The key to successful and safe weekly fasting lies in adequate hydration, proper nutrition on non-fasting days, and listening to your body's signals. It is vital for individuals with pre-existing health conditions, especially diabetes, or a history of eating disorders to avoid this practice. As with any significant dietary change, discussing your plans with a healthcare professional is the best course of action to ensure it aligns with your specific health needs and goals.

Frequently Asked Questions

Yes, fasting one day a week can contribute to weight loss by creating a regular calorie deficit. It can also help your body burn stored fat more efficiently after it depletes its glycogen reserves.

The most common side effects include headaches, hunger, fatigue, irritability, and dizziness. These are typically temporary and subside as your body adapts to the fasting schedule.

Yes, you can typically consume calorie-free beverages like black coffee, unsweetened tea, and water during a fast. These can help curb hunger and maintain hydration.

Individuals who are pregnant, breastfeeding, diabetic, have a history of eating disorders, or are under 18 should not fast without first consulting a healthcare professional.

You should break your fast gently. Start with a small, nutritious meal like a salad, soup, or a piece of fruit. Avoid large, heavy, or sugary meals that can overwhelm your digestive system.

When done in moderation, a 24-hour fast is unlikely to cause a significant metabolic slowdown. In fact, it may improve metabolic function. However, prolonged or extreme calorie restriction can potentially slow metabolism.

On your eating days, focus on a balanced and nutritious diet rich in protein, fruits, vegetables, and whole grains. This will ensure you get all the necessary nutrients and prevent nutrient deficiencies.

During a 24-hour fast, your body primarily uses stored fat for energy after glycogen stores are depleted, not muscle tissue. Lean body mass is more at risk with longer, more extreme fasting methods.