Why shouldn't you fast for 24 hours? Understanding the risks

November 9, 2025 •

4 min read

While intermittent fasting has gained significant popularity, research indicates that extended fasting can carry significant health risks for certain individuals. Understanding why you shouldn't fast for 24 hours without proper medical guidance is crucial for safe and sustainable dietary practices.

Quick Summary

A 24-hour fast can pose health risks, including severe dehydration, fatigue, and potential muscle loss. It is particularly unsafe for pregnant women, those with diabetes, and individuals with a history of eating disorders, requiring careful professional consultation.

Key Points

Significant Side Effects: A 24-hour fast can cause headaches, dizziness, and irritability, particularly for beginners, due to the body's metabolic adjustment.
Risk of Dehydration: Fasting for an extended period increases the risk of dehydration and electrolyte imbalances, as a significant portion of daily fluid intake comes from food.
Not for Everyone: Vulnerable populations, including pregnant or breastfeeding women, diabetics, and those with a history of eating disorders, should not fast for 24 hours.
Potential Muscle Loss: Prolonged fasting can lead to the breakdown of lean muscle tissue for energy, negatively affecting metabolism and making weight regain more likely.
Difficulty with Adherence: The intensity and side effects of a 24-hour fast can make it difficult to sustain long-term, leading to high dropout rates and inconsistent results.
Safer Alternatives: For many, a less restrictive approach like the 16:8 method is a safer and more sustainable way to achieve the benefits of intermittent fasting.

Understanding the Risks of 24-Hour Fasting

For many, intermittent fasting offers a promising path to weight management and metabolic health. Shorter fasts, such as the 16:8 method, are relatively easy for many healthy individuals to manage. However, extending a fast to a full 24 hours, especially on a regular basis, comes with a more significant set of risks and side effects that should not be overlooked. The decision to engage in a 24-hour fast should always be made with caution and, for many, in consultation with a healthcare provider.

Potential Side Effects and Discomforts

One of the most immediate concerns with a 24-hour fast is the array of unpleasant side effects that can occur, particularly for those new to fasting. A study of intermittent fasting participants found that a significant portion experienced side effects during their fast. These can range from mild to moderate and typically include:

Headaches: Often caused by low blood sugar levels (hypoglycemia) or caffeine withdrawal.
Irritability and Mood Swings: Hormonal changes, particularly fluctuations in cortisol, can trigger mood swings and increased irritability.
Dizziness and Fatigue: As the body switches from using glucose to fat for energy, some people experience low energy and lightheadedness.
Nausea: A feeling of nausea can accompany the hunger pangs and shifts in metabolism.
Sleep Problems: For some, fasting can disrupt the normal circadian rhythm, leading to insomnia.

Risk of Dehydration and Electrolyte Imbalances

During a fast, the body's hydration status can become compromised. A significant portion of daily fluid intake comes from water-rich foods, and abstaining from eating can easily lead to dehydration if fluid intake isn't increased to compensate. Dehydration can manifest with symptoms like fatigue, headaches, constipation, and dizziness. For fasts longer than 24 hours, the risk is even greater, potentially requiring electrolyte supplementation to prevent dangerous imbalances of minerals like sodium, potassium, and calcium.

Danger to Vulnerable Populations

For several groups of people, a 24-hour fast is not recommended and can be genuinely unsafe. These include:

Pregnant or Breastfeeding Women: These individuals have heightened nutritional needs to support fetal development or milk production. Fasting can deprive the baby of essential nutrients.
People with Type 1 or Type 2 Diabetes: Fasting can cause dangerously low blood sugar levels (hypoglycemia), especially for those on insulin or certain medications. Medical supervision is essential.
Individuals with a History of Eating Disorders: Intentional food restriction can be a trigger for disordered eating behaviors and is strongly discouraged.
Children and Adolescents: Younger individuals are still growing and require consistent calorie and nutrient intake for proper development.
Older Adults: Older individuals may be more susceptible to dehydration, malnutrition, and have existing health conditions that make fasting risky.
People on Certain Medications: Some medications, including those for blood pressure or heart disease, may require careful adjustment during a fast.

Risk of Muscle Loss and Metabolic Slowdown

While one of the goals of fasting is often fat loss, prolonged periods without food can also lead to the body breaking down muscle tissue for energy. A study on prolonged fasting found that a significant portion of the initial weight lost came from lean mass, which can negatively impact a person's resting metabolic rate and increase the risk of future weight regain. The longer the fast, the greater the potential for muscle protein breakdown. For a 24-hour fast, this effect is less pronounced than longer durations, but still a consideration, especially if done frequently.

The Challenge of Sustainability

Many people find longer fasts, like the 24-hour schedule, to be unsustainable long-term. Extreme hunger, lethargy, and other side effects often lead to high dropout rates among participants in studies. This difficulty in adherence can lead to a cycle of weight loss and regain, often called 'yo-yo' dieting, which is not beneficial for long-term health. Overeating after a fast is also a risk, potentially negating the calorie deficit from the fasting period.

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

To better understand the trade-offs, here is a comparison of a 24-hour fast versus the more common and often more sustainable 16:8 time-restricted eating method.


Feature	24-Hour Fast (e.g., Eat-Stop-Eat)	16:8 Time-Restricted Eating
Frequency	Typically done once or twice per week.	Daily, with a consistent eating and fasting window.
Intensity	More intense, with a full 24-hour period of no calories.	Less intense, with a daily 16-hour fast and 8-hour eating window.
Side Effects	More likely to cause headaches, fatigue, and irritability.	Side effects are generally milder and subside as the body adapts.
Nutrient Intake	Higher risk of nutrient deficiencies if not planned carefully during eating windows.	Easier to meet daily nutritional needs during a consistent 8-hour window.
Sustainability	Harder to adhere to long-term due to intense hunger and side effects.	Easier to integrate into a daily lifestyle and sustain over time.
Who Should Avoid	Vulnerable populations, diabetics, older adults, pregnant/breastfeeding women.	Same groups, but risks are generally lower. Still requires medical consultation.

Making a Safer Choice

For most people looking to explore the benefits of intermittent fasting, starting with a more moderate approach is a safer and more sustainable option. The 16:8 method allows for a consistent eating pattern while still providing a beneficial fasting period. For those considering more extended fasts, it is critical to consult a healthcare professional first to ensure there are no underlying health conditions that would make it dangerous. Listening to your body and prioritizing nutrient-dense meals during your eating window are key to any successful dietary strategy.

Conclusion

While some may find success with occasional 24-hour fasts, the practice is not without its significant downsides. For most, the risks of severe side effects, nutrient deficiencies, potential muscle loss, and the difficulty of long-term adherence outweigh the benefits. A more moderate form of intermittent fasting, combined with a healthy, balanced diet, is often a more effective and safer path to achieving health goals. The key takeaway is that fasting isn't a one-size-fits-all solution, and understanding why you shouldn't fast for 24 hours without careful consideration is the first step toward making informed and safe decisions about your nutrition.

Frequently Asked Questions

Individuals who are pregnant, breastfeeding, or have a history of eating disorders should not fast. It is also unsafe for those with diabetes, children and adolescents, older adults, and people taking certain medications.

The most common side effects include headaches, fatigue, dizziness, irritability, and nausea. These symptoms are often caused by the body's metabolic shift from glucose to fat for energy.

Not necessarily. Studies suggest that longer fasts are not significantly more effective than shorter ones like 16:8 and can be harder to sustain. Overall calorie intake remains the key factor for weight loss.

Yes, prolonged fasting increases the risk of the body breaking down lean muscle tissue for energy, which can negatively impact your resting metabolic rate and make weight management more difficult.

To avoid dehydration, it is crucial to drink plenty of water and other calorie-free fluids like herbal tea or black coffee. Some experts also recommend adding electrolyte supplements during longer fasts to replenish lost minerals.

If you feel unwell, lightheaded, or have severe symptoms, you should stop fasting immediately. Listen to your body and break the fast with a small, nutritious meal. If symptoms persist, seek medical advice.

For most people, a time-restricted eating method like the 16:8 fast is a safer and more manageable alternative. This involves fasting for 16 hours and eating during an 8-hour window each day.