The Difference Between Short-Term and Prolonged Fasting
While intermittent fasting (IF), involving regular short fasting windows (e.g., 16-24 hours), has been widely studied for its potential benefits, the effects of prolonged fasting (PF) are far more complex. Prolonged fasting is typically defined as abstaining from food for longer than 48 hours and is associated with a greater risk of adverse health effects. As the duration of a fast increases, the body's metabolic adaptations intensify, and so do the potential dangers. For example, a 2025 study highlighted that medically supervised water-only fasts lasting an average of 9.8 days triggered a systemic inflammatory response and platelet activation. This suggests that while shorter fasts might be manageable for some, extending them significantly raises the stakes.
The Body's Metabolic Shift in Prolonged Fasting
During a fast, the body undergoes a series of metabolic changes to maintain energy homeostasis. This process moves through several stages:
- The Postabsorptive Phase (4–18 hours): The body first uses up the glucose available from recently consumed food. Once that is gone, it begins to break down glycogen, or stored glucose, in the liver to maintain blood sugar levels.
- Gluconeogenesis (18–48 hours): After glycogen stores are depleted, the body starts producing its own glucose from non-carbohydrate sources, primarily amino acids derived from breaking down proteins, including muscle tissue.
- Ketosis (48–72+ hours): The body significantly increases the breakdown of stored fat for energy. The liver converts fatty acids into ketone bodies, which serve as an alternative fuel source for the brain and other organs. While this process, known as ketosis, can have benefits, extended periods without proper nutrient intake can become dangerous.
Common Mild to Moderate Side Effects
For those new to fasting, or even seasoned practitioners extending their fast, several uncomfortable but usually temporary side effects may arise. Many of these are linked to the body's adaptation to using fat for fuel instead of glucose.
- Headaches: Often experienced in the first few days, headaches can be a result of low blood sugar, caffeine withdrawal, or dehydration.
- Fatigue and Lethargy: As the body shifts its energy source, it is common to feel sluggish, weak, and low on energy. This can be compounded by dehydration and electrolyte loss.
- Irritability and Mood Changes: Low blood sugar and hormonal shifts can cause irritability, anxiety, and difficulty concentrating.
- Digestive Issues: Changes in eating patterns can lead to either constipation or diarrhea. Constipation can occur from a lack of dietary fiber, while diarrhea may result from the body excreting excess water and salts.
- Bad Breath: Also known as “keto breath,” this is caused by the metabolic byproduct acetone, which is released during fat metabolism and is expelled through the breath.
Significant Dangers of Prolonged Fasting
Beyond the temporary discomfort, prolonged fasting carries several serious risks that can lead to severe health complications. These are often related to the extreme metabolic stress placed on the body.
- Dehydration and Severe Electrolyte Imbalance: During the initial days of a fast, the body excretes large amounts of water and electrolytes like sodium, potassium, and magnesium. Without proper replenishment, this can lead to severe dehydration and dangerous electrolyte imbalances that can trigger heart palpitations and life-threatening arrhythmias.
- Muscle and Bone Loss: Once the body depletes its fat stores, it turns to muscle and organ tissue for energy via gluconeogenesis, leading to significant and undesirable lean mass loss. Studies on water fasting have also documented decreased bone density.
- Malnutrition and Nutrient Deficiency: Restricting food intake for extended periods can result in severe malnutrition and deficiency of essential vitamins and minerals, which can lead to frequent illnesses and other health problems.
- Refeeding Syndrome: This is a potentially fatal condition that can occur when a severely malnourished person is reintroduced to food too quickly. It causes dangerous shifts in fluid and electrolyte levels that can lead to heart failure, respiratory failure, and other severe medical issues.
- Increased Inflammation: Contrary to the hypothesis that fasting reduces inflammation, a 2025 study observed a significant increase in inflammatory markers like C-reactive protein (CRP) during prolonged water-only fasting. For individuals with existing heart and vascular conditions, this could pose a significant health risk.
- Organ Strain: Prolonged fasting can stress organs like the liver and kidneys, with studies noting elevated liver function tests in participants. The increased uric acid from muscle breakdown can also strain the kidneys and increase the risk of kidney stones.
Who Should Avoid Prolonged Fasting?
Certain individuals are at a much higher risk of harm from prolonged fasting and should not attempt it without strict medical supervision. These groups include:
- Pregnant or Breastfeeding Women: The nutritional demands of pregnancy and breastfeeding are high, and calorie restriction is not recommended.
- People with Diabetes: Fasting can cause dangerous fluctuations in blood sugar levels, especially for those on medication.
- Individuals with a History of Eating Disorders: Fasting can be a trigger for disordered eating behaviors.
- Those with Chronic Health Conditions: Individuals with heart conditions, low blood pressure, or specific kidney or liver problems should avoid prolonged fasts.
- Older Adults and Children/Adolescents: These age groups have specific nutritional needs for growth and overall health that are not met during extended fasts.
Minimizing Risks: The Right Approach to Fasting
For those who choose to engage in fasting, particularly for extended periods, medical supervision is essential. Even with shorter fasts, adopting safe practices is important to mitigate the risks.
- Electrolyte Supplementation: For any fast longer than 24 hours, supplementation with essential electrolytes like sodium, potassium, and magnesium can help prevent dehydration and imbalance.
- Gradual Refeeding: After a prolonged fast, it is critical to reintroduce food slowly to prevent refeeding syndrome. Start with small amounts of easily digestible foods and gradually increase intake.
- Prioritize a Nutritious Diet: On eating days, focus on consuming nutrient-dense, well-balanced meals to replenish the body's stores.
- Listen to Your Body: If symptoms of fatigue, dizziness, or nausea become severe, it is a sign to break the fast.
Comparison of Short-Term and Prolonged Fasting Effects
| Feature | Short-Term Intermittent Fasting (e.g., 16-24 hrs) | Prolonged Fasting (e.g., 48+ hours) |
|---|---|---|
| Metabolic Shift | Body uses stored glycogen and shifts towards burning fat for energy. | Body exhausts glycogen, burns fat, and may start breaking down muscle and organ protein for energy. |
| Common Side Effects | Mild headaches, hunger pangs, temporary fatigue, slight mood changes. | Severe fatigue, intense hunger, insomnia, nausea, irritability, mental fog. |
| Electrolyte Balance | Mild risk of imbalance, usually manageable with hydration. | High risk of severe imbalance, potentially leading to cardiac arrhythmias. |
| Muscle Mass | Less significant risk of muscle loss, especially with proper diet. | Increased risk of lean mass and bone density loss due to protein breakdown. |
| Inflammation | Potential to reduce inflammation. | Can paradoxically increase systemic inflammatory markers like CRP. |
| Refeeding Risk | Negligible risk. | High risk of dangerous refeeding syndrome. |
| Medical Supervision | Generally not required for healthy individuals. | Strongly recommended due to potential health complications. |
Conclusion
While short-term intermittent fasting may offer some health benefits for certain individuals, what are the side effects of prolonged fasting highlights a serious and potentially dangerous list of health risks. The body’s deep metabolic changes during extended fasts increase the risk of severe dehydration, electrolyte imbalances, muscle and bone loss, and malnutrition. Furthermore, recent studies have shown a concerning increase in systemic inflammation during prolonged fasts, potentially increasing cardiovascular risk. Anyone considering a prolonged fast, especially those with pre-existing health conditions, should consult a healthcare professional to assess their personal risk and ensure their safety. The risks of prolonged fasting underscore the importance of caution and medical guidance to prevent serious health consequences.
