Key Takeaways of Water Fasts Study
- Water fasts cause 2–10% weight reduction; however, the proportion of fat vs. muscle loss varies. Some studies suggest up to two-thirds of the weight loss may come from lean mass, but this can depend on fasting duration and individual metabolic factors.
- Ketone levels peak during 5–10 days of fasting; they become a major energy source. Research suggests they may also improve vascular function and reduce blood pressure, but individual responses vary.
- Some metabolic benefits, such as insulin sensitivity, may persist beyond 3–4 months post-fast if dietary habits support them. However, without proper refeeding, these benefits may diminish.
What is a Water Fast?
Fasting has been used for therapeutic purposes since ancient times, with Otto Buchinger formalizing a structured approach in the 1920s. Water fasts have gained popularity due to claims of weight loss, detoxification, and metabolic benefits. However, concerns remain about muscle loss and long-term health effects. Fasting has potential health benefits, such as rapid weight loss, improved insulin sensitivity, blood sugar regulation, and cellular health. However, long-term sustainability and more extensive human studies are needed, as most studies involve small sample sizes.
While many people pursue water fasting benefits for weight loss and metabolic improvements, one of the biggest concerns is how fasting impacts muscle loss, which could have long-term consequences for metabolism and overall health. A recent narrative review published in Nutrition Reviews by Mark Ezpeleta, Sofia Cienfuegos, and Krista A. Varady from the University of Illinois at Chicago examined the effects of prolonged water fasts on body composition, muscle retention, and metabolic health.1
Review of the Literature: What Does Science Say About Water Fasts?
Types of Prolonged Fasting
Prolonged fasting refers to fasting periods lasting several days to weeks. The two main types are:
- Water-Only Fasting: Participants consume only 2–3 liters of mineral or distilled water daily, with no food intake.2
- Buchinger Fasting: A medically supervised program popular in Europe, allowing small amounts of vegetable juice and soup, totaling 200–300 kcal daily.3
Scientific Evidence Supporting Water Fasts
Fat Loss, Ketone Production, and Cardiovascular Benefits
- Water fasts flip the metabolic switch, shifting the body from burning glucose to fats and ketones for energy. Ketone levels peak during 5–10 days of fasting.4,5
- Ketones provide metabolic benefits, including lower blood pressure and improved vascular function.6,7
- Some research suggests that ketones may improve vascular function and reduce blood pressure, but effects depend on individual metabolic responses.
Contradictory Research: The Risks of Water Fasting
Muscle Loss and Lean Mass Reduction
- A major concern with water fasts is that weight loss comes from lean muscle mass rather than fat.8 In one study, after a 10-day water fast, participants lost 7 kg (15.4 lbs.), with 4 kg (8.8 lbs.) of that being muscle. 9
- Shorter fasts (5–7 days) may preserve more muscle than prolonged fasts, particularly when resistance training or protein refeeding is incorporated
- Minor side effects of water fasts include hunger, headaches, nausea, dry mouth, and fatigue.
While water fasts can provide metabolic benefits, the significant muscle loss raises concerns about long-term sustainability. Prolonged fasting can cause significant lean mass loss, as muscle proteins supply amino acids for gluconeogenesis. However, strategic refeeding with high-protein intake and resistance training may counteract these effects. While water fasting is generally safe in medically supervised settings, prolonged fasting can lead to electrolyte imbalances (low sodium, chloride), metabolic acidosis, and altered liver function. Monitoring is essential to prevent complications
Study Overview: How Was the Research Conducted?
This narrative review examined eight human trials on water fasts, analyzing their effects on weight loss, muscle retention, blood pressure, and insulin sensitivity.
Subjects & Fasting Duration
- Participants included individuals of normal weight, overweight, and obese.
- Fasting durations ranged from 5 to 20 days.
- Some studies included Buchinger fasting, which allows minimal calorie intake.
Methods of Measurement
- Weight loss and muscle loss were assessed using dual-energy X-ray absorptiometry (DXA) and bioelectrical impedance analysis (BIA).
- Blood markers for insulin, glucose, cholesterol, and ketones were tracked.
Refeeding Period
- Some studies followed participants for several months post-fast to analyze weight regain and metabolic changes.
This structured approach allowed researchers to compare fasting effects across different populations and durations.
Results: What Happened to Muscle and Fat Loss During Water Fasting?
Key Findings:
Weight Loss: Participants lost 2–10% of body weight after fasting for 5–20 days.
Muscle Loss: Two-thirds (i.e., 66%) of weight loss came from lean muscle mass, while only one-third (i.e., 33%) came from fat.
Waist Circumference: Decreased by 3–9 cm, suggesting significant abdominal fat loss.
Blood Pressure: Systolic and diastolic blood pressure dropped by 9–14 mmHg.
Glycemic Control: Fasting glucose and insulin improved in non-diabetic participants but showed no change in diabetics.
- Longer fasts (>10 days) resulted in more severe muscle loss, raising concerns about muscle preservation.
Potential Adverse Effects of Water Fasts
Mild side effects include hunger, headaches, nausea, dry mouth, and fatigue.
More severe cases have reported electrolyte imbalances, abnormal liver function, decreased bone density, and metabolic acidosis, though these effects are less common.
Water Fasts: What Do These Results Mean?
The findings highlight both the benefits and risks of water fasts. While fasting leads to significant fat loss, the high percentage of muscle loss is concerning. Dr. Krista A. Varady, one of the study authors, summarized the key issue: “Approximately two-thirds of the weight lost is lean mass, which is a concern as lean mass is a key predictor of resting metabolic rate.” This suggests that water fasts might not be ideal for individuals aiming to preserve muscle while losing fat.
Practical Applications: How to Apply This Research
🔹 Who Should Avoid Water Fasts?
❌ Athletes or individuals focused on muscle preservation.
❌ Those prone to muscle loss, metabolic slowdown, or long-term weight regain.
How to Minimize Muscle Loss During Water Fasting
- Engage in resistance training to stimulate muscle retention.
- Break the fast with high-protein meals to rebuild muscle.
- Consider shorter fasts (5–7 days) instead of extended fasting.
- Electrolyte supplementation can help prevent excessive fluid loss.
Alternative strategies like intermittent fasting or calorie restriction may be more effective in preserving muscle mass for long-term weight loss.
Conclusion
By understanding water fasting benefits and its potential risks, individuals can make informed decisions about incorporating fasting into their lifestyle. The research is clear: Water fasts cause rapid weight loss, but at the cost of significant muscle loss. While metabolic improvements occur, most benefits disappear within months of refeeding. Individuals considering water fasts should carefully weigh the risks of muscle breakdown and metabolic slowdown.
References
1 Ezpeleta, M. et al. Efficacy and safety of prolonged water fasting: a narrative review of human trials. Nutr Rev 82, 664-675 (2024). https://doi.org/10.1093/nutrit/nuad081
2 Michalsen, A. & Li, C. Fasting Therapy for Treating and Preventing Disease – Current State of Evidence. Forschende Komplementärmedizin / Research in Complementary Medicine 20, 444-453 (2013). https://doi.org/10.1159/000357765
3 Wilhelmi de Toledo, F. et al. Fasting Therapy – an Expert Panel Update of the 2002 Consensus Guidelines. Forschende Komplementärmedizin / Research in Complementary Medicine 20, 434-443 (2013). https://doi.org/10.1159/000357602
4 Anton, S. D. et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity 26, 254-268 (2018). https://doi.org/https://doi.org/10.1002/oby.22065
5 Cabo, R. d. & Mattson, M. P. Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine 381, 2541-2551 (2019). https://doi.org/doi:10.1056/NEJMra1905136
6 Byrne, N. J. et al. Chronically Elevating Circulating Ketones Can Reduce Cardiac Inflammation and Blunt the Development of Heart Failure. Circulation: Heart Failure 13, e006573 (2020). https://doi.org/10.1161/CIRCHEARTFAILURE.119.006573
7 Zhu, H. et al. Ketogenic diet for human diseases: the underlying mechanisms and potential for clinical implementations. Signal Transduction and Targeted Therapy 7, 11 (2022). https://doi.org/10.1038/s41392-021-00831-w
8 Ogłodek, E. & Pilis, P. W. Is Water-Only Fasting Safe? Global Advances in Health and Medicine 10, 21649561211031178 (2021). https://doi.org/10.1177/21649561211031178
9 Dai, Z. et al. Effects of 10-Day Complete Fasting on Physiological Homeostasis, Nutrition and Health Markers in Male Adults. Nutrients 14 (2022).
Frequently Asked Questions (FAQ) on Water Fasting and Its Effects on Health
1. Is water fasting safe without medical supervision?
Water fasting should be done under the supervision of a healthcare provider, especially for individuals with chronic diseases like heart disease, high blood pressure, or metabolic syndrome. Without medical supervision, there is an increased risk of adverse effects such as orthostatic hypotension, uric acid buildup, and protein loss.
2. Can water fasting improve insulin resistance and blood sugar control?
Some clinical trials suggest that fasting may enhance glucose metabolism and improve insulin resistance, particularly in those with high blood sugar levels. However, these benefits may not be long-term, and older adults or those with medical conditions should consult a healthcare professional before attempting it.
3. How does water fasting benefits compare to other types of fasting?
Water fasting is one type of fasting that involves total caloric restriction, unlike intermittent fasting or the Mediterranean diet, which allows smaller meals throughout the day. A systematic review comparing fasting methods found that various fasting approaches can support weight management, but water fasting may lead to significant reduction in both body mass index and water weight.
4. What are the potential risks and adverse events associated with water fasting?
A number of adverse events in fasting studies have identified adverse effects such as dizziness, orthostatic hypotension, and muscle loss. Some small studies also report that prolonged fasting may elevate uric acid levels, increasing the risk of gout. Additionally, fasting for an extended period of time may lead to high cholesterol and affect energy metabolism.
5. Can water fasting reduce high blood pressure?
Yes, some randomized controlled trials suggest a significant reduction in systolic blood pressure among participants following a water fast. However, the impact varies across groups of people, and those with high blood pressure should only fast under medical supervision.
6. Does fasting help reduce oxidative stress and improve longevity?
Animal studies indicate that fasting may reduce oxidative stress and inflammation, potentially promoting longevity. However, more clinical trials on humans are needed to confirm these findings. A good idea for those interested in these benefits is to combine fasting with physical activity and a healthy diet.
8. Who should avoid water fasting?
Individuals with medical conditions such as diabetes, heart disease, or metabolic syndrome should avoid prolonged fasting unless done under the supervision of a healthcare provider. Pregnant women, older adults, and those at risk for orthostatic hypotension should also seek professional guidance before attempting a type of fast that involves prolonged caloric restriction.
This FAQ provides a broad overview of the effects of fasting, but always consult a healthcare professional before making significant dietary changes for health reasons.
