Water fasting, the practice of consuming only water for a specific period, has been used for centuries across various cultures for spiritual, therapeutic, and health-related purposes. A 72-hour water fast, in particular, has gained attention for its potential to trigger significant metabolic changes. This article aims to provide a comprehensive overview of the benefits and risks associated with a 72-hour water fast, offering insights for a wide range of audiences, from those with a basic understanding of health to seasoned professionals.
What is a 72-Hour Water Fast?
A 72-hour water fast involves consuming only water for three consecutive days, abstaining from all food, caloric beverages, and supplements. While some individuals may add a small amount of high-quality salt for electrolyte balance, this should be done with caution. The purpose of this practice varies from religious or spiritual reasons to weight loss, detoxification, perceived health benefits, and preparation for medical procedures.
Potential Benefits of a 72-Hour Water Fast
Metabolic Changes and Autophagy
A 3-day water fast represents an extreme intervention that can trigger profound metabolic changes. A comprehensive 2024 study published in Nature Metabolism tracked 12 healthy volunteers through a 7-day water fast, finding that significant health-altering effects appeared after three days.
One of the most significant benefits of a 72-hour water fast is the activation of autophagy. Autophagy is a cellular process where the body breaks down and recycles old, damaged cell components. This process is crucial for maintaining cellular health and preventing the accumulation of dysfunctional elements that can lead to various diseases. Research from Queen Mary University in 2024 confirmed that autophagy reaches significant levels around day 3 of fasting. Several animal studies suggest that autophagy may help protect against diseases such as cancer, Alzheimer's, and heart diseases.
Insulin Sensitivity and Blood Sugar Control
Fasting can decrease insulin resistance, making cells more efficient at managing blood glucose, which may lower diabetes risk. Research indicates that water fasting can make the body more sensitive to leptin and insulin. Greater sensitivity makes these hormones more effective. For example, being more insulin sensitive means the body is more efficient at reducing its blood sugar levels. Meanwhile, being more leptin sensitive could help the body process hunger signals more efficiently, and in turn, lower the risk of obesity.
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Weight Loss and Body Composition
Most people lose 2-6 lbs during a 3-day fast, but 70-80% of this is water weight that returns when normal eating resumes. A 72-hour water fast can lead to rapid initial weight loss. During the first 24 hours, the body primarily burns through stored glycogen (sugar) in the liver and muscles. By hour 24-36, most people enter nutritional ketosis as the liver begins converting stored fat into ketones. However, it's important to note that ≈70-80% of this weight loss is rapid water weight-including liver glycogen and its bound water that depletes early in the fast. Muscles and liver store ~500 g of glycogen, and each gram binds ~3 g of water.
While the body preferentially uses fat for fuel during a fast, most "lean mass" loss during short fasts comes from water and glycogen depletion, not actual muscle protein breakdown. Meaningful muscle loss is highly unlikely during a 3-day fast for healthy individuals. Shorter 72-hour fasts show even less nitrogen loss. Protein breakdown spikes on day 1, then plummets as ketones rise, limiting true muscle loss to a few ounces over 72 hours.
Potential Reduction in Blood Pressure
Patients with high blood pressure who experience this method under the supervision of nutritionists are expected to have blood pressure lowered. Studies have shown that longer, medically supervised water fasts may help people with high blood pressure lower their blood pressure. At the end of a study on 68 people with high blood pressure who fasted for nearly 14 days under medical supervision, 82% of these people had their blood pressure drop to a healthy level (120/80 mmHg or less). In addition, the mean reduction in blood pressure was 20 mmHg in systolic blood pressure and 7 mmHg in diastolic blood pressure.
In another study, 174 people with high blood pressure fasted water for an average of 10-11 days. At the end, 90% of people had blood pressure below 140/90 mmHg, the cut off point for diagnosing high blood pressure. In addition, the mean systolic blood pressure decreased significantly by 37 mmHG.
Reduced Inflammation and Heart Health
Scientific reviews indicate that short fasts can decrease key markers of chronic inflammation, benefiting conditions linked to heart disease, arthritis, and more. Extended fasts may help lower blood pressure, cholesterol, and triglyceride levels, supporting cardiovascular health.
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Enhanced Brain Function
Ketones generated during prolonged fasting fuel brain cells and may improve cognitive performance, memory, and mental clarity. Animal research suggests possible protective effects against diseases like Alzheimer’s. Many people report enhanced mental clarity during day 2-3 of fasting.
Psychological Benefits
Completing a 72-hour fast can boost willpower, mental resilience, and the ability to delay gratification, with some people reporting clearer thoughts and heightened senses during fasting.
Risks and Considerations
Safety Notice
A 3-day water fast is a medical intervention that can affect blood sugar, blood pressure, and electrolyte balance. A 3-day water fast can be safe for healthy adults without contraindications when done under medical supervision. However, it carries significant risks including electrolyte imbalances, refeeding syndrome, and metabolic disruptions.
It is crucial to acknowledge the potential risks associated with water fasting. The decision to attempt a 3-day fast should never be taken lightly. This approach is most appropriate for healthy adults who have extensive fasting experience, access to medical oversight, and specific health goals that require intensive intervention.
Dehydration
Although it sounds strange, a water fast could make you dehydrated. This is because roughly 20-30% of your daily water intake comes from the foods you eat. If you’re drinking the same amount of water but not eating foods, you might not be getting enough water. Symptoms of dehydration include dizziness, nausea, headaches, constipation, low blood pressure, and low productivity. To avoid dehydration, you may need to drink more than usual.
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Electrolyte Imbalance
Electrolyte management is critical for safe extended fasting. During a true water fast, only plain water is permitted. Some people add a small amount of high-quality salt for electrolyte balance, but this should be done cautiously.
Orthostatic Hypotension
Orthostatic hypotention is defined as a drop in blood pressure that happens when you suddenly stand up, and it can leave you dizzy, lightheaded, and at risk of fainting. If you experience orthostatic hypotension while fasting, you may need to avoid driving or operating heavy machinery. The dizziness and risk of fainting could lead to an accident.
Aggravation of Existing Conditions
Even if you have this diet for short, there still can be some aggravation of existing conditions:
- Gout: Water fasting can increase uric acid production, so it is one factor that causes gout attacks.
- Diabetes: Fasting may increase the risk of side effects in type 1 and type 2 diabetes.
Refeeding Syndrome
Breaking a 3-day fast improperly can be more dangerous than the fast itself. Following fasting, especially longer-term fasting, it is important to introduce food to the body slowly to prevent developing refeeding syndrome. This serious condition occurs when there are medical complications due to rapid changes in fluid and electrolyte levels.
Contraindications
Certain people should consider not fasting or should only do so under the supervision of a healthcare professional. These include children and adolescents, adults over the age of 75, people who are pregnant or nursing, and individuals with an eating disorder. Water fasting is not safe for everyone. People for whom fasting may not be safe or appropriate include:
- Those who are underweight.
- Those who have a heart condition or issues with their heart.
- Those who have type 1 diabetes.
- Those have uncontrolled migraine.
- Those who are undergoing a blood transfusion.
- Those who are taking medications.
Neurometabolic Responses to Fasting Stress
A study assessed 1H-MRS observed brain metabolite distributions over nine brain anatomic regions in healthy women before and after 72 h of fasting. The concentrations of NAA, tCho, tCr, Glx, and mI, obtained in pre-fasting, post-fasting, and non-fasting measurements, were consistent with previously published brain metabolite values in adults, demonstrating the reproducibility of the wbMRSI technique, while the use of measurements at different time points enabled significant differences to be found between pre- and post-fasting values.
In comparison to pre-fasting values, the brain regional metabolite concentrations, determined in reference to tissue water, revealed significant changes after the fasting; however, it is also noted that changes in tissue water content would impact the detected relative metabolite concentrations. The FVGM, FVWM, and FVCSF were not found to change significantly after the fasting.
The 72-h fasting induced significant decreases in glucose level, body weight, and an increase of ketone bodies that confirmed successful fasting of the volunteers. In addition, the median of BDI-2 increased significantly. With respect to single BDI items, the extent of feeling sad, blaming oneself, difficulties to make decisions, appetite, and loss of libido were increased after fasting. Furthermore, changes on concentration difficulty increased by trend.
After fasting, mood scores significantly increased. Glx decreased in all nine brain regions, tCho in eight, NAA in four and tCr in one, with Glx having the greatest change and the frontal lobes being the most affected brain region.
How to Approach a 72-Hour Water Fast
Preparation
If you have never fasted before, you should start with spending 3-4 days to prepare your body for complete fasting. You can do this by eating smaller portions at each meal or by fasting for part of the day. Transition to a nutrient-dense, low-carb diet (whole foods, avoid processed foods) in the days leading up to your fast to help the body adjust and minimize cravings.
During the Fast
During the process, you cannot eat or drink anything but water. Most people will drink 2-3 liters of water per day during the fast. Stay hydrated. Aim for 3 liters of water a day. Allowed drinks: water, mineral water, black coffee (no creamers, milks, sugars or other additives), unsweetened tea (green/black/herbal) with no milk, sugars, or additives.
Strenuous exercise should be avoided during extended fasting due to increased risk of dehydration, electrolyte imbalance, and injury from reduced coordination.
Some people will feel weak or dizzy. Therefore, in the process of fasting for weight loss, you need to avoid operating heavy machinery and driving to avoid causing an accident.
Breaking a fast early is not a failure-it's listening to your body. If you experience concerning symptoms or simply can't continue, break the fast safely using the gentle refeeding protocol.
Post-Fast
At the end of this process, you should take measures to fight cravings after 3 days of complete fasting such as:
- Drink a smoothie.
- Eat small meals throughout the day.
- Eat larger meals throughout the day when cravings decrease.
Experts recommend reintroducing food in small increments after fasting. They also recommend preparing one’s body for this by practicing it before beginning fasting. This can include dividing food intake into five smaller meals a day: breakfast, lunch, dinner, and two snacks. They also recommend that a person avoid processed foods before and after fasting. Some say that following a Mediterranean diet can help. This includes eating a diet of whole grains, fresh fruits and vegetables, legumes, and nuts. It also typically includes lean proteins, such as fish, over red meat.
Most of the initial water returns once you rehydrate and refill glycogen stores-typically 2-4 lbs within a few days.
Consider a DEXA body composition scan 1-2 weeks after your fast to assess actual body composition changes versus water weight fluctuations.
Alternatives to Water Fasting
For the vast majority of people seeking metabolic benefits, safer alternatives like 16:8 intermittent fasting or 20:4 fasting provide significant advantages with dramatically lower risks.
Intermittent fasting involves focusing on when you eat rather than what you eat. Several studies have shown that intermittent fasting can lead to weight loss. It may also have helpful metabolic effects, like improving your blood sugar control and levels of blood fats. There are several types of intermittent fasting, and one is time-restricted eating (TRE). It involves eating within a set window of time, which may last for 4, 6, or 8 hours, for example. The rest of the day you fast. The good thing about TRE is that it’s flexible - you pick the time frame that suits you. And unlike water fasting, TRE allows you to meet your daily nutrient requirements.