Should You Do A 21 Day Fast? Here’s What Science Says

The idea of a 21-day water fast can seem extreme. For 3 weeks, you consume only water, abstaining completely from food. It’s a practice that has gained attention for its potential health transformations, but it also carries significant risks that demand careful consideration. Is it a powerful reset for your body or a dangerous undertaking?

This article breaks down the science behind prolonged fasting. 

We’ll explore the evidence from clinical studies, including a key 2023 narrative review on fasts lasting 5-20 days, to give you a clear, balanced picture. We’ll cover the potential benefits, the considerable risks, and the strict rules you must follow to approach it safely.

Is A 21 Day Water Fast Safe?

A 21-day water fast is a serious commitment with significant health risks, and experts advise that no one should start the fast without medical supervision. While some medically supervised programs seem relatively safe, the 21-day water fast is not a casual diet to try on your own.

A comprehensive 2023 review in Nutrition Reviews examined 8 human trials of prolonged fasting (lasting 5-20 days). The review concluded that while these fasts can produce clinically significant weight loss, they come with notable side effects and risks.

In most of these supervised trials, the researchers did not report serious adverse events or deaths.

However, mild side effects like headaches, insomnia, dizziness, and intense hunger were common (1).

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More concerning are the physiological changes.

• Risk Of Lean Mass Loss

Two studies in the review found that approximately two-thirds of the weight lost during a water fast was lean mass (muscle and organ tissue), with only one-third being fat mass (1). 

Losing this much muscle can lower your resting metabolic rate (2), making it easier to regain weight after the fast. It can also affect function, mobility, and overall health in the long term.

• Potential Electrolyte Imbalance

Studies have shown that prolonged water fasting can cause levels of sodium and chloride to drop below the acceptable range (3). This imbalance can lead to serious health complications if not properly managed (4), highlighting some of the crucial signs to stop water fasting if not monitored.

• Refeeding Syndrome

A life-threatening risk after any period of prolonged starvation is refeeding syndrome. 

Reintroducing food too quickly can cause sudden, dangerous shifts in fluids and electrolytes (5).

This risk is why a structured refeeding plan is non-negotiable and requires professional guidance.

Many people who attempt extended fasts do so at a water fasting retreat, where medical staff can monitor vitals and intervene if necessary. For anyone considering this, safety must be the absolute top priority.

Read more: Does Fasting Help Lose Face Fat?

Why Do People Do 21 Day Fasting?

Despite the risks, people try 21-day fasts for several compelling reasons, often seeking profound changes that shorter diets may not deliver. The primary motivation is typically rapid and significant weight loss, but the appeal goes deeper.

Here are some of the key reasons people undertake such a long fast:

To Achieve Significant Weight Loss

Prolonged fasting leads to substantial weight loss. 

The 2023 review noted weight loss of 2-10% over 5-20 days (1). A 21-day fast can produce even more dramatic 21-day water fasting results, although a large portion of this is water and muscle, not just fat.

For A Metabolic Health Reset

Fasting initiates a metabolic shift from using glucose (sugar) for energy to burning fat-derived ketones. This state, called ketosis, is linked to several health benefits (6).

Studies have shown that intermittent fasting can lower blood pressure, reduce fasting insulin levels, and improve insulin sensitivity in individuals with pre-diabetes (7). 

However, the 2023 review found that any improvements in metabolic health markers achieved with fasting were not maintained 3-4 months later (1).

To Trigger Cellular Cleanup (Autophagy)

Fasting is a potent trigger for autophagy, the body’s process of cleaning out damaged cells and regenerating new, healthier ones (8). This cellular “spring cleaning” could have anti-aging effects and may help protect against certain diseases (9). However, this is still largely theoretical, and the idea requires much more research.

For Spiritual And Mental Clarity

For many, a 21-day fast is a spiritual discipline. 

Many use the practice of abstaining from food to foster mental clarity, deepen meditation, and achieve a sense of renewal (10).

These powerful motivations drive individuals to push their physical and mental limits.

However, it’s essential to weigh these desired outcomes against the well-documented risks.

To learn more about the specific metabolic shifts and health outcomes, you can explore the details of prolonged fasting benefits.

What Are The Rules For The 21 Day Fast?

The rules for a true water fast are deceptively simple but require strict adherence. 

The core principle is the complete elimination of all calories.

• Water Only: The only thing you consume is water. Aim for 2-3 liters per day, but listen to your body’s thirst signals. Distilled, mineral, or purified water is all acceptable.
• No Calories: This means no food of any kind. It also means no caloric beverages, no juice, no milk, no soda, and no broths.
• Electrolyte Supplementation: Because fasting depletes key minerals, medically supervised fasts often include electrolyte supplements, particularly sodium, chloride, potassium, and magnesium, to prevent dangerous imbalances. This electrolyte supplementation is not a “do-it-yourself” step and requires professional guidance.
• No Coffee or Tea: Even unsweetened coffee and tea contain compounds that the liver must metabolize, which can interfere with the complete digestive rest that a water fast aims to achieve. Most extended fasters will exclude these beverages.
• Medication Adjustments: Prescription medications need to be managed by a doctor. Some drugs need to be taken with food, and dosages for conditions like diabetes or high blood pressure will almost certainly need to be adjusted.

Following these rules without deviation is what defines a water fast. 

However, there are various fasting options, such as the Buchinger fast (which includes small amounts of juice and soup). All modified fasting types will have their unique requirements.

How To Prepare For A 21 Day Water Fast Correctly?

Proper preparation is critical for both the safety and success of a 21-day water fast. 

You cannot simply stop eating one day and expect your body to handle it. 

So, what is a safe way to do a 21-day water fast?

Practicing through a gradual transition is key (11).

1. Consult a Medical Professional: This is the most crucial step. 

Discuss your plans with a doctor who is knowledgeable about fasting. They can assess your health, order baseline blood work (checking kidney function, liver enzymes, and electrolytes), and help you create a safe plan. This consultation is a non-negotiable prerequisite.

2. Taper Your Food Intake: In the week leading up to the fast, gradually reduce your food intake. Start by eliminating processed foods, sugar, and caffeine. Move toward a diet of whole foods like fruits, vegetables, and lean proteins.

3. Go Low-Carb: For the final 2-3 days before the fast, adopt a very low-carbohydrate or ketogenic diet. This deprivation helps your body begin the transition from burning glucose to burning ketones, which can ease the initial side effects of the fast, like headaches and fatigue.

4. Hydrate Well: Increase your water intake in the days before you begin. Proper hydration is essential for managing the initial stages of the fast.

5. Clear Your Schedule: A 21-day fast is physically and mentally demanding. 

Do not attempt it during a period of high stress or intense physical demands; plan for rest. Many people find success at a dedicated water fasting retreat for this reason.

This preparatory phase helps soften the shock to your system and makes the initial stages of a 21-day fast more manageable.

Read more: Intermittent Fasting For Belly Fat: 5 Tips You Haven’t Tried

Who Should Avoid Doing A 21 Day Water Fast?

A 21-day water fast is not for everyone. Certain medical conditions and life stages make it incredibly dangerous. 

The following individuals should never attempt a prolonged water fast (12):

• Individuals with a history of eating disorders: Fasting can trigger disordered eating patterns and should be avoided.
• Pregnant or breastfeeding women: The nutritional demands of pregnancy and lactation are too high to support a fast.
• People with Type 1 Diabetes: While some research suggests that a modified fast could be done under close supervision (13), the risk of diabetic ketoacidosis (a life-threatening condition) is exceptionally high.
• Those with kidney or liver disease: These organs are crucial for processing waste products during a fast, and impaired function can lead to toxic buildup.
• Individuals with a low Body Mass Index (BMI) or who are underweight: Fasting can lead to severe malnutrition and muscle wasting in those without sufficient body fat reserves.
• People on certain medications: Anyone taking medicines for heart conditions, blood pressure, or seizures must have strict medical supervision, as fasting can dramatically alter how these drugs work.
• Children and adolescents: Growing bodies have high nutritional needs that cannot be met during a fast.

This list is not exhaustive. A thorough medical evaluation is the only way to determine if you are a candidate for a supervised prolonged fast.

To understand the effects on a shorter but still significant timeline, read about water fasting for a week.

What Should I Eat After 21 Days Fasting?

How you break a 21-day fast is just as essential as the fast itself. 

Reintroducing food improperly can cause refeeding syndrome, a potentially fatal condition (5). The refeeding period should be slow, gentle, and carefully planned. 

Talk to your healthcare provider about the potential need for electrolyte and/or thiamin supplementation during the refeeding period.

A general rule of thumb is to take half the number of days you fasted to return to a regular diet. For a 21-day fast, this means a refeeding period of at least 10-11 days.

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Refeeding Schedule Example:

Day 1-2: Start with tiny portions of foods that are easy to digest.

• Diluted fruit juices (like watermelon or grape).
• Clear bone broth or a simple vegetable broth.
• Start with a few ounces at a time, every couple of hours.

Day 3-5: Introduce soft, water-rich fruits and cooked vegetables.

• Fruits like melon, oranges, and berries.
• Steamed vegetables like spinach, zucchini, and carrots. Avoid fibrous, raw vegetables.
• Plain yogurt or kefir can introduce probiotics.

Day 6-8: Gradually add healthy fats and small amounts of protein.

• Avocado.
• A small handful of nuts or seeds (well-chewed).
• A small portion of fish or eggs.

Day 9+: You can begin to reintroduce whole grains and more complex meals, but keep portions small and pay close attention to how your body feels.

Throughout this period, if you experience stomach pain, bloating, or nausea, you must listen to your body, back off, and return to simpler foods. 

These are potential signs to stop the reintroduction of water fasting and slow down. 

Take note of any symptoms you experience and notify your healthcare provider right away.

If you’re curious about the best food to break a fast, check out our earlier article.

What Happens To Your Body At 21 Day Water Fasts?

A 21-day fast is a dynamic process. Your body undergoes dramatic metabolic and hormonal shifts as it adapts to the absence of food. 

A breakdown of the long-term fasting stages in 3 key phases.

Phase 1: The Glycogen Depletion Phase (Days 1-2)

In the first 18-48 hours, your body burns through its stored glucose, or glycogen, primarily located in your liver and muscles (14).

• Insulin Levels Drop: Without incoming carbohydrates, your pancreas dramatically reduces insulin secretion.
• Water Weight Loss: Each gram of glycogen is stored with 3-4 grams of water. As you burn through glycogen, you release this water, leading to rapid initial weight loss (15).
• Hunger Pangs: The hunger hormone, ghrelin, is still active, and you will likely feel intense hunger during this phase.

Phase 2: The Ketosis and Fat-Burning Phase (Days 3-10)

Once glycogen is gone, your body enters a state of deep ketosis (16).

• Fat Becomes Fuel: Your liver begins breaking down fatty acids from your body’s fat stores into ketone bodies (beta-hydroxybutyrate and acetoacetate).
• Ketones Fuel the Brain: Your brain, which usually runs on glucose, adapts to using ketones for up to 75% of its energy needs.
• Reduced Hunger: For many, hunger subsides significantly during this phase as ketones can have an appetite-suppressing effect.
• Metabolic Benefits: Blood pressure often decreases, and insulin sensitivity may improve. Studies show blood ketone levels can rise to 4- 5mmol/L and then plateau (17, 18).

Phase 3: The Protein Conservation Phase (Days 11-21+)

In this final phase of the stages of a 21-day fast, your body becomes highly efficient at running on fat and works to preserve muscle (14).

• Deepened Ketosis: Your body is now fully fat-adapted, minimizing its need for glucose.
• Muscle Sparing: The body reduces its reliance on breaking down muscle protein for glucose (a process called gluconeogenesis) to conserve essential lean tissue (19). However, some muscle loss is unavoidable.

This muscle loss is a critical consideration, as studies show it can account for a significant portion of total weight lost. Experiencing long-term 21-day water fasting results will depend heavily on preserving muscle.

• Sustained Energy: While you may feel physically weaker, mental clarity can be high during this phase.

This journey is a profound stress on the body, but it is this very stress that triggers many of the adaptive benefits that research links to fasting.

Discover more about the health impacts of consuming only water by reading about water diet benefits.

Frequently Asked Questions

• What is the purpose of a 21-day fast?

The primary purposes are to achieve significant weight loss, trigger a metabolic reset to improve markers like insulin sensitivity and blood pressure, induce cellular autophagy for rejuvenation, and for spiritual or mental clarity (20, 21).

• Does lemon water break a fast?

Strictly speaking, yes. A true water fast means consuming only water. Lemons contain a small number of calories and sugars that may trigger an insulin response and require metabolism, technically breaking the fast.

• Can I drink coffee while fasting?

Black coffee contains virtually no calories, but the caffeine and other compounds require liver metabolism. For a pure water fast focused on complete digestive and metabolic rest, coffee is typically excluded.

• Can you exercise while fasting?

Intense exercise should be avoided. Your body is under significant stress and doesn’t have the fuel for strenuous activity. Light, gentle movement like walking or stretching may be acceptable, but it’s crucial to conserve energy and avoid injury.

The Bottom Line

A 21-day water fast is a potentially powerful but high-risk intervention. While it can produce significant weight loss and metabolic improvements, it also poses grave dangers like lean muscle loss and refeeding syndrome. 

The evidence is clear: this type of fast should only be attempted under the close supervision of a qualified medical professional. Before you even consider it, speak with your doctor to understand if it is a safe and appropriate option for you.