Water Fasting: What to Know About Benefits and Risks

The possible effects of a water diet may include short-term weight changes and a simple way to practice fasting, but it also comes with important risks. As this diet involves drinking only water for a set period, it’s important to understand both the potential benefits and risks before trying it. 

Water fasting has become a popular wellness trend because it’s simple and doesn’t require special foods or meal plans. But is it really as effective as people claim? In this article, we’ll walk you through what a water diet is, how it works, its potential benefits, and the precautions to consider before deciding if it’s right for you.

Always speak to a healthcare professional before trying a water diet, particularly if you have an underlying health condition or are taking medication. 

What Is Water Fasting?

Water fasting is a type of fasting where you consume only water for a set period, without eating or drinking anything containing calories. It’s sometimes referred to as a liquid fast, although that term can also describe other fasting plans that allow low-calorie liquids. A typical water fast lasts between 24 and 72 hours, but some people choose to fast for longer under medical supervision or for religious or personal reasons. Others may follow a shorter fast before certain medical procedures, as advised by their healthcare provider.

During a water fast, your body relies on stored energy instead of calories from food. As the fast continues, it may be associated with changes in autophagy, a natural process where the body breaks down and recycles old or damaged cell components (1). While researchers continue to study its potential effects, water fasting isn’t suitable for everyone and should be approached with caution, particularly if you’re considering fasting for longer than a day.

Potential Water Diet Benefits

When discussing water diet benefits and risks, it’s important to look at both sides. Some research has suggested that water fasting may be associated with changes in certain body markers, but the evidence is still evolving. And results vary from person to person. Most studies involve short-term fasting or medically supervised participants, so water fasting shouldn’t be viewed as a guaranteed solution for weight loss or better health. 

May Affect Blood Pressure Markers

Some research has suggested that medically supervised water fasting may be associated with short-term changes in blood pressure in certain groups, including people with excess weight or elevated blood pressure. Recent studies have reported modest reductions in blood pressure following medically supervised fasting programs (2). 

However, these findings don’t mean that water fasting is a treatment for hypertension. More high-quality research is needed to make such claims. Blood pressure changes also depend on factors such as body weight, overall diet, medications, and individual health status. If you have high blood pressure, speak with your healthcare provider before trying a water fast.

May Support Weight Management

Water diet benefits are said to include weight loss, and this is one of its most popular potential perks. During a water fast, your body uses stored energy because you’re no longer consuming calories. One review found that time-restricted eating may lead to weight loss of around 3% to 5% of body weight over 1-12 months (3). Time-restricted eating in the studies reviewed involved a daily fast of 14-20 hours.

It’s unlikely to see lasting weight change from a single water fast. Short-term weight loss tends to come from water and glycogen stores, not just body fat. Maintaining long-term weight loss still depends on healthy eating habits and regular physical activity after the fast ends (4, 5).

May Affect Blood Sugar Levels

Water fasting may influence how your body responds to insulin and uses glucose. Some studies have suggested that short-term fasting may be associated with temporary changes in insulin sensitivity and blood sugar levels in healthy adults (6). 

It’s vital to note that these effects aren’t the same for everyone. People living with diabetes, particularly those who are taking insulin or glucose-lowering medications, may experience dangerous drops in blood sugar while fasting. Due to these differences, anyone with diabetes or another metabolic condition should seek medical advice before attempting a water fast.

May Influence Autophagy

Autophagy is your body’s natural recycling process, in which old or damaged cellular components are broken down and reused. This process is part of normal cell maintenance and may change during periods of reduced calorie intake.

Studies also suggest that fasting may stimulate autophagy, although much of the evidence comes from animal studies, and scientists are still exploring how these findings apply to humans. While autophagy is a normal biological process, more research is required to understand how different fasting durations influence it (1).

Cardiovascular Considerations

Some early research has suggested that medically supervised water fasting may be associated with short-term changes in certain cardiometabolic markers, such as blood pressure and cholesterol levels (2). These changes are generally seen in controlled settings, often alongside other lifestyle changes, and may coincide with weight loss. 

Although these findings are encouraging, the evidence is still limited, and water fasting shouldn’t be considered a substitute for a balanced diet, regular exercise, or prescribed medical treatment. More long-term research is needed to better understand these findings.

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Water Fast Timeline

Everyone responds to fasting differently, so there’s no single water fast timeline that applies to all. Factors such as your age, activity level, body composition, and overall health can influence how your body reacts. 

The timeline below outlines some of the changes that may occur during a typical short-term water fast. These are general observations based on current research and shouldn’t be viewed as guaranteed outcomes (7, 8).

Read more: Green Tea Intermittent Fasting: Benefits and Side Effects

Water Fasting Rules and Tips

Following a few basic water fasting rules can help you prepare for the experience and understand what to expect. While everyone’s response to fasting is different, having a simple plan before, during, and after your fast can make the process more manageable.

Before Your Fast

Prepare your body by making gradual changes rather than stopping food intake suddenly. A day or two before your fast:

• Eat balanced, nutrient-rich meals.
• Reduce your intake of alcohol, sugary drinks, and ultra-processed foods.
• Drink enough water to stay well hydrated.
• Plan your fast for a time when you don’t have demanding physical or social commitments.

During Your Fast

These water fasting tips can help you approach the fasting period more cautiously:

• Drink water regularly to stay hydrated.
• Avoid strenuous exercise and other physically demanding activities.
• Listen to your body and stop fasting if you experience any concerning symptoms.
• Keep a journal to track your hunger levels, energy, mood, and overall experience, rather than focusing only on your weight.

A water fasting app can also be useful for setting fasting timers, tracking water intake, recording your progress, and reminding you when your fasting window begins and ends. Although these apps can help you stay organised, they shouldn’t replace professional guidance.

Breaking Your Fast

How you end a fast is just as important as how you begin it. After a longer fast, start with small, easy-to-digest meals, such as soups, smoothies, or soft foods, instead of eating a large meal immediately. Gradually return to your normal eating pattern over the next day or two to give your digestive system time to adjust after the fasting period.

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The 72 Hour Water Fast (3-Day Diet)

A 72-hour water fast, also known as a 3-day water diet, involves drinking only plain water for three consecutive days while avoiding all food and calorie-containing drinks. Some people try this type of fast for personal, religious, or wellness reasons, but it should only be considered under appropriate medical supervision because longer fasts carry a greater risk of side effects and complications.

Before starting a 72-hour water fast, many people gradually reduce portion sizes and choose balanced, nutrient-rich meals to help their bodies prepare. During the fast, remaining hydrated is essential, but individual water needs can vary.

Breaking the fast is just as important as the fast itself. Rather than eating a large meal immediately, it’s generally better to reintroduce food gradually with smaller, easy-to-digest meals such as soups, smoothies, or soft foods. This gradual approach may help reduce digestive discomfort. Refeeding syndrome is a serious condition linked to rapid shifts in fluid and electrolyte levels after prolonged fasting, which is one reason longer fasts should involve medical guidance (10).

Are you curious about fasting for 3 days? Read our guide to 72-hour fasting before considering this prolonged approach.

Read more: Intermittent Fasting and Running: What to Know Before You Start

Risks and Side Effects of Water Fasting

Although water fasting may offer potential benefits, it also comes with important risks. As your body isn’t receiving calories or essential nutrients, prolonged fasting may place extra stress on the body and may not be appropriate for everyone. 

Before starting a water fast, it’s best to speak with a doctor or registered dietitian, particularly if you have an underlying medical condition or take medication.

Low Nutrient Intake

Fasting may increase the risk of orthostatic hypotension. It’s a sudden drop in blood pressure when standing up. This can cause dizziness, light-headedness, blurred vision, or even fainting, which increases the risk of falls or injury (11).

Nutritional Deficiencies

Extended water fasting means your body isn’t getting vitamins, minerals, protein, and healthy fats during that period. If prolonged fasts are repeated or last too long, your overall nutrient intake may become inadequate.

Muscle Loss

During prolonged fasting, your body may break down muscle tissue alongside fat stores to meet its energy needs. As a result, losing weight during a water fast doesn’t always mean losing only body fat. Muscle loss may also affect strength and metabolic rate (12).

Weakened Immune Function

Long periods without adequate nutrition may affect how the body supports normal immune function (13). This is one reason why extended fasting should be approached with caution.

Who Should Avoid Water Fasting?

Water fasting is generally not recommended for certain groups of people because the risks may outweigh any potential benefits. These include:

• Pregnant or breastfeeding women, who require additional nutrients to support themselves and their baby.
• People with a history of disordered eating, as fasting may trigger or worsen unhealthy eating patterns.
• Children, teenagers, and older adults, whose nutritional needs differ and who may be more vulnerable to fasting-related complications.
• People with diabetes, especially those using insulin or blood sugar-lowering medications, as it can lead to dangerously low blood sugar levels.

If you experience severe dizziness, confusion, persistent weakness, fainting, or other concerning symptoms while fasting, you should stop immediately and seek medical advice.

Frequently Asked Questions

• Can you lose weight from a water diet?

A water diet may lead to short-term weight changes as your body isn’t receiving calories during the fasting period. Much of the initial weight loss is likely to come from water, glycogen stores, and some lean tissue, rather than body fat alone. Long-term weight management still depends on consistent eating patterns, regular movement, sleep, and other daily habits after the fast. Extended water fasting should only be considered under medical supervision.

• Can you last for 40 days with only water?

Although there are documented cases of prolonged fasting, going 40 days with only water is extremely risky and isn’t recommended without specialist medical supervision. Extended fasting can increase the risk of severe dehydration, muscle loss, nutrient deficiencies, electrolyte imbalances, and other life-threatening complications. Although some people attempt long fasts for personal or religious reasons, the potential health risks are significant.

• What is the 7-day water diet?

A 7-day water diet involves consuming only water for one week while avoiding all food and calorie-containing drinks. It’s a form of prolonged fasting that carries greater risks than shorter fasts (14). A fast of this length should only be considered with medical supervision. Some people experience temporary weight loss during this period, but the amount varies, and much of the early weight loss may be water weight. Food should always be reintroduced gradually after an extended fast.

• What happens if you only drink water for 30 days?

Drinking only water for 30 days can place considerable stress on the body. Over time, prolonged fasting may lead to muscle loss, nutrient deficiencies, fatigue, electrolyte imbalances, and other life-threatening complications. Breaking such a long fast also requires care because reintroducing food too quickly may increase the risk of refeeding syndrome (10). This type of fast should not be approached without close medical supervision.

• How much water should I drink a day to lose weight?

Drinking enough water may support hydration and may help some people feel fuller before meals. General guidance suggests around 2.2 litres (9 cups) of fluids daily for most women and 3 litres (13 cups) for most men, although individual needs vary (15). Water alone is unlikely to cause meaningful weight loss, but replacing sugary drinks with water may help reduce overall calorie intake.

• Can you go 21 days with only water?

A 21-day water fast is considered an extended fast and may increase the risk of dehydration, muscle loss, vitamin and mineral deficiencies, and life-threatening electrolyte disturbances. Individual responses vary, and fasting for this long should never be approached casually. If you’re considering an extended fast, seek guidance from a qualified healthcare professional first.

• Are 40-day fasts possible?

40-day fasts have been documented, particularly in religious traditions and a small number of medically supervised cases. However, this doesn’t mean that they’re appropriate for most people. Such prolonged fasting carries serious health risks, including life-threatening complications, and shouldn’t be viewed as a routine weight-loss method. Attempting a 40-day water fast without medical supervision can be dangerous.

The Bottom Line

A water diet may lead to short-term weight changes and changes in how your body uses stored energy, but these effects vary from person to person. As this article explains, water fasting also comes with potential risks, including dehydration, muscle loss, inadequate nutrient intake, and electrolyte imbalances, particularly during longer fasts. 

Understanding how to prepare, what to expect during a fast, and how to break it gradually can help you make a more informed decision. If you’re considering a water fast, particularly one that lasts longer than 24 hours, discuss your plans with a qualified healthcare professional first.