Fasting, a method of restricting food intake, has been practiced for thousands of years.
Water fasting is a type of fast that restricts everything except water. It has become more popular in recent years as a quick way to lose weight.
Studies have shown that water fasting could have health benefits. It may lower the risk of some chronic diseases and may stimulate autophagy, a process that helps your body break down and recycle old parts of your cells (, ).
That said, human studies on water fasting are very limited. It also comes with many health risks and is not suitable for everyone.
This article gives you an overview of water fasting and how it works, as well as its benefits and dangers.
Water fasting is a type of fast where you cannot consume anything besides water.
Most water fasts last between 24 to 72 hours. You should not follow a water fast for longer than this without medical supervision.
Here are a few reasons why people try water fasting:
- Religious or spiritual reasons
- To lose weight
- For “detoxing”
- For its health benefits
- Preparing for a medical procedure
The main reason why people try water fasting is for its health benefits.
That’s because several studies have linked water fasting with some impressive health benefits. This includes a lower risk of cancers, heart disease and diabetes (, , ).
Water fasting may also promote autophagy, a process where your body breaks down and recycles old parts of your cells that may potentially be dangerous ().
Popular diets like the lemon detox cleanse are modeled after the water fast. The lemon detox cleanse only lets you drink a mixture of lemon juice, water, maple syrup and cayenne pepper, several times per day for up to 7 days ().
However, water fasting has many risks and can be very dangerous if followed for too long.
Summary: Water fasting is a type of fast in which you are not allowed to consume anything except water. It is linked with a lower risk of chronic diseases and autophagy but also comes with many health risks.
There are no scientific guidelines on how to start water fasting.
However, there are several groups of people who should not water fast without medical supervision.
This includes people with gout, diabetes (both types 1 and 2), chronic kidney disease, eating disorders, older adults, pregnant women and children (, ).
If you have never water fasted before, it is a good idea to spend three to four days preparing your body for being without food.
You can do this by eating smaller portions at each meal or by fasting for part of the day.
Water Fast (24 to 72 hours)
During a water fast, you are not allowed to eat or drink anything besides water.
Most people drink two to three liters of water per day of a water fast.
The water fast lasts a total of 24 to 72 hours. You should not water fast for longer than this without medical supervision because of health risks.
Some people may feel weak or dizzy during a water fast and may want to avoid operating heavy machinery and driving to avoid causing an accident ().
Post-Fast (1 to 3 days)
After the water fast, you should resist the urge to eat a big meal.
This is because eating a large meal after a fast may cause uncomfortable symptoms.
Instead, break your fast with a smoothie or smaller meals. You can start introducing larger meals throughout the day as you feel more comfortable.
The post-fast phase is especially important after longer fasts. This is because you may be at risk of refeeding syndrome, a potentially fatal condition where the body undergoes rapid changes in fluid and electrolytes ().
This phase normally lasts a day, but people who fast for three or more days may need up to three days before they feel comfortable eating larger meals.
Summary: A water fast usually lasts 24 to 72 hours and is followed by a post-fast phase. If you’re new to water fasting, you might want to spend three to four days preparing your body to be without food by reducing your portion sizes or fasting for part of the day.
Water fasting has been linked with a variety of health benefits in human and animal studies.
Here are a few health benefits of water fasting.
It May Promote Autophagy
Autophagy is a process where old parts of your cells are broken down and recycled ().
Several studies have found that autophagy may help protect against diseases like cancer, Alzheimer’s disease and heart disease (, , 12).
For example, autophagy may prevent damaged parts of your cells from accumulating, which is a risk factor for many cancers. This may help prevent cancer cells from growing (13).
Research from animal studies consistently finds that water fasting helps promote autophagy. Animal studies also show that autophagy may help extend lifespan (, , ).
That said, there are very few human studies on water fasting, autophagy and disease prevention. More research is needed before recommending it to promote autophagy.
It May Help Lower Blood Pressure
Research shows that longer, medically supervised water fasts may help people with high blood pressure lower their blood pressure (, ).
In one study, 68 people who had borderline high blood pressure water fasted for nearly 14 days under medical supervision.
At the end of the fast, 82% of people saw their blood pressure fall to healthy levels (120/80 mmHg). Additionally, the average drop in blood pressure was 20 mmHg for systolic (upper value) and 7 mmHg for diastolic (lower value), which is significant ().
In another study, 174 people with high blood pressure water fasted for an average of 10 to 11 days.
At the end of the fast, 90% of people achieved a blood pressure lower than 140/90 mmHg — the limits used to diagnose high blood pressure. Additionally, the average fall in systolic blood pressure (upper value) was a substantial 37 mmHG ().
Unfortunately, there are no human studies that investigate the link between short-term water fasts (24 to 72 hours) and blood pressure.
It May Improve Insulin and Leptin Sensitivity
Insulin and leptin are important hormones that affect the body's metabolism. Insulin helps the body store nutrients from the bloodstream, while leptin helps the body feel full (, ).
Research shows that water fasting could make your body more sensitive to leptin and insulin. Greater sensitivity makes these hormones more effective (, , , ).
For example, being more insulin sensitive means your body is more efficient at reducing blood sugar. Meanwhile, being more leptin sensitive could help your body process hunger signals more efficiently and, in turn, may lower your risk of obesity (, ).
It May Lower the Risk of Several Chronic Diseases
There is some evidence that water fasting may lower the risk of chronic diseases like diabetes, cancer and heart disease (, , ).
In one study, 30 healthy adults followed a water fast for 24 hours. After the fast, they had significantly lower blood levels of cholesterol and triglycerides — two risk factors for heart disease ().
Several animal studies have also found that water fasting may protect the heart against damage from free radicals (, 28).
Free radicals are unstable molecules that can damage parts of cells. They are known to play a role in many chronic diseases ().
Moreover, research on animals has found that water fasting may suppress genes that help cancer cells grow. It may also improve the effects of chemotherapy ().
Keep in mind, there are only a handful of studies that look at the impact of water fasting in humans. More research on humans is needed before making recommendations.
Summary: Research shows that water fasting may lower the risk of many chronic diseases and promote autophagy. However, most research is from animal studies or are short-term. More studies are needed before recommending it.
Although water fasting may have some benefits, it comes with health risks.
Here are a few dangers and risks of water fasting.
You May Lose the Wrong Type of Weight
Because a water fast restricts calories, you will lose a lot of weight quickly.
In fact, research shows that you may lose up to 2 pounds (0.9 kg) each day of the 24- to 72-hour water fast ().
Unfortunately, a lot of the weight you lose may come from water, carbs and even muscle mass.
You May Become Dehydrated
Although it sounds strange, a water fast could make you dehydrated. This is because roughly 20 to 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, then 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 ().
You May Suffer From Orthostatic Hypotension
Orthostatic hypotension is common among people who water fast ().
It is defined as a drop in blood pressure that happens when you suddenly stand up, and can leave you dizzy, lightheaded and at risk of fainting (, , 34).
If you suffer from orthostatic hypotension while fasting, then you may need to avoid driving or operating heavy machinery. The dizziness and risk of fainting could lead to an accident.
If you experience these symptoms during a water fast, then this fast may not suit you.
Water Fasting May Worsen Several Medical Conditions
Although a water fast is relatively short, there are a few conditions that may be aggravated by water fasting.
People with the following medical conditions should not water fast without first seeking advice from their doctor:
- Gout: Water fasting may increase uric acid production, a risk factor for gout attacks (, 35).
- Diabetes: Fasting may increase the risk of adverse side effects in type 1 and type 2 diabetes (36).
- Chronic kidney disease: Water fasting may cause further damage to the kidneys in people with chronic kidney disease ().
- Eating disorders: There is some evidence that fasting may encourage eating disorders like bulimia, especially in teenagers ().
- Heartburn: Fasting may trigger heartburn, as your body may continue to make lots of stomach acid without any food to digest ().
Summary: Although water fasting may have some health benefits, it also comes with many risks and dangers. For example, water fasting could make you prone to muscle loss, dehydration, blood pressure changes and a variety of other health conditions.
Water fasting is not an effective way to burn fat.
Although it can help you lose weight rapidly, most of the weight you lose will most likely come from water, carbs and muscle mass, rather than fat ().
Not to mention, water fasts come with plenty of health risks that are easily avoidable.
These fasts provide similar health benefits but can be followed for much longer as they allow you to eat food (, 41).
Summary: A water fast is not an effective way to burn fat, as it takes several days before your body starts to burn fat for fuel. However, other types of fasting can offer you the benefits of fasting and weight loss with fewer risks.
Water fasting is a popular method of fasting that may have some health benefits.
However, most of the health benefits of water fasting are seen in animal studies. Human studies might not show the same effect.
Water fasting also comes with several risks, especially if you fast for longer than three days or have medical conditions like gout, chronic kidney disease or diabetes.
If you want the health benefits of fasting, try safer methods like intermittent fasting and alternate-day fasting. These fasts allow you to eat some food, which makes them easier to follow long-term.