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. For example, it may lower the risk of some chronic diseases and stimulate autophagy, a process that helps your body break down and recycle old parts of your cells (1Trusted Source, 2Trusted Source). That said, human studies on water fasting are very limited. Moreover, it 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. What is water fasting? Water fasting is a type of fast during which you cannot consume anything besides water. Most water fasts last 24–72 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 to improve their health. In fact, several studies have linked water fasting with some impressive health benefits, including a lower risk of certain cancers, heart disease, and diabetes (1Trusted Source, 2Trusted Source, 3Trusted Source). Water fasting may also promote autophagy, a process in which your body breaks down and recycles old, potentially dangerous parts of your cells (4Trusted Source). 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 (5Trusted Source). However, water fasting has many risks and can be very dangerous if followed for too long. Summary Water fasting is a type of fast during which you’re not allowed to consume anything except water. It’s linked with a lower risk of chronic disease and autophagy, but it also comes with many risks. How do you water fast? There are no scientific guidelines on how to start water fasting. However, several groups of people should not water fast without medical supervision. This includes people with gout, diabetes (both types 1 and 2), eating disorders, older adults, pregnant women, and children (6Trusted Source). If you have never water fasted before, it’s a good idea to spend 3–4 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 (24–72 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 during a water fast. The water fast lasts for 24–72 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 (7Trusted Source). Post-fast (1–3 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 in which the body undergoes rapid changes in fluid and electrolyte levels (8Trusted Source). This phase normally lasts a day, but people who fast for 3 or more days may need up to 3 days before they feel comfortable eating larger meals. Summary A water fast usually lasts 24–72 hours and is followed by a post-fast phase. If you’re new to water fasting, you might want to spend 3–4 days preparing your body to be without food by reducing your portion sizes or fasting for part of the day. Potential benefits of water fasting Both human and animal studies have linked water fasting to a variety of health benefits. Here are a few health benefits of water fasting. May promote autophagy Autophagy is a process in which old parts of your cells are broken down and recycled (4Trusted Source). Several animal studies suggest that autophagy may help protect against diseases like cancer, Alzheimer’s, and heart disease (9Trusted Source, 10Trusted Source, 11). 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 (12). Animal studies have consistently found that water fasting helps promote autophagy. Animal studies also show that autophagy may help extend life span (1Trusted Source, 3Trusted Source, 13Trusted Source). 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. May help lower blood pressure Research shows that longer, medically supervised water fasts may help people with high blood pressure lower their blood pressure (14Trusted Source, 15Trusted Source). 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 or less). Additionally, the average drop in blood pressure was 20 mmHg for systolic (the upper value) and 7 mmHg for diastolic (the lower value), which is significant (14Trusted Source). In another study, 174 people with high blood pressure water fasted for an average of 10–11 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 (the upper value) was a substantial 37 mmHG (15Trusted Source). Unfortunately, no human studies have investigated the link between short-term water fasts (24–72 hours) and blood pressure. 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 (16Trusted Source, 17Trusted Source). Research shows that water fasting could make your body more sensitive to leptin and insulin. Greater sensitivity makes these hormones more effective (18Trusted Source, 19Trusted Source, 20Trusted Source, 21Trusted Source). For example, being more insulin sensitive means your body is more efficient at reducing its blood sugar levels. Meanwhile, being more leptin sensitive could help your body process hunger signals more efficiently, and in turn, lower your risk of obesity (22Trusted Source, 23Trusted Source). 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 (2Trusted Source, 24Trusted Source, 25Trusted Source). 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 (26Trusted Source). Several animal studies have also found that water fasting may protect the heart against damage from free radicals (2Trusted Source, 27). Free radicals are unstable molecules that can damage parts of cells. They are known to play a role in many chronic diseases (28Trusted Source). Moreover, animal research has found that water fasting may suppress genes that help cancer cells grow. It may also improve the effects of chemotherapy (29Trusted Source). Keep in mind, only a handful of studies have analyzed the effects of water fasting in humans. More research in 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 or short-term studies. More studies are needed before recommending it. Dangers and risks of water fasting Although water fasting may have some benefits, it comes with health risks. Here are a few dangers and risks of water fasting. 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 a 24- to 72-hour water fast (7Trusted Source). Unfortunately, a lot of the weight you lose may come from water, carbs, and even muscle mass. May become dehydrated 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 (30Trusted Source). 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 (31Trusted Source). May experience orthostatic hypotension Orthostatic hypotension is common among people who water fast (32Trusted Source). It’s 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 (7Trusted Source, 32Trusted Source, 33). 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. If you experience these symptoms during a water fast, 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 healthcare provider: Gout. Water fasting may increase uric acid production, a risk factor for gout attacks (7Trusted Source, 34Trusted Source). Diabetes. Fasting may increase the risk of adverse side effects in type 1 and type 2 diabetes (35). Eating disorders. There is some evidence that fasting may encourage eating disorders like bulimia, especially in teenagers (36Trusted Source). Summary Although water fasting may have some health benefits, it 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.