Water fasting is a common form of fasting that involves drinking only water for a specific period of time. Although forms of fasting have been historically practiced for religious and spiritual reasons, it has more recently become a popular wellness trend, with some claiming that fasting can boost weight loss and promote overall health. However, is it effective, and more importantly, safe?
Keep reading for more about water fasting, including what it is, how it works and whether it may be worth a try.
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What Is Water Fasting?
Water fasting is a practice that requires abstaining from food altogether and consuming only water, sometimes for up to several days at a time.
Some people try water fasting to improve or maintain health. Other purported health claims include reduced inflammation, decreased oxidative stress and increased lipolysis, which is the breakdown of lipids.
Who Should Not Water Fast?
Water fasting is not recommended for everyone, according to Kaytee Hadley, a Richmond, Virginia-based functional medicine dietitian. In particular, Hadley notes that the following groups should avoid water fasting:
- Children
- Teenagers
- People who are pregnant or breastfeeding
- Individuals with certain health conditions, including gout or insulin-dependent diabetes
- Adults over 75 years of age
- People with a history of disordered eating
It’s also best to talk to a doctor before trying prolonged fasts, as going 24 hours or more without food can be dangerous for some people.
How Does Water Fasting Work?
Just as its name implies, water fasting simply involves restricting your food intake and drinking only water. Here is a closer look at some of the common approaches and what you should eat when it’s time to break your fast.
Water Fast
There are several different ways to try water fasting, which vary based on the amount of time that you fast. 16/8 fasting is a popular, more manageable form of intermittent fasting, which involves limiting your food intake to an eight-hour window each day and drinking only water for the remaining 16 hours.
You can also try water fasting for any specific amount of time, depending on your personal preferences and schedule. However, fasting for longer periods (such as 24 to 72 hours at a time) can be dangerous for some individuals and is not typically recommended unless directed by a doctor.
While fasting, it’s important to drink enough water to stay hydrated. It’s also generally advised to avoid strenuous exercise and stick to lighter activities, such as yoga. If your job is physically demanding, it’s best to plan your fasts around your schedule to prevent negative side effects.
Post-Fast
After completing your fast, following a balanced and nutritious diet is key. Try sticking to nutrient-dense foods like fruits, vegetables, nuts and legumes and be sure to get enough protein, particularly plant-based sources or fish, which can help maintain lean body mass. Staying hydrated by drinking plenty of water can also help support overall health after your fast.
It might also be helpful to aim for smaller, lighter meals immediately after a fast and gradually introduce larger meals. This is especially important for longer fasts, as it can help prevent refeeding syndrome, a serious condition that can occur as a result of fluid and electrolyte shifts following a period of starvation.
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Potential Benefits of Water Fasting
When done properly, water fasting might offer some health benefits, especially for supporting cardiometabolic health, enhancing insulin sensitivity and boosting autophagy, according to Ro Huntriss, a registered dietitian based in London and chief nutrition officer at Simple, an intermittent fasting app. “Some also suggest that water fasting promotes mental clarity and focus,” adds Huntriss.
Here are a few of the potential perks, according to experts.
May Improve Blood Sugar Levels
Fasting might help lower blood sugar levels by boosting insulin sensitivity, says Melanie Murphy Richter, a functional dietitian based in Santa Monica, California. This could help improve your body’s ability to efficiently use insulin, which is the hormone responsible for regulating blood sugar levels. “This may aid conditions like prediabetes and type 2 diabetes,” says Murphy Richter.
A 2023 review in Nutrition Reviews found that prolonged fasting (characterized by consuming little to no caloric food or beverages) ranging from five to 20 days could reduce fasting blood sugar and insulin levels while improving insulin resistance. Researchers also noted that it could reduce levels of hemoglobin A1c (a marker of long-term blood sugar control) in adults with normal blood sugar levels.[1]Ezpeleta M, Cienfuegos S, Lin S, Vasiliki P, Gabel K, Varady KA. Efficacy and Safety of Prolonged Water Fasting: A Narrative Review of Human Trials. Nutr Rev. 2023:nuad081. Similarly, another small, recent study concluded that prolonged fasting may be more effective for improving the release of insulin and glucose tolerance (or the body’s response to sugar) compared to short-term fasts.[2]Solianik R, Židonienė K, Eimantas N, Brazaitis M. Prolonged Fasting Outperforms Short-Term Fasting in Terms of Glucose Tolerance and Insulin Release: A Randomised Controlled Trial. Br J Nutr. 2023;130(9):1500-1509.
May Increase Weight Loss
Water fasting may lead to weight loss and fat loss, according to Murphy Richter. She explains that it can also help break weight loss plateaus by triggering ketosis, a metabolic state in which the body burns fat for fuel instead of sugar. “This produces ketones, which serve as an alternative energy source for the brain and other organs,” says Murphy Richter.
In one very small study, participants lost an average of over 13 pounds and experienced significant reductions in body fat after completing an eight-day water fast.[3]Ogłodek E, Pilis W. Is Water-Only Fasting Safe?. Glob Adv Health Med. 2021;10:21649561211031178.
Still, research shows that while fasting may cause weight loss, it can also cause significant loss of muscle mass. This may be due to the increased breakdown of muscle proteins that occurs with prolonged fasting.
Promotes Autophagy
One of the main benefits of water fasting is its ability to kickstart a process called autophagy, says Murphy Richter. This process occurs when the body begins, “to clean out old and dysfunctional cells, making way for the generation of new, healthier cells,” she says.
Research suggests that autophagy may help slow aging and increase longevity. It might also aid in the prevention of several conditions, including neurodegenerative disorders and metabolic disease.[4]Bagherniya M, Butler AE, Barreto GE, Sahebkar A. The Effect of Fasting or Calorie Restriction on Autophagy Induction: A Review of the Literature. Ageing Res Rev. 2018:47:183-197.
Could Help Protect Heart Health
Fasting can reduce several risk factors of heart disease, notes Murphy Richter, explaining that it’s been shown to reduce blood pressure levels in people with hypertension and may also improve cholesterol by reducing levels of LDL (bad) cholesterol while increasing HDL (good) cholesterol.
One 2021 review reported that water fasting could significantly reduce blood pressure levels in people with hypertension, or high blood pressure. However, researchers also noted that more studies are needed to understand the effects of fasting for hypertension.[5]Hammoud S, Kurdi M, van den Bemt BJF. Impact of Fasting on Cardiovascular Outcomes in Patients With Hypertension. J Cardiovasc Pharmacol. 2021;78(4):481-495.
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Risks of Water Fasting
Though water fasting might be linked to some benefits, it’s not clear whether these benefits last long-term. In fact, one review reported that the metabolic benefits of prolonged fasting were no longer observed three to four months after the fast was completed, even when weight loss is maintained.[1]Ezpeleta M, Cienfuegos S, Lin S, Vasiliki P, Gabel K, Varady KA. Efficacy and Safety of Prolonged Water Fasting: A Narrative Review of Human Trials. Nutr Rev. 2023:nuad081.
Furthermore, water fasting might also be associated with several risks. “Skipping meals means missing out on vital nutrients that sustain our bodies,” says Hadley. She notes that this can lead to several serious short-term side effects, including:
- Low blood sugar
- Sudden drops in blood pressure
- Fatigue
- Dizziness
- Nausea
“When done long term, water fasting can be dangerous and even fatal without medical supervision because of nutrient and electrolyte imbalances,” says Hadley. For this reason, it’s best to talk to a health care professional before trying fasting and stop if you experience any of the symptoms above.
If you’re interested in trying water fasting, starting slow might be the best approach, according to Johannes Uys, a general practitioner at Broadgate General Practice in London. “Beginners can get used to longer water fasts of 24 to 72 hours by first fasting for shorter periods before slowly building themselves up until they can reach 24 hours without eating,” says Dr. Uys. “This is especially important if you are used to eating large amounts on a regular basis.”
A few other tips to help you get started with water fasting include:
- Gradually reduce your intake: “Start by reducing your portion sizes, cutting out snacks and slowly eliminating the number of meals you eat throughout the day,” says Dr. Uys. “Some people find that this is enough for them to get the results they’re looking for, so a taper is a great way of gradually testing the waters until you find a method that works for you.”
- Drink more water: “A lot of people are surprised to find out that we receive around 20% to 30% of our daily hydration from the food we ingest,” notes Dr. Uys. He explains that it’s important to drink more water than you usually would while fasting, which can help make up for the water you would normally get from foods.
- Talk to a doctor: If you have diabetes or another health condition that affects your gut, metabolism or heart, checking in with a doctor first can help reduce some of the risks associated with fasting, according to Dr. Uys.
Is Water Fasting Safe?
While water fasting may be associated with several possible benefits, Huntriss and Murphy Richter both emphasize that it’s definitely not suitable for everyone. “The safety of water fasting depends on various factors, including individual health conditions and medical history,” says Murphy Richter. Specifically, water fasting may not be safe for people who are pregnant or breastfeeding, older adults, those at risk of eating disorders and people with certain health conditions, including immunodeficiency disorders.
Water fasting can also alter the water-mineral balance in the body, according to Shinta Galej, a renal dietitian based in New York. Galej notes that this can cause issues like hyponatremia (low sodium levels), which could be dangerous or even fatal. Water fasting can increase levels of uric acid as well, which is harmful for those with kidney disease and gout, says Galej.
Additionally, keep in mind that prolonged fasting periods (ranging from around 24 hours to 72 hours) can be dangerous if done without medical supervision. They can also cause your body to store extra fat in response to starvation, which may negate many of the potential health benefits of fasting. Therefore, it’s best to stick to shorter fasts (lasting around eight to 24 hours or less) unless advised by a doctor.
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When to See a Doctor
Talk to a doctor before trying water fasting if you have any underlying health conditions, such as diabetes. You should also check with a doctor before fasting if you’re taking medications or supplements. Not only do some medications and supplements need to be taken with food to prevent stomach irritation or nausea, but skipping meals could also increase the risk of electrolyte imbalances if you’re taking medications for heart disease or high blood pressure.
Water fasting may be associated with several benefits including increased weight loss, improved heart health and better blood sugar control. However, it’s not a good fit for everyone and in some cases, it can be dangerous. If you’re interested in trying water fasting, talk to a doctor or registered dietitian to determine if it’s a safe option for you and whether it can help you meet your goals.
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