Water fasting: benefits, safety, and how-to

Water fasting means consuming only water — no food at all — for a set period of time. It's an extreme approach that some people try for weight loss or other potential health benefits, but it isn't safe for everyone, and most of what we know about it comes from studies conducted under medical supervision. 

Water fasting is eating nothing and drinking only water — no food, and no non-caloric drinks like coffee or tea, which other fasting approaches sometimes allow. 

Experts define general "fasting" in slightly different ways, but most agree it means restricting food or calorie intake for a longer-than-usual period — anything from an overnight fast to several days without calories. Water fasting is a stricter version of that: water only, for the entire fast. 

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Water fasting causes several metabolic changes, including depletion of stored glycogen, and production of ketones. It may also affect something called  autophagy, but its timing and health significance in humans are not well established:

• Metabolic shift: A metabolic shift is a change in the body's fuel source. When a person stops eating, the body uses up its stored glucose and switches to relying on stored fatty acids and fatty-acid–derived ketones for energy instead.

• Autophagy: Autophagy — which literally translates to "self-eating" — is a cellular cleanup process in which cells break down and recycle damaged or unnecessary components. This is thought to potentially contribute to healthier cells, though researchers are still working out how significant this effect is in humans during shorter fasts.

Water fasting creates a large calorie deficit, so weight can drop quickly. However, the loss includes water and lean tissue as well as fat. Researchers are also studying how fasting affects cellular processes such as autophagy, but it has not been shown that water fasting prevents disease through this mechanism.

Research on the benefits of water fasting is still limited. Studies have been small, short-term, and conducted under medical supervision, so it is unclear whether the findings apply broadly or last over time. Even so, some of the research points to a range of potential health benefits, including:

• Reduced blood pressure among people with hypertension 

• Less perceived stress 

• Weight loss (including water, fat, and some lean tissue)

• Temporary changes in blood glucose and blood lipids

• Greater sense of well-being  

Looking beyond water-only fasting, other approaches — such as time-restricted eating and alternate-day fasting — have been studied more extensively and may offer certain health benefits. 

 A review of 99 randomized trials found that these approaches may lead to:

• Modest weight loss

• Small improvements in some measures of heart and metabolic health

• Slightly greater short-term weight loss with alternate-day fasting compared with daily calorie restriction

These findings should not be assumed to apply to prolonged water-only fasting, which was not included in the review. It's worth noting that most studies on water fasting involve a week or more of fasting, so it isn't clear whether shorter water-only fasts offer the same benefits. It's also not established that water-only fasting provides benefits beyond what other,  less extreme forms of fasting can offer.

Water fasting carries real risks alongside its potential benefits, and research has documented a number of adverse events and unwanted side effects.

These include :  

• Dehydration

• Elevated blood uric acid (which is a risk factor for gout)

• Low blood sodium (which may lead to headaches, nausea, poor balance, and more) 

• Fatigue

• Insomnia

• Weakness or lightheadedness

• Upset stomach

• Back pain

• Electrolyte disturbances and loss of lean tissue 

Research has also tied water fasting to serious adverse events. In one review of 768 medically supervised fasting visits, two people required hospitalization — one for dehydration and one for severely low sodium.

Nearly all research on water fasting involves regular health monitoring in a medical clinic or other healthcare facility — people in these studies aren't fasting for days at home, alone. 

Most of the studies have also been small, involving just a handful of adults. Because of this, the medical community still needs more research to fully understand water fasting's risks and benefits — and until then, doing an extended water fast without medical supervision carries meaningfully more uncertainty than what the research reflects. 

If you're considering a water fast, talk with a healthcare provider first. Home precautions cannot reproduce the screening, laboratory testing, medication management, and monitoring used in clinical studies, and prolonged water fasting should not be attempted without medical supervision.

• Limit the length of your fast: The risk of adverse events and side effects rises with how long the fast lasts, and there's no single "safest" duration that applies to everyone — another reason a provider's input matters.

• Skip strenuous exercise: Intense activity during a fast may increase the risk of dizziness, dehydration, fainting, and poor recovery. Choose light activity and stop if you feel weak or lightheaded 

Water-only fasting may be unsafe for many people. It should not be attempted without individual medical guidance, particularly by people who:

• Are pregnant or breastfeeding

• Have a current or past eating disorder

• Are underweight, malnourished, or experiencing severe muscle wasting

• Have advanced kidney or liver disease or unstable or severe heart disease

• Take medications that may need to be adjusted or taken with food

Even outside these groups, it's worth talking with a healthcare provider before starting any water fast, however short.

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Water fasting means drinking only water and eating no food. This extreme approach may offer some health benefits, but it isn't safe for everyone, and most of what's known about it comes from medically supervised research.

Water fasting may offer some health benefits, but it also carries real risks, and researchers haven't yet worked out how to do it optimally — or whether it offers any advantage over less extreme approaches, like intermittent or alternate-day fasting.

• Water fasting works by affecting your metabolism and changing your body’s energy source, and it also may lead to autophagy, or a cellular cleanup process.

• Small studies have reported short-term weight loss and lower blood pressure in some people. Effects on blood sugar, cholesterol, kidney function, and long-term health remain unclear. Water fasting also carries risks of side effects such as fatigue, weakness, lightheadedness, and nutritional deficiencies. In severe cases, hospitalization for dehydration or sodium deficiency may be necessary.

• Check with a healthcare provider before attempting a water fast. Do not attempt a prolonged fast without medical supervision, and avoid strenuous activity while fasting.