Luisa Torres, PhD
Luisa is a science communications manager at LifeOmic. She is a neuroscientist and science writer covering topics related to aging, metabolism, and brain health.

Luisa Torres, PhD
Luisa is a science communications manager at LifeOmic. She is a neuroscientist and science writer covering topics related to aging, metabolism, and brain health.


What is it | Health benefits | Safety | Working out while water fastingTips

What is water fasting?

Water only fasting— also known as a “zero-calorie diet”— is a type of intermittent fasting that requires you to avoid food intake for sometime, usually between 16-72 hours, consuming only water in the process. No solid foods are allowed during a water fast, and liquid calories from juices and sodas are also off the table.

When you perfom water fasting your body uses your stored body fat as energy once it’s depleted the sugars it stored after your last meal. This leads to weight loss and other health benefits. 

No solid foods are allowed during a water fast, and liquid calories from juices and sodas are also off the table. You should only drink water during a water fast.

Why drink water?

Water is a natural appetite suppressant, and it helps improve kidney function by removing waste and toxins from your blood. Without water, your body can’t convert your stored body fat into energy. In fact, burning fat costs the body more water than many other processes. Well-hydrated cells also have greater insulin sensitivity, which may reduce your risk of insulin resistance, prediabetes and type-2 diabetes. 

Health benefits of a water fast

Water fasting is an acute form of stress for the body. Most of what we know about the health benefits of water-fasting has come from studies of intermittent fasting methods such as alternate day fasting and prolonged fasting. These methods have potential health benefits for slowing the progression of chronic diseases such as Alzhemier’s and Parkinson’s.

Health benefits of short-term water fasting

Water fasts between 16 and 24 hours fall into the category of short-term water fasting or time-restricted eating, which involves consuming all of your meals within a time range. For example, you could choose to have your meals and calories between 10 am and 6 pm and drink only water when you are not eating. Time-restricted eating is flexible, but it’s particularly beneficial if you eat your meals before sunset. This helps you eliminate bad habits like late-night snacking, and it improves your sleep and insulin sensitivity. 

Intermittent fasting concept flat vector illustration, eating window vector illustration,dieting concept, healthy eating flat vector illustration
Short-term water fasting can help you lose weight, improve insulin sensitivity, reduce blood pressure and reduce your risk of breast cancer and other chronic conditions.

Water fasting benefits include:

Water fasting leads to weight loss

In the only long-term study (1 year) of time-restricted eating conducted to date, those who ate within an 8-hour window dropped 9% of their body weight after 12 months, while those who ate without the time restriction lost 7.2%. Most other studies of time-restricted eating show 3-4% weight loss within 8-12 weeks. A more recent study showed that participants who ate in an 8-hour window lost 2.3 kg more in 14 weeks than those eating over 12 or more hours. 

Water fasting improves insulin sensitivity and reduces blood pressure

Eating within a 6-hour period was shown to improve insulin sensitivity,blood pressure, and oxidative stress in men with prediabetes. These health benefits occurred even in the absence of weight loss. Greater insulin sensitivity and low inflammation are associated with reduced risk of heart disease, diabetes and cancer.

An 8-hour eating window also reduces blood pressure, improves mood and reduces symptoms of depression. 

Water fasting reduces breast cancer risk

Short-term water fasting, and particularly avoiding food consumption during the evening, is associated with lower risk of breast cancer. Taking in calories late at night is linked to higher levels of inflammatory molecules in circulation. Starting your fast earlier in the evening is also associated with lower breast cancer recurrence.

Water fasting reduces heart disease risk factors

Nighttime eating decreases the quality and quantity of sleep, which may lead to high blood pressure and heart disease. Short-term water fasting also improves LDL cholesterol and triglycerides, and reduces the risk of coronary artery disease.

Are short water fasts safe? 

Short term fasting is safe and well-tolerated. Those most likely to benefit are normal weight or slightly overweight people looking for an intermittent fasting style they can adopt for life.

Health benefits of a long water fast

Alternate day fasting is the most studied method of intermittent fasting in humans and it’s a form of long-term water fasting. It involves alternating between days in which you perform water fasting and days in which you eat normally. This type of fast is usually medically supervised and it’s for people looking to jumpstart serious weight loss or reduce risk of chronic disease.

Some health benefits of long-term water fasting include:

Weight loss, reduced cholesterol, reduced blood pressure and improved body composition 

Studies of strict water only alternate day fasting are limited, but participants lose weight and show reductions in blood pressure, low density cholesterol, triglycerides and aging markers compared to ad libitum eating. Another study found that this method of long-term water fasting reduced fat mass and preserved more lean mass compared to continuous energy restriction.

Shot of a doctor examining a man with a blood pressure gauge
Water fasting can help you reduce risk factors for heart disease, including high blood pressure.

Although shorter fasts are beneficial, there are health benefits of going beyond 48 hours without food intake. These include reducing heart disease risk factors, losing weight, rejuvenating your immune system, addressing autoimmune disease, attacking cancer cells, or tapping into autophagy to rid your body of damaged cell components. One way of doing a long water fast is through extended fasting, which involves going with no food consumption for 3-5 days. These types of fasts are usually done under medical supervision. Patients in studies of medically supervised water-only fasting undergo comprehensive physical and psychological tests before participating. Most people drink two to three liters of water per day during a water fast longer than 24 hours. 

Studies of long-term, medically supervised water-only fasting are limited. Benefits of long-term water fasting include: Reduced body weight and oxidative stress, improved blood pressure, improved lower tract urinary function, and improved quality of life during chemotherapy

Long-term water fasting  might also ‘prime’ the gut to regrow short-chain fatty acid-producing gut bacteria. Short-chain fatty acids help control inflammation in the gut and can ameliorate symptoms of gut inflammatory conditions such as inflammatory bowel syndrome. They also cause blood vessels to dilate, lowering blood pressure.

Is long-term water fasting safe?

Medically supervised water-only fasting on alternate days has been shown to be well tolerated for up to 6-months. However, studies are limited and we recommend doing these types of fasts under the guidance of a healthcare provider. Water only fasting on alternate days is great for the first several months of a serious weight loss plan. Some people are able to maintain it long-term, but many fall back to time-restricted eating after they’ve lost most of the weight.

Multi-day fasts are generally well tolerated, but they need to be medically supervised. Replenishing fluid and electrolyte levels is important during a long water fast to avoid dehydration.

Some side- effects that have been reported with water fasts lasting from 2-22+ days include mild-to-moderate fatigue, nausea, insomnia, headache, dizziness, indigestion and back pain. There are many health risks associated with extended fasts including low blood pressure. Other adverse events that have been reported in studies are high uric acid production and hyponatremia after an 8-day water fast. Uric acid promotes oxidative stress in cells and can also lead to problems such as high blood pressure and nephritis, which is why those with renal disease should not do extended water only fasting. Hyponatremia, or low levels of sodium in the blood, is another serious adverse event and a potentially fatal condition. 

Long-term water fasting can be a stressor on the body, and just like with anything in life, more is not always better. Long term studies of repeated cycles of long-term water fasting are lacking and it’s unknown what the long term health risks of restricting calories for extended periods could be. Talk to your healthcare provider and make sure you are under medical supervision before attempting an extended water fast. 

Water fasting variations

If you dislike the taste of plain water, you may drink sparkling water or water flavored with thyme or mint leaves during a water fast. You may also try a lemon detox cleanse in which you drink water mixed with the juice of half a lemon and a teaspoon of honey. This ‘cleanse’ is associated with weight loss and reduced cholesterol and triglycerides. 

General safety precautions

Who should not water fast

Pregnant women, women planning to become pregnant, breastfeeding women, underweight people, those with nutrient deficiencies, and those who have or have had an eating disorder, should not perform water fasting beyond 12-14 hours overnight.

Seek professional medical advice from an eating disorder specialist if fasting is damaging your perceptions of your body and/or food. 

 If you take medications, seek medical advice before a water fast

Talk to your healthcare provider before you start water only fasting. Always take your meds as prescribed, but make sure they are safe to take on an empty stomach. 

If you suffer from Type 1 or Type 2 diabetes

Fast under the medical supervision of a healthcare provider who can adjust your medications in accordance with your fasting schedule.

Can you work out during intermittent fasting?

Yes! If your water fast is under 36 hours, you don’t have to change your exercise regimen, though you’ll have to get used to working out without a pre-meal snack. If you are afraid you’ll get dizzy, try a lighter workout until you get used to it. Studies have found no changes in performance during fasted exercise if workouts are under 60 minutes. 

You don’t have to change your exercise regimen during short fasts, but you’ll have to get used to working out without a pre-meal snack.

If you are doing multi-day water only fasting pay attention to hydration and electrolyte supplementation, and modify your routine depending on how you are feeling. For example, you might do your usual workout on the first day of a 3-day water fast, and do a lighter workout during the remaining two days. 

Extended water only fasting could impact your exercise performance. Fasting for 4 days decreased capacity to perform endurance exercise– fasted participants felt fatigued faster than non-fasted ones. Another study showed that cyclists felt fatigued after 24h fasts. If you need some energy before a workout during a long water fast, try this tip from LIFE Apps fasting coach Marisa Moon: put a dash of maple syrup and sea salt in your water to have a tiny bit of glucose and enough sodium to get through your workout.

Intermittent fasting plus exercise increases fat burning 

There are health benefits to working out while you perform water fasting. Scientific evidences shows that combining water only fasting and exercise makes you burn more fat. Regular intermittent fasting already makes your body use more lipids than other sources of energy, such as sugars, and adding exercise could speed up this process. An analysis of 46 scientific studies found that there are more free fatty acids circulating in the bloodstream of people who exercise while practicing intermittent fasting compared to those who exercise after a meal. This suggests that intermittent fasting mobilizes more triglycerides from fat tissue to use as fuel during workouts, thus burning more fat. Another study also showed that exercising while performing water fasting increased lipolysis– the breaking down of fat molecules. This study also found higher concentrations of stress hormones and enzymes that triggered fat burn and increased fatty acids in the bloodstream. 

Water only fasting lowers insulin levels, while exercise increases adrenaline and cortisol. This combination increases fat-burning. In contrast, eating before exercise increases insulin levels, (especially if you break your fast with something high in carbs) which prevents your body from deriving energy from stored fat. 

Tips for practicing short-term water fasting

You should aim to develop an intermittent fasting schedule you can maintain over time. A 16-18 hour daily water fast is a great goal most people can work up to! Here are some tips:

Start slow 

Even a 12-hour overnight water fast can be a challenge, especially if you like to snack after dinner. Your first water fast should be between 12-16 hours overnight.


Add hours gradually

Once you’re ready to do a longer water fast, add hours gradually. Do 1-hour increments once a week.

Take note of how you feel

After your first water fast, take note of how you felt and adjust accordingly. If your first one felt too difficult, try a shorter water fast until you get used to it.

Avoid snacking between meals

Avoid snacking in between meals to work your fasting muscle during the day. Skipping snacks will make water only fasting easier!

Aim for an early dinner most days of the week

snacking or eating dinner late at night prevents you from burning fat as you sleep. Eating with the sun gives your body enough time to process your dinner before you go to sleep and your metabolism changes to burning stored fat. This will help you sleep better! 

Eat mindfully before you start your water fast

Water only fasting can help you lose weight if you adopt a healthy diet. Limit carbs and added sugars during your last meal of the day. The more carbs and added sugars you eat, the faster you will feel hungry and the harder it will be to perform water fasting.

Pay attention to what you eat after a water fast 

As a general rule, avoid processed foods and refined carbs in your first meal to avoid feeling lethargic and hungry shortly after. Increase your lean protein intake to improve satiety and muscle mass. Maximize the health benefits of water only fasting and lose weight by eating natural foods eliminating added sugars. Consuming added sugars is strongly linked to insulin resistance and metabolic dysfunction. In addition to weight gain, added sugar intake leads to high blood pressure, cardiovascular diseases, inflammation and fatty liver disease

Salad with grilled chicken breast, avocado, pomegranate seeds and tomato on white background. healthy diet food. Top view
Avoid processed foods and refined carbs and increase your protein intake to stay full longer.

Use sleep to your advantage

 Go to bed earlier if you think hunger will strike at night. Try to get at least 7 hours of sleep. Chronic sleep deprivation increases one’s risk for cognitive deficits, obesity, type-2 diabetes, heart disease, depression and suicide. Eating when you should be sleeping and infusing your brain with artificial light at the wrong times can cause you to be out of tune with your natural rhythms, which in turn affects the quality and quantity of your sleep, your ability to exercise the next day, and your ability to lose weight.

Exercise in the morning! 

It’s safe to work out during water only fasting and exercise will distract you from hunger. If you think you’ll get dizzy, try a lighter workout while you adjust.

Stop your fast early and seek medical advice if you feel unwell

If you feel dizzy or faint, or have a headache at any point during your fast, drink a tall glass of water with a pinch of pink salt to replenish electrolyte levels. If this doesn’t help, end your fast early. These are signs you should never ignore. There’s always another day to fast. 

Have patience with yourself!

You may not complete your water only fasting goal every time, and that’s ok. You can always try again!


Luisa Torres, PhD

I'm a science communications manager at LifeOmic and the editor of this blog. I am a neuroscientist and science writer interested in covering topics related to aging, metabolism, and brain health. I have written for NPR's blogs 'Shots', 'Goats and Soda', and 'The Salt'.

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