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Water Fasting: Do The Potential Benefits Outweigh The Risks?

Article written and reviewed by Cyrus Khambatta, PhD
Published September 8, 2021

What Is Water Fasting?

Water fasting is a strict form of intermittent fasting in which certain days (or groups of days) throughout the week are designated as ‘water fasting days’, in which you consume nothing besides water.

This strategy does take some inspiration from the science behind intermittent fasting, which has shown well-documented and evidence-based benefits for weight loss, your immune system, and your metabolic health.

However, some of the more extreme versions of water fasting (in which fasts last for up to 72 hours or even longer), are supported by very little research and may present serious health risks.

In this article, we’ll explore water fasting in the context of intermittent fasting — a tried and true method for weight-loss and improving your overall health — and explore where current water fasting recommendations may go a little too fa

Common Water Fasting Methods

There are currently three very popular methods of water fasting: full-day fasts, 3-day water fasts, and alternate-day fasting.

  • Full-day fasting includes one or two days throughout the week in which all meals and snacks are replaced with water, and augmented with water throughout the day.
  • 3-day water fasting is a more intense form of this same fast. Instead of one or two days, with eating windows in between, there are three full days in which water replaces all meals and snacks.
  • Finally, alternate-day fasting is a more rigorous version of full-day fasting, in which every second day, all meals and snacks are replaced with water.

Why Do People Try Water Fasting?

In some cases, people choose to fast for extended periods of time for personal or religious reasons, like in the case of Ramadan or other religious ceremonies.

However, we’ll primarily focus on water fasting as a health strategy and its various purposes.

The Science Behind Intermittent Fasting

The health benefits of intermittent periods of calorie restriction are actually not new information. In fact, these studies date back more than 75 years and suggested that periods of calorie restriction increase health and longevity.

In fact, calorie restriction is so powerful, it’s the only known intervention that has been proven to extend longevity in many species studied to date.

It’s for this reason that the science behind intermittent fasting emerged. As it turns out, the human body enters a biological state called autophagy during long periods without food.

In this state, your body switches from burning recently-digested food to burning excess glucose, fat, and protein, which can contribute to weight loss as well as improve your immune system and overall health.

In addition, for people who have unhealthy habits (like excess eating, drinking, or drug use), this period of time also offers a form of detox, in which you are no longer consuming harmful substances or amounts of substances.

So there’s a very real benefit to any form of intermittent fasting (not just water fasting), especially for those struggling with weight and its many ensuing complications.

The Popularization of Water Fasting

It was relatively recently that the research of Alan Goldhamer led to the popularization of water fasting, a system in which closely supervised patients would undergo a longer fast in order to help with weight loss, cardiovascular health, and a number of other health challenges.

And in a medically supervised environment, the results were tangible, yielding mostly positive results with doctors able to intervene in the case of any nutritional deficiencies or medical complications.

Is Water Fasting Safe?

This question depends on the length and intensity of your water fast.

If you are water fasting using the ‘full-day fast’ or ‘alternate-day fast’ strategies, you are just as safe as with any other form of intermittent fast, and only have an elevated risk if you have gout, type 1 or type 2 diabetes, eating disorders, or are elderly, sick, or pregnant/breastfeeding.

However, if you are following a ‘3-day water fast’, your risks can be far more severe even if you are completely healthy, and the science has not yet determined whether this extended fasting period provides commensurate benefits.

It is highly recommended NOT to undergo a fast of any longer than 72 hours without medical supervision.

The (Potential) Benefits

The direct health benefits of any intermittent fast are autophagy, which helps burn fat and reduce oxidative stress in your body, as well as potentially making it easier to maintain a calorie deficit (though this is not guaranteed).

Reducing oxidative stress has been proven to help improve your immune function in the short-term, and reduce your risk for early aging and long term diseases like Alzheimer’s.

And if you’re currently struggling with the many complications of being overweight (which include high blood pressure, high cholesterol, high blood glucose, and increased risk of chronic diseases), weight loss to your target weight can be incredibly powerful.

It’s worth noting, however, that these benefits can be achieved through less intense forms of intermittent fasting (like 16:8 intermittent fasting, or 24 hours fasts) combined with an improved diet.

There is little research on whether or not these benefits are significantly increased during fasts longer than 24 hours.

The Risks Involved

If you are following either full-day water fasting or alternating day water fasting, your risks of side effects are exactly the same as with any form of 24-hour intermittent fast, and can include:

  • Fatigue
  • Headaches
  • Crankiness
  • Digestive trouble
  • Overeating, binging and refeeding after the fast

However, if you’re attempting to follow three-day water fasts or fasting for even longer periods of time, you can expose yourself to all of these risks even more severely, along with the possibility of:

  • Fainting
  • Migraines
  • Dizziness
  • Intense mood swings
  • Hormonal imbalances
  • And even feelings of starvation due to depletion of carbohydrates, electrolytes, and other key nutrients

These risks can definitely be mitigated by following a water fast with a trained medical supervisor, but it’s worth assessing if this is the most effective way to get the most out of your fast.

Alternatives To Intense Water Fasting

If you’re looking to lose weight and improve your overall health without some of the risks of the more intense forms of water fasting, there are three other key strategies that we recommend:

  • Switching to a low-fat, plant-based, whole-food diet
  • Increasing your movement and exercise across the board
  • Combining less intense forms of intermittent fasting with the options above

The Best Diet for Weight Loss and Health

If you’re looking to lose weight and improve your overall health in the long term (especially if you’re dealing with any form of diabetes), the research points to a low-fat, plant-based, whole-food diet that’s high in natural carbohydrates.

Compared to some other dietary strategies like keto, which have short term results but can be disastrous to your long term health, this plant-based strategy has been proven to help lose weight and lower your insulin resistance without those negative side effects.

So one very possible option is to simply change the type of food, rather than your eating habits.

Ramp Up the Exercise

Another way to match the benefits of fasting and water fasting is to simply ramp up your physical activity.

Whether that means increasing the intensity of your existing workouts, adding moderate physical activity like walking and swimming to days you might not have exercised before, or simply making sure you hit your step count, the research is clear: exercise can make an impact.

The Wide World of Intermittent Fasting

Water fasting isn’t the only way to take advantage of autophagy and the benefits of intermittent fasting. In fact, there are a number of different intermittent fasting and eating patterns that might work just as well for you without posing the same risks.

For example, many people find success with 16:8 fasting, in which eating and fasting are spread out throughout each day, rather than choosing a specific day to not eat.

Tea fasting or green juice fasting are another option, which take advantage of the health benefits of green teas and herbal teas, or low calorie juices while keeping your body in autophagy.

Or, you can combine intermittent fasting and moderate water fasting with any (or all) of the strategies above, for a three-pronged strategy that can yield results fast.

Our Take: Do The Potential Benefits Outweigh The Risks?

When it comes to water fasting as a form of moderate intermittent fasting, the benefits are tangible, and as long as you follow the body of well-established, well-documented research, you should be able to improve your health without negative side effects.

However, in terms of longer, more extreme solutions like a multiple day or longer water fast (even with medical supervision), we think that the research actually points to different solutions, like switching to a low-fat, plant-based, whole-food diet, or increasing your exercise.

Combined with moderate intermittent fasting, these dietary and lifestyle changes can produce very quick results for your metabolic wellness, your blood glucose (blood sugar), your weight, and many other aspects of your overall health.

If you’re uncertain about which of these dietary and lifestyle changes might be right for you, you can reach out to our team of Mastering Diabetes coaches.

Working together with these diabetes care experts and a trusted certified dietitian, you can tailor your dieting and lifestyle changes to your body and your specific needs.

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About the author 

Cyrus Khambatta, PhD

Cyrus Khambatta, PhD is a New York Times bestselling co-author of Mastering Diabetes: The Revolutionary Method to Reverse Insulin Resistance Permanently in Type 1, Type 1.5, Type 2, Prediabetes, and Gestational Diabetes. He is the co-founder of Mastering Diabetes and Amla Green, and is an internationally recognized nutrition and fitness coach who has been living with type 1 diabetes since 2002. He co-created the Mastering Diabetes Method to reverse insulin resistance in all forms of diabetes, and has helped more than 10,000 people improve their metabolic health using low-fat, plant-based, whole-food nutrition, intermittent fasting, and exercise. Cyrus earned a Bachelor of Science in Mechanical Engineering from Stanford University in 2003, then earned a PhD in Nutritional Biochemistry from the University of California at Berkeley in 2012. He is the co-author of many peer-reviewed scientific publications. He is the co-host of the annual Mastering Diabetes Online Summit, a featured speaker at the Plant-Based Nutrition and Healthcare Conference (PBNHC), the American College of Lifestyle Medicine Conference (ACLM), Plant Stock, the Torrance Memorial Medical Center, and has been featured on The Doctors, NPR, KQED, Forks Over Knives, Healthline, Fast Company, Diet Fiction, and the wildly popular podcasts the Rich Roll Podcast, Plant Proof, MindBodyGreen, and Nutrition Rounds. Scientific Publications: Sarver, Jordan, Cyrus Khambatta, Robby Barbaro, Bhakti Chavan, and David Drozek. “Retrospective Evaluation of an Online Diabetes Health Coaching Program: A Pilot Study.” American Journal of Lifestyle Medicine, October 15, 2019, 1559827619879106. https://doi.org/10.1177/1559827619879106 Shrivastav, Maneesh, William Gibson, Rajendra Shrivastav, Katie Elzea, Cyrus Khambatta, Rohan Sonawane, Joseph A. Sierra, and Robert Vigersky. “Type 2 Diabetes Management in Primary Care: The Role of Retrospective, Professional Continuous Glucose Monitoring.” Diabetes Spectrum: A Publication of the American Diabetes Association 31, no. 3 (August 2018): 279–87. https://doi.org/10.2337/ds17-0024 Thompson, Airlia C. S., Matthew D. Bruss, John C. Price, Cyrus F. Khambatta, William E. Holmes, Marc Colangelo, Marcy Dalidd, et al. “Reduced in Vivo Hepatic Proteome Replacement Rates but Not Cell Proliferation Rates Predict Maximum Lifespan Extension in Mice.” Aging Cell 15, no. 1 (February 2016): 118–27. https://doi.org/10.1111/acel.12414 Roohk, Donald J., Smita Mascharak, Cyrus Khambatta, Ho Leung, Marc Hellerstein, and Charles Harris. “Dexamethasone-Mediated Changes in Adipose Triacylglycerol Metabolism Are Exaggerated, Not Diminished, in the Absence of a Functional GR Dimerization Domain.” Endocrinology 154, no. 4 (April 2013): 1528–39. https://doi.org/10.1210/en.2011-1047 Price, John C., Cyrus F. Khambatta, Kelvin W. Li, Matthew D. Bruss, Mahalakshmi Shankaran, Marcy Dalidd, Nicholas A. Floreani, et al. “The Effect of Long Term Calorie Restriction on in Vivo Hepatic Proteostatis: A Novel Combination of Dynamic and Quantitative Proteomics.” Molecular & Cellular Proteomics: MCP 11, no. 12 (December 2012): 1801–14. https://doi.org/10.1074/mcp.M112.021204 Bruss, Matthew D., Airlia C. S. Thompson, Ishita Aggarwal, Cyrus F. Khambatta, and Marc K. Hellerstein. “The Effects of Physiological Adaptations to Calorie Restriction on Global Cell Proliferation Rates.” American Journal of Physiology. Endocrinology and Metabolism 300, no. 4 (April 2011): E735-745. https://doi.org/10.1152/ajpendo.00661.2010 Bruss, Matthew D., Cyrus F. Khambatta, Maxwell A. Ruby, Ishita Aggarwal, and Marc K. Hellerstein. “Calorie Restriction Increases Fatty Acid Synthesis and Whole Body Fat Oxidation Rates.” American Journal of Physiology. Endocrinology and Metabolism 298, no. 1 (January 2010): E108-116. https://doi.org/10.1152/ajpendo.00524.2009