Often, we’re told happiness and self-improvement can come only through addition—it’s the consumer culture we live in.

There is nothing inherently wrong about this. Eating healthy food, supplementing, and taking some time to better yourself through exercise or meditation are all additive actions that (hopefully) add something to our lives and help us achieve our best.

But, there may also be another side to human enhancement, one that involves restriction, rather than addition.

By restriction, we are referring to the practice of fasting, the (recently mainstream) practice of abstaining from food for a period of time. Everyone from Silicon Valley CEOs to medical professionals are recommending fasting as the next best thing in human optimization, the key to peak mental and physical performance.

This ancient health practice is getting a shot in the arm with science and data. Here’s how you can try water fasting, and what to expect when you do.

Water Fasting 101

Most proponents of fasting claim that humans evolved to go without food for prolonged periods of time. Fasting is natural, in this way.

In fact, it was probably a necessity. Food wasn’t always available. It was hard to obtain. No supermarkets in the paleolithic era. For this reason, our bodies seem to have several mechanisms that allow us to survive prolonged periods of time without taking in external energy sources (food). The ability to oxidize fat for fuel as well as the ability to produce and use ketones are two such pathways. Without them, we’d often be hangry, or worse, dead.

Substantial benefits occur when we are able to activate these conserved evolutionary pathways through fasting, making the body more robust.

Many systems become stronger, more resilient. But how do we tap into these systems in modern day, with a burger joint on every corner?

One way is through water fasting. As the name suggests, water fasting is the practice of going without food or any calorie-containing beverages for a period of around 24 - 72 hours. The only thing permitted? Good old H20. Some claim that coffee, sparkling water, and other non-caloric beverages are allowed. We'll leave the rules up to you on this one.

People partake in water-only fasts for various reasons—medical and healing purposes, for spirituality and religious reasons, weight loss, mental clarity, longevity, and “gut resetting.” While it’s been commonly used for obesity treatment since around 1915,Kerndt1982 fasting has health benefits above and beyond just weight loss.

Water Fasting vs. Intermittent Fasting and Time Restricted Eating

“Fasting” is a broad term that really just means “not eating” and therefore can mean a variety of things to different people. You’ll hear fasting mentioned online and in the media, with little specificity on what protocol is actually being employed. Several iterations of fasting are popular, so let’s differentiate them a bit.

Intermittent fasting (IF) refers to a “reduced meal frequency.” Again, while time lengths can vary, it often takes the form of a periodic (hence, intermittent) period of fasting that last around 24 - 48 hours. Some people do one weekly, some monthly. Intermittent fasting can also take the form of alternate-day fasting, where you eat every other day, or a "fasting mimicking" protocol where calories are drastically reduced for a few days at a time.

Time-restricted feeding (TRF) or time-restricted eating (TRE) involves eating within a certain window and then fasting for the rest of the day and night. Popular protocols for IF/TRF include 16:8, 18:6, and 20:4—where the first number refers to the fasting period and the latter, the feeding period.

Water fasting, on the other hand, typically involves a bit longer of a fasting period. 24, 48, and 72-hour water-only fasts are most common, but some might even push a fast out to 5 or 7 days. Some people may do these 1 - 7 day water fasts once a month or a few times per year. Water only fasting can be seen as a more extreme version of IF/TRE that might have unique and distinct benefits from the daily practice of a shorter fasting window.

Benefits of Water-Only Fasting

Three square meals a day—this is commonly accepted as the “normal” paradigm of daily food consumption. If this suits your lifestyle and you enjoy it, then there is nothing wrong with this way of eating. It’s your health, and your life.

However, there is something to be said about occasionally restricting ourselves using things like water fasting and IF/TRE. Giving a little “shock” to the body with an extended fast can be a good thing, especially when we’ve trained it to constantly expect incoming energy. Think of fasting as training for cells—it’s a process by which we can make them stronger by first stressing them out a bit. This and other health benefits of fasting have been a topic of study for decades.

Autophagy: Cellular Garbage Man

You may have heard this buzzword in most nutrition and health spheres: autophagy. Often compared to some sort of recycling or garbage disposal system, autophagy (meaning “self-eating”) is a conserved process that involves the degradation of organelles, proteins, and other large molecules from the body, recycling some of their components while getting rid of others.Parzych2014

Why do we need autophagy? Throughout daily life, some molecules become damaged or senescent (they stop dividing and become useless). We need to get rid of these lingering damaged molecules somehow, and this is where autophagy comes into play. Cellular “trash” is taken up by compartments called autophagosomes, delivered to breakdown machines known as lysosomes, and then basically ripped apart. From the waste products, new building material is created, and used to aid in cellular repair, regeneration, and immune system regulation.

Fasting is one way to activate autophagy and clear out our body's damaged machinery.

While our ability to measure autophagy in humans is elementary at the moment, there have been a few studies in rodent models that document the induction of autophagy in response to fasting. For instance, after 24 - 48 hours of fasting (only water allowed), autophagy was enhanced in neurons in the brains of mice. This was confirmed by the increase in the number of garbage-clearing autophagosomes throughout the brain.Alirezaei2010 Similar periods of water-only fasting in mice (24 - 48 hours) are sufficient to upregulate autophagy in the liver,Martinet2006 kidney, muscle, pancreas, thymus, and even the lens of the eye.Mizushima2004

24 - 48 hours in mice represent a much longer time period in humans—so take these results with caution. The same robust autophagy benefits in humans might only arise via longer fasting periods.

Nevertheless, autophagy may still be activated in response to similar fasting time periods in us humans—just at lower levels and after a bit longer period of time.

Reducing Oxidative Stress

Oxidative stress, sometimes also mentioned along with inflammation, is a process by which highly-reactive molecules in our body cause damage to cells and DNA. This is often a result of an imbalance between pro-oxidants and antioxidants and a highly activated immune system.

Oxidative stress is known to play a role in the development and worsening of many medical conditions like heart disease, chronic kidney disease, neurodegenerative disease, and cancer.Liguori2018 It’s even claimed to be one of the causes of aging.Salmon2010

Can fasting lower oxidative stress? The research indicates a strong possibility.

Since one of the known sources of reactive oxygen species and inflammation is the metabolism of food, it seems likely that short term food restriction will lower production of these molecules that are released during metabolism.

An 11-day water fast was shown to significantly reduce oxidative stress markers in adults; this was accompanied by several other benefits including enhanced kidney function.Mojto2018 Another study demonstrated that, while oxidative stress was not reduced, 48 - 60 hours of fasting made cells more resistant to oxidative stress induced by chemotherapy.Lee2012 There are several published case reports that autoimmune diseases, characterized by risk factors like chronic inflammation, can be completely resolved with 7 - 24 days of water-only fasting.Fuhrman2002

The benefits of water fasting may partly be owed to ketones. It has been shown that the ketone body Beta-hydroxybutyrate (BHB), whether given exogenously or increased through fasting/calorie restriction or a ketogenic diet offers significant protection against oxidative stress. It does this by increasing the expression of genes related to protective stress responses and antioxidant pathways.Shimazu2013 This may be why supplementing with exogenous ketones has been shown to reduce inflammatory processes in human and animal cells,Veech2017 and why ketogenic diets show such promise for improving medical conditions characterized by dysregulated metabolism.Veech2004

Weight Loss and Body Composition

Short-term water fasting has many benefits, but weight loss might not be one of the most robust.

In 24 - 48 hours, most of the weight you lose will be the result of shedding a significant amount of water weight and/or a minor loss in muscle glycogen, along with smaller amounts of muscle and fat.

For long term fat loss, a more prolonged (medically supervised) water-only fast may be needed, or a dedicated regimen of intermittent fasting undertaken for a significant length of time.

Yet, some studies do demonstrate significant weight loss after short term water fasts.

After a 24-hour water-only fast, healthy adults experienced a decrease in weight which was maintained for up to 48 hour post-fast.Horne2013 Larger changes in body mass are seen with longer fasts. In an older study consisting of 46 obese participants who were put on a 14-day fast, average weight loss was 7.6 kilograms.Gilliland1968 A similar time period in rats led to significant weight loss, body fat loss, and an increase in relative lean mass (meaning more fat was lost).Huth1959

Metabolic Benefits: From Blood Sugar to Mitochondria

Perhaps one of the most widely-accepted uses of therapeutic water-only fasting is for metabolic diseases like diabetes.

In fact, people who report taking part in routine, periodic fasting have a lower risk of diabetes, lower glucose, and a lower body mass index.Horne2012 Studies have shown that diabetic patients improve in several health markers after intermittent or modified fasting regimens.LiC2017,Arnason2017 It could be expected that similar benefits would be achieved with a water-only fast.

Profound changes in metabolic biomarkers have also been observed after water-only fasting. In one study, 30 adults fasted for 24 hours (water only) and experienced increases in human growth hormone, hemoglobin, red blood cell count, hematocrit, and total cholesterol, along with decreased triglycerides and body weight. Many of these are associated with metabolic health and longevity.Horne2013

Another metabolic benefit of fasting is the ability to promote a better capacity for “fat burning.”

You may have heard this concept explained or described as “metabolic flexibility.” This refers to the ability to “switch” energy metabolism between oxidizing glucose and fat in response to different states of nutrient availability and physiological stress.Harvie2017

For instance, after a carbohydrate-containing meal, you might quickly start using the available nutrients efficiently to generate energy. But, when stressed with a prolonged fast, you’re also able to respond by increasing fat oxidation—you’ve become metabolically flexible.

Without external sources of energy (like glucose), the body is forced to oxidize its own fatty acids. This has the added benefit of reducing insulin and glucose levels. After fasting for 72 hours, plasma levels of free fatty acids and 3-hydroxybutyrate are increased in healthy young men.Stannard2002 A 60-hour water-only fast has also been shown to result in significant adipose tissue breakdown and free fatty acid release, indicating lipid metabolism was highly active.Jensen2001

Fasting and Cardiovascular Health

High blood pressure (hypertension) and inflammation / oxidative stress wreak havoc on the blood vessels, heart, and other organs. Having a healthy cardiovascular system is essential for both high performance and prevention of chronic disease.

Routine, periodic fasting may have the potential to lower these risk factors. In one study, people who routinely fasted had a lower risk of coronary artery disease than those who reported never fasting.Horne2008 These were the same “fasters” who had a lower diabetes risk as well—go figure.

Longer, water-only fasts have been shown to dramatically reduce blood pressure. 13 days of medically supervised water-only fasting in people with borderline hypertension resulted in 82% of them achieving a normal or “ideal” blood pressure by the end of the regimen. On average, they reduced systolic blood pressure by 20mmHg and diastolic pressure by 7mmHg. Those with the highest blood pressure tended to improve the most.Goldhamer2002

In another similar study, 10 - 11 days of water only fasting led to an average reduction in blood pressure of 37 (systolic) and 13 (diastolic) mmHg and by the end of the fast, all of the participants on blood pressure medications were able to stop taking them. Talk about a cheap alternative to prescription meds.Goldhamer2001

Mood and Well Being

Fasting anecdotes are ripe with claims of increased mental clarity, alertness, and even enhanced cognition during the fast.

Another benefit cited by many is that they simply stop “worrying” about food. They’re no longer a slave to hunger, and feel like they have a greater control over their appetite and their life.

Water-only fasts (even intermittent fasting or TRF/TRE) can teach a self discipline and resilience that may be just as beneficial as the physiological changes.

But, the studies haven’t been done yet to make firm conclusions about the mental benefits of fasting. One study did, however, note an increase in physical and emotional well being, an absence of hunger feelings, and a general acceptance of the “feasibility of fasting for 4 - 21 days” in around 93% of the participants.Wilhelmi2019 This is in contrast to chronic caloric or energy restriction, where most people report just generally feeling miserable.

So…see for yourself. Try a shorter water fast, then work up as your resilience and metabolic tolerance build. This will likely accompany the internal changes (metabolic flexibility) with fasting, which might inhibit the hangry feelings you once felt after three hours without a snack (we speak from experience).

Is Water Fasting Safe?

There are published studies on fasts lasting from 60 days to hundreds of days.Runcie1970 There's even one impressive case of a morbidly obese man who underwent a medically supervised 382 day water-only fast with no ill effects.Stewart1973 These are obviously extreme cases, of course, but they nonetheless indicate that for most healthy (or unhealthy) adults, a 24 - 72 hour period without food is perfectly safe (assuming adequate fluid intake) and achievable. While fasting is far from mainstream medical advice, it’s becoming a more widely used strategy for health promotion.

It might not be the greatest idea to engage in certain activities on your water only fast—high intensity exercise, for example. Each individual will have a different tolerance and response, different limits to what they can and can’t do while fasting. Remember, this is a self-experiment in which biomarkers should be tested.

Who Should and Shouldn’t Fast?

A big concern among those opposed to the concept of fasting is that this practice could promote erratic eating patterns, binging, and a poor mood or outlook among participants.

This may be less of a worry for overweight individuals, who were shown to improve several markers of body image perception and binge-eating disorders after an alternate-day energy restriction regimen (this was not a water-only fast).Harvie2017 In contrast, individuals at risk for eating disorders should be more careful when planning to make water-only fasts part of their routine, or avoid them all together.

Certain side effects may occur during water only fasts, and you should be aware of some in order to know what to expect. A study analyzing hundreds of water-only fasts produced a list of side effects including fatigue, insomnia, nausea, headache, abdominal pain, diarrhea, palpitations, and other minor disturbances.Finnell2018 None of these, however, required hospitalization nor were considered serious.

Athletes, at least prior to competition, might want to stay away from water-only fasting. Some athletes swear by periodic fasts to reset and promote recovery. As a training tool, it’s likely a feasible approach when done right (similar to something like carb cycling, which times refeed days with workouts). However, there is no doubt that a prolonged water-only fast will affect athletic performance, at least in the short term.

Others who shouldn’t fast? Teenagers in the growing stage, pregnant women, and those who have concerns about losing too much muscle mass (older individuals) may stand to do more harm than good with a complete fast.

Everyone else probably has at least a bit to gain (or perhaps...lose) from a shorter-term, water-only fast. Most of us are far from metabolically “optimal” (if there is such a thing), and fasting could help bring us closer to that point.

What to Expect on a Water Fast

Everyone’s fasting experience will be different, but the body responds to fasting in some predictable ways. It’s good to know beforehand what to expect during your period of food restriction, and how best to prepare.

First suggestion—drink more water than you think you need. How much? This will depend on several factors, so we won't make a strict recommendation here. This might require you to increase how much you normally drink, however. This is because a good portion of our total fluid intake (about 20% - 30%) comes from food, which you won’t be eating during your fast. Also, make sure to take in some minerals (added to your water or through a supplement) to prevent electrolyte imbalances.

How to prepare for a water fast? If you’re an experienced intermittent faster, then water-only fasting might be the next step up.

It may be a good idea to gradually decrease your portion sizes in the day(s) leading up to the fast or experiment with longer fasting windows during your TRF/TRE regimen.

This strategy will build up your “fasting” ability along with metabolic flexibility.

Weight loss is the greatest during the initial fasting period, largely due to sodium excretion and the fluid that comes with it.Bloom1960 Studies have documented a progressive loss of sodium and potassium in the urine after three days of fasting. This might be accompanied by a weight loss of 0.9 kg/day in the first week of fasting, which slows to about 0.3 kg/day in the second week (assuming the fast is this long).Kerndt1982 To repeat the earlier recommendation—get some minerals in during your fast.

Since you’re not eating, you may experience feelings of hunger.

The best way to curb those cravings? Exogenous ketone supplements (and endogenously produced ketones reduce ghrelin—the hunger hormone.Stubbs2018 If water isn’t cutting it, you may need a little help fro an exogenous ketone supplement to get you through that fast.

Refeeding Right

For some, the logical next step after a successful water-only fast is a trip to the local buffet. You earned it, right? Wrong.

While bingeing on dumplings and Kung Pao chicken might seem heavenly, this type of refeed is not recommended. It’s best to resist the urge to eat a big meal, as this may lead to gut discomfort or gastrointestinal distress. After all, your stomach has likely shrunk a bit in size, and your digestive system has been idling for a few days. Easy does it.

Instead, opt for a smaller meal. Maybe a smoothie alongside some nuts. Soup works well for some. Find what works for you.

Evolution’s Hack

If you’ve been intermittent fasting or practicing time-restricted feeding and have experienced the myriad benefits that come with these “rituals,” perhaps water-only fasting is a logical next step on your journey toward self-optimization.

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