Today’s post is about dry fasting. I’ve covered plenty of other aspects of intermittent fasting , including recommendations around longer fasts , but lately I’ve gotten enough questions about this particular angle that I thought I’d address it.
Dry fasting is going without both food and fluid. That means no coffee, no tea, no broth, and no water or liquid of any kind (except the saliva you manage to produce). It’s an extreme type of fast whose fans and practitioners are adamant that it can resolve serious health issues. But does it? Is it safe? And what kind of research is available on it?
Where Does the Idea of Therapeutic Dry Fasting Come From?
The main proponent of dry fasting is a Russian doctor named Sergei Filonov. Filonov is still practicing from what I can tell, somewhere in the Altai mountains that span Central Asia. I found a very rough English translation of his book—Dry Medical Fasting: Myths and Realities . Difficult to read in full because it’s not a professional translation, but manageable in small chunks.
His basic thesis is that dry fasting creates a competitive environment between healthy cells, unhealthy cells, and pathogens for a scarce resource: water. The dry fast acts as a powerful selective pressure, allowing the strong cells to survive and the weak and dangerous cells to die off. The end result, according to Filonov, is that the immune system burns through the weak cells for energy and to conserve water for the viable cells, leading to a stronger organism overall. He points to how animals in nature will hole up in a safe, comfortable spot and take neither food nor water when recovering from serious conditions, illness, or injuries that prevent them from moving around. But when they’re able to move while recovering from more minor issues, they’ll drink water and abstain from food. I’m partial to this naturalistic line of thought, but I don’t know if the claims about animal behavior during sickness are true.
Another claim is that dry fasting speeds up fat loss relative to fasts that include water. There may be something to this, as body fat is actually a source of “metabolic water” —internal water the body can turn to when exogenous water is limited. Burning 100 grams of fat produces 110 grams of water, whereas burning the same amount of carbohydrate produces just 50 grams of water.
Are There Any Dry Fasting Studies?
Unfortunately, we don’t have many long term dry fasting studies. In fact, we have one 5-day study in healthy adults. For five days, ten healthy adults refrained from eating food or drinking water. Multiple physiological parameters were tracked daily, including bodyweight, kidney function, heart rate, electrolyte status, and circumference of the waist, hip, neck, and chest.
Participants lost weight (over 2 pounds a day) and inches off of various circumferences, including waist, hip, neck, and chest. The drop in waist circumference was particularly large—about eight centimeters by day five. Blood pressure, heart rate, oxygen saturation, sodium and potassium levels, creatinine, and urea all remained stable throughout the study. Creatinine clearance—which can be a marker of muscle breakdown but also a normal artifact of fasting —increased by up to 167%.
The most voluminous research we have on dry fasting is the Ramadan literature. During the month of Ramadan, practicing Muslims complete a daily dry fast—from sunup to sundown—every single day. They eat no food and drink no fluids during daylight hours, which, in the countries where Islam originally arose, run about 15-16 hours. These are shorter dry fasts than the 5-day fast detailed above.
What happens to health markers during Ramadan? Mostly good things.
- Almost everyone loses body fat . Few lose muscle . There’s no sign  of overt dehydration.
- In fatty liver patients, Ramadan fasting lowers  blood glucose, insulin levels, inflammatory markers.
- In obese and overweight subjects, Ramadan fasting lowers  inflammatory markers, body weight, and insulin resistance.
- In obese adults, Ramadan fasting improves  the lipid profile.
- Athletic performance is compromised during Ramadan (like impaired maximal force production of the muscles ), though not as much as you’d expect .
A 15- or 16- hour dry fast isn’t very extreme, even in the hot climates of the Near East. Two or three day-long dry fasts, particularly in hot weather, is another thing entirely. What works and is safe across 16 hours might not be safe or effective over three or four days.
I wonder if there’s a genetic component to dry fasting tolerance, too. Have populations who’ve spent thousands of years in hot, dry, desert-like climates developed greater genetic tolerance of periods without water? I find it likely, though I haven’t seen any genetic data one way or the other. It’s an interesting thing to ponder.
Is Dry Fasting Safe?
Obviously, skipping water can be dangerous. While we’ve seen people go without food for as long as a year  (provided you have enough adipose tissue to burn, take vitamins and minerals, and are under medical supervision), going without water is a riskier proposal. The number I’ve always heard was three weeks without food, three days without water, though I’ve never really seen it substantiated or sourced.
One reason I’m skeptical of “three days” as a hard and fast rule is that most cases of people dying of dehydration occur in dire circumstances. People are lost out in the wilderness, hiking around in vain trying to find their way back to the trailhead. They’re thrown in jail after a night out drinking and forgotten by the guards for three days. They’re spending 24 hours dancing in a tent in the desert on multiple psychoactive drugs. These are extreme situations that really increase the need for water. Your water requirements will be much higher if you’re hiking around in hot weather bathing in stress-induced cortisol and adrenaline, or dancing hard for hours on end. Very rarely do we hear of people setting out to abstain from water on purpose for medical benefits, water on hand in case things go south, and ending up dehydrated. Part of the reason is that very few people are dry fasting, so the pool of potential evidence is miniscule. I imagine this last group will have more leeway.
Still, if you’re going to try dry fasting, you have to take some basic precautions.
6 Precautions To Take When Dry Fasting
1. Get Your Doctor’s Okay
Sure, most will be skeptical at best, but I’d still advise not skipping this step—particularly if you have a health condition or take any kind of medication. Diuretics (often used for blood pressure management), for one example, add another layer to this picture.
2. No Exercise
Avoid anything more intense than walking. For one, the hypohydration will predispose you to middling results, increasing cortisol and reducing testosterone. Two, the hypohydration may progress rapidly to dehydration. If you’re going to exercise during a dry fast, “break” the fast with water first and then train.
3. Keep It Brief
Yes, there was the 5-day study, but those people were being monitored by doctors every single day. I’d say 16-24 hours is a safe upper limit and probably provides most of the benefits (as Ramadan literature shows). Any longer, buyer beware. (And, of course, make sure you get fully hydrated in between any dry fasts you might do.)
4. Fast While You Sleep
Ramadan-style probably isn’t ideal from a pure physiological standpoint. The length (16 hours) is great, but the eating schedule is not. Those who observe Ramadan fasting ritual often wake up before sunrise to fit in food. They may stay up late to eat more. They go to sleep in a well-fed state, never quite taking advantage of the 8 hours of “free” fasting time sleep usually provides (and, of course, that’s not what their fasting practice is about). For a health-motivated dry fast, on the other hand, you should take advantage of it.
5. Take Weather Into Account
Hot, humid weather will generally cause the most water loss. Cold, dry weather will cause the least. Adjust your dry fasting duration accordingly.
6. Listen To Your Body
I’ve said this  a million times, but it’s especially worth saying here. If you’re not feeling well during the dry fast, listen to your instinct rather than your agenda. (And don’t begin a dry fast when you’re ill. That should go without saying.) This is an optional tool. There are hundreds of other ways to serve your health and well-being. Don’t lose the forest through the trees because you’re drawn to a practice that feels more radical. Approach it smartly, but let your body’s intuition be the final arbiter.
That’s it for me. I haven’t done any dry fasting, not on purpose at least, and I’m not particularly interested in it for myself, but I am interested in your experiences. Do any of you do dry fasting? What have you noticed? What do you recommend?
As always, if you have any questions, direct them down below. Thanks for reading!
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