What Is the Purest Water for a Water Fast?
Written by Dr. Nathan Gershfeld, D.C., founder of Fasting Escape and former staff doctor at True North Health Center.
Distilled water and reverse osmosis filtered water are the two purest options available for drinking water, and both are appropriate for water-only fasting. Every other common water source — tap, bottled, alkaline, filtered pitchers, and well water — falls short in purity and is not what I recommend during a fast.
Here is why purity matters more during a fast than at any other time, and how to choose the right water for your situation.
Why Water Purity Matters More During a Fast
When you are eating, your body processes a wide variety of inputs — food, beverages, environmental exposures — and handles impurities through its normal detoxification pathways. During a water-only fast, water becomes your only intake. There is nothing else coming in to dilute or buffer whatever is in that water.
This means that whatever is in your water is what your body is working with, exclusively, for the duration of the fast. Chlorine, fluoride, heavy metals, pharmaceutical residues, dissolved salts — during a fast, your body’s sensitivity to all of these increases. Some people notice that tap water tastes noticeably different during a fast than it does when they are eating normally. That sensitivity is real, not imagined.
For this reason, the standard I hold for water during a supervised fast is the same standard used in medical and laboratory research settings: water that is as close to pure H₂O as possible.
Distilled Water: The Gold Standard
Distillation is the most complete purification method available outside of a laboratory. The process works by boiling water and collecting the steam as it condenses. Since contaminants — minerals, chlorine, fluoride, heavy metals, volatile organic compounds — are either too heavy to vaporize or do not boil at the same temperature as water, they are left behind. What you collect is water that is as close to pure hydrogen and oxygen as you can get.
Distilled water is used in hospitals, research facilities, and clinical fasting programs precisely because its purity is not a matter of debate. It has been the water of choice at the True North Health Center, where the most rigorous water-only fasting research has been conducted.
If possible, I recommend distilled water for fasting — and honestly, for everyday drinking as well.
A note on plastic jugs: If you are purchasing distilled water rather than distilling it yourself, pay attention to the container. Polycarbonate plastics can contain bisphenol A (BPA), an endocrine disruptor that can leach into water, particularly when the container is exposed to heat. Choose BPA-free containers, keep them out of direct sunlight and warm environments, and do not reuse them. I have personally tasted plastic in certain brands — Ramon brand being one example — so the concern is not theoretical. Kroger brand, as one example, uses BPA-free containers.
The mineral leaching myth: A common objection to distilled water is that it leaches minerals from the body. This is not accurate. The body obtains minerals from food, not from water. During a fast, the body draws on its own nutrient reserves, and a properly supervised fast includes regular laboratory testing to monitor those reserves. If mineral levels drop to a point that warrants concern, the fast is modified or terminated. Distilled water does not accelerate that process.
Reverse Osmosis: An Excellent Alternative
Reverse osmosis works by forcing water through a semi-permeable membrane under high pressure. The membrane allows water molecules to pass through while blocking dissolved salts, heavy metals, minerals, bacteria, and viruses. A well-maintained reverse osmosis system removes up to 99% of contaminants, producing water that approaches distilled water in purity.
For people who prefer a permanent home filtration solution rather than purchasing distilled water or owning a distiller, an under-sink reverse osmosis system is an excellent investment. The water is consistently pure, readily available, and eliminates the need to transport heavy jugs.
The one caveat with reverse osmosis is maintenance. The filters need to be changed on schedule. If your RO water starts tasting off or unusual, check the filters before assuming there is a problem with the water source itself.
For water-only fasting, reverse osmosis water is fully appropriate and what I recommend when distilled water is not practical.
What I Do Not Recommend
Tap water: Municipal water treatment is designed to make water safe for the general population, not to produce clinical-grade purity. Treatment processes address bacteria, sediment, and some chemicals, but pharmaceutical residues, chloramine, fluoride, and disinfection byproducts are commonly present. The quality also varies significantly by location.
Bottled water: Regulated similarly to tap water in most countries, bottled water is essentially municipal water in a plastic container. It is not significantly purer than tap water and adds the plastic exposure concern on top of it.
Brita and pitcher filters: These address chlorine and some heavy metals but leave dissolved minerals, salts, and many organic contaminants in the water. They are a meaningful improvement over unfiltered tap water for everyday use but fall well short of what I recommend for fasting.
Alkaline water: The marketing around alkaline water is not supported by meaningful clinical evidence. The body has its own sophisticated buffering systems that regulate pH within a very narrow range regardless of what you drink. Adding alkaline minerals to water and calling it therapeutic is not consistent with how physiology works. Pure water, like distilled water, has a neutral pH — and that is exactly what I want during a fast.
Well water: Purity varies enormously depending on geography, local industry, and well construction. Without regular independent testing, you simply do not know what is in it. I do not recommend well water for fasting without verified testing results.
What I See Clinically
During fasting, taste perception becomes noticeably more acute. This is part of the taste bud reset that occurs during extended fasting — the same process that makes whole, natural foods taste remarkably good after a fast ends. That heightened sensitivity applies to water as well. People who never noticed anything unusual about their tap water often find it difficult to drink during a fast. The chlorine, the mineral taste, the slight chemical undertone — all of it becomes more noticeable when water is all you are consuming.
This is one of the practical reasons I specify distilled or reverse osmosis water for the people I supervise. It removes one unnecessary variable from an already demanding process.
The Practical Recommendation
If you are preparing for a water-only fast, here is what I suggest:
For the most convenient and cost-effective long-term solution, install an under-sink reverse osmosis system. It produces pure water on demand and pays for itself quickly compared to purchasing distilled water by the jug.
If you are starting a fast soon and do not have a filtration system yet, purchase BPA-free distilled water from a reputable grocery store brand, store it away from heat and sunlight, and do not reuse the containers.
Either way, do not fast on tap water, bottled water, or filtered pitcher water if you can avoid it. During a fast, water quality is not a minor detail — it is your entire nutritional environment.
Book your initial consultation here if you are considering a supervised water fast and want to discuss preparation in detail.
Frequently Asked Questions
Can I use a Brita filter during a water fast? I do not recommend it. Brita filters reduce chlorine and some heavy metals but leave many dissolved contaminants in the water. During a fast, when water is your only intake, purity matters more than usual. Distilled or reverse osmosis water is a better choice.
Is alkaline water good for fasting? There is no meaningful clinical evidence that alkaline water provides benefits during a fast. The body regulates its own pH through well-established buffering systems. I recommend pure water — distilled or reverse osmosis — rather than water with added minerals or alkaline substances.
Does distilled water remove minerals from my body? No. This is a common myth. The body gets its minerals from food, not from water. During a fast, mineral levels are monitored through regular lab work, and the fast is adjusted if needed. Distilled water does not accelerate mineral loss.
What is the best home filtration system for fasting? An under-sink reverse osmosis system is my top recommendation for home use. It produces water close to distilled quality, is cost-effective over time, and eliminates the need to purchase and transport jugs of water.
Can I drink well water during a fast? Only if you have recent independent lab results confirming its purity. Well water quality varies widely and is not guaranteed. Without verified testing, I would not recommend it for fasting.
How much water should I drink during a water fast? Generally, drink to thirst. Most people consume between two and three liters per day during a fast, but individual needs vary. Your supervising doctor will give you specific guidance based on your health status and how the fast is progressing.
See all the conditions we work with at Fasting Escape
Written by Dr. Nathan Gershfeld, D.C. Doctor of Chiropractic with 14 years of clinical experience in fasting and natural health. Dr. Gershfeld spent 6 years at the True North Health Center as an intern and staff doctor, then founded Fasting Escape, first as an in-person fasting clinic and now as a remote program bringing doctor-supervised fasting to people at home.