Key takeaways
Reverse osmosis removes microbes, lead, salts and most PFAS very effectively, but it also strips out calcium and magnesium
WHO has raised concerns about long-term drinking of demineralised water, but evidence of harm in people eating a normal diet is weak
For athletes, minerals come mainly from food and electrolyte drinks, so RO water is fine if your diet is good and you replace sweat losses
Reverse osmosis (RO) systems have moved from labs and desalination plants to kitchen sinks. Some people swear by them for "pure" water. Others claim RO water leaches minerals from your body. Neither extreme matches the evidence.
What reverse osmosis does
An RO filter pushes water under pressure through a membrane with pores of roughly 0.0001 micron. Water passes through; most dissolved substances don't. Home units usually add a carbon pre-filter (to protect the membrane from chlorine) and often a post-filter for taste.
CDC's guide to household water treatment rates RO as very highly effective against protozoa, bacteria and viruses, and says it removes common chemical contaminants including sodium, chloride, copper, chromium and lead. It may also reduce arsenic, fluoride, nitrate, sulfate, calcium, magnesium and potassium. For PFAS, US EPA research finds RO and nanofiltration membranes are typically more than 90% effective.
| Removed well | Partly reduced | Notes | |
|---|---|---|---|
| Microbes | Protozoa, bacteria, viruses | Membrane damage or poor maintenance reduces this | |
| Metals and salts | Lead, copper, chromium, sodium, chloride | Arsenic (depends on form) | Check certification for each claim |
| Nitrate, fluoride | Yes | Varies by system | |
| PFAS | Typically over 90% | EPA lists RO among effective options | |
| Calcium, magnesium | Mostly removed | This is the trade-off | |
| Chlorine taste | Via carbon pre-filter | Not the membrane itself |
Do you need RO?
Most people on a safe, regulated supply do not. RO earns its keep when you have a known problem: lead plumbing you can't replace yet, PFAS above local limits, high nitrate in a well, or salty or brackish water. Check your local data first, as explained in our water quality guide. If a cheaper certified carbon filter handles your specific contaminant, it may be enough.
The WHO view on demineralised water
In 2005 WHO published Nutrients in drinking-water, based on an expert meeting about desalinated and demineralised water. One chapter, by František Kožíšek of the Czech National Institute of Public Health, reviewed the health risks of drinking demineralised water. Its conclusion was that unremineralised demineralised water is not ideal drinking water. The expert group also pointed to observational studies linking calcium and magnesium in water with lower heart disease death rates, and supported remineralising desalinated supplies.
WHO's later background document on hardness (2011) is more cautious. It says the protective link with magnesium or hardness "does not prove causality", proposes no minimum or maximum mineral levels, and still recommends that utilities replacing a supply with demineralised water consider adding calcium and magnesium back.
So the fair reading is:
- Remineralisation is sensible for public supplies built on desalination, and WHO supports it.
- Water can matter for people whose diet is marginal in calcium or magnesium.
- For most people, food supplies the bulk of these minerals, and there is no good evidence that drinking RO water harms someone eating a normal diet.
- Claims that RO water "leaches minerals from your bones" aren't supported by trials.
Infants are a special case: WHO-cited reports describe problems when infant drinks were made with very low-mineral water. Follow formula instructions and local health advice.
Remineralisation options
- Built-in mineral cartridges that add calcium and magnesium after the membrane.
- Mineral drops, which work but vary in dose.
- Simply eating well: dairy or fortified alternatives, leafy greens, nuts, seeds, legumes and whole grains.
None of this has been shown to improve health outcomes in people with a good diet, so treat it as taste and reassurance rather than medicine.
Cost and waste water
RO systems cost more up front and need membrane and filter changes. They also waste water. US EPA says a typical point-of-use RO system generates five gallons or more of reject water for each gallon treated, with inefficient units up to ten. WaterSense labelled systems must send 2.3 gallons or less down the drain per gallon. If you're buying one, look for NSF/ANSI 58 certification and a WaterSense label.
RO water for athletes
No study shows RO water changes VO2max or endurance performance compared with other safe water. What matters is drinking enough and replacing sodium when you sweat heavily. ACSM advises avoiding fluid losses above about 2% of body weight, and notes that electrolyte and carbohydrate drinks can beat water alone in some situations.
- Short or easy sessions such as zone 2 runs: plain RO water is fine.
- Long, hot or very sweaty sessions such as HYROX training or summer long runs: add sodium through a sports drink, electrolyte tabs or salty food. See heat training.
- Don't overdrink: drinking large volumes of low-mineral water during long efforts without sodium raises the risk of low blood sodium (hyponatraemia).
- Supplements for performance are a separate question, covered in our caffeine and nitrate guide.
The bigger levers for your heart are fitness and blood pressure; see VO2max and heart disease and estimate where you stand with the VO2max calculator. Wellness gadgets in general deserve a sceptical eye, as our look at red light therapy shows, while free habits like morning daylight are easy wins. If your home is damp, that's another environmental factor worth fixing; see mold exposure.
Frequently asked questions
Is reverse osmosis water bad for you?
Not for most people eating a varied diet. RO water is very low in minerals, and WHO recommends remineralising demineralised water used for public supply, but food provides most of your calcium and magnesium anyway.
Does reverse osmosis remove PFAS and lead?
Yes. US EPA research finds RO membranes are typically more than 90 percent effective for PFAS, and CDC lists lead among the contaminants RO removes. Choose a system certified to NSF/ANSI 58.
How much water does a reverse osmosis filter waste?
According to US EPA, a typical home RO unit sends five gallons or more down the drain for each gallon of treated water. WaterSense labelled systems must waste 2.3 gallons or less.
Should athletes drink RO water?
It is fine as long as you eat well and replace sodium in long or hot sessions. Do not rely on any water, filtered or not, for electrolytes during heavy sweating.
Sources
- World Health Organization. Nutrients in drinking-water. Geneva, 2005 (including Kožíšek F. Health risks from drinking demineralised water).
- World Health Organization. Hardness in drinking-water. Background document for the Guidelines for Drinking-water Quality, WHO/HSE/WSH/10.01/10/Rev/1, 2011.
- World Health Organization. Guidelines for drinking-water quality: fourth edition incorporating the first and second addenda. Geneva, 2022.
- CDC. A guide to drinking water treatment technologies for household use. cdc.gov.
- US EPA. Reducing PFAS in drinking water with treatment technologies. Science Matters, epa.gov.
- US EPA. Reducing PFAS in your drinking water with a home filter. Fact sheet, 2024.
- US EPA WaterSense. Point-of-use reverse osmosis systems. epa.gov/watersense.
- American College of Sports Medicine, Sawka MN, et al. Exercise and fluid replacement. Position stand. Med Sci Sports Exerc, 2007.