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Myths

Does RO water strip out minerals you need?

Separating the real science from the mineraliser add-on upsell.

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If you've stood in a purifier showroom, you've probably heard it: RO strips out the healthy minerals, said with a knowing pause, right before a mineraliser or 'alkaline' cartridge is sold as the fix. It's one of the few purifier claims that starts from something true — which is exactly what makes it work as a sales line, and exactly why it's worth pulling apart rather than dismissing outright.

Reverse osmosis membranes really do strip out most dissolved minerals — calcium and magnesium included — along with the nitrates, heavy metals and salts they're built to remove. That part isn't myth. What's oversold is what comes next: whether the stripped water actually harms you, and whether the cartridge bolted on to fix it does much nutritionally.

What reverse osmosis strips out

Reverse osmosis forces water through a membrane fine enough to block bacteria and heavy metals along with calcium, magnesium, sodium and the other dissolved ions that make up total dissolved solids (TDS) — the minerals and salts that give water its taste and body. Run without blending, a membrane routinely brings TDS down into the single or low double digits, close to what water-treatment researchers call demineralised.

This isn't disputed — it's membrane physics, not marketing spin. Most purifiers sold in India soften the effect with a TDS controller that lets a small stream of untreated source water bypass the membrane and blend back in, or with a mineral cartridge downstream. A unit with neither, fed low-TDS source water, can hand you water close to distilled.

India hasn't settled this internally either. In December 2021 the National Green Tribunal ordered RO purifiers banned wherever source water was already below 500 mg/L TDS, reasoning that stripping minerals from water that didn't need heavy treatment was wasteful. The Supreme Court stayed the order three months later after purifier makers appealed, and years on there's still no national rule — proof that how much mineral loss is acceptable remains genuinely contested.

What the WHO research shows

The most cited WHO document on this is a 2004 technical monograph by researcher František Kozisek, written as background for a revision of the WHO drinking-water guidelines. Its status matters: it's an expert background paper, explicitly marked as a draft not meant for citation, not a binding standard — a distinction most marketing that quotes it skips.

What it found, drawing mostly on animal experiments and some human studies from the former USSR, is real: rats on distilled or very low-TDS water for up to a year showed higher urine output, disrupted electrolytes and hormonal changes; human volunteers on low-TDS water showed roughly 20% higher urine output on average, with more sodium, potassium, calcium and magnesium lost in it. A few population studies also linked low-mineral water to higher cardiovascular risk, though the paper flags that several underlying studies are old and don't meet modern standards.

  • Cooking with low-mineral water pulls minerals out of food rather than adding any — reported losses run up to 60% for magnesium and calcium.
  • WHO's own 1996 guidelines evaluated calcium and magnesium but declined to set a minimum or maximum limit for either, citing insufficient data.
  • No country has adopted a binding minimum TDS standard on health grounds; the widely quoted 'minimum 100 mg/L TDS, 30 mg/L calcium' figures trace to an unpublished 1980 WHO-commissioned paper never formally adopted.

Where the real risk sits

The straightest reading: the mechanism is real — low-TDS water measurably increases mineral loss through urine and through food cooked in it — but proof that this causes disease in someone eating a normal diet doesn't exist. Water isn't most people's major source of calcium and magnesium to begin with; food is. That's a genuinely mixed verdict, not an all-clear or a warning.

Strip away the branding and the risk sorts into three groups.

  • Most people on a normal, varied Indian diet: negligible risk. Water is a minor mineral source next to food, and an adequate diet covers the shortfall several times over.
  • People whose diet already runs low on dairy, pulses and greens, or leans heavily on processed food: a bypass or mineraliser is a reasonable, low-cost hedge.
  • People with high fluid turnover in heat — labourers, athletes, anyone sweating for hours and rehydrating mainly with RO water. A litre of sweat carries roughly 1–1.5 g of sodium plus meaningful potassium, magnesium and calcium; replacing that fluid with near-zero-TDS water and nothing else is the scenario the research actually flags as risky, not the office-desk RO bottle.

What a mineraliser cartridge adds back

Mineraliser and 'alkaline' cartridges sit downstream of the RO membrane and work by dissolution: water flowing past chunks of calcium carbonate and magnesium oxide, sometimes with potassium chloride, picks up ions as it passes, raising both TDS and pH. Chemically, this is a legitimate way to put back some of what the membrane removed.

The gap is scale. A cartridge that lifts water to, say, 30–50 mg/L calcium and a lower 10–20 mg/L magnesium adds roughly 90–150 mg of calcium and 30–60 mg of magnesium across three litres a day — against ICMR-NIN's recommended 1,000 mg calcium and 370–440 mg magnesium for an adult. That's a real but modest top-up, a tenth to a fifth of a day's requirement, not a substitute for a glass of milk or a serving of greens.

Copper and alkaline add-ons

Copper cartridges trade on the old Indian practice of storing water overnight in copper vessels, and do add measurable ionic copper, which has real antimicrobial action against some bacteria given enough contact time. What doesn't hold up is the specific marketed language — boosted immunity, better digestion, glowing skin — none of it backed by controlled human trials. BIS caps copper in drinking water at 0.05 mg/L, because excess copper irritates the liver; it's not something to maximise.

Alkaline water's broader claims — better hydration, slowed ageing, disease prevention — rest on thinner ground still. Blood pH is tightly regulated by the kidneys and lungs regardless of what you drink, so alkaline water doesn't meaningfully change the body's pH. A few small studies show modest short-term differences in things like blood viscosity, and some cellular-ageing markers have improved in mice, but neither has produced a human trial supporting the claims as sold.

The bottom line

RO does strip minerals — that part of the pitch is true. Whether it matters to you depends far more on your diet and how much you sweat than on which cartridge is bolted onto your purifier. A mineraliser adds real minerals in modest amounts; a copper or alkaline add-on is mostly ordinary chemistry doing something plausible, dressed in claims that outrun the evidence. If your meals already cover calcium and magnesium, the cartridge is a nice-to-have. If they don't — or your days involve serious sweat — the fix is a better diet or electrolytes, not a fancier cartridge.

Common questions

Does drinking RO water cause mineral deficiency?

Not on its own, for someone eating a reasonably varied diet — food supplies the large majority of daily calcium and magnesium, and water is a minor contributor even when fully mineralised. The real risk sits with people whose diet is already low in these minerals, or who drink large volumes of near-zero-TDS water while losing minerals another way, such as heavy sweating.

Should I cook with RO water or tap water?

Use whichever water is safe to cook with, not necessarily the lowest-TDS one — low-mineral water pulls calcium and magnesium out of food as it cooks rather than adding any. If your tap or borewell water is otherwise safe, or boiled, it's a fine choice for the pot; save the RO output for drinking, where the concern is contaminants, not minerals.

Why does RO water taste flat compared to tap or bottled water?

Taste comes largely from dissolved minerals and salts — TDS — which give water body and a faint sweetness or tang. Strip TDS down toward zero and water tastes noticeably flatter, even though it's perfectly safe; it's part of why most purifiers blend some source water back in or add a mineraliser, for taste as much as nutrition.

Is it cheaper to add a pinch of salt or an electrolyte sachet than buy a mineraliser cartridge?

For sweat-driven electrolyte loss, yes — an oral rehydration or electrolyte sachet replaces sodium and potassium far more precisely and cheaply than any cartridge. Mineraliser cartridges are really about restoring calcium and magnesium and improving taste, not replacing electrolytes lost to heavy exertion, so the two solve different problems rather than competing.

Does bottled 'mineral water' avoid this problem entirely?

Only if it genuinely retains natural minerals rather than being purified and then artificially topped up, which some brands do without much disclosure. Checking the label's TDS and mineral content tells you more than the word 'mineral' on the bottle — some bottled water is barely different from RO output.

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