5 Minerals RO Water Purification Removes and Whether the Deficiency Actually Matters

What RO filtration actually removesReverse osmosis works by forcing water through a semi-permeable membrane with pores small enough to block dissolved salts, heavy metals, and bacteria. That same thoroughness catches minerals the body needs. The five that disappear in measurable quantities are calcium, magnesium, potassium, sodium, and fluoride. A 2019 review published in the journal Nutrients found that RO-treated water typically retains less than 10 mg/L of calcium and less than 5 mg/L of magnesium, compared to 40 to 80 mg/L and 10 to 30 mg/L respectively in untreated groundwater across most Indian cities. The membrane does not distinguish between arsenic and calcium. It removes both.
Calcium and magnesium: the two that matter mostOf the five, calcium and magnesium carry the most physiological weight. Calcium drives bone density, nerve signalling, and muscle contraction. Magnesium is a cofactor in over 300 enzymatic reactions, including those governing blood pressure regulation and blood sugar control. The World Health Organization's 2011 guidelines on drinking water quality flagged demineralised water, the category RO output falls into, as a potential contributor to cardiovascular risk, specifically because of low magnesium intake. A Czech study cited in that WHO report tracked populations switching to soft, demineralised water and found statistically higher rates of hypertension and ischaemic heart disease compared to populations drinking mineral-rich water. The mechanism proposed: magnesium in drinking water is absorbed at a higher rate than magnesium from food, so its removal has an outsized effect relative to its quantity.
Potassium, sodium, and fluoridePotassium loss through RO is real but rarely clinically significant. The average Indian diet, dal, sabzi, bananas, coconut water in the south, delivers potassium in quantities that dwarf what drinking water contributes. The same logic applies to sodium. Unless someone is on a medically restricted sodium diet and drinking eight or more glasses of RO water daily, the sodium removed by filtration is not the variable that matters. Fluoride is the complicated one. In areas where groundwater fluoride is dangerously high, parts of Rajasthan, Andhra Pradesh, and Telangana have recorded fluoride levels above 3 mg/L, well above the BIS safe limit of 1 mg/L, RO removal is protective. In cities where municipal water is already fluoridated to safe levels, RO strips out fluoride that was doing useful work for dental health. Whether fluoride removal is a benefit or a loss depends entirely on your source water.
How much do you actually get from water anyway?The WHO estimates that drinking water contributes roughly 5 to 20% of daily calcium and magnesium intake for most adults, the rest comes from food. For someone eating a balanced diet with dairy, green leafy vegetables, and legumes, the mineral loss from RO water is a rounding error. For someone whose diet is already low in these minerals, a pattern more common than assumed in urban India, where processed food consumption has risen sharply, RO water removes one more source from an already thin margin. A 2017 study in the Indian Journal of Community Medicine found magnesium deficiency in approximately 42% of urban adults sampled in Delhi, attributing it to both dietary shifts and increased consumption of processed foods. That number was collected before RO penetration in urban households reached its current scale.
Does remineralisation fix it?Several RO systems sold in India now include a remineralisation cartridge as a final stage, adding back calcium and magnesium after filtration. The mineral concentrations these cartridges deliver vary by brand and cartridge age, and no Indian regulatory body currently mandates testing or labelling for the mineral output of domestic RO units. If the cartridge is fresh, it helps. If it has not been replaced on schedule, and most household cartridges are not, it contributes little. The more reliable fix is simpler: eat food that contains calcium and magnesium. Ragi, rajma, spinach, and sesame seeds deliver more of both in a single serving than a litre of even well-mineralised water.
The real question RO water raises is not whether the filtration is harmful but what else in the diet is doing the work those minerals used to do. When the answer is a lot, the stripped water is a footnote. When the answer is not much, the stripped water is one more gap in a diet that was already running short.