Reverse osmosis (RO) is a water purification technology that uses a semi-permeable membrane to remove dissolved solids and contaminants from water. This highly effective process results in water that is significantly demineralized, meaning it contains very few naturally occurring minerals. The low mineral content is the primary difference between RO water and standard tap or spring water, leading many users to question if this purity itself can cause digestive upset. The concern is whether drinking water stripped of its mineral content directly causes diarrhea or other acute gastrointestinal problems.
The Chemical Composition of Reverse Osmosis Water
Reverse osmosis systems are designed to push water through a dense membrane that blocks the passage of most dissolved substances. This filtration process dramatically reduces the level of Total Dissolved Solids (TDS), which is the measure of inorganic salts and organic matter present in water. RO filtration typically reduces this measurement by 90 to 99 percent, often leaving the final water with a TDS as low as 5 to 15 milligrams per liter (mg/L).
This process removes not only contaminants like heavy metals and salts but also naturally occurring minerals that the body uses, such as Calcium, Magnesium, and Potassium. Since carbon dioxide gas can pass through the membrane, the resulting demineralized water often has a slightly acidic pH, typically ranging between 6.0 and 6.5.
The World Health Organization (WHO) has noted that demineralized water lacks the minimum recommended concentrations of minerals, suggesting that drinking water should ideally contain a minimum of 100 mg/L of dissolved salts. Specifically, the WHO suggests minimum levels of 20 mg/L for Calcium and 10 mg/L for Magnesium. This lack of mineral contribution has prompted the inclusion of remineralization stages in many modern RO systems to restore a balanced mineral profile.
Direct Physiological Effects on the Digestive System
For a healthy individual, the consumption of demineralized RO water does not typically result in acute diarrhea. The body possesses robust mechanisms to regulate water and electrolyte balance, primarily through the kidneys and the digestive tract. The large intestine is extremely efficient at reabsorbing water and maintaining homeostasis.
When demineralized water enters the digestive system, the body’s internal environment, which is rich in electrolytes, must balance the low-solute water. This process involves the intestine adding electrolytes to the water before absorption, drawing them from the body’s reserves.
Some individuals may report minor, temporary digestive changes, such as slightly soft stools, especially when switching suddenly from mineral-rich water. This mild change is usually temporary as the body adjusts to the different water composition. Long-term, the low pH of RO water has been linked to potential gastrointestinal irritation, which may contribute to issues like chronic gastritis or acid reflux in sensitive individuals.
Contamination Risks and System Maintenance
The most frequent cause of genuine gastrointestinal distress, including diarrhea, experienced by reverse osmosis users is not the purity of the water but rather the microbial contamination of the system itself. RO systems, particularly the storage tank, tubing, and pre-filters, can become breeding grounds for bacteria if neglected. This microbial growth forms a sticky layer known as a biofilm.
If the pre-filters—which protect the main RO membrane—are not changed on schedule, bacteria can multiply and eventually contaminate the purified water stored in the tank. Pathogens from this contamination are the source of waterborne illnesses that cause true symptoms of diarrhea, stomach cramps, and fever.
Regular maintenance is essential to prevent biofilm formation and subsequent illness. This includes following the manufacturer’s schedule for replacing sediment and carbon pre-filters, typically every 6 to 12 months. Periodic sanitization of the entire system, including the storage tank and lines, is also necessary to eliminate any established biofilm.