The question of whether drinking purified water can cause dehydration is a common concern for many people. This worry stems from the idea that highly filtered water, stripped of its natural mineral content, might “leach” essential minerals from the body’s cells. Addressing this belief requires understanding the physical properties of purified water and how the body manages the delicate balance of water and electrolytes.
Defining Purified Water and Its Characteristics
Purified water is defined by its extremely low concentration of dissolved substances, measured as Total Dissolved Solids (TDS). To earn the “purified” label, water must contain less than 10 parts per million (ppm) of TDS, a significantly lower threshold than most tap or natural spring waters. This level is achieved through intensive filtration processes that remove nearly all types of contaminants, including chemical pollutants and microorganisms.
The most common methods used are reverse osmosis (RO) and distillation. RO forces water through a semipermeable membrane, removing between 92% and 99% of dissolved inorganic minerals and salts. Distillation works by boiling the water into steam and then condensing it back into a liquid, leaving virtually all dissolved solids behind. This leaves a product that is almost pure H2O but lacks trace minerals like calcium, magnesium, and potassium naturally present in other water sources.
The Mechanics of Osmosis and Hydration
Hydration at the cellular level is governed by osmosis. This is the movement of water across a semipermeable membrane, like a cell wall, from an area of lower solute concentration to an area of higher solute concentration. The goal is to equalize the concentration of dissolved particles, or solutes, on both sides of the membrane. These solutes are primarily electrolytes—charged minerals like sodium, potassium, chloride, and calcium—dissolved inside and outside of cells.
When you drink water, it is absorbed into the bloodstream, becoming part of the body’s extracellular fluid. The concentration of solutes in this fluid determines the direction of water movement into or out of your cells. If the extracellular fluid has a higher concentration of solutes than the fluid inside the cells, water moves out of the cells, causing them to shrink. Conversely, if the extracellular fluid has a lower concentration of solutes, water moves into the cells to dilute the contents, which is the process of hydration.
The body has robust mechanisms to maintain a stable, balanced internal environment, known as homeostasis. A primary regulator of this balance is the kidney, which constantly monitors the concentrations of water and electrolytes in the blood. The kidneys adjust the amount of water and minerals excreted in urine to keep the body’s fluids within a very tight range. This homeostatic control acts as a buffer against minor fluctuations caused by the water a person drinks.
The Actual Health Impact of Drinking Purified Water
The notion that purified water causes immediate, acute dehydration is largely unfounded for a person with normal kidney function and a balanced diet. While purified water is hypotonic—meaning it has a lower solute concentration than the body’s fluids—the volume consumed during a typical day is processed quickly by the kidneys. The body’s homeostatic mechanisms are highly effective at regulating the osmotic balance, mitigating any immediate, dramatic shift in cellular fluid levels from a glass of purified water.
However, the long-term consumption of purified water as a person’s exclusive source of hydration introduces a different, more relevant concern: mineral deficiency. Because this water lacks its own mineral content, it contributes no electrolytes to the body’s reserves. If a person’s diet is already deficient in minerals like calcium and magnesium, relying solely on purified water can exacerbate this deficiency over time. The World Health Organization has noted that continuous consumption of water low in minerals may lead to a lower intake of essential elements.
In extreme or prolonged cases, consuming large volumes of low-mineral water can dilute the body’s electrolytes, which may lead to symptoms such as tiredness, headache, and weakness. This condition is an issue of electrolyte imbalance rather than simple dehydration. To mitigate this long-term risk, a person can ensure their diet is rich in mineral-containing foods or consider mineral supplementation. Alternating purified water with a mineral-rich source, or using remineralization drops, supports the body’s mineral balance while providing contaminant-free water.