Explainer · Water quality

Low-salt water does not harm your health

After more than 20 years of professional work in Mexico's drinking-water field, we answer a question surrounded by myths.

The basics

What is the difference between dissolved salts and dissolved minerals in drinking water?

Both terms are synonyms. They are inorganic compounds. Most of them dissociate when dissolved in water; those that dissociate are called electrolytes. To demineralize water is the same as to reduce the concentration of dissolved salts in it.

Why do most people who study chemistry-related fields believe that drinking demineralized water is harmful?

Students in those fields have always been warned not to drink the demineralized water used in the laboratory. They have been given various reasons not to do so: that it causes diarrhea; that it robs the body of minerals; that it causes acidity, cell swelling…

Considering that demineralized water for laboratory use has been considerably more expensive than drinking water, the warning has paid off.

The history of bottled water

When did low-salt bottled waters appear massively on the market?

For decades, various brands of bottled water were sold, from very diverse sources: a natural surface water body —a river or lake—, a shallow well, or a deep one. The mineral content varied widely; if the water came from rain or snowmelt, the content was very low. However, its market was not large and the topic did not draw much attention.

In the 1990s, Coca, Pepsi and other large companies, such as Grupo Danone, began selling bottled water whose mineral content was low. It was at this moment that the topic gained importance.

Why did these international companies offer low-salt bottled water?

Being international companies, it was important to standardize the taste. It is not true that water is tasteless: two perfectly potable waters can differ enormously in flavor, and that depends on the type and concentration of dissolved salts in each one. These companies achieved taste standardization by reducing the dissolved-salt content. We do not know whether there was another reason, but the one mentioned is very important.

What technology did these companies use to reduce the dissolved-salt concentration?

They used reverse osmosis, a process that began to be applied in the 1970s and had been substantially improved in recent years.

What salt concentration did the bottled water they offered have?

The concentration of dissolved salts in water is measured with a parameter called total dissolved solids (TDS). The first brands these companies offered in the Mexican market had a TDS of 70 mg/L. A reverse-osmosis unit produces treated water with a TDS of around 10 mg/L; therefore, to obtain 70 mg/L it was necessary to blend RO-treated water with "raw water" —untreated— in the right proportions.

What did the smaller bottlers in Mexico do?

Very soon, the smaller water bottlers followed the same steps as the large ones: they began to treat the water with reverse osmosis. They did so because the public began to associate the taste of low-salt waters with the idea that they were potable. This is mistaken, since potable water can contain very diverse TDS concentrations, but it was a reality that was hard to resist.

Why did many companies reduce the salt content even further?

Some of the small companies, out of ignorance, bottled the water straight from the osmosis unit, with a TDS of between 5 and 10 mg/L. If low-salt water had been harmful, many of their customers would have suffered anything from discomfort to health damage, but that did not happen. In fact, some of the international companies began to sell their bottled water with equally low TDS.

Given that it is not harmful, can low-salt water offer any health advantage?

Just as some have claimed that low-salt waters are harmful, others have claimed they are a source of benefits —for example, that they hydrate better or help detoxify—. This is probably also untrue.

The only advantage of fully RO-treated water is that it greatly reduces the probability that compounds or microorganisms capable of causing health damage are present, because they do not permeate reverse-osmosis membranes. A company that does not osmosis-treat all of its water —because it blends raw water with treated water— can produce perfectly potable water, but it will face a greater challenge in guaranteeing the absence of contaminants in the raw water.

What does the evidence say?

Why have so many online publications appeared warning about the dangers of low-salt water?

The internet is not a platform that makes it easy or guarantees finding valid scientific knowledge. Anyone publishes anything, and this turns it into a battlefield of claims whose criteria are not always scientific or ethical. To access knowledge, one must turn to the right sources: one of them is scientific publications; another is the analysis of facts through common sense or reasonableness.

What do scientific publications and health authorities say?

There is no shortage of scientific publications on the health effects of low-salt drinking water, but there has been no conclusive evidence for or against this type of water. Both the scientific and medical communities agree on the importance of hydration and of water purity, but there is no consensus on the benefits or harms that low-salt water might cause.

The World Health Organization (WHO) has noted the importance of minerals in nutrition, but states that they need not come from the water one drinks. Neither the WHO nor the health authorities of the most developed countries —such as the U.S. FDA— have issued alerts or bans regarding the consumption of low-salt water. In fact, drinking-water standards do not set minimum limits for TDS. Many brands of purifiers —from well-known companies— are currently sold that are installed in kitchens or dispensers and that osmosis-treat all the water they deliver.

What can common sense say about low-salt water as a beverage?

An indirect but weighty argument is that many human groups drink water that comes exclusively from rain or snowmelt. The TDS content of these sources is the same as, or even lower than, that of RO-treated water, and these groups show no lower life expectancy nor any morbidity or mortality associated with the mineral content of the water they drink.

Another argument that reinforces the previous one is that the low-salt water sold in bottles on a massive scale for more than two decades has not been the cause of any recorded harm. Companies like Coca and Pepsi have more than enough resources to research what they offer and to avoid putting their business at risk.

Some will give no value to this claim; they may point out that the soft drinks from these companies have caused health damage. However, this cannot be considered a definitive judgment: the harm caused by soft drinks lies in the amount each individual consumes, and refined sugar cannot be blamed either, since from a logistical standpoint it has been essential to urban life in large cities.

The body’s physiology

What happens from the standpoint of cellular water-balance physiology?

Cell membranes are permeable to water. Osmotic forces drive water into or out of cells to maintain osmotic balance. Although the composition of the intracellular fluid is very different from that of the extracellular fluid, both have the same osmolality —that is, the same total solute concentration—, of around 9 grams per liter. When a solution has this concentration it is called isotonic; if higher, hypertonic, and if lower, hypotonic.

If the extracellular fluid were very hypotonic, the amount of water entering the cell would be very large and would damage it. However, we must remember that drinking water is practically as hypotonic as fully demineralized water.

Can low-salt water provide fewer minerals than the body requires?

As long as the diet is adequate, communities that drink very low-salt waters have shown no deficiency in any particular mineral. An analysis of the mineral content of foods shows that, with access to an adequate diet, they provide a substantially greater amount than water can. On the other hand, without access to a good diet, it is impossible to make up a mineral deficiency with what water can provide.

Demineralized water absorbs CO₂ from the air and becomes acidic. Can that acidity harm health?

Demineralized water is eager to dissolve compounds; because of that eagerness it absorbs carbon dioxide (CO₂) from the air, which forms carbonic acid (H₂CO₃) and acidifies the water. Completely pure water has a pH of 7.0; upon dissolving CO₂ from the air, the pH can drop to values near 5.5. This value corresponds to a weak acid and bears no comparison to the acidity of the stomach, rich in hydrochloric acid (HCl), which produces a pH between 1.0 and 3.0.