Houston, I Have a Problem, and It Is in My Glass

Why weeks of heat can dismantle a perfectly healthy body, how nearly pure water quietly becomes an accomplice, and why the expensive mineral water from the supermarket is watched less closely than the water from your own tap.

A healthy man, a hot summer, a stalling engine

After a few days of that heat, something was off. Wherever I was, indoors or out, morning or night, I felt dull and drained, as if I were standing half a step beside myself. My head worked through cotton wool. My calves twinged for no reason. And I am not the candidate where any of this makes sense. I am 56, I do not touch a drop of alcohol, I take no drugs, I eat sensibly, I move every day. Careless with my body I am certainly not, quite the opposite. I have supplemented testosterone for close to 20 years, I take a magnesium complex and a whole range of other good things, because I give my body what everyday life withholds from it. A man who tops himself up that meticulously is the last one you would suspect of a plain mineral deficiency. A body kept that way has no reason to quit in the middle of summer.

This one mistake I genuinely did not have on my radar, and I admit it openly. Of all things, it was sodium, the simplest electrolyte there is, that slipped past me while I watched testosterone and magnesium down to the milligram. My first reflex was the wrong one, accordingly. I thought of sleep, of circulation, of the weather, of all the vague suspects we like to blame our own fatigue on. But anyone who knows how I think knows that I pull the cause out of every problem, my own body included. Then the obvious answer occurred to me, and it was almost embarrassing in its simplicity. For years I have been drinking almost nothing but filtered water. 2 to 3 liters a day. And across 2 weeks in which I sweated around the clock, I had been taking something away from my body without ever putting it back. No exotic illness. No mineral deficiency out of a tropical medicine textbook. Just the most banal piece of arithmetic in the world, which I had failed to do for 2 weeks straight.

This piece is for everyone who does the same. For the many who drink only filtered or very pure water, replace nothing, neither salt nor minerals, and then wonder why the heat drains them. It is also for those who mistake their expensive supermarket water for the gold standard. Both groups carry the same error of reasoning, just at opposite ends.

Pure sounds good, but it means empty

Let us start with the filtered water, because that is exactly the mistake I made myself. Filtered water is not an enemy in itself. A good reverse osmosis unit or a decent filter pulls out of the water everything we all want gone, residues, heavy metals, the assorted junk that lurks in soils and pipes. That is a real gain. Only the same process also strips out what the body needs, and nobody selling you such a unit ever mentions that part.

The World Health Organization published a strikingly blunt paper on this. Frantisek Kozisek compiled the risks of demineralized water for the WHO in 2005, and the core message is uncomfortable for the purity crowd: very low-mineral water does not count as ideal drinking water, and anyone who drinks it long term may get too little of what a good water actually delivers. Magnesium and calcium above all. There are even recommendations that drinking water should contain at least 20 milligrams of calcium and 10 milligrams of magnesium per liter. Pure water sits at nearly zero.

This is not gray theory. In the Czech Republic and Slovakia, around the years 2000 to 2002, many households began treating their tap water at home by reverse osmosis. Within weeks to months, complaints surfaced that looked suspiciously like acute magnesium deficiency, cardiovascular disturbances, fatigue, weakness, muscle cramps. Read that list again and hold it next to my 2 weeks. Dull, drained, twinging calves. I had reenacted the textbook on my own body without noticing.

Magnesium is the secret lead here. It sits in countless enzymatic reactions and steadies the heart rhythm. Above all it keeps the muscles quiet, which is why a shortage so readily announces itself as a calf cramp and a fluttering heart. A good mineral water delivers a noticeable amount of it, a pure water almost none. Anyone who switches to low-mineral water over the years, and on top of that, like most of us, does not exactly eat lavishly magnesium-rich meals, slowly draws down his reserves without a single warning light coming on. Right up to the day the heat calls in the last reserve.

And now the part that stings, if you are honest. Pure water even leaches out what is already there. Cook vegetables in low-mineral water and the minerals migrate from the vegetables into the water, obeying the osmotic gradient, and end up down the drain instead of inside you. The Otto Sapiens, that variant of Homo Sapiens who believes he knows everything because he once listened to an audiobook about it, carries his reverse osmosis unit through the kitchen like a holy relic and proudly announces that he now drinks only the purest of the pure. So he has, under certain circumstances, lost twice over. He takes in barely any minerals, and he rinses the leftovers out of his food along the way. Then he stands there in high summer with cramping calves and blames the heat.

The tipping point is in your sweat

Pure water alone does not make a healthy person sick right away. The body is a tough system, it regulates a great deal away. The tipping point only arrives when a second factor joins in, and this summer supplied it. Heat.

Sweating is first and foremost a loss of table salt, not of pure water. Sweat carries sodium out of the body at concentrations of roughly 10 to 90 millimol per liter, varying widely from person to person, while it is far more frugal with potassium at 2 to 10 millimol per liter. On a 40-degree day when you sweat by the liter, you therefore lose sodium above all, in amounts that add up to several grams over the hours. If you then refill that loss with pure water, you do exactly the wrong thing. You give back volume, but no sodium. The concentration in your blood keeps falling. You quench the thirst and dilute yourself at the same time.

The body fights back, and it does so with an elegant emergency program. Under heat and sodium loss, the adrenal gland releases more aldosterone, a hormone that orders the kidney to hold on to sodium. Sounds like rescue. Only the same measure comes at a price, because aldosterone in turn drives up the excretion of potassium. So anyone who pushes in a lot of sodium during heat to plug the one hole tears open a second. The balance is a mobile, and you cannot pull on one thread without setting everything else swinging.

This is precisely why a long stretch of heat is more dangerous than a single hot day. On one afternoon you lose sodium and win it back at dinner, and the balance evens out overnight. But when the record days drag on across 2 weeks, without a break, without a cool night, you give back a little less each day than you sweat out, and the deficit grows slowly, without any single moment being dramatic. It is a creeping minus, not a bang. And creeping deficits are the treacherous ones, because no single day ever looks like the culprit.

What too little sodium really does to the body

Now it gets serious, and now we go deep. Sodium is not just any mineral. It is the most important ion outside the cells, the substance on which practically all nerve conduction and muscle contraction hangs. Every nerve impulse, every action potential, every heartbeat rests on sodium being concentrated high outside and low inside, and shooting through the cell membrane in split seconds. Sodium also governs how much water stays in the bloodstream, and with it the blood pressure and the volume. Without the sodium gradient the whole shop stands still.

When the sodium level in the blood drops below roughly 135 millimol per liter, medicine speaks of hyponatremia. So that the number means something: the healthy range lies between 135 and 145, below 135 the deficiency begins, and below 125 it turns serious. This is the most common electrolyte disorder among hospital patients of all, and it is more widespread than most people suspect. Around 5 percent of all adults sit below the mark, among the over-65s it is already 20 percent, among hospital patients up to 30. This is no fringe phenomenon for extreme athletes, it sits in the middle of the population.

Why does it strike the elderly so much more often? Because 2 protective systems fade with the years. The one is the sense of thirst, which grows blunter with age, so that some people simply drink too little and then derail all the faster in the heat. The other is the fine control through the antidiuretic hormone, also called vasopressin, by which the body steers how much water the kidney holds back. When that system falls out of step, through medication or a weakening kidney, the body keeps water it ought to excrete, and dilutes itself. In a 30-year-old the organism still cushions this. In a 75-year-old who eats too little at 38 degrees and dutifully drinks his pure water instead, the balance often tips within a few days.

The mechanism behind it is as plain as it is brutal. When sodium in the blood falls, the concentration of dissolved particles outside the cells drops, the osmolality. Water always follows toward where more is dissolved, so it streams out of the diluted blood and into the cells. Everywhere in the body this is only half a problem, because the cells can give way. Only in one place they cannot, inside the skull. The brain sits in a rigid bony capsule, it cannot swell without pressing against its own wall. The astrocytes, those support cells that mediate between blood and nerve tissue, soak themselves full, and the brain begins to swell. This is where the first symptoms come from, headache, nausea, confusion, loss of concentration, in severe cases seizures and coma.

The brain is not defenseless, though, and here lies a twist worth knowing. It adapts. Within hours it flushes electrolytes out of its cells, over days then also organic substances like myo-inositol, taurine and glutamine, to bring its own concentration back in line with the diluted surroundings. The swelling shrinks, the symptoms ease, the person feels roughly normal again. You might think the organ has cleverly saved itself. It has saved itself, yes, but in doing so it has maneuvered into a second trap. Because now the brain sits at an artificially low salt level it has grown used to. Whoever now pulls the sodium back up too fast, say through rushed infusions in the emergency room, reverses the osmotic pull. Water shoots abruptly out of the brain cells, they shrink, and in the pons and neighboring regions the myelin sheath of the nerve fibers collapses. This osmotic demyelination, once called central pontine myelinolysis, is a nightmare. It shows up days later with slurred speech, difficulty swallowing, movement disorders, parkinsonian rigidity, in the worst case with paralysis of the whole body or death. This is why clinicians correct chronic hyponatremia in tiny steps, no more than 10 millimol per day, in high-risk patients rather 4 to 6. The slow drop is dangerous, the fast retrieval is just as dangerous. A body once knocked out of balance forgives no haste in the other direction.

The real reason I am writing this, though, is not the dramatic full catastrophe. That lands in the clinic and gets treated. The quiet version is what is dangerous. Because chronic hyponatremia, even the mild kind, even the seemingly symptom-free kind, is precisely not harmless, as was long assumed. It costs attention, it makes you dizzy and tired, and it throws off your gait. It raises the tendency to fall, it eats at muscle mass, and over time it even attacks the bones, with a measurably higher risk of osteoporosis and fractures. This is exactly the zone most of us move through without knowing it. Not in a coma, but in the gray area of a dull head, powerless legs and a concentration that no longer grips. People put it down to stress, to poor sleep, to the weather. When the cause is sitting in the glass.

You can even actively drink the problem into being, and that is the bitter irony of the especially health-conscious. Someone who has heard that drinking a lot is healthy, and pours liters of pure water into himself as a result, entirely without salt, drives his sodium down not only passively through sweating but dilutes it further from the inside. In extreme cases, in endurance athletes who drink only plain water during a long race, this ends as full-blown water intoxication with brain swelling. It need never go that far in daily life for anyone. But the principle is the same, only in slow motion. Too much pure water plus too little salt produces, over days, exactly the gray area I was stuck in for 2 weeks.

The answer is salt and tomato, not abstinence

As complicated as the physiology is, the remedy is just as plain, and that is the good news. You need no powders, no colorful sports drinks, no subscription box of electrolyte sachets. You need salt, and you need potassium from real food.

The sodium you win back through ordinary table salt, and on a sweaty day the pinch may be considerably more generous than the diet apostle allows. Half a gram to a gram of salt per liter of water is no cardinal sin when you sweat heavily, it merely balances out what went through your skin. Emphasize the word balance. You fill a hole, you do not dig one.

The potassium, in turn, does not come from the water, where there is almost none of it, but from your food. The daily requirement lies around 3500 to 4000 milligrams, and on hot days rather at the upper end, because the aldosterone-driven kidney pulls it up. The most elegant solution is already sitting on plenty of plates anyway. A tomato with a little salt is nearly the perfect summer electrolyte, sodium from the salt, potassium and water from the fruit, plus a bit of magnesium. A banana, a handful of potatoes, an avocado do the same. No marketing, no sticker with the word electrolyte on it, just foods that existed long before the wellness aisle was invented.

Hands off the colorful health salts

And now the warning I fell for myself, before I knew better. On the shelf stand the potassium-based salts, sold as diet salt, as heart salt, as the healthier salt, often in friendly yellow or green, with a label that looks like a clean conscience. These salts replace part of the table salt with potassium chloride. And that is exactly what makes them the wrong tool for our purpose, for 2 reasons, of which the second is dangerous.

First, they deliver potassium, not sodium. Anyone who wants to replace the sweated-out sodium during heat grabs at nothing with them and on top pours in still more of the very mineral the kidney is already excreting in excess. It is like filling the wrong tank.

Second, and this is the point where a harmless error becomes a risk. Potassium in the blood is tightly regulated, because it hangs directly on the heart rhythm. A healthy person with healthy kidneys clears an excess without trouble. But someone with reduced kidney function, and many know nothing of their early kidney weakness, because the creatinine still looks normal, can no longer get rid of potassium cleanly. On top of that come medications that brake exactly this excretion, ACE inhibitors, angiotensin blockers, potassium-sparing diuretics, plus the painkillers from the NSAID group. In this constellation a supposedly healthy salt can trigger hyperkalemia, a dangerously high potassium level. And that one is treacherous, because at first it barely makes symptoms, then muscle weakness and tingling, and in the worst case cardiac arrhythmias up to cardiac arrest. This is precisely why the British health authority, for one, expressly advises that older people, diabetics, pregnant women, kidney patients and people on the medications named above should not reach for potassium-based salt.

The perfidious part is how many people belong to this risk group without knowing it. An emerging kidney weakness makes no complaints for a long time, the creatinine in the blood panel can still sit in the green while the actual filtering performance has already declined. Someone who at 60 has slightly raised blood pressure and takes a common drug for it, who now and then swallows a painkiller on the side, may already be collecting several risk factors without ever having heard of them. For exactly this person the friendly yellow health salt is no harmless alternative. It is sold to him as the better choice and is in truth the one ingredient he can least control.

So that no false impression arises here, because I am not manufacturing fear. For the healthy person with healthy kidneys these salts are not poisonous, on the contrary, the large Chinese salt-substitute trial even showed that a mix of 75 percent table salt and 25 percent potassium chloride can lower blood pressure and stroke rates. The point is a different one. As a remedy against sodium loss in high summer they are useless, and they can seriously harm the wrong people. Whoever uses them should know what he is doing, and not trust the yellow label just because it looks so healthy.

The real cheek is on the label you never read

Let us stay at the shelf, because that is where the second message of this piece waits, and it surprised me almost more than the sodium business did. Anyone who has had enough of his filtered water and reaches for the expensive mineral water, in the belief that this is now the safe and monitored option, is mistaken. In Germany, bottled mineral water falls under a different set of rules than tap water, and the stricter of the two is, of all things, the one from the tap.

Tap water falls under the drinking water ordinance, mineral water under the mineral and table water ordinance. For tap water, limits are set for around 30 substances, for mineral water only for around 17. And in a direct comparison the bar gapes wide apart. For uranium, tap water may contain at most 10 micrograms per liter, while for ordinary mineral water there is no legal uranium limit at all. For boron, the limit for tap water is 1 milligram per liter, for mineral water 5.5, more than five times as much. For fluoride it stands at 1.5 against 5.0 milligrams. And for pesticides, where tap water must hold a sharp value of 0.1 micrograms per active substance, there is simply no limit for mineral water. On top of that comes the difference in monitoring. Tap water is tested shortly before it reaches the consumer, a large utility measures several times a day. Mineral water is tested at the source, not in the bottle that stands on the shelf months later.

You have to let that dissolve on your tongue. The water I buy for good money in glass bottles and drink with the feeling of doing myself something good is watched more loosely than the water I flush the toilet bowl with in the morning. I sit there with my popcorn in the front row and watch this inversion of logic go about its work.

There is a trick almost nobody knows, and it exploits exactly this gap. Water labeled suitable for the preparation of infant food must meet distinctly stricter limits, among others for arsenic, uranium and nitrate. The lawmaker, you see, trusts the infant with something he denies the adult, real purity. So anyone who wants to drink low in contaminants, without wrestling with lab values himself, reaches for the baby water. It is a small absurdity that a 56-year-old reaches for the water meant for a newborn in the supermarket, because it is the only one the state actually keeps its fingers on. But it works.

And since we are on the subject of price, a word on the belief that expensive equals good. With water this holds even less than anywhere else. There are luxury waters that go across the counter for around 219 dollars a liter, ennobled with Swarovski crystals, and of all things this water is treated by reverse osmosis, meaning demineralized. You pay a week’s wages for empty water in a glittering bottle. Other brands ask more than 1000 dollars a liter, a designer edition of a French brand cost around 100 dollars a bottle, and the auction record belongs to a water in a bottle made of 24-karat gold. Blind tastings regularly show that these noble waters do not even taste better. Expensive means, with water, neither safer nor richer in minerals, and often not even better-tasting. It usually means only that you pay for the bottle and get the water thrown in for free, minus, unfortunately, the best part of it.

America does not do it better, only with other agencies

Anyone who thinks this is a German peculiarity, I am glad to disappoint. In the USA the situation is structurally the same, only the acronyms change. There the environmental agency EPA watches over the tap water, backed by the Safe Drinking Water Act, and this act regulates more than 90 contaminants and obliges every utility to an annual public water report. Bottled water, by contrast, falls under the food agency FDA, treated as a packaged food, and the Safe Drinking Water Act expressly does not apply to bottled water.

The consequences are well documented, and they are unpleasant. The Government Accountability Office, the audit arm of the United States, concluded as far back as 2009 that the FDA protection for bottled water is often less stringent than the EPA protection for tap water. Between 2002 and 2008, bottled water was recalled 23 times, mostly over excess levels of arsenic and bromate. The testing intervals speak a language of their own. Bottled water need only be tested once a week for coliform bacteria, municipal tap water 100 times a month or more. Bottled water producers do not even have to use certified labs, and need not report their results to the FDA, even when a value breaks the standard. And around a quarter of American bottled water is nothing other than bottled tap water anyway, some of it treated further, some of it not.

For fairness, there is one exception in bottled water’s favor, and it concerns lead. Because many old houses still have lead pipes, the EPA limit for lead in tap water is less strict than the FDA value for bottled water. On lead, the bottled water pulls ahead. In almost everything else the picture stays the same, on both sides of the Atlantic. The water we distrust the least is monitored the least.

A tool that does the work for you

You do not have to memorize all this, and you do not have to pore over ordinance tables either. We live in 2026, and there is a tool that tells you in 2 minutes whether your water of choice is any good. Take the name of your mineral water, type in the values from the label, and have them checked. Which language model you use for it is secondary. Any reasonably intelligent large language model, whatever its origin, should be able to solve this task, provided it can research on the internet and pull up current values. It is not about the brand, it is about you asking the question at all.

But now comes the warning, and it weighs more than the tool itself. A language model is only ever as smart as the question you put to it. A vague, badly built prompt delivers not knowledge but half-knowledge, and half-knowledge is more dangerous than none, because it feels like certainty and is wrong all the same. Whoever simply asks a model whether this water is healthy gets an agreeable answer without substance and afterward mistakes it for verified. This is exactly why I give you the prompt ready-made, word for word, instead of leaving you to guess. Just copy it:

“I live in the USA and drink 2 to 3 liters of the mineral water [ENTER NAME] every day. Please research the current analytical values of this source online, in particular sodium, magnesium, calcium, bicarbonate, as well as arsenic, uranium, boron, nitrate and fluoride. Compare these values with the EPA standards for tap water and note whether the product stood out in any recent test by a consumer organization. Tell me, first, whether the water supplies enough sodium and magnesium for an adult who sweats a lot in the heat, and second, whether it is conspicuously loaded with any of the contaminants. Cite your sources and give the date of the analysis.”

Whoever lives in Germany swaps the EPA standards for the drinking water ordinance and the mineral and table water ordinance. 2 minutes, one prompt, and you know more about your water than the cashier who drags it across the scanner.

Whoever drinks only pure water and wonders has built it himself

Now the uncomfortable part, and I mean it as coldly as it sounds. If you spend weeks in record heat drinking almost nothing but pure or filtered water, replace nothing, neither salt nor minerals, and then wonder about fatigue, head pressure and calf cramps, then you have built this state yourself, with some consistency. That is no accusation, it is a description. The body keeps mercilessly honest books. It takes what you give it, and it loses what you fail to replace. It knows no good intentions, it knows only the balance sheet.

And here lies the real mistake, the one bigger than salt and water. We have unlearned how to listen to our own bodies, and listen to the label instead. Pure sounds healthy, so we drink pure water. Expensive sounds safe, so we buy the priciest bottle. Healthy is printed on the yellow pack, so we sprinkle the potassium salt. In all 3 cases we replace physiology with marketing and delegate a judgment only our own metabolism can make to an advertising department. The body takes no part in this vote. It keeps calculating stubbornly with what actually arrives, no matter what the front of the bottle says.

The good news in it is that this very honesty also supplies the solution. What you built yourself, you can dismantle again. A pinch more salt on sweaty days, a tomato alongside, a mineral water that earns the name, and when in doubt the reach for the baby water instead of the priciest bottle on the shelf. It takes no more than that. That is the whole science, boiled down to a shopping list.

The embarrassingly logical conclusion

That leaves the man from the beginning, meaning me, 56, perfectly healthy, whose strength drained away for no reason in the middle of high summer. After 2 weeks I did not need long to find the problem, once I had stopped looking for it in the weather. It was not in the climate and not in my age, nor in some diffuse ailment. It sat in a plain piece of arithmetic I had failed to do for 2 weeks, and in a glass I had taken to be harmless.

I put a pinch of salt in the water, I ate tomatoes again, I rethought my drinking water. 2 days later I stood upright beside myself again instead of half a step off. It was no epiphany, it was bookkeeping. And that is perhaps the most sobering thing about this whole story. The problem had been there the entire time, visible and banal, right in front of me. Houston would have seen it at once. I took 2 weeks, because I kept staring into the heat when I should have been staring into my water glass.

References

• Kozisek, F. (2005). Health risks from drinking demineralised water. In Nutrients in Drinking Water (pp. 148 to 163). World Health Organization. https://aguaenmexico.com/wp-content/uploads/2021/02/WHO-HEALTH-RISKS-FROM-DRINKING-DEMINERALISED-WATER.pdf
• Giuliani, C., & Peri, A. (2014). Effects of Hyponatremia on the Brain. Journal of Clinical Medicine, 3(4), 1163 to 1177. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4470176/
• Rondon-Berrios, H. (2025). Hyponatremia. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK470386/
• Filippone, E. J., et al. (2023). Morbidity Associated with Chronic Hyponatremia. Journal of Clinical Medicine. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9917626/
• Seay, N. W., et al. (2023). Hyponatremia Demystified: Integrating Physiology to Shape Clinical Practice. https://pmc.ncbi.nlm.nih.gov/articles/PMC9993811/
• Neal, B., et al. (2021). Effect of Salt Substitution on Cardiovascular Events and Death. New England Journal of Medicine, 385, 1067 to 1077. https://www.nejm.org/doi/full/10.1056/NEJMoa2105675
• Marklund, M., et al. (2024). Potassium-Enriched Salt Substitutes: A Review of Recommendations in Clinical Management Guidelines. Hypertension, 81, 400 to 414. https://www.ahajournals.org/doi/10.1161/HYPERTENSIONAHA.123.21343
• U.S. Government Accountability Office. (2009). Bottled Water: FDA Safety and Consumer Protections Are Often Less Stringent Than Comparable EPA Protections for Tap Water (GAO-09-610). https://www.gao.gov/assets/a291471.html
• Natural Resources Defense Council. (2026). Bottled Water vs. Tap Water. https://www.nrdc.org/stories/bottled-water-vs-tap-water
• Verbraucherzentrale. Trinkwasser, Mineralwasser, Tafelwasser, was sind die Unterschiede? [Tap water, mineral water, table water: what are the differences?]. https://www.verbraucherzentrale.de/wissen/lebensmittel/gesund-ernaehren/trinkwasser-mineralwasser-tafelwasser-was-sind-die-unterschiede-13225
• Federal Institute for Risk Assessment (BfR). (2006). Maximum levels for boron and fluoride in natural mineral waters should be based on drinking water regulations (Opinion No. 024/2006). https://www.bfr.bund.de/cm/343/hoechstmengen_fuer_bor_und_fluorid_in_natuerlichen_mineralwaessern_sollten_sich_an_trinkwasserregelungen_orientieren.pdf