For a healthy adult with working kidneys, thirst and pale urine are good-enough guides to how much water to drink, and food counts toward the total alongside what you drink. The eight-glasses-a-day rule most people carry is a round number, not a measured need: the best measurement of how much water people turn over found need varies so much with body size, activity, and climate that it produced prediction equations, not a target. Electrolyte drinks are the right tool for heavy fluid loss, the two cases that matter being diarrheal illness and prolonged hard sweating, and for everyday sipping they add little over plain water.
For most people the bigger acute risk is drinking too much during a long event, not too little, which can dilute blood sodium dangerously. Buy electrolytes for a reason, not as a daily habit: a pinch of kitchen salt does the same job on a hot training day for about a cent.
Findings & Outcomes
What It Is
Hydration is drinking to your body's needs. Electrolytes are the salts that leave with the water when you lose a lot of it, chiefly sodium; potassium leaves too, but in a narrow band that ordinary food already replaces. Almost everything useful and everything cautionary about the topic turns on one distinction: an ordinary day, when water and meals cover you, against heavy fluid loss from illness or hard sweating, when sodium has to be replaced as well.
The popular numbers are not backed by the measurements. No single daily target holds up, and the figure most people quote for overnight water loss has no primary measurement behind it. The findings below set out where the evidence is solid and where it is thin.
What the Evidence Shows
The evidence is solid at the two extremes and soft in the middle. At one end, a full day without fluid measurably dulls memory, alertness, and mood, and drinking water reverses it. At the other end, drinking well past thirst through a long event can dilute blood sodium far enough to kill. The ordinary day sits in the middle, the glass count and the electrolyte stick, where the evidence is thinnest and the marketing heaviest.
Two things explain most of that soft middle. When researchers searched the whole literature for trials testing whether drinking more water improves health, they found only a handful, and the one outcome that held up clearly was fewer recurring kidney stones. And almost every positive result for electrolytes in rested people comes from a single design: give a fasted person a liter of a drink, then measure urine for a few hours to see how much was held. That is a fluid-retention surrogate, not energy, focus, or performance, and several of these trials were paid for by companies that sell the drinks. Sodium does raise short-term retention, which is why salt matters after heavy sweating and adds little on a rested day when there is nothing extra to hold. Each finding below is placed at the strength of its own evidence, with its funding named.
The Research & Studies
Everything here is based on the research we have collected and checked, sorted into groups and ordered with the strongest evidence first. Click any claim to open the studies behind it.
How it works
The universal eight-glasses-a-day target rests on no measurement
The best measurement ever made of how much water people actually use found no single right number. Across 5,604 people in 26 countries, need varied with body size, activity and climate, and the study produced equations, not a daily target.
Water turnover measured by the doubly-labeled-water method in 5,604 people aged 8 days to 96 years across 26 countries varied with age, body size, body composition, physical activity, athletic status, pregnancy, socioeconomic status, latitude, altitude, temperature and humidity. People in lower-income countries turned over more water than those in higher-income ones. The output is a set of prediction equations with no single number inside them. At 3,729 women and 1,875 men it is the best sex-represented finding on the page, though the factors it identifies travel together in a pooled real-world sample, so none is cleanly isolated.
Who this may not transfer to:Both sexes across the whole lifespan, which makes this the best-represented finding on the page. Country coverage is uneven, so a reader from an under-represented region sits further from the data the equations were fitted to than the sample size suggests.
Since there is no universal target, use the signal you already have. For a healthy adult with working kidneys, thirst plus the water in meals is the instruction, and it adjusts itself to the heat, the exertion and the body the equations say matter.
The study · 1
Yamada et al., variation in human water turnover associated with environmental and lifestyle factors · Science 2022;378(6622):909-915
Sweat sodium varied fourteen-fold between runners in the same race, while potassium barely moved
How salty your sweat is varies enormously from person to person. In one marathon, runners in the same race and weather differed about fourteen-fold in sweat sodium, while potassium barely moved between them.
Across 157 runners in one marathon, sweat sodium averaged 42.9 plus or minus 18.7 mmol/L and ranged from 7.0 to 95.5, a fourteen-fold spread in the same race and the same weather. Potassium sat in a narrow band, 6.0 plus or minus 0.9 mmol/L. Women in that sample averaged 33.9 mmol/L against 44.0 in men, and one runner in five was above 60 mmol/L, none of them women. Everyone was an experienced, heat-acclimatized marathoner, which lowers sweat sodium relative to someone who trains indoors for forty minutes, so the sample sits at the lower, better-adapted end.
Who this may not transfer to:141 of the 157 were men, so the average is close to a male average, and the women's figure comes from 16 people. Everyone in it was an experienced marathon runner, and therefore heat-acclimatized, which lowers sweat sodium relative to someone who trains indoors for forty minutes.
This is why there is no single right amount of salt to replace. If your skin and cap crust white after a long hot session you are likely a heavy loser and salt earns its place; if they do not, salted food and plain water already cover you.
The study · 1
Lara et al., interindividual variability in sweat electrolyte concentration in marathoners · J Int Soc Sports Nutr 2016;13:31
Individual characteristics explained only 17 to 23% of who loses the most salt
There is no simple way to predict who sweats out a lot of salt. A large analysis found that a person's age, ethnicity, diet and how hard they sweat explain almost none of it, so no rule of thumb tells you where you fall.
Across 1,944 sweat tests from 1,304 people, regression models explained only 17 to 23% of the variation in whole-body sweat sodium. Age group, race or ethnicity, humidity, exercise duration, hydration status and dietary sodium had no significant effect; season, energy expenditure, exercise mode, sweating rate, body mass and sex did register. Every author is employed by PepsiCo R&D at the Gatorade Sports Science Institute, which sells the category the finding is used to discuss. It is a retrospective compilation of tests collected for other purposes, drawn mostly from athletes who sought out sweat testing, so nothing was controlled and the sample skews trained and already worried about salt.
Who this may not transfer to:Sex was a significant predictor, with men higher, but the paper's abstract does not give the sample's own sex composition. The population is people tested at a sports science institute rather than the general public, so it is fitter and more athletic than a reader picking up an electrolyte stick.
Because the obvious predictors fail, the only way to actually know your sweat sodium is to measure it. That is worth doing only if a lot rides on it: hours of training in heat, with cramping. For everyone else, the sign on your skin is enough to act on.
The study · 1
Baker et al., explaining variation in sweat sodium concentration · J Appl Physiol (1985) 2022;133(6):1250-1259
Sodium in a drink raised short-term fluid retention by up to about 24%
Adding sodium to a drink makes you hold onto a bit more of it for a couple of hours, up to about a quarter more than plain water. What that buys you is hours of urine measurement, not energy, focus or performance.
In young adults the beverage hydration index rose with beverage sodium, independent of glucose or amino acids: the highest, a 60 mmol/L drink, reached 1.24 against water at 1.00 by two hours, about 24% more fluid retained. In older adults the 30 mmol/L drink led, at 1.20. The outcome is urine volume over two to four hours and nothing else. It was partly funded by an unrestricted gift from the company that makes the two amino-acid drinks that scored highest, and twelve people per age group is small.
Who this may not transfer to:Twelve per age group with a near-even sex split, which is too few to test for a difference between men and women. The two age groups behaved differently enough that the authors recommend a four-hour rather than two-hour index for older adults, so the 24% figure belongs to the young group.
Sodium does help you retain fluid, which is exactly why salt matters when you have sweated a lot out. It is not a reason to add sodium on a rested day, when there is nothing extra to hold onto and the measured benefit is hours of urine, not anything you would feel.
The study · 1
Clarke et al., a randomized trial to assess beverage hydration index in healthy older adults · Am J Clin Nutr 2019;109(6):1640-1647
For everyday hydration at rest, electrolytes alone did not consistently beat plain water
When electrolytes alone were compared with plain water in people sitting at rest, they did not reliably beat it. Drinks that also had carbohydrate or protein did a little better, and this trial was funded by a soft-drink company.
The authors' own sentence: adding electrolytes alone, in the range found in sports drinks, did not consistently improve the hydration index versus water. Carbohydrate-electrolyte and dipeptide-electrolyte drinks did beat water, both reaching 1.15. It was funded by The Coca-Cola Company. The outcome is again four hours of urine. The same paper reports that electrolyte content made the largest single contribution to the index, so 'electrolytes do nothing' is not the finding; what it found is that electrolytes on their own did not consistently beat water at rest. Fifteen of nineteen participants were men, all young, all fasted and at rest, so nothing here describes exercise or heat.
Who this may not transfer to:Fifteen of the nineteen were men and all were young, so this sits close to a young-male result despite the mixed label. Everything was measured at rest in a fasted state, so nothing here describes what happens during or after exercise.
At rest, plain water is the sensible default and an electrolyte drink adds little you can measure. Save the sodium for when you have actually sweated it out.
The study · 1
Millard-Stafford et al., the beverage hydration index, influence of electrolytes, carbohydrate and protein · Nutrients 2021;13(9):2933
Coffee matched plain water for hydration, total body water 113.5 versus 113.3 lb (51.5 versus 51.4 kg)
Coffee does not dehydrate you. When habitual coffee drinkers spent three days on four cups a day, their total body water and daily urine output were no different from three days on water.
In a counterbalanced crossover, 50 men who habitually drank 3 to 6 cups a day completed two three-day trials, one on four cups of coffee a day at 4 mg/kg caffeine and one on water, with food, fluid and activity controlled. Total body water, measured by deuterium dilution, did not differ between them (113.5 versus 113.3 lb; 51.5 versus 51.4 kg), nor did 24-hour urine volume (2,409 versus 2,428 mL) or any blood or urine hydration marker. Urinary sodium ran higher on coffee. Everyone was a male habitual drinker at a moderate dose, and tea was not tested; one author is employed by PepsiCo.
Who this may not transfer to:All 50 participants were men and habitual coffee drinkers at a moderate dose, so the result speaks to regular coffee drinkers rather than to occasional ones or to women, and it was measured for coffee rather than tea.
If you drink coffee regularly, it counts toward your fluids like any other drink. The mild diuretic nudge is one a regular drinker has already adapted to, so there is no need to drink an extra glass of water to cover a cup of coffee.
The study · 1
Killer, Blannin and Jeukendrup, moderate daily coffee intake and hydration, a counterbalanced crossover in a free-living population · PLoS One 2014;9(1):e84154
The overnight fluid-loss figure traces to a teaching exercise, not a measurement
The figure you have heard for how much water you lose overnight, usually a liter or so, has no measurement behind it. It appears to trace back to a teaching exercise written to show students how such a loss would be estimated, not to a study that weighed anyone.
The number in general circulation has no primary measurement we could locate. The document it descends from is indexed under the educational-models heading and its own opening describes it as a theoretical exercise for students, so it has no participants. We could not obtain the full text, and tracing the popular figure to it is our own reading of the citation trail rather than something the document claims; it has been cited only a handful of times in two decades, which is hard to square with being the origin of a globally repeated number, so the fair position is that the figure's provenance is unclear.
Who this may not transfer to:There are no participants, so there is no sex basis and no group this transfers from. A number with no one behind it transfers to no one.
You do lose water overnight through breath and skin, so a dry mouth on waking is normal. Treat any specific volume you are quoted as an estimate, not a deficit you must drink back before you have even felt thirsty.
The study · 1
Weissenberg, insensible water loss during sleep, a theoretical exercise · Adv Physiol Educ 2005;29(4):213-5
The only continuous overnight measurement fell about 12 W/m2 and was never converted to milliliters
The one study that actually monitored water loss through a whole night reported it in a physics unit and never turned it into milliliters. In a neutral room the loss fell as the night went on rather than holding steady, which is the opposite of what the popular figure assumes.
The single study that tracked skin evaporation continuously across a night reported watts per square meter and never converted it to a volume. In thermoneutral conditions evaporation fell across the night by about 12 W/m2 alongside falling skin and core temperature; in warm conditions it held level. Evaporation also stopped in bursts during each REM period. Thermoneutral here means a chamber at 89.6–93.2°F (32–34°C), neutral for an uncovered sleeper and far warmer than a bedroom, so none of these numbers describe a person under a duvet at 64.4°F (18°C). It is a 1977 paper with no replication we could find.
Who this may not transfer to:The abstract says only in man, in the generic sense, and gives no number of participants and no sex split. Sweat gland output and thermoregulatory response both differ with sex and with age, so we do not know who this was measured in or who it transfers to.
The study · 1
Henane et al., variations in evaporation and body temperatures during sleep in man · J Appl Physiol Respir Environ Exerc Physiol 1977;42(1):50-5
At rest, doubling the electrolyte dose did no better than a single dose
Doubling the electrolyte dose at rest did nothing extra over the single dose. This is the trial often quoted as proof electrolytes are pointless, but its 'not necessary' refers to the double dose, and the single-strength drink did beat plain water.
The double-strength drink performed no better than the single. The single-strength drink beat plain water on urine output and lost less weight (0.8 lb (0.38 kg) lost versus 1.3 lb (0.57 kg) on water, p=0.02); at double strength the difference was not significant (0.9 lb (0.43 kg) lost, p=0.08). Funded in part by the tablet's manufacturer. In the paper's discussion, 'additional' means the double-strength dose rather than electrolytes as such. Everything measured is urine, body weight and blood pressure over four hours in eighteen young, exercise-trained adults sitting still; nothing about how anyone felt, thought or performed was recorded, and nobody older or sedentary was tested.
Who this may not transfer to:Eighteen young exercise-trained adults with a near-even sex split, too few to test for a difference between them. No exercise, no heat, and nobody older or sedentary, which is precisely the population the rested-state conclusion gets applied to.
Read precisely, this says more is not better at rest, not that electrolytes never help. A single ordinary dose is plenty when you need one, and at rest you usually do not.
The study · 1
Pence and Bloomer, impact of Nuun electrolyte tablets on fluid balance in active men and women · Nutrients 2020;12(10):3030
Digestion
In childhood diarrhea, oral rehydration salts cut unscheduled IV drips, odds ratio 0.59
For a stomach illness with diarrhea, oral rehydration salts are the one product that beats plain water. Their exact sugar-to-salt ratio is built to be absorbed together in the gut, which is why the formula, not the branding, is what does the work.
In children hospitalized with acute diarrhea, a reduced-osmolarity oral rehydration solution needed fewer unscheduled IV infusions than the standard WHO formula (odds ratio 0.59, 95% CI 0.45 to 0.79, pooled across 8 trials with no heterogeneity), with lower stool output and less vomiting and no excess hyponatremia. Changing the sodium and glucose concentrations changed the clinical result, which identifies the formula as the working part. This compares two rehydration formulas rather than rehydration against plain water, and it is in hospitalized children, not adults managing a bug at home; the glucose-linked sodium cotransport mechanism is textbook physiology, and the review has not been updated since 2002.
Who this may not transfer to:The review reports no sex breakdown across its included trials, and every participant was a hospitalized child. Adults with milder illness at home are the group most readers belong to and the group this was not measured in.
Keep a couple of sachets of a WHO-formula oral rehydration solution in the cupboard before you need them. A sugar-free electrolyte stick is a different product and does not do this job, so it is not a substitute during illness.
The study · 1
Hahn, Kim and Garner, reduced osmolarity oral rehydration solution for treating dehydration caused by acute diarrhoea in children · Cochrane Database Syst Rev 2002;(1):CD002847
Cognition
After 24 hours without fluid, rehydrating restored memory and mood, with a liter working best
After a full day with no fluid at all, memory, mood and alertness dropped, and drinking water afterwards brought them back, with a liter working best. That is recovery from deliberate dehydration, a state most people are never in.
After 24 hours without fluid, portrait-memory scores fell from 34 to 27, vigor from 11.8 to 9.2 and self-esteem from 7.8 to 6.4, while fatigue and total mood disturbance rose. Drinking plain purified water then improved processing speed, working memory and mood, and 1,000 mL beat 500, 200 and none, measured 90 minutes later. The fluid was plain water with nothing added, which is the part left out when the study is used to sell a product. Funded by the National Natural Science Foundation of China with no industry involvement; all participants were young, healthy students, so older adults with declining thirst and kidney concentrating ability are not represented.
Who this may not transfer to:Equal numbers of men and women in every group, which is better balanced than most work on this page. All were young, healthy, non-habitual caffeine drinkers from one college in Hebei, so older adults, whose thirst and renal concentrating ability both decline, are not represented.
This says drinking water helps a dehydrated person, which it does. It says nothing about topping up someone already normally hydrated, so do not read a 24-hour-restriction result as a reason to drink past thirst on an ordinary day.
The study · 1
Zhang et al., effects of water restriction and supplementation on cognitive performances and mood among young adults · Nutrients 2021;13(10):3645
Drinking on waking has been tested only after a 12-hour fluid restriction, never after an ordinary night
Drinking water on waking has been tested, but only after the evening's fluid was held back first, so the trials measure recovery from a small overnight shortfall rather than the value of the glass itself.
In one trial 64 young adults went 12 hours overnight without fluid and were then randomized to 500, 200, 100 or no milliliters on waking, with cognition and mood measured; rehydrating on waking improved them. A separate 12-week trial had Japanese adults increase daily water intake and tracked health markers. Neither isolates the specific habit of a glass of water first thing after an ordinary night, and the restriction trial withheld evening fluid rather than testing the routine as most people practice it. The samples are young or general adults, so the evidence for a habit recommended to everyone is thin.
Who this may not transfer to:The Baoding trial reports no sex split for its 64 young adults, and both studies were in young or general adults, so older people, whose thirst blunts with age and who are the group most often told to drink on waking, are not represented.
It is a good habit for its own sake: cheap, pleasant, and a fixed anchor for a morning routine. Keep it because you like it, not because a number says you woke up in deficit.
The studies · 2
Zhang et al., different amounts of water supplementation after 12 h water restriction, cognition and mood in young adults · Int J Environ Res Public Health 2020;17(21):7792
effect of increased daily water intake and hydration on health in Japanese adults · Nutrients 2020;12(4):1191
Kidney Stones
Only 18 trials tested drinking more water; fewer kidney stones and modest weight loss held up
When researchers looked for trials testing whether drinking more water improves health, they found only 18 in the entire literature. Two things held up: fewer recurring kidney stones, and modest weight loss. Most of what water is popularly credited with has never been tested.
A systematic search screened 1,464 records and found 18 randomized trials testing a change in daily water intake against any health outcome. Ten reported at least one positive result and eight were negative. Two outcomes held up: recurrent kidney stones, about 15 fewer stone events per 100 people over five years, and weight loss. Single trials each hinted at benefit for migraine, urinary tract infection, diabetes control and blood pressure. The review does not pool the trials statistically and calls both the amount and quality of evidence limited; most trials recruited people who already had the condition, so the kidney-stone result belongs to recurrent stone formers rather than the general reader.
Who this may not transfer to:The included trials span varied populations and the review reports no overall sex breakdown, so the balance across the 18 is unknown. Kidney stone trials skew male and weight loss trials skew female, which pulls the two surviving findings toward different readers.
If you form kidney stones, drinking more water is one of the few water habits with a randomized trial behind it and worth doing deliberately. Beyond that narrow case, there is no measured reason to push intake past thirst for energy, skin or mood.
The study · 1
Hakam et al., outcomes in randomized clinical trials testing changes in daily water intake, a systematic review · JAMA Netw Open 2024;7(11):e2447621
When Electrolytes Actually Help
Two situations call for electrolytes. The first is diarrheal illness, the one case where a made product beats plain water. Oral rehydration salts use a glucose-to-sodium ratio the gut absorbs together, so the formula, not the branding, is the working part. A sugar-free electrolyte stick is a different product and does not do this job. Keep a couple of sachets of a WHO-formula oral rehydration solution in the cupboard before you need them, because a stomach bug is the worst time to go shopping.
The second is prolonged heavy sweating, in heat or a long hard session, where enough sodium leaves that replacing it matters. A liter of average sweat carries off around 990 mg of sodium, and a quarter teaspoon of table salt in your bottle covers that for a fraction of the price of a branded stick. On an ordinary day at a desk, neither situation applies. The powder is convenient and pleasant, and on a hot training day it will get sodium into you, but for daily sipping a glass of water does the same job.
How It Works
Anatomy of the Practice
1When you are thirsty
Thirst rises with plasma osmolality, the saltiness of the blood, and in a healthy adult with working kidneys it tracks that closely enough that no counting is needed. The kidneys hold water back or let it go by concentrating or diluting the urine, keeping the balance steady, and the meals you eat supply a large part of the day's total on top of what you drink.
2When you sweat
Water leaves through the skin and sodium leaves with it. How much sodium is the part that varies enormously from one person to the next, while potassium leaves in a narrow band that ordinary food replaces. A long hot session is when a heavy salt loser drops several times what the person beside them does.
3When you replace it
Plain water and salted food restore most people. Sodium in a drink raises how much fluid you hold in the short term, which is what the electrolyte research measures. The one situation where a specific product beats water is diarrheal illness, where oral rehydration salts use a set sugar-to-salt ratio the gut absorbs together.
There is no lookup table for whether you are a heavy salt loser. The white crust on your skin and cap after a long hot session is the usual sign, and cramping late in a hard event points the same way. If neither shows, salted food and plain water already cover you.
Drink to thirst, not to a schedule, and never ahead of your losses.
Ways to Do It
Water is free and thirst is the instruction, so most of this ladder costs nothing. Salt from your own kitchen covers a hard-sweat day, oral rehydration salts are a cheap cupboard staple for illness, and a branded electrolyte product is the one optional purchase, worth it for convenience, not for a result the free version cannot match.
For a healthy adult with normal kidneys, thirst tracks the blood closely enough that no counting is needed, and ordinary meals and drinks supply a large share of the total. If you would rather have a habit than wait on a signal, keep a glass where you work and drink it when you notice it.
Sweat potassium sits in a narrow band that ordinary food already covers, so it needs no supplement. Sodium is the one that varies between people and the one to attend to, and on most days a normally salted diet supplies more than you lose.
A liter of average sweat carries off about 990 mg of sodium, and a quarter teaspoon of table salt is roughly 590 mg. Stirred into the bottle you already carry, that is the same intervention the retention studies tested, without the packaging. This is the day to read the heat exposure page alongside this one.
Diarrheal illness is the one case where a made product beats water, because oral rehydration salts use a glucose-to-sodium ratio the gut absorbs together. A sugar-free electrolyte stick is a different product and does not do this job. Buy a couple of sachets before you need them, because a stomach bug is the worst time to go shopping.
Convenient, portable and pleasant, and it will get sodium into you on a hot training day. On a rested day, which is most days, a glass of water does the same job. Judged as a convenience you are buying, not a better result, it holds up fine.
Go Deeper
- Heat exposure: the sweat losses this page's sodium advice is written for, and the guardrail the two views share.
- Kidney stones: the one outcome where drinking more water has a randomized trial behind it, and how much to aim for if you form them.
- Sodium reduction and the DASH diet: the other side of sodium, where most people already take in more than they lose.
- Recurrent UTI: another condition where a single trial found drinking more water helped.
The Chinese Medicine View
Body fluids are a named vital substance in Chinese medicine, Jin Ye. Jin are the light, watery fluids that move at the surface and emerge as sweat, tears and saliva; Ye are the denser fluids that lubricate the joints, spine, brain and marrow. Both originate in food and drink, and the tradition assigns water metabolism to the organs: the Spleen and Stomach transform what you take in, and the Kidney governs the fluids at the deepest level, opening and closing the gate that keeps some fluid and passes the rest. So hydration is built from what the digestion can transform, not from volume alone.
That organ picture carries a caution in both directions. The tradition warns against dryness, and equally against overloading the Spleen and Stomach with more fluid than they can move, which is understood to breed internal damp and sluggishness. Drink to thirst and no further is close to what it would advise anyway, and the classical unease about large volumes of cold water belongs here too: iced drinks are held to tax the digestive fire that transforms fluid, most of all in someone already cold, damp and sluggish. Room temperature or warm is the classical preference, and hot water with meals remains ordinary practice across much of China for this reason. Western science has not measured whether beverage temperature changes digestion in a way a person would notice; we present it as what the tradition holds.
The two views meet cleanly on sweat. Chinese medicine reads heavy sweating as carrying off the Defensive Wei Qi along with the fluid, which is why the classics instruct that you never induce sweating in someone who is bleeding or blood-deficient. That classical caution, that over-sweating exhausts the Qi and fluids, is the same guardrail heat exposure needs, which modern medicine states as dehydration and cardiac strain. Read as causal accounts the two languages stay separate, and neither one validates the other, but the practical instruction they arrive at is the same.
Cautions For This Practice
Everything to be aware of is here, in one place. This practice suits most healthy people; a few situations call for real care.
Overdrinking during endurance events diluted blood sodium, odds ratio 4.2, with at least twelve deaths
Drinking beyond thirst during prolonged exercise can dilute blood sodium enough to cause swelling of the brain and lungs. At least twelve confirmed deaths are documented in the consensus literature, in settings from football practice to military training to an 11.8 mile (19 km) bike ride, and the authors state athletes continue to die of it. In Boston Marathon runners, substantial weight gain during the race, the sign of overdrinking, carried an odds ratio of 4.2 (95% CI 2.2 to 8.2) for hyponatremia; the highest odds in the study were for a racing time over four hours (7.4, 2.9 to 23.1). Intake was recalled by questionnaire after the race. Smaller athletes, slower finishers and longer events carry more risk. Women appeared higher-risk until weight gain and race time were accounted for, after which sex was not an independent factor.Almond et al., hyponatremia among runners in the Boston MarathonHew-Butler et al., statement of the third international exercise-associated hyponatremia consensus development conference
Four weeks of daily electrolyte use was broadly tolerated, with bloating and cramping at higher doses
Over four weeks of daily use, the group-by-time differences were in tolerability, bloating and cramping, and in some blood markers (mean corpuscular volume, estimated glomerular filtration rate and potassium), which the authors describe as little to no change rather than none. Forty adults were randomized four ways, ten per group, with visits at 0, 2 and 4 weeks. It was funded by the manufacturer of the product it tested, everyone was tested outside physical stress, and ten per arm is far too few to separate men from women.Bloomer et al., randomized trial to assess the safety and tolerability of daily intake of an allulose amino acid-based hydration beverage
Drinking well beyond your losses is the bigger danger
Over hours of exercise, taking on fluid faster than you sweat it out dilutes blood sodium, which is exercise-associated hyponatremia: headache, nausea, swelling and confusion that feel like dehydration, which is the trap, because the instinct is to drink more. It has killed people. The correction endurance medicine settled on is to drink to thirst rather than on a schedule, and not to drink ahead of your losses. Smaller athletes, slower finishers and longer events carry more of the risk, and if these signs appear during a long event, stopping and getting checked is the right response, not another cup.
Heart failure, kidney disease, and medications that change your fluid needs
If you have kidney disease, heart failure or liver disease, or you take a diuretic, lithium or an SSRI, your fluid and sodium amounts are a medical decision rather than general advice. Talk to a doctor about the right amounts for you rather than following any rule of thumb, including ours.
Oral rehydration salts are for illness, not a sports drink
The WHO-formula oral rehydration solution is built to rehydrate during diarrheal illness, where its exact sugar-to-salt ratio is the point. In infants, and in anyone with prolonged vomiting or diarrhea, it is the right tool and a doctor is the right call if it does not settle. Reaching for it as an everyday sports drink misreads what it is for.
Older adults, and thirst as a signal
Thirst blunts with age, so an older adult often does better keeping a glass in sight than waiting for the signal, which is the one group where drinking to thirst is not enough on its own. Persistent heavy thirst with high urine volume runs the other way and is worth getting checked rather than drinking through, since it is one of the common first signs of diabetes.
Start slow, be smart, read the research, and consult a professional if you have any concerns. This is here to inform your choice, not make it for you.
Common Questions
How much water should you drink a day?
There is no established universal number. The strongest evidence, water turnover measured by the doubly labeled water method across dozens of countries, produced prediction equations based on body size, body composition, activity and climate rather than a target. For a healthy adult with working kidneys, thirst plus the water in ordinary meals and drinks covers it, and pale urine is a good-enough check.
Is "eight glasses a day" backed by evidence?
It is a round number, not a finding. The best measurement of how much water people turn over gives equations tied to your body and climate instead of a single quota. Most healthy people are neither dehydrated nor short of some fixed count; thirst and meals keep them in range. Dehydration does happen, but it shows up in heat, in illness, and in older adults whose thirst has dulled, not on an ordinary day at a desk.
Do you need an electrolyte drink if you are not an athlete?
On most days, no. Every positive result for electrolytes in rested people is a urine retention measurement over a few hours, not a health outcome, and the situations where sodium matters are prolonged exercise, heavy sweating or heat, and diarrheal illness. On a hot training day a pinch of salt in your bottle does the same job as a stick for about a cent.
Do coffee and tea dehydrate you?
No, for regular drinkers. In a controlled crossover, 50 male habitual coffee drinkers spent three days on four cups a day and three days on water, and their total body water came out the same, 113.5 against 113.3 lb (51.5 against 51.4 kg), with daily urine output no different either, 2,409 against 2,428 mL. Every participant was a man, so the result speaks to regular male coffee drinkers, and tea was not tested. Coffee counts toward your fluids like any other drink. Caffeine has a mild diuretic effect that a regular drinker has already adapted to, so there is no need to add a glass of water to cover a cup of coffee.
Can you drink too much water?
Yes, and for most people it is the larger risk. Over hours of exercise, drinking faster than you sweat can dilute blood sodium far enough to cause swelling of the brain and lungs, and it has killed athletes. The early signs feel like dehydration, which is the danger. The instruction that endurance medicine settled on is to drink to thirst rather than to a schedule, and not to force fluid ahead of your losses.
Does drinking water first thing in the morning do anything?
It has been tested only in a narrow form. The nearest trials withheld evening fluid first, or imposed 12 to 24 hours of deliberate restriction, so they measure recovery from a deficit rather than the glass itself. Nobody has measured it in someone who slept normally and drank normally the evening before. The habit is cheap, pleasant and harmless, and nothing here argues for stopping it.
How do I know if I am a heavy salt loser?
The white crust on your skin, cap and clothes after a long hot session is the usual sign, and cramping late in a hard event points the same way. A sweat sodium test can measure it, but individual characteristics predict only 17 to 23% of where a person sits, so the number is worth it only if you train for hours in heat and keep cramping. For everyone else, salted food and plain water cover you.
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Thomas Dehli, Founder & Editor, Sacred Lotus
Sacred Lotus has published Chinese medicine reference material since 2001. Integrative pages are held to the same standard as the herb and formula library: cite the source, grade the claim at its real strength, and say where the research has not looked. This page is educational and it is not medical advice. Last reviewed and updated August 9, 2026.
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