Hydration: Importance of Water and Dehydration Prevention
Linas JuozėnasShare
Hydration
Water supports circulation, temperature control and everyday function. Good hydration means meeting changing needs without forcing more fluid than the body can handle.
Your fluid needs change with activity, weather, food, health and life stage. A comfortable day indoors, a long walk in the heat and an illness with vomiting require different decisions.
This guide explains how water balance works, how to interpret European intake recommendations and how to make drinking practical. It also separates useful signs from unreliable tests, and explains why dehydration and excessive drinking both deserve attention.
The aim is a routine that supports your life: accessible drinks, informed choices and adjustments when circumstances change.
Food and other drinks contribute alongside plain water.
Thirst, activity, access and health all affect the plan.
Electrolytes do not make overdrinking safe.
What water does in the body
Water is present inside cells and in the fluids surrounding them, including blood plasma. The proportion of body mass that is water varies with age and body composition; a single percentage cannot describe every person.
It provides the environment for many biochemical reactions, helps transport dissolved substances and enables the kidneys to excrete waste in urine. Water also contributes to saliva, tears, joint fluid and the fluid around the brain and spinal cord. Metabolic reactions produce some water within the body as well.[1]
A solvent, with important limits
Water dissolves many substances, including salts and sugars. It does not dissolve everything: many fats travel in blood with proteins or within lipoprotein particles. Calling water a “universal solvent” can obscure how transport actually works.
Water participates directly in reactions such as hydrolysis, in which water helps split a chemical bond. It also supports the conditions in which enzymes and cells function.
Water and heat loss
Working muscles release heat. Sweating can help remove that heat when sweat evaporates from the skin. Sweat that drips away represents fluid loss but provides much less evaporative cooling.
Humidity, airflow, clothing and the surrounding temperature affect how well evaporation works. Drinking supports fluid balance, but it cannot make every hot environment safe or remove the need to reduce heat exposure.
Enough water supports normal function
More is not automatically better once needs are met. Extra water does not make the kidneys cleanse the body without limit, and it does not replace food, sleep or medical care when those are needed.
Useful hydration is about maintaining an appropriate balance over time, not achieving the largest possible daily total.
Thirst is part of a responsive control system
The body monitors both the concentration of dissolved particles in its fluids and signals related to blood volume and pressure. Thirst helps motivate drinking when more water is needed.
The regulation of drinking involves a network of brain regions, rather than a single isolated “thirst centre.” Signals from the mouth and digestive tract also help anticipate incoming water before all of it reaches the circulation. Research on thirst circuitry has clarified this rapid feedback, with much mechanistic work conducted in animals.[2]
Hormones help regulate retention
Vasopressin, also called antidiuretic hormone or ADH, is made in the hypothalamus and released from the posterior pituitary. It helps the kidneys conserve water. Aldosterone, produced by the adrenal glands, influences sodium retention as part of the wider regulation of fluid balance.
The kidneys adjust water and solute excretion; they do not simply dispose of any amount of water immediately. Exercise, illness and medicines can change these processes.
Use thirst without ignoring circumstances
For many healthy adults with ready access to drinks, thirst is a useful everyday guide. It is not a reason to ignore a drink when you want one, nor a reason to drink continuously when you do not.
Older people, someone unable to obtain a drink independently, or a person with an individual medical plan may need a more structured approach. An athlete expecting long gaps between drink opportunities also benefits from preparation.
Think of thirst as useful information within a setting. A workable plan combines bodily signals with access, activity demands and any relevant health guidance.
Daily recommendations include water from food
EFSA sets adequate intakes for total water: plain water, other beverages and moisture in food combined. These values apply to moderate environmental temperatures and activity levels; they are population references, not exact requirements for every individual.[3]
| Group | Total water per day |
|---|---|
| Adult women | 2.0 L |
| Adult men | 2.5 L |
| Pregnancy | About 2.3 L, including an additional 300 mL above the adult female reference. |
| Breastfeeding | About 2.7 L, including an additional 700 mL above the adult female reference. |
The pregnancy and breastfeeding totals above are calculated from EFSA's stated additions. They do not mean drinking those amounts of plain water on top of usual food and beverages.
Turn a reference into a practical routine
Food contributes different amounts of water depending on what you eat. Soup, fruit, vegetables, yoghurt and cooked dishes can all contribute. There is no need to calculate every gram of food moisture during an ordinary day.
Keep drinks available, have them with meals if that suits you, and respond to thirst. During heat, exercise or fluid-losing illness, reassess the situation instead of treating the daily reference as a ceiling or a compulsory quota.
Some goals need individual guidance
Fluid targets used to help prevent recurrent kidney stones can differ from a general daily reference. NIDDK guidance emphasises adequate liquid intake while advising an individual discussion, particularly when kidney function is impaired.[4]
If you have been given a fluid restriction or a specific replacement plan, use that plan. Heart, kidney or other conditions can make generic advice to “drink more” inappropriate.
The useful question is whether your usual routine meets your needs comfortably, and what changes when your activity or health changes.
Water, food and other drinks all contribute
Safe plain water is a practical default for everyday thirst. Still and sparkling water both contribute fluid. Preference matters: a drink you enjoy and can access is easier to include in a regular routine.
Other beverages can provide fluid as well as energy, nutrients or caffeine. Their usefulness depends on the reason for drinking them.
| Drink or food | Potential role | What to consider |
|---|---|---|
| Plain water | Everyday fluid intake and many ordinary exercise sessions. | Use a safe supply; no special mineral formula is required for routine hydration. |
| Tea and coffee | Contribute to fluid intake. | Caffeine tolerance, sleep and the amount consumed still matter. |
| Milk or suitable alternatives | Fluid alongside nutrients. | Products differ in protein, sugar and fortification; choose for your dietary needs. |
| Water-rich foods | Contribute fluid as part of meals. | Food moisture varies, so total water intake is not the same as bottled-water intake. |
| Sports drinks | Can combine fluid, carbohydrate and sodium for demanding activity. | Composition and serving size vary; they are not automatically needed for a short session. |
| Oral rehydration solution | A specified glucose-electrolyte mixture for relevant fluid losses, especially diarrhoea. | Use the product as directed. It is not interchangeable with an ordinary sports drink. |
Coffee does not simply cancel out its water
A controlled study in habitual male coffee drinkers found similar hydration measures with moderate coffee intake and water under the studied conditions.[5] That finding does not establish that every caffeine dose suits every person, but it contradicts the idea that each coffee necessarily causes a net fluid deficit.
Juice and sweetened drinks also contain water. Their sugar and energy content affect how they fit into an everyday diet; WHO recommends limiting free sugars.[6] During prolonged exercise, carbohydrate may have a deliberate fuelling role.
Alcohol should not be used as a rehydration strategy. Drinking water alongside it does not remove alcohol's effects or make it an appropriate recovery drink.
Choose the drink to meet the actual need, rather than assuming that the longest list of added ingredients makes a product more hydrating.
Hydration cannot be diagnosed from one colour or number
Thirst, a change in urination, dry mouth or an unexpected change in body mass can prompt a closer look at fluid intake and losses. These signs are not specific enough to identify every fluid problem or its severity.
Urine gives context, not a complete answer
Urine colour reflects concentration as well as pigments from food, supplements, medicines and illness. A darker first morning sample can differ from urine later in the day. Pale yellow can be a useful everyday observation, but clear urine is not a compulsory target.
A methodology review cautions against interpreting a single spot urine concentration as a direct measure of hydration status.[7] The sample reflects recent intake, kidney responses and timing. Repeated comparable observations are different from a one-off reading.
| Measure | Useful information | Important limit |
|---|---|---|
| Thirst and recent history | Whether you want fluid and whether intake or losses have changed. | Symptoms and access vary; thirst alone cannot diagnose a medical fluid disorder. |
| Urine appearance or concentration | Information about the urine produced at that time. | A sample does not directly describe every body-fluid compartment. |
| Body-mass change around exercise | A practical estimate of short-term net fluid change under controlled conditions. | Wet clothes, food, urine and other mass changes affect the result. |
| BIA body-water estimate | A model-based estimate informed by electrical properties. | A consumer percentage is not a clinical test for dehydration. |
| Clinical assessment and blood tests | Can help distinguish water, sodium, volume and kidney-related problems. | Interpretation depends on symptoms, history and the specific question. |
Be especially careful in older adults
A Cochrane diagnostic review found that commonly used signs and simple tests—including thirst, urine colour and some BIA measures—should not be relied on individually to identify water-loss dehydration in older people.[8]
Blood sodium is a concentration, not a measurement of total body sodium or total body water. A normal value does not rule out every kind of volume depletion.
If symptoms persist, seek an explanation rather than repeatedly drinking until a preferred colour or device score appears.
Hydration affects performance, with important variation
Losing body water during activity can increase cardiovascular and thermal strain. The effect depends on the amount and type of loss, exercise duration and intensity, environmental conditions and the person's starting state.
A comprehensive physiology review describes evidence for impaired endurance performance around or beyond a 2% body-mass loss attributable to dehydration, while emphasising the mechanisms and conditions involved.[9] That figure is not a universal switch between safe and dangerous, or a diagnostic stage for every illness.
Percentages need a reliable starting point
For a hypothetical 70 kg adult, 2% of body mass is 1.4 kg. The arithmetic is simple; establishing what caused a measured loss is less simple. Changes over days can involve food, glycogen and other tissues as well as water.
A short exercise-related change measured under comparable conditions is more informative about fluid than an unrelated comparison between two morning weights.
Thinking and mood are not governed by one threshold
A meta-analysis found a small average impairment in cognitive performance with dehydration, with larger effects in some tasks and studies involving greater water deficits.[10] The results varied across studies. It is too strong to claim that every small shortfall necessarily impairs everyone's memory or decision-making.
Headache, tiredness and difficulty concentrating can have several causes. A drink may be helpful when fluid is needed, but these symptoms do not prove dehydration.
Similarly, there is no good reason to dismiss hunger as merely thirst. Regular food, rest and hydration each have their own role. Use the whole situation to decide what support is needed.
Before and during exercise, use an adaptable plan
Begin with ordinary fluid balance
Maintain your usual access to drinks and meals before activity. If you are thirsty or have recently lost substantial fluid, address that before adding a demanding session. Avoid forcing a large volume immediately before starting simply to “get ahead.”
Check how long the activity may last, the weather, expected effort and where drinks will be available. A short local outing needs a different plan from a long event or remote route.
Thirst and planning serve different needs
Drinking to thirst is a useful approach for many recreational situations. More deliberate planning can help during prolonged, hot or demanding exercise, especially when opportunities to drink are limited. A review comparing these strategies explains why the context and performance goal matter.[11]
A plan should remain responsive to conditions, stomach comfort and actual losses. It should not compel you to finish a predetermined volume regardless of what happens.
Avoid net weight gain from drinking
Sports guidance recommends individualising fluid replacement and avoiding excessive intake.[12] During endurance exercise, gaining weight because you are taking in more fluid than you lose is a warning that the plan may be excessive.
There is no single hourly amount that is right for all bodies and all weather. Copying another participant's bottles or using a fixed amount every few minutes can miss your needs in either direction.
Practise the practical details
Try the bottle, drink and access arrangements in training before relying on them in a long event. Small, comfortable drinking opportunities may be easier to manage than infrequent large amounts.
Personalised hydration strategies consider the person, exercise and environment.[13] Adjust the activity itself when heat, lack of supplies or symptoms make the original plan unsuitable.
How to estimate sweat loss during a familiar session
When a more detailed exercise plan is useful, a short-session sweat estimate can help. It works best with a reliable scale, similar dry clothing, recorded drinks and any urine passed, and no substantial unrecorded food intake.
Weigh immediately before and after the activity, towel dry before the final measurement and keep the method consistent. Wet clothing can hide some of the apparent loss. Repeat in representative conditions rather than assuming one session describes every season.[12]
A worked example
A hypothetical person starts at 70.0 kg and finishes at 69.4 kg after one hour. They drink 0.5 L and pass 0.1 L of urine during the measurement period.
Estimated sweat loss = 0.6 + 0.5 − 0.1 = 1.0 L. Estimated sweat rate is therefore 1.0 L/h for that particular session. Their net body-mass loss was 0.6 kg; the two numbers describe different things.
What the calculation leaves out
Respiratory losses, fuel use, scale error and other mass changes affect precision, especially in longer events. Sweat-testing guidance emphasises sound collection and interpretation because both sweating rate and sweat sodium concentration vary.[14]
The result helps estimate access needs and compare conditions. It is not an instruction to replace every estimated millilitre immediately or to drink 1.0 L/h in every activity.
Body mass alone cannot diagnose blood sodium problems. A person who becomes unwell needs an appropriate assessment even if their weight has decreased.
Electrolytes and carbohydrate have different roles
Electrolytes are charged particles involved in fluid distribution and nerve and muscle function. Sodium and chloride are major electrolytes lost in sweat; the amount lost depends on both sweat volume and its composition.
For many ordinary days and shorter exercise sessions, water and normal meals provide a workable approach. A specialised drink is not automatically needed whenever you sweat.
When sodium replacement becomes more relevant
Long or repeated sweaty sessions, particularly in heat, may make deliberate sodium replacement useful. The plan needs to account for the food and drinks already being consumed. An electrolyte label alone says little about the actual dose.
Check the amount of sodium per serving and the amount of water the product is intended to be mixed with. A drink high in potassium is not necessarily an appropriate substitute for a sodium-containing drink when sodium replacement is the purpose.
White marks on clothing can suggest salty sweat, but they do not quantify a person's losses. Even formal sweat testing needs careful interpretation.[14]
Carbohydrate is fuel
During prolonged or demanding exercise, carbohydrate can support energy supply. A drink may be chosen to deliver fuel and fluid together, or these needs may be met separately through drinks and food. Sports-nutrition guidance distinguishes fuelling needs according to the activity.[15]
A sugar-free product is therefore not automatically the better sporting choice. Conversely, a carbohydrate drink may be unnecessary when the aim is simply to satisfy thirst during a short session.
Cramps need a broader explanation
Exercise-associated cramps are not proof of a single electrolyte deficiency. An evidence review describes multiple contributing factors and supports an individual approach.[16] Repeated cramping calls for attention to the activity, fatigue and relevant health factors, not just a larger supplement dose.
Adding sodium does not make excessive fluid intake safe. Sports drinks and salt supplements cannot reliably prevent exercise-associated hyponatraemia if overdrinking continues.[17]
After exercise, combine fluids with food and recovery
After an ordinary session, many healthy people can recover fluid balance through drinks and meals over the following hours. The urgency changes when losses were substantial, another session is soon, or the person feels unwell.
Use thirst, the session history and any carefully measured body-mass change together. A meal can provide sodium and other nutrients while drinks replace water.
Replacement and retention are not identical
Some of what you drink is later excreted in urine. Controlled rehydration research shows that drink volume and sodium content interact to influence fluid retention.[18]
This explains why a sports professional may plan more than the estimated net deficit across a recovery period when rapid restoration is needed. It does not justify an immediate large bolus or treating a generic litres-per-kilogram formula as appropriate for every situation.
Allow comfortable drinking opportunities and consider the fluid in meals. Water with food, or another suitable drink alongside a meal, can be part of recovery. Fluid is only one need: energy, protein, rest and relief from heat may matter too.
Do not pursue a scale number at any cost
Body mass changes with food, urine, fuel stores and measurement conditions. Do not keep drinking simply to force an exact return to a previous number.
If the person gained weight during a long event, drank large amounts or has headache, vomiting or unusual behaviour, automatic fluid replacement may be the wrong response. Use the symptom guidance below and seek appropriate assessment.
Review what the session taught you
Was the drink accessible? Was the amount comfortable? Were conditions different from expected? These observations can improve the next plan more than adding a recovery product without a clear purpose.
Manage the heat load as well as the water loss
Hot weather, humidity, direct sun, protective clothing and limited airflow can all increase the challenge of exercising or working. Sweat loss is only part of the problem: the body also needs a way to release heat.
IOC recommendations for events in the heat emphasise preparation for the expected conditions, health assessment, cooling, clothing and event planning alongside hydration.[19] A bottle cannot compensate for a pace or environment that overwhelms cooling.
Adjust the activity
Where possible, choose a cooler time or place, reduce intensity, allow more recovery and make drinking opportunities practical. Build familiarity with hotter conditions gradually rather than treating the first hot day like a cool-weather session.
Changes should account for the person and activity. Heat tolerance differs, and a person who is ill or not accustomed to the conditions may need a more conservative plan.
Make workplace protection practical
Heat guidance for workplaces includes accessible water, rest and cooling opportunities, and changes to work demands when conditions become more difficult.[20] These arrangements are organisational responsibilities as well as personal habits.
Someone operating equipment or doing physical work should be able to stop, obtain a drink and cool down when needed. Protective clothing and limited breaks need to be considered in the plan, rather than assuming a reminder to drink solves the exposure.
Recognise when ordinary hydration advice is insufficient
Heat exhaustion and heatstroke require attention to symptoms and cooling, not just the amount drunk. A person with heatstroke may still be sweating.
Confusion, collapse or markedly altered behaviour during heat exposure is an emergency. Do not wait for dry skin or a particular home thermometer reading. The action guidance in section 13 explains the immediate response.[21]
Vomiting and diarrhoea can change the replacement needed
Gastrointestinal illness can cause losses of both water and electrolytes. The problem is different from ordinary thirst after a meal or a short walk.
Oral rehydration solution, or ORS, contains a specified balance of glucose and salts. Glucose and sodium absorption in the intestine helps water absorption. WHO guidance supports ORS for preventing and treating dehydration from diarrhoea in appropriate cases.[22]
ORS and sports drinks are not interchangeable
Sports drinks are formulated mainly for exercise-related fluid and fuel needs. Their glucose and electrolyte balance can differ substantially from ORS. A sports drink should not be assumed to replace a medically appropriate ORS when substantial gastrointestinal losses need treatment.
CDC travel-medicine guidance distinguishes packaged ORS for more serious fluid loss from the use of ordinary preferred fluids in milder situations.[23] The choice depends on the illness and person, not simply which bottle says “electrolytes.”
Prepare the product accurately
Use the exact water volume specified on the packet and a safe water source. Packets are not all designed for the same volume. Do not add extra salt or sugar, make a stronger mixture, or guess with a “pinch of salt.” Follow the instructions for preparation, storage and use.
If the person is alert and can swallow, small, repeated amounts may be easier to tolerate than a large drink. Persistent vomiting or inability to maintain intake needs clinical advice; severe symptoms should not be managed by continuing to try different drinks.
Know when to seek help
Very little urine, ongoing inability to keep fluids down, marked weakness, worsening symptoms or concern about an infant or frail person warrants prompt medical assessment. Confusion, collapse or reduced consciousness requires emergency help.[24]
Intravenous treatment, when needed, must be selected for the actual fluid and electrolyte problem. A routine “hydration drip” is not a substitute for finding the cause of significant illness.
Avoid overdrinking and recognise serious symptoms
Hyponatraemia means an abnormally low sodium concentration in blood. It has several possible causes. During or after prolonged exercise, excessive intake of water relative to losses, together with impaired water excretion, is an important mechanism.
The exercise-associated hyponatraemia consensus describes the risks of drinking beyond need and the contribution of vasopressin-mediated water retention.[25] The problem can occur with sports drinks as well as plain water.
Keep the prevention principle clear
Use thirst or an appropriately individualised plan, rather than forcing a fixed volume. Sodium supplements cannot cancel the effects of continuing to overdrink. Weight gain during exercise is a warning, but weight loss does not exclude hyponatraemia.[17]
Symptoms such as headache, nausea and weakness overlap with dehydration and heat illness. Serious or worsening symptoms after prolonged exercise need medical evaluation; symptoms alone cannot tell you to give more water.
| Situation | Appropriate response |
|---|---|
| Ordinary thirst, otherwise well and fully alert | Drink a comfortable amount, and rest or cool down if needed. |
| Persistent vomiting, very little urine, marked weakness or worsening illness | Arrange prompt medical assessment. Do not rely on a home hydration score. |
| Confusion, collapse, seizure or reduced consciousness | Call emergency services. Do not force oral fluids or give a drink to someone who cannot swallow safely. |
| Suspected heatstroke | Get emergency help and begin cooling immediately; move to a cooler place, remove excess outer clothing, wet the skin and fan while following emergency instructions. |
In Lithuania and throughout the EU, the emergency number is 112.[26] Elsewhere, use the local emergency number.
Stay with a person who is seriously unwell. Heatstroke requires rapid cooling and emergency care; sweating does not rule it out.[21] When sodium imbalance is possible, tell the medical team about recent activity and roughly how much fluid was consumed.
These actions are a guide to recognising urgency, not a way to diagnose the cause at home. The right treatment for one fluid disorder can worsen another.
Build a routine that works across changing circumstances
Most everyday hydration does not need elaborate tracking. A clear routine can prevent access problems while leaving room for thirst, preferences and changing needs.
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Make drinks easy to reach.
Keep a suitable drink available at home, work or during travel. Include comfortable opportunities with meals and between activities.
-
Count the whole routine.
Food, milk, tea and other drinks contribute. Avoid adding a full plain-water quota on top because you misread a total-water reference.
-
Anticipate unusual demands.
Plan access for heat, longer activity or travel. Use a more specific strategy when losses or health circumstances justify it.
-
Review several clues together.
Consider thirst, intake, recent losses, symptoms and comparable observations. Avoid chasing continuously clear urine or a device percentage.
-
Adapt when the situation changes.
Use appropriate ORS guidance for illness, seek help for concerning symptoms and follow any individual fluid plan.
Older adults and people needing assistance
Reduced thirst, difficulty obtaining drinks, swallowing problems or concerns about toileting can interfere with intake. ESPEN recommends treating older people as at risk of low-intake dehydration and supporting adequate drinking.[27]
Offer preferred drinks regularly, provide suitable containers and practical assistance, and address toileting access. Swallowing difficulties need an individual approach. A blanket instruction to drink more is not enough when the barrier is physical or environmental.
Pregnancy and breastfeeding
The European reference values account for increased needs in these stages. Keep drinks accessible and respond to thirst, while following individual advice if pregnancy or another condition complicates fluid balance.
Forcing extra fluid beyond need is not established as a way to increase milk production. A Cochrane review found the direct evidence limited and did not establish a benefit from advising extra fluids.[28] Concerns about feeding deserve appropriate support rather than pressure to consume ever more water.
Infants and children
Adult targets and sports strategies do not apply to infants. WHO advises that exclusively breastfed babies under six months do not need additional water, including in hot weather.[29]
Prepare infant formula exactly as directed. Do not dilute it to provide extra water or change the powder-to-water ratio.[30] Seek individual advice about illness, feeding or fluid concerns in a young baby.
Older children need age-appropriate access to drinks and adult support during activity or illness. Do not assume a child will always notice and communicate a developing problem.
Medical fluid plans come first
Some conditions or treatments require specific fluid or electrolyte guidance. If you have a prescribed limit or replacement plan, ask how to adapt it during heat, exercise or illness. A commercial drink label cannot make that decision for you.
Clear answers for everyday hydration
Do I need two litres of plain water every day?
Not as a universal rule. European reference values concern total water from food and drinks, and individual needs change with circumstances.
Does tea or coffee count?
Yes, these drinks contribute fluid. Caffeine dose, tolerance and effects on sleep still matter; a drink’s water content does not make every amount suitable.
Should my urine always be completely clear?
No. Urine colour varies with timing and other factors. Continuously clear urine is not a required goal, and a single colour cannot diagnose fluid balance.
Am I already dangerously dehydrated when I feel thirsty?
Usually, thirst is an ordinary regulatory signal. Its significance depends on symptoms, recent losses, access to fluid and health; it is not a measure of severity by itself.
Do I need electrolytes for every workout?
No. Water and regular meals are often sufficient for shorter or ordinary activity. Longer sweaty sessions may justify a more individual fluid and sodium plan.
Can a sports drink prevent hyponatraemia?
Not if you keep drinking more fluid than the body can handle. Electrolytes do not make overdrinking safe.
Does a sweat-rate estimate tell me exactly how much to drink?
No. It helps describe losses in a particular session. Use it with conditions, thirst, tolerance and the purpose of the activity rather than treating it as a compulsory rate.
Are sports drinks and oral rehydration solutions the same?
No. Their formulations and purposes differ. Use an appropriate ORS and its instructions when gastrointestinal fluid losses call for it.
Does losing weight during a workout mean I lost fat?
A rapid change often reflects net fluid changes and other short-term factors. It is not a direct measure of fat loss or a reason to restrict drinking.
Can a person with heatstroke still be sweating?
Yes. Confusion, collapse or altered behaviour in the heat requires emergency action even if the person is sweating. Begin cooling and get help.
Sources and scope
Evidence and guidance checked: 5 September 2026. Sources include European dietary reference values, sports-medicine consensus statements, hydration research and guidance on illness and heat.
The main audience is adults. Brief sections on pregnancy, breastfeeding and children highlight situations where adult exercise advice does not transfer directly. Numerical examples explain calculations; they are not individual prescriptions.
Clinical dehydration, changes in circulating volume and blood sodium disorders overlap but are not identical. The article therefore avoids diagnosing severity from a fixed percentage, urine colour or a consumer device.
Research findings are interpreted within their methods and populations. A short laboratory study, an athlete review or a diagnostic study in older people does not provide a universal drinking rule.
- Popkin BM, D’Anci KE, Rosenberg IH. Water, hydration, and health. Nutrition Reviews. 2010;68(8):439–458.Review of body-water functions, regulation and health; broader claims about extra water require outcome-specific evidence. Return to text ↑
- Zimmerman CA, Leib DE, Knight ZA. Neural circuits underlying thirst and fluid homeostasis. Nature Reviews Neuroscience. 2017;18(8):459–469. DOI: 10.1038/nrn.2017.71.Mechanistic review of thirst and anticipatory regulation; much of the circuit-level evidence comes from animal research. Return to text ↑
- EFSA Panel on Dietetic Products, Nutrition, and Allergies. Scientific Opinion on Dietary Reference Values for water. EFSA Journal. 2010;8(3):1459. DOI: 10.2903/j.efsa.2010.1459.Total-water references include food and beverages and assume moderate conditions. Return to text ↑
- National Institute of Diabetes and Digestive and Kidney Diseases. Eating, Diet, & Nutrition for Kidney Stones. Patient guidance.Fluid intake in stone prevention, with individual advice and kidney-function considerations. Return to text ↑
- Killer SC, Blannin AK, Jeukendrup AE. No Evidence of Dehydration with Moderate Daily Coffee Intake: A Counterbalanced Cross-Over Study in a Free-Living Population. PLOS ONE. 2014;9(1):e84154. DOI: 10.1371/journal.pone.0084154.Controlled study in habitual male coffee drinkers; the findings do not establish suitability of every caffeine dose. Return to text ↑
- World Health Organization. Healthy diet. Public-health guidance, updated 26 January 2026.Dietary adequacy and variety, with distinctions between free sugars and sugars within intact foods. Return to text ↑
- Cheuvront SN, Kenefick RW, Zambraski EJ. Spot Urine Concentrations Should Not be Used for Hydration Assessment: A Methodology Review. International Journal of Sport Nutrition and Exercise Metabolism. 2015;25(3):293–297.Explains timing and interpretation problems with isolated urine-concentration measurements. Return to text ↑
- Hooper L, Abdelhamid A, Attreed NJ, et al. Clinical symptoms, signs and tests for identification of impending and current water-loss dehydration in older people. Cochrane Database of Systematic Reviews. 2015;4:CD009647. DOI: 10.1002/14651858.CD009647.pub2.Diagnostic review showing that common individual signs and simple tests should not be relied on to diagnose water-loss dehydration in older people. Return to text ↑
- Cheuvront SN, Kenefick RW. Dehydration: physiology, assessment, and performance effects. Comprehensive Physiology. 2014;4(1):257–285. DOI: 10.1002/cphy.c130017.Distinguishes osmotic and volume effects, measurement variation and exercise-performance findings. Return to text ↑
- Wittbrodt MT, Millard-Stafford M. Dehydration Impairs Cognitive Performance: A Meta-analysis. Medicine & Science in Sports & Exercise. 2018;50(11):2360–2368. DOI: 10.1249/MSS.0000000000001682.Average cognitive effects varied by task, water deficit and experimental design; a small deficit does not imply a uniform response. Return to text ↑
- Kenefick RW. Drinking Strategies: Planned Drinking Versus Drinking to Thirst. Sports Medicine. 2018;48(Suppl 1):31–37.Reviews how duration, environment, access and performance goals influence exercise drinking strategies. Return to text ↑
- McDermott BP, Anderson SA, Armstrong LE, et al. National Athletic Trainers’ Association Position Statement: Fluid Replacement for the Physically Active. Journal of Athletic Training. 2017;52(9):877–895. DOI: 10.4085/1062-6050-52.9.02.Individual fluid replacement, assessment and the risks of both inadequate and excessive intake during activity. Return to text ↑
- Maughan RJ, Shirreffs SM. Development of individual hydration strategies for athletes. International Journal of Sport Nutrition and Exercise Metabolism. 2008;18(5):457–472.Personalised strategies that account for exercise, environment and individual circumstances. Return to text ↑
- Baker LB. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Medicine. 2017;47(Suppl 1):111–128. DOI: 10.1007/s40279-017-0691-5.Measurement methods and sources of variation. The worked calculation in this article is an original hypothetical example. Return to text ↑
- Burke LM, Hawley JA, Wong SHS, Jeukendrup AE. Carbohydrates for training and competition. Journal of Sports Sciences. 2011;29(Suppl 1):S17–S27. DOI: 10.1080/02640414.2011.585473.Sports-nutrition review. Fuel needs and carbohydrate timing vary with the demands of training and competition. Return to text ↑
- Miller KC, et al. An Evidence-Based Review of the Pathophysiology, Treatment, and Prevention of Exercise-Associated Muscle Cramps. Journal of Athletic Training. 2022;57(1):5–15. DOI: 10.4085/1062-6050-0696.20.Supports an individual, multifactorial interpretation rather than diagnosing an electrolyte deficit from cramps alone. Return to text ↑
- Bennett BL, Hew-Butler T, Rosner MH, Myers T, Lipman GS. Wilderness Medical Society Clinical Practice Guidelines for the Management of Exercise-Associated Hyponatremia: 2019 Update. Wilderness & Environmental Medicine. 2020. DOI: 10.1016/j.wem.2019.11.003.Prevention and clinical management. Sodium supplementation cannot prevent hyponatraemia when excessive drinking continues. Return to text ↑
- Shirreffs SM, Taylor AJ, Leiper JB, Maughan RJ. Post-exercise rehydration in man: effects of volume consumed and drink sodium content. Medicine & Science in Sports & Exercise. 1996;28(10):1260–1271.Controlled study of fluid retention; drink volume and sodium content interact. Its protocol is not a universal home-treatment instruction. Return to text ↑
- Racinais S, et al. IOC consensus statement on recommendations and regulations for sport events in the heat. British Journal of Sports Medicine. 2023;57(1):8–25.Heat preparation and event management, including hydration, cooling, clothing and environmental conditions. Return to text ↑
- National Institute for Occupational Safety and Health. Workplace Recommendations: Heat Stress. Occupational-health guidance.Access to water, rest, cooling and adjustment of work demands. Used here for general prevention principles, not local legal requirements. Return to text ↑
- National Institute for Occupational Safety and Health. Heat-related Illnesses. Symptoms and first-aid guidance, updated 3 March 2026.Heatstroke can involve dry skin or profuse sweating; rapid cooling and emergency care are priorities. Return to text ↑
- World Health Organization. Oral rehydration salts. WHO/FCH/CAH/06.1. 2006.Glucose-electrolyte oral rehydration for diarrhoeal fluid losses in appropriate cases. Return to text ↑
- Centers for Disease Control and Prevention. Travelers’ Diarrhea. CDC Yellow Book; updated 24 March 2026.Distinguishes ORS from other fluids according to loss severity and explains preparation with the indicated water volume. Return to text ↑
- MedlinePlus Medical Encyclopedia. Dehydration. Patient information, reviewed 1 July 2025.Symptoms and indications for prompt or emergency medical assessment. Return to text ↑
- Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference. British Journal of Sports Medicine. 2015;49(22):1432–1446. DOI: 10.1136/bjsports-2015-095004.Title shortened. International consensus on mechanisms and prevention of exercise-associated hyponatraemia. Return to text ↑
- European Commission. 112—the EU’s emergency phone number. European emergency-service information.112 is available throughout the European Union, including Lithuania. Return to text ↑
- Volkert D, Beck AM, Cederholm T, et al. ESPEN practical guideline: Clinical nutrition and hydration in geriatrics. Clinical Nutrition. 2022;41(4):958–989.European clinical guidance on low-intake dehydration risk and practical support for drinking in older people. Return to text ↑
- Ndikom CM, Fawole B, Ilesanmi RE. Extra fluids for breastfeeding mothers for increasing milk production. Cochrane Database of Systematic Reviews. 2014;6:CD008758. DOI: 10.1002/14651858.CD008758.pub2.Limited, low-quality direct evidence; no established benefit from advising extra fluids beyond physiological needs. Return to text ↑
- World Health Organization. Breastfeeding. Questions and answers, updated 2 October 2024.Exclusive breastfeeding and the absence of a routine need for added water before six months, including hot weather. Return to text ↑
- Centers for Disease Control and Prevention. Infant Formula Preparation and Storage. Infant-feeding guidance, updated 14 May 2026.Use the specified water-to-formula ratio; dilution and overconcentration create nutritional and health risks. Return to text ↑
General education. Fluid restrictions, medical replacement plans and significant symptoms require individual guidance.