Water: the overlooked foundation of health and performance

Nutrition conversations often start with protein, fat and carbohydrate. They quickly become conversations about identity too: keto, low-carbohydrate, carnivore, pescatarian, vegetarian or vegan. Each pattern can be followed well or poorly. None changes the basic fact that every person depends on water.

Water is an essential nutrient, but calling it a macronutrient can cause confusion because it does not provide energy in the way protein, fat and carbohydrate do. A more useful description is that water is the medium in which much of human physiology happens. Food contributes some water, drinks contribute the rest, and the body continually balances intake against losses through urine, sweat, breathing and the gut.

That balance is dynamic. A routine that feels adequate on a cool rest day may not match a long session in the heat, an illness with fever, or a day when access to drinks is limited. Hydration is therefore less about chasing one perfect number and more about noticing context.

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Circulation: water is the main component of blood plasma. It helps maintain the volume through which oxygen, nutrients, hormones and waste products are transported.

Temperature control: working muscles produce heat. Blood carries some of that heat towards the skin, and sweat can remove heat as it evaporates. Both processes depend on available body water and are affected by the environment.

Cells and chemistry: water provides the setting for metabolic reactions and helps maintain the concentration of dissolved substances inside and outside cells. Sodium and potassium are part of this tightly regulated electrical and fluid system. They also support nerve signalling and muscle function.

Kidneys and waste: the kidneys filter blood, return needed water and solutes to the circulation, and excrete waste in urine. They work with thirst and hormones to adjust water balance rather than simply letting every drink pass straight through.

Digestion and function: water contributes to saliva, digestive secretions and stool consistency. Normal fluid balance also supports physical and cognitive function, although tiredness, headache or poor concentration are non-specific symptoms and should not automatically be labelled dehydration.

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The NHS gives 6 to 8 cups or glasses of fluid a day as a general population guide. It is not a personalised prescription, and it includes drinks other than water. Moisture in food contributes too. The NHS also notes that more fluid may be needed during pregnancy or breastfeeding, in hot environments, during long periods of physical activity, and during or after illness.

Individual need varies with body size and composition, habitual diet, food moisture, age, weather, altitude, clothing, heat acclimatisation, exercise intensity and duration, sweat rate, illness and alcohol intake. Medicines that change urine output and conditions affecting the kidneys, heart or hormones can change the calculation again.

This is why a fixed daily litre target can be falsely reassuring for one person and excessive for another. Population guidance is a starting point. The circumstances of the day, and any medical guidance already given, matter more than an internet target.

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Exercise increases heat production. Sweating helps remove that heat, but sweat loss varies widely between people and between sessions. Body size, intensity, duration, temperature, humidity, clothing, acclimatisation and genetics all contribute. This is why two people completing the same workout may finish with very different fluid losses.

A change in body mass before and after a longer session can help estimate the scale of sweat loss when measurements are made under comparable conditions and account for drinks and urine during the session. It is still an estimate, not a universal drinking instruction. Food and drink in the gut, clothing, scale error and the fuel and water used or produced during exercise all add noise. One session also cannot define every future session, especially when the weather or intensity changes.

The practical aim is to avoid starting exercise deliberately underhydrated and to avoid gaining weight by forcing down fluid during exercise. People training for prolonged events, working in heat, or with a history of heat illness or exercise-associated hyponatraemia need an individual plan from an appropriately qualified sports dietitian or clinician.

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Dehydration broadly means losing more fluid than is taken in. Thirst, a dry mouth, headache, tiredness, dizziness, darker urine and passing urine less often can occur, but none is unique to dehydration. Vomiting, diarrhoea, fever, heavy sweating and hot conditions can increase risk. Older adults and young children are more vulnerable, although this article is written for adults.

More fluid is not always safer. Drinking faster than the body can excrete water can dilute blood sodium. During prolonged exercise this is called exercise-associated hyponatraemia. It has occurred with water and with sports drinks because the central problem is excessive fluid relative to losses. Early symptoms such as headache, nausea or lethargy overlap with heat illness and dehydration, while confusion, reduced consciousness or seizures are emergency signs.

Hydration problems are not all the same. Water loss, salt-and-water loss and fluid overload can produce different patterns and require different clinical responses. That is why serious symptoms should not be managed by guessing between plain water and electrolytes.

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At home, use several clues together rather than turning one sign into a score.

Thirst: notice it, but place it in context. Thirst can be useful during ordinary daily life and many exercise settings, while age, illness, access to drinks and some medicines can alter the signal.

Urine: pale yellow urine is a useful day-to-day prompt. Darker urine or passing less than usual can accompany dehydration. However, urine colour is a prompt, not a diagnosis. A first-morning sample is normally more concentrated, and vitamins, medicines, foods, alcohol, exercise and illness can change colour or frequency. The association with hydration is weaker in older adults. Colourless urine can follow high fluid intake and is not a goal.

Recent context: consider heat, exercise, travel, alcohol, fever, vomiting or diarrhoea. For prolonged exercise, a repeatable pre-session and post-session body-mass comparison may add information, but it should not be converted into a rigid prescription from a single workout.

If the clues disagree, symptoms persist, or urine is red, brown or otherwise unusually coloured, seek clinical advice rather than assuming hydration is the explanation.

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For everyday activity and many shorter or easier sessions, water alongside normal meals is generally sufficient. Food supplies sodium, potassium and other minerals, and the body regulates their blood concentrations closely. An electrolyte product is not an automatic requirement for everybody who exercises.

Drinks containing sodium and carbohydrate can be relevant when exercise is prolonged or hard, sweat losses are high, conditions are hot, recovery time is short, or food is not practical. The useful composition and amount depend on the person, session, diet and environment. A drink containing electrolytes does not cancel the danger of overdrinking.

Vomiting and diarrhoea can cause losses of water, sugar, salts and minerals. NHS guidance recommends asking a pharmacist about an appropriate oral rehydration solution. A sports drink is not automatically equivalent to a clinically formulated oral rehydration solution.

Electrolytes are not a harmless insurance policy. They add substances that may be inappropriate for some people, and they cannot correct a clinically important sodium or potassium abnormality without diagnosis and supervision. Anyone with significant heart, kidney or liver disease, an endocrine condition, or an existing fluid restriction should follow their healthcare professional's advice rather than a generic sports routine.

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A routine blood panel does not diagnose hydration. It can show a pattern that prompts questions about fluid balance, but the same pattern can have several explanations.

Urea is produced when the body processes protein and is cleared mainly by the kidneys. Its concentration may rise when circulating fluid volume is reduced, but it can also change with protein intake, gastrointestinal bleeding, severe liver disease and kidney clearance. Urea alone cannot separate those causes.

Creatinine and eGFR are interpreted together. Creatinine reflects muscle creatine turnover, and creatinine-based eGFR is an estimate rather than a direct measurement. Hydration and kidney function matter, but so do muscle mass, meat or protein intake and supplements. Current evidence shows that creatine supplementation can cause a modest rise in serum creatinine without a corresponding fall in measured filtration, which is another reason the result needs context. Recent strenuous exercise may also complicate interpretation through changes in muscle metabolism and plasma volume.

Sodium and potassium are concentrations in blood, not measures of how much water or electrolyte is stored in the whole body. A normal result does not prove optimal hydration. Sodium can change with the balance of salt and water and with kidney or endocrine conditions. Potassium can change with kidney function, severe vomiting or diarrhoea, and some medicines. Damage to blood cells during collection or handling can falsely raise the reported potassium result. Abnormal results require clinical interpretation and should never trigger a do-it-yourself instruction to drink more water, restrict water or take electrolytes.

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Fluid shifts can change the concentration of substances measured in blood without changing the total amount in the body. This is often called haemoconcentration when plasma volume falls, or haemodilution when it rises. The effect is real, but it is neither uniform nor specific enough to create a hydration score.

Haematocrit and haemoglobin may appear higher when plasma volume is lower. Haematocrit is also affected by posture, sampling technique, heat acclimatisation, exercise and individual baseline. Exercise itself can shift plasma volume for some time.

Red blood cell count can also be higher when plasma volume is lower. Other causes of a high red blood cell count include altitude, smoking, lung or heart conditions and bone marrow disease.

A raised result in any of these markers cannot be assumed to be dehydration, and a normal result does not rule fluid imbalance out.

Albumin is a major plasma protein. Concentration can make a result appear higher, but albumin is also shaped by inflammation, liver function, malabsorption and protein loss through the kidneys or intestines. Low albumin is not proof of overhydration, and high albumin is not a direct hydration test.

Bilirubin comes from red blood cell breakdown and is processed by the liver. Bilirubin is not a hydration biomarker, and an elevated result should not be explained as dehydration or expected to resolve simply by drinking water. One narrow exception is Gilbert syndrome: dehydration can trigger an isolated unconjugated bilirubin rise in some people with the condition. Raised bilirubin has other possible causes, including haemolysis, liver disease and bile-flow problems, so it belongs in the wider result pattern. Yellowing of the skin or whites of the eyes needs urgent medical assessment and should not be attributed to hydration.

Trends under comparable test conditions are often more informative than a single isolated number. Even then, abnormal results need review alongside symptoms, medical history, medicines, examination and other tests.

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A broadly safe routine is deliberately unremarkable: drink regularly across the day, respond to thirst and context, include the fluid in food and other drinks, and plan access to drinks when heat or longer exercise will increase losses. Avoid both deliberate restriction and forced overdrinking. Follow the preparation instructions supplied for a blood test rather than trying to manipulate a result by drinking unusually large amounts beforehand.

People with significant heart, kidney or liver disease, an endocrine condition, or an existing fluid restriction should follow their healthcare professional's advice. The same applies during pregnancy or when taking medicines that affect fluid balance. This article cannot replace individual clinical guidance.

Call 999 if someone is confused, has a seizure, collapses, loses consciousness, is difficult to wake, has difficulty breathing, or has very hot skin with signs of heatstroke. During or after exercise, these symptoms can have several causes, including heatstroke and exercise-associated hyponatraemia. Do not assume the person simply needs more water.

Contact NHS 111 urgently for persistent dizziness on standing, much less urine than usual, rapid breathing or a fast heartbeat, unusual drowsiness, repeated vomiting or an inability to keep fluids down. After prolonged exercise, new headache, nausea, weakness, swelling or puffiness, particularly after drinking large amounts, also needs prompt assessment.

Blood testing can add useful context by measuring kidney-function, blood-count and liver-related markers, depending on the panel. It does not diagnose hydration, prescribe a fluid target or explain an abnormal result on its own.

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