Water: the overlooked foundation of health and performance

Nutrition discussions often begin with protein, fat and carbohydrate. Before long, the conversation becomes a debate about which dietary pattern is best. Water tends to receive less attention, despite being essential to every one of those approaches.

Water does not provide energy in the way the three main macronutrients do. It provides the environment in which much of our physiology takes place. We obtain it from drinks and food, and continually lose it through urine, sweat, breathing and the gut.

The body is constantly adjusting that balance. A reasonable intake on a cool rest day may be inadequate during prolonged exercise in the heat. Equally, forcing down large volumes because more seems healthier can create a different problem.

Hydration needs context.

Sources:[1][2]

Water is the main component of plasma, helping maintain the circulating volume that carries oxygen, nutrients, hormones and waste products around the body.

It is central to temperature regulation. Working muscle produces heat. Blood flow carries heat towards the skin, and sweat removes heat as it evaporates. Temperature, humidity, clothing and available body water all influence how effectively that system works.

Inside and outside cells, water provides the medium for chemical reactions and the distribution of dissolved substances. Sodium and Potassium contribute to fluid balance and the electrical gradients required for nerve and muscle function.

The kidneys filter the blood, recover needed water and solutes, and excrete waste. They work with thirst and hormonal signals to adjust excretion as circumstances change. Drinking a glass of water does not simply add a fixed amount to a tank and then pass straight through.

Water also contributes to digestive secretions and stool consistency. Normal fluid balance supports physical and cognitive function, but headache, fatigue and poor concentration have many possible causes. They should not automatically be explained as dehydration.

Sources:[2][3][5]

The NHS guide of six to eight cups or glasses of fluid daily is a population starting point. It includes drinks other than plain water, and food also provides moisture.

Requirements vary with body size and composition, diet, age, weather, altitude, clothing, exercise intensity and duration, sweat rate, heat acclimatisation and alcohol intake. Fever, vomiting and diarrhoea can change losses substantially. Pregnancy and breastfeeding can increase requirements too.

Medicines that affect urine output and conditions involving the heart, kidneys or endocrine system can change what is appropriate. A person with a prescribed fluid restriction should not replace that advice with a general internet target.

A fixed litre goal can therefore be too much for one person and too little for another. The day's circumstances and existing medical guidance matter more than reaching an arbitrary number.

Sources:[1][2][4][23]

Two people can complete the same session and lose very different amounts of sweat. Intensity, duration, heat, humidity, clothing, acclimatisation, body size and individual physiology all contribute.

Comparing body mass before and after a longer session can help estimate the scale of loss. Measurements need to be made under comparable conditions, with drinks and urine during the session accounted for.

The result is still an estimate. Food and fluid in the gut, wet clothing, scale error, and the fuel and water used or produced during exercise introduce uncertainty. One session also cannot determine requirements for every future session, especially when the weather or intensity changes.

The practical aim is to avoid deliberately starting underhydrated and to avoid gaining body mass by forcing down excess fluid during exercise. A prolonged event, demanding work in heat, previous heat illness or exercise-associated hyponatraemia may justify an individual plan with a sports dietitian or clinician.

Sources:[4][5][6]

Dehydration broadly describes a shortfall in body water. Reduced circulating volume, sometimes termed hypovolaemia, is related but not identical; water and salt losses can occur in different proportions.

Thirst, dry mouth, headache, tiredness, dizziness, darker urine and reduced urine output may accompany dehydration. They are not specific enough to establish the diagnosis on their own. Illness, fever, gastrointestinal losses and heavy sweating can increase risk. Older adults are particularly vulnerable, although this article concerns adults generally.

Excessive intake can also be dangerous. If water is consumed faster than the body can excrete it, the sodium concentration in blood may fall. During or shortly after exercise, this may represent exercise-associated hyponatraemia.

It can occur with water or sports drinks. Adding some sodium to a drink does not cancel the risk created by excessive fluid intake relative to losses and impaired water excretion.

Early symptoms such as headache, nausea and lethargy can overlap with dehydration or heat illness. Confusion, seizures or reduced consciousness are emergency signs. Guessing whether someone needs more water or more salt is not an adequate response to serious symptoms.

Sources:[6][8][23][25]

Thirst is useful in ordinary daily life and many exercise settings. Its reliability can be affected by age, illness, medicines and whether drinks are readily available.

Pale yellow urine is a useful everyday prompt. Darker urine and passing less than usual can accompany dehydration, but first-morning urine is normally more concentrated. Foods, vitamins, medicines, exercise, alcohol and illness can alter colour or frequency. The association between urine colour and hydration is also less reliable in older adults.

Colourless urine is not a target. It may simply reflect recent high fluid intake.

Consider what has happened recently: heat exposure, travel, exercise, alcohol, fever, vomiting or diarrhoea. Where appropriate, repeated body-mass comparisons around exercise can add information, but should not become a rigid drinking prescription based on one workout.

If symptoms persist, the clues disagree or urine is red, brown or otherwise unusual, seek clinical advice. Blood in urine must not be dismissed as a hydration issue.

Sources:[1][5][7][22][23]

For everyday activity and many shorter or easier sessions, water with normal meals is generally adequate. Food provides Sodium, Potassium and other minerals. An electrolyte drink is not an automatic requirement for anyone who exercises.

A drink containing Sodium and carbohydrate may be useful when exercise is prolonged or demanding, sweat losses are high, the weather is hot, recovery time is limited or food is impractical. The formulation and amount depend on the person, event, diet and environment.

Sports drinks and oral rehydration solutions are not interchangeable. Vomiting and diarrhoea can produce losses of water and salts; a pharmacist can advise on an appropriate oral rehydration product where indicated.

Electrolytes are physiologically active substances. They are not a harmless insurance policy, particularly for someone with kidney, heart or liver disease, an endocrine condition or a fluid restriction. A clinically significant Sodium or Potassium abnormality requires diagnosis and supervised management, not a supplement chosen from the laboratory flag.

Sources:[1][5][6][23][26]

A routine panel does not diagnose hydration. It may reveal a pattern that raises questions about fluid balance, but several different mechanisms can produce similar results.

Urea is produced during protein metabolism and cleared mainly by the kidneys. It can rise when kidney perfusion or circulating volume is reduced. Protein intake, gastrointestinal bleeding, kidney clearance and severe liver disease can also alter it. Urea alone cannot distinguish those explanations.

Creatinine reflects creatine metabolism, particularly in muscle. It is used in equations that estimate glomerular filtration rate, or eGFR. That estimate is not a direct measurement of filtration.

Muscle mass, recent meat intake, supplements, acute illness and kidney perfusion can affect interpretation. Creatine supplementation can modestly raise serum Creatinine without a corresponding fall in measured filtration. Recent hard exercise can add further changes through muscle metabolism and plasma-volume shifts.

Those possibilities are reasons to interpret a result carefully, not reasons to ignore an unexpected deterioration. Clinical history, medicines, previous results, examination and sometimes additional blood or urine tests may be needed.

Sources:[8][9][10][11][12][13]

Sodium and Potassium results are concentrations in a blood sample. They do not directly measure total body water or total electrolyte stores.

Sodium can change with the relationship between salt and water, kidney function and hormonal regulation. A normal Sodium result does not prove ideal hydration, and a low value is not automatically a reason to eat more salt or restrict fluid without advice.

Potassium can change with kidney disease, medicines and gastrointestinal losses, among other causes. Damage to blood cells during collection or handling can release Potassium and produce a falsely high result. The possibility of a collection problem needs proper assessment because genuinely abnormal Potassium can be important.

Do not use an isolated result to decide independently to drink more, drink less or take electrolytes. The same flag can have different causes and require very different responses.

Sources:[8][9][10][14][25]

When plasma volume falls, some measured concentrations can appear higher even if the total amount in the body has not increased. This is haemoconcentration. Expansion of plasma volume can produce the opposite effect, haemodilution.

Haemoglobin, Haematocrit and red blood cell count may therefore be affected by fluid shifts. Haematocrit is also influenced by posture, sampling, recent exercise, heat acclimatisation and the individual's baseline.

A raised red cell count has other possible causes, including smoking, altitude, lung or heart disease and bone marrow conditions. A high result should not automatically be attributed to dehydration. A normal result does not exclude fluid imbalance either.

There is no reliable hydration score hidden in a Full Blood Count. The concentration effects are real, but neither sufficiently uniform nor specific for that purpose.

Sources:[15][16][17]

Albumin is a major plasma protein. Concentration changes can influence its measured value, but inflammation, liver function, malabsorption and protein loss through the kidneys or intestines also matter. Low Albumin is not proof of overhydration, and high Albumin is not a stand-alone test for dehydration.

Bilirubin is produced from the breakdown of red blood cells and processed by the liver. It should not be treated as a hydration marker or assumed to normalise simply by drinking more water.

Gilbert syndrome is a specific exception worth understanding: dehydration can trigger an isolated unconjugated Bilirubin rise in some people with this benign inherited condition. Even then, a new or unusual result needs to fit the established clinical picture.

Other explanations for raised Bilirubin include haemolysis, liver disease and bile-flow obstruction. Yellowing of the skin or whites of the eyes requires urgent medical assessment rather than an assumption about hydration.

Sources:[18][19][20][21]

Drink regularly, respond to thirst in context and remember that food and other drinks contribute. Plan access to fluid when heat, travel or longer activity will make that difficult. Avoid deliberate restriction and forced overdrinking.

For a blood test, follow the supplied preparation instructions. Drinking unusually large volumes to improve a result can change the sample rather than make it more representative.

Anyone with significant heart, kidney or liver disease, an endocrine disorder or a prescribed fluid limit needs individual advice. Pregnancy and medicines affecting fluid balance may also change what is appropriate.

Depending on the panel, Praetor tests may include Kidney Function, Full Blood Count, Albumin and Bilirubin. These results can contribute to assessment, but no panel diagnoses hydration or sets an individual fluid target.

Sources:[1][4][23][26]

Call 999 if someone is confused, has a seizure, collapses, loses consciousness, is difficult to wake, has difficulty breathing or has signs of heatstroke, including very hot skin with neurological symptoms. During or after exercise, these findings can have several causes. Do not assume that giving more water is the answer.

Seek urgent advice through NHS 111 for persistent dizziness on standing, markedly reduced urine output, rapid breathing or heartbeat, unusual drowsiness, repeated vomiting or inability to keep fluids down. Following prolonged exercise, new headache, nausea, weakness or swelling, particularly after drinking large amounts, also needs prompt assessment.

Symptoms, examination and the wider circumstances guide the response. A laboratory result cannot replace that assessment.

Sources:[6][23][24][25]