Hydration advice circulates with unusual confidence given how little of it comes from research, and the underlying physiology is well understood.
The regulation
Body water is regulated by mechanisms that detect blood concentration and volume.
Which trigger thirst and adjust kidney water retention through hormonal signalling.
The system is precise, maintaining concentration within a narrow range under ordinary conditions.
Where the eight glasses figure came from
Traced back to a recommendation that included water contained in food, with the food component subsequently dropped in repetition.
Which means the widely repeated figure omits a substantial portion of what the original recommendation counted.
Health authorities generally publish total fluid intake recommendations that include all beverages and food, and these vary by age, sex and conditions.
What counts
All beverages contribute, including tea and coffee.
The diuretic effect of caffeine at typical intakes is modest and does not produce net fluid loss.
Which means the common advice that coffee dehydrates is not supported at ordinary consumption.
Food contributes substantially, particularly fruit, vegetables and anything cooked in water.
Thirst
An effective signal in healthy adults under normal conditions.
Which means drinking to thirst is generally adequate, and the claim that thirst indicates existing dehydration is not well supported.
Thirst sensitivity does decline with age, which is why older adults are advised to drink to a schedule rather than to thirst.
Assessing hydration
Urine colour is a rough indicator, with pale yellow generally indicating adequate hydration.
Which is affected by vitamins, medications and foods, so it is approximate.
Very concentrated dark urine, reduced output, dizziness on standing and dry mouth are the practical signs of inadequate intake.
Overhydration
Genuine and occasionally serious.
Drinking substantially more than the kidneys can excrete dilutes blood sodium, which can cause confusion, seizures and, in severe cases, death.
Which has occurred in endurance events and in water-drinking challenges.
The risk is low under ordinary circumstances and real at extremes.
Exercise
Fluid and electrolyte requirements rise with sweat loss, which varies enormously between individuals and conditions.
Guidance has shifted from encouraging maximum intake toward drinking to thirst for most recreational activity.
For prolonged intense exercise in heat, structured intake including electrolytes has evidence.
Specific conditions
Kidney disease, heart failure, certain medications and some hormonal conditions all affect fluid handling.
Which means general advice does not apply, and intake should be discussed with a doctor.
Anyone with persistent excessive thirst, greatly increased urination, or unexplained changes in either should see a doctor, since these can indicate conditions requiring treatment.
Electrolytes
Sodium, potassium and other electrolytes are lost in sweat and are required for fluid balance.
Which means replacing water alone after very heavy sweating can dilute blood sodium.
For ordinary activity and diet, dietary intake is adequate and electrolyte supplementation is unnecessary.
For prolonged intense activity in heat, structured replacement has evidence.
Older adults
Thirst sensitivity declines with age, and kidney concentrating ability reduces.
Which makes dehydration more likely and less well signalled.
Regular scheduled drinking rather than waiting for thirst is generally recommended, and it matters particularly during illness and hot weather.
Illness
Fever, vomiting and diarrhoea all increase fluid loss substantially.
Which is why oral rehydration solutions exist, containing a specific balance of glucose and electrolytes that improves absorption.
Signs of dehydration in children and older adults warrant prompt medical attention, since deterioration can be rapid.
Water quality
Tap water in most developed countries meets standards more stringent than those applying to bottled water.
Which is published in annual quality reports by suppliers.
The environmental cost of bottled water is substantial relative to any measurable benefit in those contexts.
Caffeine and fluid
Studies of habitual caffeine consumers find no meaningful net fluid loss at ordinary intakes.
Which means tea and coffee count toward fluid intake, contrary to widespread advice.
Very high acute intakes in people unaccustomed to caffeine do produce a diuretic effect.
Practical approach
Drinking to thirst, with attention during illness, heat and exercise, and having water accessible.
Which is what the physiology supports, and it is considerably less effortful than tracking.
Children
Have higher fluid requirements relative to body weight and are more vulnerable to dehydration during illness.
Which is why reduced wet nappies or urine output is a specific warning sign warranting medical attention.
Sports drinks
Formulated for prolonged intense exercise, containing carbohydrate and electrolytes.
Which is appropriate for that context and unnecessary for ordinary activity, where they add sugar without benefit.
For sessions under about an hour, water is adequate for most people.
Practical summary
Drink to thirst, count all beverages and food, pay more attention during illness, heat and exercise, and pay more attention with increasing age.
Which is what the physiology supports and it is considerably simpler than tracking targets.
Anyone with a kidney, heart or hormonal condition should follow the advice specific to it rather than general guidance.