Hydration: Separating Evidence From Marketing

Total fluid requirement varies considerably with body size, activity, temperature and diet, and a meaningful proportion of intake comes from food rather than drinks.

Thirst works

In healthy adults, thirst is a well-calibrated mechanism that responds to changes in blood concentration long before any harm occurs.

Drinking to thirst is adequate for most people in most circumstances. The idea that thirst indicates you are already dangerously dehydrated is not supported by physiology.

Exceptions exist and matter: older adults have a blunted thirst response, young children rely on carers, and certain medical conditions and medications affect fluid balance.

Urine colour as a rough guide

Pale yellow generally indicates adequate hydration. Dark concentrated urine suggests drinking more would be sensible.

The caveat is that some vitamins, particularly B vitamins, colour urine brightly and make the guide useless. Certain medications and foods do the same.

Completely colourless urine throughout the day may indicate drinking considerably more than needed, which is generally harmless but unnecessary.

All fluids count

Tea, coffee, milk, juice and soft drinks all contribute to fluid intake. The idea that caffeinated drinks cause net dehydration is not supported at typical consumption levels; the diuretic effect is modest and outweighed by the fluid volume.

Food contributes meaningfully. Fruit, vegetables, soups and many cooked foods carry substantial water.

When requirements genuinely rise

Prolonged exercise, particularly in heat, increases losses substantially through sweat. Endurance activity beyond an hour or two also involves electrolyte losses that plain water does not replace.

Fever, vomiting and diarrhoea increase losses and are the common situations where deliberate rehydration matters, especially in children and older adults. Oral rehydration solutions are designed for this and outperform water alone.

Hot climates and physically demanding work raise requirements considerably.

Overhydration is real, if uncommon

Drinking far more than the kidneys can excrete dilutes blood sodium, which can cause serious illness. Cases have occurred in endurance events where participants drank heavily throughout.

The practical implication is not to force fluid intake substantially beyond thirst, particularly during prolonged exercise, and to consider electrolyte replacement rather than water alone in those settings.

On the products

Electrolyte powders and sports drinks are useful for prolonged intense exercise and largely unnecessary for ordinary daily life or short workouts.

Alkaline, structured and enhanced waters are marketed with claims that lack good supporting evidence. Ordinary water is adequate.