🥗 Nutrition · 4 min read

Fluid intake: what lies behind it

What the body uses water for

In adults, water makes up roughly half to two thirds of body mass, depending on age and body composition. Muscle tissue contains considerably more water than fat tissue, which is why the share can differ substantially between two people of the same weight.

It is used as a transport medium in the blood, as a solvent for metabolic processes, for excretion via the kidneys and for temperature regulation through sweating. Losses arise not only via urine and sweat but also through breathing and the skin — and these imperceptible losses rise in dry air and during physical exertion.

Where the fluid comes from

A common error is counting only drinks. In fact a considerable share of fluid intake comes from solid foods: vegetables and fruit consist largely of water, as do soups and cooked dishes. Added to that is a small amount of so-called metabolic water, produced by metabolism itself.

Caffeinated drinks count too. The formerly widespread idea that coffee and tea remove fluid from the balance on net is considered outdated for habitual consumption — the diuretic effect exists but is smaller than the amount taken in.

The two-litre figure

Figures in the region of roughly one and a half to two litres of drinks a day come from population reference values and partly account separately for fluid from solid food. They are orientation figures under average conditions — not in heat, not during physical work, not during sport.

Actual requirements vary correspondingly widely. More useful in practice than a fixed number are ordinary everyday indicators: the frequency of urination and the colour of urine give rough orientation, though the latter is also influenced by supplements and individual foods.

Sweating and training

How much someone sweats differs several-fold between people and additionally depends on intensity, duration, temperature, humidity, clothing and heat acclimatisation. General figures such as “half a litre per hour of training” therefore describe an order of magnitude at best.

Anyone wanting more precision can weigh themselves before and after a session under identical conditions and add the amount drunk. The weight difference approximates the fluid loss — a simple method, though one that applies only to the situation actually measured and would have to be redetermined in heat or under different loads.

Electrolytes

Besides water, sweat contains dissolved salts, chiefly sodium and chloride, and smaller amounts of potassium, calcium and magnesium. How salty sweat is differs between individuals and is sometimes visible as white marks on dark clothing.

For ordinary gym sessions these losses are made up again through normal eating; salt is plentiful in usual foods. For very long exertion, high temperatures or several sessions a day, the topic is discussed at greater length in sports science. This article names no specific amounts — that depends too heavily on the situation and, in case of doubt, belongs under professional guidance.

Drinks differ

For fluid balance alone, what a drink is made of is initially secondary. For everything else it is not: fruit juices, soft drinks and many mixed drinks contribute energy that easily disappears from records, because what is drunk is noted less often than what is eaten.

Sports drinks typically contain carbohydrate and sodium in matched concentrations. They were developed for longer endurance efforts, in which both are lost or needed in appreciable amounts. During an hour-long strength session in an air-conditioned gym that situation generally does not apply — there it is simply water with additions nobody requires.

Alcoholic drinks occupy a special position: they contribute energy and act as a diuretic at the same time. For fluid balance they are therefore not a sensible figure to count on.

Both directions have limits

Drinking too little is the more common everyday situation, but the other direction exists too: very large amounts drunk in a short time can unbalance the body’s salt levels. This is described chiefly at endurance events, when large quantities of plain water are drunk over hours.

Both are edge cases, not everyday worries. Anyone who feels uncertain, notices persistent strong thirst, passes unusually much or little water, or has complaints should have that assessed medically rather than managing it through drinking volume. The same applies with kidney or heart conditions and with medication acting on fluid balance — there, individual instructions apply that expressly supersede general figures from an article.

Last reviewed: 14.08.2026