Thirst always tells you late

The water bottle is still nearly full in the afternoon. Your head aches and the work is dragging, even though thirst has not reminded you of anything yet.

Decide the drinking rhythm in advance

Dehydration starts before thirst is felt. By the time thirst announces itself, a mild deficit is already running. So drinking should not be left to the feeling.

Water absorbs slowly. A large amount drunk at once does not fix the situation any faster, so a small amount regularly and often works better. The rule of thumb is about a decilitre every quarter of an hour. In heat and under hard exertion the interval shortens. Drink by the clock, not by thirst.

It helps to tie the rhythm to something that repeats through the day anyway. A break, a move, or a change of task reminds you more reliably than your own sense of it.

Fluid is lost at rest too

About 65 per cent of the human body is fluid. It leaves continuously through sweating, breathing and urination, including on a quiet day. At rest an adult needs about 2.5 to 3 litres of fluid a day. Some of that comes with food, so not all of it has to be drunk. The daily intake is what keeps fluid balance normal. Fluid is not only lost under exertion. Breathing and urination continue whatever the day contains.

The need rises in heat and in movement

The need for fluid is not constant. It rises in heat, in cold and in heavy movement. Fatty and protein-rich food also raises it. So the same daily amount is not enough every day. A day spent moving demands more than a day spent still, and that is worth accounting for while you are filling the bottles. Cold weather does not make the need smaller, even though sweating feels lighter.

A warm drink absorbs better than a cold one

Drink temperature affects absorption. Cold fluid absorbs less well than warm. In heat, on the other hand, a cold drink relieves the sensation of heat. Both are true, so a cold drink suits cooling down and a warmer one suits getting fluid in efficiently.

The deficit shows in how you feel and in urine colour

Dehydration does not start dramatically. Early signs are thirst, dizziness, headache, weakness, lethargy, loss of appetite and absent-mindedness. They are ordinary symptoms, which is why they are rarely connected to drinking. Thirst is only one signal among the others.

Urine colour is a more concrete measure. The darker the urine, the more fluid has been lost and the worse the work goes. A pale colour tells you the drinking rhythm has been enough.

A bottle that empties steadily tells you more than a full bottle in the evening.

Sources

• Selviytymisen käsikirja (Survival Handbook, 2024), p. 117. A public guide of the Finnish Defence Forces.