Food, drinks and where daily fluid intake actually comes from

Three sources rather than one

Total water intake arrives from drinks, from the water contained in food and from water produced by the body as it breaks down what it has eaten. The third source is small and genuinely exists, which is a reasonable illustration of how the accounting is more complicated than the popular framing suggests. Food contributes a meaningful share, and that share varies enormously depending on whether a diet contains many soups, fruits and vegetables or few.

Someone eating a diet high in water-containing foods therefore requires less from drinks than someone eating a drier one, without noticing the difference. This variation is one of several reasons a universal figure for daily drinking cannot be correct for everyone.

The drinks that were wrongly excluded

Tea and coffee were long excluded from fluid totals on the basis that caffeine increases urine output, and that reasoning has not held up well. The diuretic effect at ordinary consumption levels is modest, and the fluid in the drink more than offsets the additional output for habitual consumers. Tolerance develops to the effect, which is why occasional drinkers may notice it and regular ones generally do not.

The practical conclusion is that these drinks contribute to total intake, which contradicts advice that many people still repeat confidently. This is a useful example of a plausible mechanism producing an incorrect conclusion when the size of the effect was not checked.

Alcohol behaves differently

Alcohol suppresses a hormone that controls water retention, which increases urine output by more than the volume of the drink in many cases. This is the mechanism behind the familiar experience of frequent urination during drinking and the thirst that follows the next morning. The effect depends on concentration, so drinks with higher alcohol content produce it more strongly than weaker ones of the same volume.

Describing this as the cause of a hangover is an oversimplification, since several other mechanisms are involved and the picture remains incomplete. The narrower point is that alcohol should not be counted towards fluid intake in the way other drinks are.

Losses that are easy to overlook

Water is lost through urine, through sweat, through the digestive tract and continuously through breathing and through the skin without any visible sweating. That last category is substantial and largely unnoticed, and it increases in dry air, at altitude and during illness with fever. Losses through sweat vary enormously between individuals doing identical activity, which is why generic replacement advice fits people badly.

Illness involving vomiting or diarrhoea produces losses that matter clinically, particularly in young children and older adults. Those situations warrant medical advice rather than general reading, since the assessment of dehydration is a clinical judgement.

What the accounting is good for

Understanding the sources mainly explains why people with very different drinking habits can both be perfectly well hydrated. It also explains why appetite and fluid intake tend to fall together during illness, which is one reason illness raises the risk of dehydration. None of this supports counting anything, and the attempt to track total intake precisely is more elaborate than the situation warrants.

For anyone with a condition affecting fluid balance, the relevant guidance comes from a clinician and takes precedence over any general account. For everyone else the accounting is chiefly useful for making sense of why the popular rules contradict each other so often.