Recommended daily fluid intake is stated with confidence and stated differently by almost every source. The variation reflects a genuine difficulty in specifying a number that applies to anyone in particular.
Requirements vary widely between people and days
Water loss depends on body size, ambient temperature, humidity, altitude, activity level and how much of the day is spent outdoors.
A sedentary day in a cool office and a physically demanding day in heat can differ severalfold in fluid requirement for the same person.
Any single figure therefore has to be either an average that fits few individual days or a range wide enough to be uninformative.
Food contributes a substantial share
Fruit, vegetables, soups, and cooked grains carry considerable water, and the metabolism of food produces a further small amount.
Someone eating a diet high in whole plant foods obtains noticeably more water from food than someone eating mostly dry processed items.
Which is why guidance sometimes refers to total water intake and sometimes to drinks alone, and why the two figures are frequently compared as though they were the same measure.
Regulation is tight and automatic
Blood concentration is sensed continuously, and small increases trigger both thirst and hormonal signals that instruct the kidney to conserve water.
The system operates within a narrow range and adjusts urine concentration across a wide span to hold that range steady.
For healthy adults with reliable access to fluid, this machinery handles the problem without conscious intervention, which is the strongest argument against fixed targets.
Thirst has genuine limits
The thirst response becomes less reliable with age, and older adults may not register mild dehydration as thirst.
It also lags during heavy exercise and in hot conditions, where sweat losses can outpace the sensation, and illness involving fever, vomiting or diarrhoea shifts the picture substantially.
These are the situations in which deliberate attention to intake is recommended, and they are also the situations most often generalised into universal advice.
More is not straightforwardly better
Excess intake is normally excreted without difficulty, so moderate overconsumption is mostly inconsequential.
Very large intakes over short periods can dilute blood sodium, which is uncommon but documented in endurance events and is a serious condition.
Anyone with kidney disease, heart failure, or a condition affecting fluid balance is given individual targets by their clinical team, and general guidance is not intended to override those.