We are all roughly two-thirds water. It makes up about three-quarters of our lean body mass, about 10% of our fat, and in terms of the amount of time we could live without consuming it, water is the most essential of our nutrients. But exactly how much water should we be taking in each day?

You might expect science to have provided a reliable answer to such a question - water is a fundamental constituent of life, after all. But with the highly evolved ability of the human body to regulate water so exquisitely - and with lots of individual variability in the optimum intake - there is no definitive answer for the amount of water one person or another should get each day.

In fact, the best guidance is simply to follow the natural call of the body when more fluid is needed: just follow your thirst.Yet the question of amount persists, and there is misinformation in abundance. Vested interests come up with highly questionable ideas about how we should all be drinking more water. Even well-respected sources cite daily intake amounts that lack good scientific evidence to support them

How much water is in the human body?

The principal chemical making up the human body is water (H₂O), roughly comprising two-thirds of the body because humans show considerable variability in body composition. The average young man has a percentage water composition anywhere between 50-70% of their body weight.

A similar range is seen between early and later years - infants have 75% of their body weight accounted for by water, whereas the proportion in older people is just 55%. Variability in the overall water composition of the body is mostly due to differences explained by age, sex and aerobic fitness. These affect the ratio between.

How is water regulated by the body?

Without water, there can be no life at all, let alone human life. From the earliest origins of life on earth to continued survival today, organisms have adapted to avoid dehydration.

Healthy humans can survive only a matter of days without water intake, and water loss through illness that does not get replaced can quickly prove dangerous in vulnerable people such as the very young or old.

We have evolved fantastically effective physiological mechanisms for maintaining our bodies' fluid homeostasis (water balance). 

The two main mechanisms for maintaining water balance are:

• Thirst - this tells us when we need to take in more fluid
• Urine output - the kidneys regulate the surplus, or deficit, of the water we consume by either emptying it into the urinary bladder or holding onto it in the blood plasma.

The kidneys also regulate the balance of electrolytes such as sodium and potassium in the body fluids. The kidneys receive hormonal signals to conserve or release water into the urine following the brain's detection of alterations in the concentration of the solutes in the blood (changes in the plasma osmolality), via tiny changes in cell size according to the amount of water inside versus outside cells.

The brain's response to plasma osmolality is also partly responsible for the sensation of thirst that drives us to replace lost water.Water is lost by other means aside from the action of the kidneys. Total water loss from the body is accounted for by the following means, each of which shows a range of variability:

• Urinary - water regulation by the kidneys, with excess fluid excreted into the urine being estimated at 500-1,000 mL per day
• Respiratory - water lost from the lungs as we breathe out, about 250-350 mL per day
• Fecal - excreted in our solid waste, estimated at 100-200 mL per day
• Other "insensible water loss" - other unnoticed water loss is via evaporation from the skin, which increases with greater sweating. Sedentary loss is somewhere between 1,300-3,450 mL per day but can range from 1,550-6,730 mL per day with sweating due to physical activity.

The '8 by 8' water mantra: 8 times 8oz glasses a day.

The start of the millennium saw the widespread repetition of the idea that we needed to drink at least eight glasses of water a day. However, the origins of this mantra, and scientific support for it, remain elusive, even though it is still widely cited.

Even the UK's National Health Service (NHS), through its NHS Choices website, once gave the recommendation to drink up to eight glasses of water a day - although it has since changed its recommendation to "plenty of water" for "quenching your thirst at any time."

The one-size-fits-all mantras of daily water intake do seem to be retreating a little, however, or are at least including the idea that other fluids aside from water can contribute to the "8 by 8" recommendation.

Other questions are currently refocusing the ideas about optimum hydration, too. Are marathon runners drinking way too much water, for example? And is there any real role in the fight against obesity to be played by drinking more plain water?

How much is too much? Water intake and hyponatremia

One of the major questions about water intake concerns exercise, and this is the area saturated by marketing messages from manufacturers of bottled beverages.

Within the guidelines issued for normal daily intake in healthy adults, there is also guidance on how much to drink during exercise, but it is controversial.

Messages about water intake during exercise could even be proving dangerous in the case of high-endurance exercise. A study of runners taking part in the 2002 Boston Marathon, for example, estimated that almost 2,000 of the participants would have had some degree of hyponatremia (abnormally low levels of sodium in the blood).

The study also found that some 90 finishers in that event might have had critical hyponatremia. These abnormally and potentially dangerously low sodium levels in the blood plasma at the end of the race were put down to excessive fluid consumption, as evidenced by weight gain while running.

Hyponatremia is a real danger whether too much fluid is taken in during exercise, or even as a result of the "eight glasses a day" guidance for regular intake.

Written by Markus MacGil

Hamro Doctor