Around 60% of a human body is water: blood 83%, muscle 75%, brain 75%, skin 72%, bone 22%, fat 10% water. We need water for every biological function. And then we excrete it.
So, we’re constantly having to replenish our bodies’ water content. Every day – many times a day – we must drink water. If we drink no water at all, we’ll be dead in three days.
The European Food Safety Authority (EFSA) recommends a daily fluid intake of 2500ml for men and 2000ml for women.
Around 20% of our fluid intake comes from the food we eat. Therefore, when it comes to the amount we drink, 2000ml (for men) and 1600ml (for women) is adequate.
The eight glasses are based on the assumption that an average drinking glass holds between 200ml and 250ml; that’s 1600 to 2000ml.
“Eight ounces eight times a day” is a mantra often heard, especially in the US, where eight lots of eight fluid ounces equals 1892ml. In the UK, eight lots of eight fluid ounces equals 1818ml.
So, why eight glasses?
Is this just a catchy mnemonic?
Approximately 8-10% of a mammal’s water is obtained through the body’s aerobic metabolism, which is the oxidisation of carbohydrates.
This is a very small percentage compared to many other animals, including birds, who rely on a higher percentage of metabolic water. Birds excrete waste products from the blood in the form of uric acid, passed via the anus, as opposed to the highly diluted urine that mammals excrete by the urethra.
This contrast in the way mammals and birds maintain a healthy level of hydration brings home the fact that we rely heavily on water intake. Unlike migrating birds, who are for long periods hydrated exclusively by metabolic water, we must consume a high volume of water in order to function effectively.
Dr Caroline Edmonds, Professor of Experimental Psychology, has led numerous studies surrounding the effects of hydration levels on cognition. One area of her research was a study of the expectations associated with good hydration – an investigation into the extent to which the benefits of hydration can be attributed to a person’s preconceptions. A study in which one group of subjects was made aware of the nature of the experiment, and the second group was told that the study was part of research into the effects of repeated cognition tests, showed that expectation made no impact at all.
In 2013, Dr Edmonds et al found that water ingestion enhanced cognitive functions such as reaction time, concentration, memory, happiness, and alertness. Even when subjects were already adequately hydrated, they showed improved cognition immediately after drinking water.
Because healthy kidneys are capable of excreting as much fluid as necessary – as much as 1 litre per hour – it’s unlikely that you could ever drink too much water. In those rare cases of fatal hyperhydration, death was caused by sodium deficiency due to over-dilution.
All day, every day, your body is using water to process food, nourish trillions of cells, produce neurotransmitters and hormones, cushion joints, regulate temperature, and excrete waste.
We’re all familiar with the sensation of thirst and recognise it as a signal that we need to take in fluid. But when we feel thirsty, we’re already dehydrated.
Maybe there’s some sense in saying “drink eight glasses of water a day”. This advice gives us an idea not only of how much we should drink, but also of how often. We’re continually losing water, so we should be continually rehydrating.
So – drink a glass of pure, chilled water eight times a day, and you’ll be on the right track to good hydration.
And don’t worry too much about the size of the glass!