How much water do we need?
Everyone knows humans need water and we can’t survive without it. We’ve all heard we should be aiming for eight glasses, or two litres of water a day.
This target seems pretty steep when you think about how much water that actually is, and don’t we also get some water from the food we eat?
We asked five medical and sports science experts if we really need to drink eight glasses of water a day.
All five experts said no. Here are their detailed responses:
Karen Dwyer, nephrologist
You only need to drink to thirst. The best gauge of your hydration level is the colour of your urine. You should aim for light yellow in colour; if very dark, then you’re dehydrated and need more water; if clear (like water) then you don’t need so much water.
Excessive water intake can be dangerous, particularly in those with heart conditions.
The kidney has a remarkable ability to concentrate water so if you are ‘‘getting dry’’, the kidney will concentrate the urine and send a message to the brain to drink more.
Vincent Ho, gastroenterologist
is contained in prepared foods’’, a fact that is often overlooked. We do get a lot of our water intake from the foods we consume. Cauliflower and eggplant, for example, are 92 per cent water.
A one-size-fits-all approach is unlikely to be helpful. Healthy adults may not need to drink an additional eight glasses of water a day. On the other hand, persons with certain diseases or living in very hot climates may require larger intakes of fluid.
Michael Tam, GP
Eight glasses, which is just less than two litres of water, is very roughly the basal water required by a fasting, well adult per day, who is doing nothing at all (for example, staying in hospital), with no special losses (such as vomiting or diarrhoea).
In day-to-day life, we usually have additional losses (exercise, or sweating during a hot day), and we receive water from other sources.
There are the obvious ones from our diet such as beverages, and juicy and moist foods, such as fruit and vegetables. Less obvious is water from the metabolism of food.
The conversion of fats, carbohydrates, and proteins to energy in our bodies all produce water.
Rather than focusing on the number of glasses, simply drink fluids when thirsty. Aiming for more water (especially in place of sweetened drinks) is often a good idea to improve health.
Jon Bartlett, sport scientist
A person’s daily water requirements are highly individual and dependent upon a number of internal and external factors.
While eight glasses of water a day is recommended as a base requirement to meet daily physiological needs, the actual volume of water required in a day is dependent on one’s day-to-day activities, health, and the climate in which they reside.
Research shows even just a mild level of dehydration can negatively affect both mental and physical performance.
This is further accentuated for individuals who are highly active and who live in hot environments. A simple and easy reminder to ensure you are drinking enough is to drink to thirst, and for days when activity levels are higher than normal or in hotter environments to increase the regularity of drinking and the total volume.
Toby Mundel, exercise scientist
Many factors will determine how much water (via all foods and fluid, not just water!) your body needs. These include body size and composition (weight, muscle and fat), how much you sweat (physically active, hot or humid environment, too much clothing) or urinate (taking certain medication, being at high altitude), your health (having fever, vomiting or disease) or status (pregnant, breast-feeding), and diet (high-water content foods, carbohydrates).
For most healthy adults rarely feeling thirsty and having light yellow (or colourless) urine usually confirms adequate water intake. Other helpful tips include drinking a glass of low-calorie fluid before and with every meal (to distinguish hunger from thirst), and drinking low-calorie fluid before, during and after physical activity (especially if you sweat). Although rare, drinking too much fluid can also have negative health consequences so more is not necessarily better.
Disclosure statement: Toby Mundel has received research funding from the Gatorade Sport Science Institute and Neurological Foundation of New Zealand, which has included research on hydration.