If I had to nominate one pervasive myth in nutrition, it would be the popular advice that we should drink at least eight glasses of water a day. Of course our bodies need water, otherwise we would die from dehydration. But the amount needed is extremely variable and depends on a person’s body size, physical activity levels, climate and what types of food they are eating.
Water makes up about 60% of an adult’s body weight and is an essential nutrient, more important to life than any others. Water helps regulate body temperature, carries nutrients and waste products throughout the body, is involved in blood transport, and allows many metabolic reactions to occur. It also acts as a lubricant and cushion around joints, and forms the amniotic sac surrounding a foetus.
It is believed that the “eight glasses” myth was a US Recommended Dietary Allowance dating back to 1945. The guide said a suitable allowance of water for adults was 2.5 litres a day, but most of this water could be found in prepared foods. If that last, crucial part is ignored, the statement could be interpreted as clear instructions to drink eight glasses of water a day.
Even a comprehensive search of the scientific literature finds no evidence to support the eight-glasses-a-day advice. The clear reason that evidence for such prescriptive advice doesn’t exist is that a person can get all the water they need without consuming a single glass.
Drinks like soft drink, fruit juice, tea and coffee, milk, and foods like fruit, yoghurt, soups, and stews all have appreciable amounts of water that contribute to fluid intake.
Australian dietary recommendations also bust the eight-glass myth as the official Nutrient Reference Values state “there is no single level of water intake that would ensure adequate hydration and optimal health for the apparently healthy people in the population.”
Don’t be concerned about seeing coffee listed as a fluid – the “coffee makes you dehydrated” mantra is another myth that needs to be busted. Drinks such as coffee, tea and cola do have a mild diuretic effect from the caffeine but the water loss caused by this is far less than the amount of fluid consumed in the drink in the first place. It’s only alcoholic drinks which have a dehydrating effect.
So how do you know if you are drinking enough water? Well. You can check this for yourself every few hours. If your urine is lightly coloured or clear, you’re drinking enough. If it’s dark, then you should drink more. How simple is that?