Health Myth: How Many Glasses of Water should we Drink?
And yet the effects of water in the diet are complex and there is not a little misinformation out there, as the recent New Scientist series on health myths so strikingly demonstrated. For a start, the much-quoted maxim that we should all drink eight glasses of water a day is not based on any scientific evidence.
Author Caroline Williams speculates that the ‘eight times eight rule’ – drink eight eight-ounce glasses of water a day – may have had its origins in a 1945 recommendation by the US National Research Council (NRC). This suggested that adults should consume a millilitre of water for each calorie of food, adding up to about 2.5 litres per day for men and two litres for women, which is approximately the same amount as eight eight-ounce glasses.
Williams also points out that we gain a lot of our daily water requirement from our food and that, of course, other drinks also provide the water that we need. Although certain drinks like tea and coffee and alcohol may be diuretic, the volume of water in such drinks more than compensates for any diuretic quality they may have.
Of course, our water consumption and the things that we eat do have an effect on our bodies for all sorts of reasons. For instance, drinking water instead of sugary drinks (even fruit juices) and alcohol can help cut our calorie intake, and also rehydrate us when we have over-indulged.
Dieticians suggest that drinking water through the day can help stop us snacking by making us to feel full. Intriguingly, in a study on water-induced thermogenesis, researchers found that drinking cold water caused an increase in energy expenditure in both men and women, as the body sought to warm the water to normal body temperature. The effects of elevated metabolism began some ten minutes after consuming the water and peaked 20-30 minutes later. However, before you rush to the ice maker, note that this phenomenon is unlikely to have more than a minimal effect on body weight in the average person.
Drinking water and cutting down our salt intake can also reduce water retention, a common
cause of bloating. Fluid retention occurs when the body cannot get rid of excess water and may cause swelling in the legs, ankles, hands, feet and abdomen. It happens for a variety of reasons, but most commonly due to premenstrual syndrome in women, too much salt in our diet or as a side effect of medication.
Can drinking too much water be bad for you? Yes, excess water consumption can lead to a condition known as water intoxication and to a related problem resulting from the dilution of sodium in the body – hyponatraemia. It is most commonly seen in infants under six months of age – for instance when baby formula is consistently over-diluted – and sometimes in athletes. Vigorous exercise causes us to sweat heavily, and we lose both water and electrolytes. Water intoxication and hyponatraemia result when a dehydrated person drinks too much water without the accompanying electrolytes.
But don’t worry. The kidneys of a healthy adult can process 15 litres of water a day! Typical adults are unlikely ever to suffer from water intoxication, so long as we don’t imbibe a huge volume of water at any one time.
And if you’re running a marathon, remember to take those electrolytes on board, along with the fluid.