Posted by: Bystander PJ9 OCT 2013
I recently commented on an NHS leaflet that offers tips on “better eating” based on popular beliefs that lack good evidence, such as the supposed need to consume five portions of fruit and vegetables a day (PJ, 8 June 2013, p684) and the alleged benefits of a hearty breakfast (PJ, 20 July 2013, p94).
A further tip is: “Have plenty to drink, including water.” At least, the leaflet does not repeat the common belief that we should all drink at least 2L of water a day. Again, this is a claim with no evidence base to support it.
The amount you should drink depends on a variety of factors, including your level of exercise and the ambient temperature and humidity, all of which can increase loss of water through perspiration. Other factors that should be taken into account are kidney disease, adrenal problems and the use of diuretics.
Our bodies normally tell us when we need to drink, and it is not a good idea to bully people into consuming large amounts of water when the circumstances may not require it.
Unfortunately, pressure to drink plenty of water has led some people to believe either that the more you drink the healthier you will be or that you should drink 2L of plain water on top of all your other daily drinks.
But drinking a lot of water quickly can push the balance of electrolytes beyond safe limits, sometimes fatally. The main risk is dilutional hyponatraemia, in which body fluids become deficient in sodium.
An excessive water intake causes a drop in levels of sodium, which is found mostly in extracellular fluids. To balance the levels, water moves into the cells, causing them to swell. Most cells can handle this, but brain cells cannot, and the result can be a potentially fatal disturbance in brain functions.
Mark Twain wrote: “Water, taken in moderation, cannot hurt anybody.” True, but water taken in excess certainly can.