Ignore guidance on water intake

By Bystander

A year ago (PJ, 8 June 2013, p684) I pooh-poohed the popular opinion that we should drink at least 2 litres of water a day.

I have since learnt that the US Mayo Clinic is even more specific about water intake, claiming that men should drink 13 cups of “total beverages” every day, and women nine cups. Since a standard US cup holds 236.6ml, that means 3.08 litres for men and 2.13 litres for women. 

But not a scrap of empirical evidence exists to support such claims. My suggestion was that we should rely on our own bodies to know when we need to drink. I also pointed out that pressure from supposed health experts may scare people into drinking water to excess, with the risk of addling their brains through dilutional hyponatraemia.

Support for my sceptical view now comes from an Australian study, reported in the US Proceedings of the National Academy of Science. This shows that drinking beyond thirst can indeed confuse the brain.

The researchers persuaded subjects to exercise until they lost about 1 per cent of their bodyweight through sweating. The subjects were then put into a magnetic resonance imaging (MRI) scanner, given water to drink and asked to keep drinking even when no longer thirsty.

As the subjects persevered, it became obvious that they were finding it harder and harder to swallow more water. And their brain scans showed that the motor cortex, which permits swallowing, was being forced to work overtime. Areas of the brain that inhibit swallowing began to wake up and send commands to stop drinking, so that the motor cortex had to work even harder to turn on the swallowing switch.

This clearly indicates that our brains know when we should drink and when we should stop drinking. As I suggested, we should ignore the unproven views of alleged experts and rely on the wisdom of our own bodies.