B efore you read this article, can I just check you’ve drunk enough water today? You might want to refill your bottle because, remember, if you wait until you feel thirsty, you may already be dehydrated. No one is sure where this advice came from, but it’s all over the internet. “Nowadays this is not considered sensible,” says Stuart Galloway, an associate professor in physiology, exercise and nutrition at the University of Stirling. “As humans, we have this homeostatic system, so when we need water, we feel thirsty.” Drinking when you are thirsty, he says, maintains your body’s water level within about 1-2% of its ideal state. “For most people, this is absolutely fine. Even for athletes, a loss of around 1% is considered to have negligible impact upon performance. So, although thirst may not kick in until you have lost some body water, this is not necessarily a bad thing.”