The heat of the day is oppressive, but you’ve taken care to bring along a full Camelbak plus a couple of extra bottles of water. A nice apple lines your pack. You’ve been sweating profusely. This does not faze you because you follow the rule of drinking water before you need it. So you know that you are well-hydrated and you still have plenty to drink.
Then it hits you: a headache combined with nausea. Oh, this is easy, you say to yourself. “I’ll just drink more water.” You slake your thirst but the headache and nausea do not go away as they usually do. Hmm. Maybe you need something to eat? You pull out your apple and reduce it to its core. The syndrome is getting worse. You throw up your emergency meal and the water you drank. What is going on?