Water
About 70% of the fat free mass of the human body is made of water.

To function properly, the body requires between one and seven liters of water per day to avoid dehydration; the precise amount depends on the level of activity, temperature, humidity, and other factors.

Most of this is ingested through foods or beverages other than drinking straight water.

It is not clear how much water intake is needed by healthy people, though most experts agree that 8-10 glasses of water (approximately 2 liters) daily is the minimum to maintain proper hydration. For those who do not have kidney problems, it is rather difficult to drink too much water, but (especially in warm humid weather and while exercising) it is dangerous to drink too little.

People can drink far more water than necessary while exercising, however, putting them at risk of water intoxication, which can be fatal.