Water is not just a transporter of nutritional elements it also plays an active role, intervening in metabolic reactions.
For example, water enables hydrolysis reactions like those of proteins, lipids and carbohydrates, which require water molecules. The H20 molecule is consumed in these metabolic reactions.
But other metabolic reactions can, on the other hand, produce and release water molecules; this is referred to as "metabolic water" or "endogenous water." The amount of water produced depends on whether it derives from the oxidation of proteins, lipids or carbohydrates.
So, within the framework of a balanced diet of a sedentary person, the body is capable of producing 250-350 ml of metabolic (or endogenous) water a day.
Related on Nestlé Waters:
Are you sure you drink enough?
Obesity and drinking water
Water for diabetic diet
Sources: (1) EFSA, 2010 - IOM, 2004 ; (2) Pivarnik J.M., CRC Press, 1994