Water is more critical to the maintenance of life than food, i.e., although humans can live without food for weeks and even months (depending on fat stores), without water death occurs within days.
In fact, water is the only substance necessary to all life. Many organisms can live without air, but none can live without water.
Water is the largest single component of the body.
In an adult male, weighing 70kg, it amounts to 60% of the body weight (i.e., 42 litres). In women, the value is slightly lower (around 55%). This results from the fact that women have more body fat (adipose tissue) than men and fat contains very little water (around 10%by weight).
Therefore, in general, as the fat content of the body increases, the water decreases.
The proportion of body water varies with age, being at 94% in a five month old foetus, at 75% in an infant, and at 50% in the elderly.
Loss of more than 20% of body water may cause death, and a loss of only 8% may lead to severe disorders and illness.
This is not because water is more important than other nutrients, but because water as such is not stored in the body, nor can the body conserve it to a sufficient degree. Regular intake, therefore, is mandatory to compensate for daily losses and to maintain bodily functions.