Water is an essential nutrient for life, but how much of it we should drink is an unanswered question.  Despite the commonly quoted figure of 8 glasses a day, there is little scientific basis for that figure.  The complexity arises from the fact that a number of factors determine our water needs.  These include: 

• Exercise habits.  Sweat loss needs to be replaced.

• The environment.  Hot and humid weather and higher altitudes cause water loss.

• Other illnesses such as fever, diarrhea, or vomiting cause water loss.

• Medical illnesses.  In the case of some conditions, as kidney stones or low blood pressure upon standing,  dehydration is to be avoided

• Dietary habits.  In studies, the average person takes in about ⅓ of daily water intake by drinking plain water.  Other sources include other beverages and many fruits and vegetables.  Lettuce, leafy greens, cucumbers, bell peppers, summer squash, celery, berries, and melons have a very high water content.  More recent studies also report that it is not true that caffeinated beverages cause dehydration.

• Body size.  Larger persons need more water.

For most people, the best guide to water intake is our own thirst mechanism.  In some cases, particularly in the elderly, the thirst mechanism is felt to be unreliable, so there is a greater risk of dehydration.