Water and sodium balance: a nutritional goal.

Back in the 5th century BC, Empedocles suggested that the biosphere was composed of four elements: earth, wind, fire and water. These four components of life are currently known under names different from those dear to the early philosophers. ‘Earth’ contains the nutritional substrates including vitamins and trace elements. ‘Wind’ refers to the respiratory gases supporting cell life or produced as byproducts of metabolic processes occurring within the cell. ‘Fire’ is the energy resulting from substrate oxidation. ‘Water’ is the only element that is still called by its original name owing to it being the original ‘milieu’ in which life was made possible. In this chapter we will review some basic aspects of water and electrolyte metabolism emphasizing those that bear a relation to major areas of clinical nutrition such as body composition, undernutrition, nutrition assessment and refeeding.