The water produced as an end product of the oxidation of energy-containing molecules such as carbohydrates, fats and proteins.
Metabolic water has indeed a small part of the total water in many animals compared with free drinking water and the water contained in food. Although it may be small and insignificant in many animals, it is essential in the survival of some animals that reside in dry lands or in habitats where water sources are not available. For instance, the chief source of water for kangaroo rats (thriving in desert) is metabolic water. They never have to drink; the metabolic water produced within their cells is sufficient for their survival.
See also: metabolism.