Metabolic water refers to water created inside a living organism through their metabolism

Metabolism

Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

, by oxidizing energy-containing substances in their food. Metabolism of fat produces about 1020 grams of water per 100 grams, protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 41.3 grams, and starch

Starch

Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

 55.1 g.http://www.encyclopedia.com/doc/1O39-metabolicwater.html

Some organisms, especially xerocoles

Xerocole

Xerocoles are animals adapted to live in a desert.Under the scorching rays of sun, the arid and baked sands of deserts may appear to be devoid of animal life. But surprisingly a large number of animal species may thrive there. The animals are either drought evaders or drought resistors. Drought...

, animals living in the desert

Desert

A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...

, rely exclusively on metabolic water. Migratory birds must rely exclusively on metabolic water production while making non-stop flights. Humans, by contrast, obtain only about 8-10% of their water needs through metabolic water production.

In mammals, the water produced from metabolism of protein roughly equals the amount needed to excrete the urea

Urea

Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

 which is a byproduct of the metabolism of protein. Birds, however, excrete uric acid

Uric acid

Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates such as ammonium acid urate. Uric acid is created when the body breaks down purine nucleotides. High blood concentrations of uric acid...

and can have a net gain of water from the metabolism of protein.