Metabolic water

Metabolic water refers to water created inside a living organism through their metabolism, by oxidizing energy-containing substances in their food. Metabolism of fat produces about 110 grams of water per 100 grams, protein 41.3 grams, and starch 55.1 g.

Some organisms, especially xerocoles, animals living in the desert, 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 which is a byproduct of the metabolism of protein. Birds, however, excrete uric acid and can have a net gain of water from the metabolism of protein.