Metabolic water

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Metabolic water refers to water created inside a living organism through their metabolism, by oxidizing energy-containing substances in their food. Animal metabolism produces about 100 grams of water per 100 grams of fat,[1] 42 grams of water per 100 g of protein and 60 grams of water per 100 g of carbohydrate.[1][2][3]

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.[4][5] Humans, by contrast, obtain only about 8-10% of their water needs through metabolic water production.[6]

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.[6] Birds, however, excrete uric acid and can have a net gain of water from the metabolism of protein.