Ask the Expert: How does burning protein free up water?

A reader sent in this question regarding our continuing conversation about how birds burn proteins to conserve water during long distance migrations:

"It's not immediately clear to me how burning intracellular proteins is going to automatically "free up" much water. I do know that burning fat makes more metabolic water, from scratch, per gram fuel burned. But I haven't read the paper." -Sven

So we asked Dr. Christopher G. Guglielmo to provide a response and here is what he had to say:

"Dear Sven, Thank you for your comment and you are correct that on a dry mass basis oxidation of fatty acids produces more metabolic water than the oxidation of amino acids (about 1.05 versus 0.4 g water per g oxidized). The key thing to keep in mind is that in the living animal, lean tissue protein has a large amount of associated or bound water, while fat remains nearly anhydrous. The result is that when wet tissue protein is catabolized it liberates about 5 times more total water per kJ of energy (metabolic plus bound) than fat. This explains why birds should shift towards oxidizing more protein when they are facing dehydration."

Dr. Guglielmo is an Associate Professor in the Department of Biology and Advanced Facility for Avian Research at the University of Western Ontario. His research on protein utilization during long distance migration was recently featured in Science Magazine. You can listen to the interview here.