What process lets hibernating bears use urea-derived nitrogen to make amino acids?

The process that lets hibernating bears use urea-derived nitrogen to make amino acids is urea nitrogen salvage. It helps recycle nitrogen that would otherwise be lost as metabolic waste.

During hibernation, urea is produced and transported to the gut, where microbes break it down into ammonia. The bear can then use that nitrogen to synthesize amino acids, including essential amino acids such as valine and leucine. This is especially important because the animal is not eating protein while it lives on stored fat.

Urea nitrogen salvage helps explain how bears can remain inactive for months without suffering the severe muscle wasting expected from prolonged fasting. It works with other adaptations, including low metabolic activity, reduced movement, and the recycling of body materials. The process is not the same as glycogen breakdown, ketone-body synthesis, or lactic-acid cycling: those describe other metabolic pathways, whereas urea nitrogen salvage specifically concerns nitrogen recovery and amino-acid production.