Hibernating bears recycle urea to help meet their protein needs.
Urea is a nitrogen-containing waste compound produced when the body breaks down proteins. Most mammals excrete it in urine, but hibernating bears can recycle nitrogen from urea and use it to help make new amino acids and proteins. This helps them maintain muscle during months without eating or drinking.
The adaptation is especially important because bears remain in their dens for long periods while relying mainly on stored body fat. They normally do not urinate or defecate during hibernation, and retained waste contributes to a hardened fecal mass called a fecal plug. Their bodies therefore conserve and reuse materials that other mammals would usually lose.
Urea recycling does not mean that bears obtain all their energy from waste. Stored fat remains the major fuel source, while urea recycling helps supply nitrogen for protein maintenance. This is also why “urine” can be misleading as an answer: the relevant compound being recycled is urea, a component of urine, not urine as a whole.
The same physiology helps explain why bears emerge from hibernation with far less muscle loss than a comparably inactive human or other non-hibernating mammal.