From the new scientist:
During their hibernation, the bears neither defecate or urinate. This would normally mean that nitrogenous wastes during that time would cause poisoning to the urinal system. However this it does not do. The bear solves its nitrogenous waste problem by a form of recycling." The hibernating bears body diverts nitrogen from pathways that synthesise urea into pathways that generate amino acids and new proteins. And it does this by glycerol (produced when fats are metabolized) and recycled nitrogen as the building blocks
To remain in good physical condition for months without eating requires change in a bear’s body chemistry. Some of the physiological changes in healthy hibernating bears would be a real problem for people. How hibernating bears remain healthy in winter and how their physiological adaptations might be useful in human medicine are subjects of study at some of North America’s leading medical research centers. For example, bears living off their accumulated body fat have hibernating cholesterol levels more than twice summer levels and more than twice as high as the normal cholesterol levels of most humans. Yet bears exhibit no hardening of the arteries or formation of cholesterol gallstones. Medical studies reveal hibernating bears produce a bile juice, ursodeoxycholic acid that may help them avoid problems with gallstones. When this acid is given to people it dissolves gallstones and eliminates the need for surgery. Hibernating black bears also exhibit reduced kidney function. They do not urinate for months but still do not poison their bodies with waste products such as urea. Urea is somehow broken down and its nitrogen is reused to build protein. The ability to build protein while fasting allows black bears to maintain their muscle and organ tissue throughout the winter. They only use up fat. Evidence is accumulating that hormone-like substances control physiological changes in hibernating black bears. These substances also produce hibernation-like effects when injected into other species—both other hibernators and non-hibernators, suggesting potential human medical applications.