Animals

Black bears, unlike true hibernators that burn protein reserves, use their fat reserves for energy. Their urea, from the urine, is recycled back into the production of muscle tissue, and a bear may gain muscle mass in winter although it may lose 20 to 40 percent of its weight. Most of this is from the fat reserves gained during the summer and fall. One interesting side note is that humans on a starvation diet have problems with the formation of gallstones. Bears produce an acid in called ursodeoxycholic acid that is stored in their gallbladders. This acid prevents gallstones, enhances digestion and enables the bears to lose fat, not muscle mass, during their winter slumber. Though some black bears are taken illegally for their ursodeoxycholic acid, a black market product, a synthetic form of this acid has been created using cow bile. The product is used to prevent or dissolve gallstones.