A by-product of fat metabolism is glycerine. This is used by the bear to combine with waste nitrogen from protein metabolism to rebuild amino acids for the bear to use. This prevents urea from building up in the blood to poisonous levels and forcing the bear to urinate or perish. In fact, during hibernation, protein metabolism is at a surprisingly high rate. Despite being inactive for months on end, the bear does not lose any muscle mass or otherwise weaken.
Some bears seem to wake up with better muscle definition than they went to sleep with.
In diabetic humans, they cause the grave condition called diabetic coma. In small mammals, the build up of ketones is believed to be the trigger for awakening. But bears do not build up significant levels of ketones, despite living purely on their body fat. How they manage this is still something of a mystery, but apparently they recycle ketones into new fat. If humans or other mammals are forced into prolonged inactivity, they lose calcium from their bones and eventually risk osteoporosis.