Bear Hibernation
The process of hibernation is remarkably fine-tuned in its metabolic recycling efficiency. It is another tribute to the almost eerie evolutionary perfection that bears have achieved. It is worth remembering that bears have been bears a lot longer than we have been humans. In many respects, we are still not done yet.
The basis of bear metabolism while under hibernation is the burning of body fat. When fat is burned, no residues are left, only carbon dioxide and water, and fat yields almost twice the energy as protein or carbohydrate. This high energy density and its solid form make it an efficient calorie storage medium. The carbon dioxide gets breathed out, but the water stays in the bear's bloodstream. The approximately 4,000 calories a bear burns each day yields about a pint and a half of water, enough for the bear's metabolic needs. Usually, the balance between water generation through fat-burning and water loss through breath and evaporation is so accurate that the bear neither needs to drink nor urinate all winter long. Since the bear doesn't eat anything, it doesn't need to defecate either. A certain amount of excess water is likely to be stored in the disappearing fatty tissue. This is lost after the bear wakes up, causing the bear to seem to quickly get skinnier after hibernation ends. Occasionally, the balance doesn't quite work so well and the bear may need to get up to get a drink or urinate to restore it.
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. Humans and other mammals can recycle nitrogen into amino acids via synthesis by intestinal bacteria but the process is at best only 20% efficient. The bear's ability to recycle nitrogen is essentially 100% efficient. 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.
When other mammals burn large amounts of fat; toxic by-products called ketones build up in the blood. In diabetic humans, they
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