Hibernating animals are in a state of starvation. As a result, they obtain their life energy in the same way as any animal that is not consuming nutrients. First (and just like any human on a planned diet) fat stores are converted to glucose and other basic proteins used to maintain normal physiologic functions. But once all available fat stores are gone the body starts to breakdown muscle. This process results in muscle wasting and the buildup of toxic levels of nitrogen, in the form of urea, in the blood. Once muscle stores are gone though, the body no longer has any reservoir of energy left to utilize. Humans cannot survive long in this state. Bears, on the other hand, go for months without urinating and never develop the complications of toxic urea levels or muscle wasting. They are able to accomplish this because they have microbes in their gut that convert urea into a form of nitrogen that can be used to make new amino acids--the basic building blocks of protein. These new proteins can then either be used for energy or be used to maintain muscle mass. Interestingly, it has recently been discovered that the human gut possesses these microbes as well, Unfortunately, we humans are not nearly as good as bears at recycling urea.
The good news is that humans do not need this physiologic process to successfully hibernate. Astronauts that undergo torpor will not be under the state of starvation that other hibernating animals experience due to a medical practice called Total Parenteral Nutrition. Total Parenteral Nutrition, or TPN, is a routine medical process where a person is feed intravenously, bypassing the usual process of eating and digestion. Astronauts in hibernation will receive a nutritional formula that contains all the glucose, amino acids, lipids, vitamins and dietary minerals they need to maintain normal, healthy bodily functions. This avoids the conversion of fat and muscle stores that leads to toxic urea levels. In addition, astronauts will still excrete urine through a suprapubic or indwelling foley catheter, allowing the kidneys to process and excrete urea in a normal matter.
Not that maximizing urea recycling in humans would be without its benefits. In fact, it could help us solve one of the major health complications associated with prolonged space flight, muscle atrophy. If we were able to boost the recovery and conversion of urea in humans the same way that hibernating bears do we could potentially delay or even reverse muscle atrophy. This would not only help keep hibernating astronauts healthier, but aid in every space mission NASA conducts.
- Dr. Talk