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
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