If you have a basic understanding of the science of fasting, you know humans can live without food for months. If you don't, you can read about the world record for the longest human fasting experiment - 382 days - as well as the 44-day fast of David Blaine from 2004, which he did in public view.
But, one thing is to go without food for weeks or months at end. We can survive on water alone, provided that we have sufficient fat stores to derive energy from; and most of us do. And another thing is to go without water. We can't survive without water for more than a few days. But other organisms can. And they fascinate me.
Studying human metabolism from Lehninger's biochemistry textbook, I learned that some species of bears can survive without food and water for up to 8 months. Here I'm about to give you a brief overview of the biochem behind it all.
Metabolic 'Magicians' - Extreme Physiology (Simplified)
Grizzly bears and a few other species of black and brown bears can hibernate for 5 - 8 months per year, provided that they are not in a warm climate. According to Lynn Rogers:
"Once considered not true hibernators because of their high body temperatures in winter, black bears are now known to be highly efficient hibernators. They sleep for months without eating, drinking, urinating, or defecating."
In warmer climates bears do not usually hibernate; but those living in colder climates hibernate between the middle of September until the middle of April each year. To successfully go through such an extraordinary physiologic feat, they need to fatten up in the months prior to hibernation.
Bears often gain up to 180kg (approximately 400 lbs) before going into hibernation. According to Lehninger's biochemistry textbook, they eat ~9,000 kcals/day in early summer and ~20,000 kcals/day near hibernation. To consume so many calories they often have to spend almost 24 hours per day eating (give me some!). During hibernation, their metabolic needs are around 6,000 kcals/day.
What happens in Hibernation
As I wrote in the past, the only source of energy for hibernating grizzly bears is their body fat (no water, no nothing).
To get 6,000 kcals/day for energy from fat, they upregulate beta-oxidation (an extreme one, if I may):
"During this process an acyl-CoA molecule which is 2 carbons shorter than it was at the beginning of the process is formed. Acetyl-CoA, water and 5 ATP molecules are the other products of each beta-oxidative event, until the entire acyl-CoA molecule has been reduced to a series of acetyl-CoA molecules." [source]
So, through beta-oxidation they get the water needed to remain hydrated. The glycerol backbone released from the hydrolysis of triglycerides (from fat tissue) is used for gluconeogenesis - in order to deliver glucose to the tissues that cannot metabolize other substrates.
What's more interesting to me is how they evolved to manipulate urea in a beneficial way. According to nps.gov:
"The urea produced from fat metabolism (fatal at high levels) is broken down and the resulting nitrogen is used by the bear to build protein, which allows them to maintain muscle mass and organ tissues. Bears lose fat and may actually increase lean-body mass while hibernating due to this nitrogen recycling. Bears may lose 15-30 % of their body weight during hibernation."
Moreover, as per a recent 2013 paper by Stenvinkel and colleagues they keep good muscle tone through shivering thermogenesis. If we could only do this...
Interesting Notes (from bear hibernation):
- higher testosterone production in the context of hypothalamic hypothyroidism
- slower production of feces without defecation
- extremely high cholesterol levels (due to high fat oxidation) without experiencing cardiovascular events. According to Lynn Rogers:
"Medical studies have shown that bears in winter produce a bile juice, ursodeoxycholic acid, that may help them to avoid problems with gallstones. When given to people, this acid dissolves gallstones, eliminating the need for surgery. Black bears also greatly reduce their kidney function in winter."
- lack of bone mass loss and inexistent signs of osteoporosis due to prolonged state in a non-weight bearing position:
If we find out the mechanism behind this and if we can replicate the findings in other organisms and ultimately in humans, it could lead to the discovery of an efficient strategy for space travels (astronauts experience bone-mass loss due to hypogravity) and not only!
- impaired wound healing in uremia (and in the context of dietary caloric deprivation - I would add)
Stenvinkel and colleagues (2013) explore all these mechanisms in their paper Metabolic Magicians of Definite Interest to the Nephrologist, so make sure to check it out.
Ending Thoughts
Some species of bears seem to have developed an extremely efficient ability to rely on nothing but their own fat stores during hibernation. More interesting to me is the lack of negative effects (and apparent positive effects) that support these mechanisms. This area of research requires intense further interest from scientists because if we're able to discover how they work in detail, we may be able to apply our findings to enhance human health.
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Credits for Images: Skeeze via Pixabay and Cruithne9, CC BY-SA 4.0, via Wikimedia Commons.
#extreme #physiology #biochemistry
Cristi Vlad, Self-Experimenter and Author