I was reading on lipid metabolism the other day when I got to a very interesting “fact box” in Lehninger Principles of Biochemistry. The title was “Fat Bears Carry out Beta Oxidation in Their Sleep”.
It describes that many animals depend on their fat storage for energy production when they are in hibernation and it says that one of the most pronounced adjustments of fat metabolism occurs in hibernating grizzly bears.
Did you know that grizzly bears are in hibernation for as long as 7 months per year?
Bears, Hibernation and Beta Oxidation
Besides, unlike many hibernating animals, bears are able to keep their body temperature between 32 and 35 degrees Celsius (which is very close to the normal body temperature of 37 degrees Celsius).
The caloric daily consumption during hibernation in bears is approximately ~6,000 kcals. Keep in mind that they don’t eat, drink, urinate, and defecate for months at a time. Humans are able to go without food for very long periods of time. However, it would not be wise to do so.
According to Lehninger, the sole fuel for grizzly bears in hibernation is their body fat. Through beta-oxidation they are able to yield enough energy for homeostasis, for protein synthesis, as well as for membrane transport (among other processes).
The oxidation of fat yields consistent amounts of water that keeps the animal hydrated and it makes up for the loss of water through breathing. They also use the glycerol that is released from degrading TAG (triglycerides) to convert it in glucose through the process called gluconeogenesis (synthesis of glucose).
It is very interesting how these bears evolved to reabsorb the urea which results from amino-acid breakdown and use it to make new amino-acids. This helps them to minimize the loss of lean muscle during this prolonged period of inactivity. Stenvinkel et al. (2013) say that bears may also use shivering to keep a good muscle tonus and that they develop hypothalamic hypothyroidism and increased testosterone production in hibernation.
Good to know is that they load with enormous amounts of energy in their body fat storage before going to hibernation. According to Lehninger’s Biochemistry bears consume approximately 9,000kcals/day in late spring/beginning of the summer and as the hibernation period approaches they go for ~20,000kcals/day eating almost 24 hours a day. I wonder what their diet consists of.
Important to note that the textbook says about the large amounts of TAG that are formed from the huge intake of carbohydrates during the fattening-up period (do they eat berries?). So, the textbook says that they store body-fat largely from lipogensis.
Shall we assume that the fat they eat does not tend to accumulate so much? How is this quantifiable in humans?
Research Studies on Bears
There is not too much research on the subject, or I was not able to find it. However, there are some articles that are worth looking at. There is this one by Hissa et al. (1998) conducted on captive bears for 5 years. They followed the seasonal changes in plasma nitrogenous compounds of European brown bears. Have a look.
Then there is also this interesting master’s thesis from 2013 done by Hill for the University of Tennessee, Knoxville where the author does a literature review on the subject and then talks about the experiment she conducted on wild bears from the Great Smoky Mountains National Park and New Jersey as well as on captive bears from facilities in Tennessee and North Carolina.
She discusses about body mass variation, plasma triglycerides, BOHB, as well as a series of hormonal variations in the studied bears between their hibernating and active periods. Interesting to note is how she says that leptin expression in subcutaneous fat is not changed by season.
Another very recent article (2014) discusses how polar bears show genome wide signatures of bio-energetic adaptation to life in the arctic environment. The researchers basically compared polar bears with brown bears and saw differences in their mitochondrial and nuclear genomes. One of their findings was the difference in nitric oxide production which is better “handled” by polar bears. They conclude that polar bears may be able to better control “trade-offs between energy production as ATP vs. thermogenesis“.
By far the most interesting paper is the one conducted by Stenvinkel, Jani, and Johnson (2013) entitled Metabolic Magicians of Definite Interest to the Nephrologist. It is like a praise article to the bears where the authors talk about:
– survival and hibernation in bears
– marked reduction in GFR yet minimal azotemia during hibernation (the re-absorption process I’ve been telling you earlier)
– unparalleled ability for muscle preservation during hibernation
– impaired wound healing in Uremia
– why are the bears protected from vascular disease
– preservation of bone mass during hibernation.
All of these subjects are treated in detail in their paper. This is definitely a must read for those interested in ketosis, as well as for those interested in fasting for longer periods of time.
It is clear that these bears have genetically evolved to optimize the fat-oxidation process as well as the muscle preservation process. We can see that different species of bear can be genetically different.
I would be interested to see how many of the processes and adaptations are valid for humans. This also raises the question of nutrition optimization based on geographic location. Should the people living in cold places eat the same diets like the ones in warmer locations?
I now understand why Dr. Jack Kruse says to:
Live like a polar bear and eat like a great white shark!
1. Hissa, R., Puukka, M., Hohtola, E., Sassi, M. L., & Risteli, J. (1998, January). Seasonal changes in plasma nitrogenous compounds of the European brown bear (Ursus arctos arctos). In Annales Zoologici Fennici (Vol. 35, No. 4, pp. 205-214). Helsinki: Suomen Biologian Seura Vanamo.
2. Hill, E. M. (2013). Seasonal Changes in White Adipose Tissue in American Black Bears (Ursusa americanus).
3. Welch, A. J., Bedoya-Reina, O. C., Carretero-Paulet, L., Miller, W., Rode, K. D., & Lindqvist, C. (2014). Polar Bears Exhibit Genome-Wide Signatures of Bioenergetic Adaptation to Life in the Arctic Environment. Genome biology and evolution, 6(2), 433-450.
4. Stenvinkel, P., Jani, A. H., & Johnson, R. J. (2013). Hibernating bears (ursidae): metabolic magicians of definite interest for the nephrologist. Kidney international, 83(2), 207-212.
5. David Nelson (2008). Lehninger Principles of Biochemistry.