Re: Can humans hibernate especially if they are fat?

Dear Donald,

thank you very much about your question about hibernation. As the subject is a bit on the fringe of human anatomy and physiology, I needed some time for my answer. My apologies for the delay.

The short answer to your question is: No, humans cannot hibernate. Hibernation is a complex process involving profound changes in an animal's physiology, hormonal regulation and metabolism. We humans have lost the ability to perform the entire intricate process of going into, maintaining, and coming out of hibernation. It is interesting to note, though, that some “modules” of the hibernation regulatory chain are still in place in humans.

In trying to give you a more extensive answer, I assume that your class already knows about the basic facts of hibernation. Reference [1] gives a good overview, including the Wikipedia article on the subject. A very important fact here is that the lowering of body temperature is not a passive process. Instead, the body's internal thermostat is set to a different aim point, e. g., 5 °C instead of 37 °C. If it becomes colder outside, the body takes active measures to maintain this set temperature. There is no known mechanism in humans that could set our internal thermostat to any other temperature than 37 °C (or thereabouts), so we lack a crucial prerequisite for hibernation.

The decisive factor that enables species to hibernate is a specialized form of fatty tissue, the so-called brown adipose tissue. It has a unique feature: This tissue can burn fat generating almost nothing but heat. This is significant, since in all other tissues, heat is only generated as a by-product of other biochemical processes or as a result of muscle activity. Brown adipose tissue does this by means of a specific protein (uncoupling protein 1, UCP-1) which inactivates the last step in the respiratory chain. Without UCP-1, the respiratory chain would lead to the formation of ATP, the universal cellular energy store. With the last step inhibited, the energy generated by burning fat dissipates as heat. When hibernator animals build up their energy stores in the run-up to the hibernation period, they deposit the extra calories almost exclusively in the form of brown fat. At the end of hibernation, the brown adipose tissue initiates the re-warming of the body by its ability to generate heat. Now while hibernator animals have significant depots of brown fat in several anatomical locations (most notably between the shoulderblades), humans have very insignificant stores of this specialized tissue. We can only generate significant amounts of heat by muscle activity, i. e., shivering. So not only in terms of our internal thermostat but also anatomically, we humans are ill-equipped to hibernate. In technical terms, we are incapable of “non-shivering thermogenesis”. The large amounts of fatty tissue in overweight people are useless for hibernation, as this “yellow” adipose tissue is unable to generate heat. Otherwise we could loose weight simply by turning down the heating during winter or taking a walk in the cold with inadequate clothing. Sadly, this is not an option for us. See reference [2] for an extensive description of the properties and significance of brown adipose tissue. The article is available free over the Internet.

While the subject of hibernation itself does not seem to have been the subject of extensive research in the past, the biochemical and hormonal processes involved have aroused interest in other fields, especially medicine. For example, there seems to be a single signaling molecule responsible for inducing hibernation in animals (hibernation inducing factor, HIT – [3]). Its actions are manifold, and independently of actual hibernation, one of these activities seems to be the induction of an energy-saving behavior in a number of tissues. This is of interest when you want to preserve oxygen-starved tissues, e. g., after a heart attack, a trauma or in cancer treatment [4]. In another development, NASA is taking a view towards inducing hibernation in human astronauts for long interplanetary trips [5]. The article also contains a nice and concise discussion of some more recent developments in hibernation research. The book on human hibernation may not be closed after all.

[1] Overview on hibernation:
http://www.answers.com/topic/hibernation

[2] An extensive article on the properties of brown adipose tissue (one paragraph describing the biochemistry of hibernation in detail); contains a link to a .pdf version
http://physrev.physiology.org/cgi/content/full/84/1/277

[3] The first description of HIT:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=568785&query_hl=4&itool=pubmed_docsum
(only the abstract, the full text is unavailable over the Internet)

[4] “Hibernation on demand” about possible medical applications
http://www.fhcrc.org/about/ne/news/2005/04/21/roth.html

[5] “Spiegel online” article about possible “human hibernation”
http://service.spiegel.de/cache/international/spiegel/0,1518,393746,00.html