Dave wrote SNIP >In short, no. Unlike bears and other similar mammals, we are not able to >metabolize fat effectively without a carbohydrate input. And, we do not >store enough carbohydrate (as glycogen) in our bodies to use much stored >fat as an energy source. SNIP >Peter, I think your skepticism about the insulating value of fat is >misplaced. Never mind there is not air trapped in it ... blubber is a very >good insulator. Just ask a whale. Remember, they are mammals, also. SNIP G'Day Thanks Dave for the analysis. I agree that my use of "solidly built" and "body mass" wasn't very precise and probably I'm too precious about describing a body as "fat". So I'll talk about mass when describing energy sources for example a gram of carbohydrate vs. a gram of fat and I'll use length when talking about insulators for example comparing the insulation of an inch of blubber with an inch thickness of clothing. Many of my friends who have plenty of muscle and fat could last all day in water that had me mildly hypothermic in an hour or two, even while I was wearing a dry suit on one occasion. Hence my doubts. I've just done a quick check on thermal conductivities of muscle and fat using Google scholar. Their conductivities when isolated are only different by a factor of x2 while being greater than air by a factor of 10. In living tissue their 'effective' conductivities appear to overlap and to be 10 to 40 time greater than air. However this latter study seemed to depend on the degree of vascularisation. The last reference below at the bottom of this post gives a list of biological thermal conductivities. Given the conductivities involved, I'm finding it hard to grasp how human subcutaneous fat, blubber, whatever, is a sufficiently good insulator, to explain the survival of inadequately clothed people in the sea for three days. Its ability to insulate is certainly necessary but I still wonder if the reason that low fat bodies can't take the cold is also due to lack of the energy that fat may be able to provide. Regarding metabolising fat we might not be very good at it but we can still use it as an energy source although its last in speed of use in the chain from: blood sugar, glycogen in muscle, carbohydrate and then fat. From your comments it sounds as if we may not be able to metabolise fat fast enough? I can see that's true for using fats as an energy source in exercise but is it true for maintaining body heat? Maybe a useful emergency food would be a combination of carbohydrate and fat, but I suspect people with plenty of fat would win hands down - the premise being that they can store more in their bodies than most kayakers could in a dry bag or PFD. I'm no expert but can't say I'm convinced yet. BTW this isn't a surreptitious plug for Mars bars and I don't own shares in any chocolate factories. Some references - the first acknowledges the value of fat as an insulator in channel swimmers but also references the factor of two conductivity difference from muscle. http://www.nature.com/nature/journal/v178/n4524/abs/178087b0.html http://www.nature.com/nature/journal/v168/n4282/abs/168918a0.html http://users.ece.utexas.edu/~valvano/research/Thermal.pdf http://www.ingentaconnect.com/content/mksg/sms/2004/00000014/00000002/art000 03 Abstract on fat metabolism All the best, PeterO *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Fri May 21 2010 - 08:14:29 PDT
This archive was generated by hypermail 2.4.0 : Thu Aug 21 2025 - 16:31:41 PDT