Probably the most common complaint about the seasons in Utah is how fast they change. Instead of a gentle decline from the summer heat into gentle cool days of autumn, we get- BAM!- a 40 degree drop in temps and a snowstorm. Actually I’m not sure that it’s all that different in this regard from the places I’ve lived previously- Colorado, Massachusetts, Pennsylvania- but I sure like to whine about it.
At any rate, if I were in denial before, I’m faced with reality now: summer is ending. The Living Year is starting to wind down, preparing to tolerate, survive or slumber through Winter in anticipation of the next Living Year.
There’ll be plenty of cool changes to keep me busy the next month or so, but the biggest change hit me hard this morning- the COLD. When I started pedaling at 6:30AM it was 29F. When I returned to the car- an hour forty minutes later- it was 35F. The whole ride- up & down- I was shivering, tense and out-of-sorts, unable to lock into a decent thermal equilibrium.
Every year I have a tough time with the first few really cold rides. It’s not that I’m a cold-pussy; come December I’ll regularly be riding pre-dawn with temps in the 20’s. But it’s those first few rides where my body can’t warm up. This happens every year. And it bugs me because if it’s just psychological then I’m being a wimp. At yet it feels so real, I’ve always wondered if there’s some sort of adaptive process, or “getting in shape” for the cold that’s at work in me physiologically.
Tangent: For some strange reason, the part of my body that hurts the most when exposed to cold air is the tippity-top of my ear. Not my toes or my fingertips, or my nose, or any of the normal places. Just that tops of the ears.
When I go skiing in the winter I’m always awestruck by guys skiing bareheaded or in baseball caps, skiing along, or just sitting on the lift, smiling away without a care in the world. Aren’t their ears killing them? How can they stand it??
Well, it turns out I was right: Tests on winter athletes and specifically winter swimmers show that regular exposure to cold over periods ranging from 10 to 30 days induce significant metabolic changes to the human body that improve the body’s conservation of heat, and extend the time of cold-exposure tolerated before the onset of shivering.
What Keeps You Warm?
The body’s generation of heat is called thermogenesis. There are 2 kinds of thermogenesis.
Shivering Thermogenesis (or just "shivering" in layman's terms) works, producing between 10 and 15 KJ/minute, but it has the significant downside of impairing coordination, or “skilled performance.” It’s really only functional as a “bridge” to non-shivering thermogenesis. And if it doesn’t bridge you to non-shivering thermogenesis in about 30 minutes or so, it’s more of “plank-walk” to hypothermia… Shivering is controlled by the hypothalamus, that part of the brain that also regulates sub-conscious or half-conscious bodily functions such as heartrate and breathing.
Non-Shivering Thermogenesis is the real ticket; that’s what’ll keep you warm outside for the long-haul. And the part of your body that plays the most important role in this process is the rather gross-sounding Brown Fat, or Brown Adipose Tissue. 
Brown Fat isn’t the fat on your gut or your ass (that’s White Fat); it generally surrounds blood vessels and internal organs and comprises less than 2% of your total body mass. (In infants, Brown Fat comprises more like 5% of total body mass, due to the challenges of keeping a smaller body- with its higher surface area-to-volume ratio- warm.) Brown fat is packed with fat cells and capillaries, and its primary function is keeping you warm.
A Wee Bit O’ Chemistry
The fat cells in Brown Fat are packed with an unusually high number of mitochondria. Mitochondria are like little mini-cells within a bigger, complex, eukaryotic cell (the kind of cells that complicated creatures like us, moose, turtles and black-headed grosbeaks have) and their primary job is to generate energy for the cell, in the form of adenosine triphosphate, or ATP (structure diagram below right). ATP is the biochemical fuel that powers every cell in our bodies.
In Brown Fat the ATP-producing reaction is altered by a special kind of protein in the mitochondrial membrane wall called an uncoupling protein, which causes the reaction to produce heat instead of ATP.
The primary regulator of this heat-producing reaction is a hormone called noradrenaline (structure diagram left), which acts to depolarize, or reduce the voltage across the cell membranes, and thereby accelerate the uncoupled-protein-modified, would-be ATP-producing reaction.
How We Adapt to Cold
Cold-weather athletes show enhanced levels or noradrenaline after about 10 days of repeated exposure to cold temperatures, and during that same period blood flow is modified to provide greater flow to critical internal organs and Brown Fat concentrations.
So I wasn’t imagining things; we really do adapt to the cold every year. We just have to tough it out for a couple of chilly weeks to do so, which- depending on how much pre-dawn riding I do this Fall- probably means I’ll be comfortably cold-adapted sometime in mid-to-late October.
Thanks for posting this!
ReplyDeleteI knew about shivering as a bridge and the use of brown fat for thermogenisis but didn't really understand the adaptation, especially the period of adaptation, until reading this :)