Cold Exposure, Seasons, and Metabolic Rate: We Are Homeotherms, After All.

 

(Note: This is an old-ass post that I’m just trying to finish so that I have some form of communication with the outside world due to my internet restriction. I hope you find some value in it)

Though the “heyday” for cold exposure, under the super-silly name of “cold thermogenesis,” has passed us, there’s still periodic chatter about the subject. This leads to interesting conversations, namely about how valuable it is or can be and how best (time/temp/duration) to expose one’s self to it. There are those who eventually turned to polar bears, but the vast majority of folks found the results to be less than advertised. Nobody became a shredded, body fat-proof human being as a result of hanging out in cold tubs.

In my own experiments, I pushed the pendulum pretty far, even managing to give myself mild hypothermia for the sake of “science” (read: being thickheaded, or awesome depending on who you ask [But if you ask me, awesome]). For me, it was merely hormetic: being warm weather urban folk, I found that I can now tolerate cold better than I could before. I’d still prefer a sweat lodge to an igloo, but the latter is less disgusting now.

That said I was having a discussion on twitter a couple months years ago with a woman who was stating that, as a result of her recent bout of “cold exposure,” meaning going outside in the freezing winter of middle America, that she was now eating a LOT of food just to warm up. Thus cold exposure must be obesogenic. This got me thinking: we’re circadian animals; is there a differentiated response to cold exposure depending on the season?

Big bags of Chemical Reactions leading to Heat

So humans are homeotherms, but we tolerate deep cold much better than heat. While we can handle a drop in body temperature of nearly ~18*F, we can handle only an increase of ~9*F. I’d even seen numbers as low as 7*F during my graduate research. The point is that cold sucks but it won’t kill us quite so quickly. We’re constantly generating heat, which is dissipated at the skin and through breathing, primarily. During especially cold periods, our smarter than us brain will constrict blood flow to the skin, reduce heart rate,  and shunt blood back toward our trunk to maintain organ function. This is the mammalian diving reflex when it happens quickly, but we do the same things (minus the blood shift mentioned in the link above) due to “dry” cold exposure. One of my clients, a cardiologist, has told me that during vein work in the limbs, blood flow can be cut off for nearly 2 hours without damage to the tissue. This is probably a result of this inbuilt dive reflex: if your core is freezing cold without heat generation for 2 hours, you’re probably dead…so who cares about the limbs?

Anyway, we have a deep thermic “floor” relative to our low thermic “ceiling.” However, during winter, we experience changes that set us up to survive. We tend to put on a bit of weight coming into the season and the thermic effect of food is one of the ways in which we keep ourselves warm. It has also been demonstrated that our sleeping metabolic rate is modestly higher in winter than in summer. As a result, winter “raises the floor” for cold exposure; our body, being smarter than us, only has so many compensations to account for cold exposure. We’re already running hotter in the winter, meaning that any further deepening of cold exposure has to be weighed to some degree: too much energy expenditure during the dry season runs the risk of exceeding environmental energy supply. Thus reducing activity and eating what is available both maintains body weight (or reduces the rate of loss) AND helps to maintain body temperature due to the thermic effect of food.

But we’re generally in thermal-neutral environments; does that make a difference to our cold respones? Remember, we’re circadian beasts. The literature shows a distinctly differentiated metabolic responses cold exposure in the summer versus the winter. One study has demonstrated that when exposing individuals to cold in the winter, an 11.5% increase in metabolism is observed. Compare this to the only 7% increase seen during the same circumstances in the summer and you’ll begin to understand why extra cold exposure in the winter may very well be obesogenic. The combination of already increased metabolic rate (modestly, both free living and sleeping) AND a significantly increased response to cold exposure trips the energetic negative feedback loop. For the same reasons that too much exercise makes you hungry (lots of energy expenditure), too much cold during the winter makes you hungry. So contextually, she was right.

However, and here’s the biggie, she’s lean. She used to be obese but now she’s lean. Lean individuals have been shown to have a disproportionate response to cold exposure as compared to obese individuals. We’re not insulated, so we have to increase our core temperature somehow. Given the above, we have some evidence in men that cooler temperatures results in compensatory overeat ad libitum. To quote the article:

 Overeating under ad libitum circumstances at 16°C attenuated the decrease in rectal core body temperature.

Further, recent literature has shown that obese individuals’ brown adipose tissue (BAT) does not function as well as lean individuals and “The increase in cold-induced BAT glucose uptake rate after weight loss was not statistically significant.” That may also be contributing the the “formerly obese now lean but overeating in the winter when cold” thing mentioned above.

So was she right? Here’s my takeaways:

  1. If you’re lean, cold seems to cause more aggressive energy expenditure vis á vis shivering and BAT activation.
  2. Cold exposure in winter seems to cause more aggressive metabolic response.
  3. Longer duration, aggressive cold exposure in the summer and more mild (relatively) and shorter exposure in the winter seems to better “match” the seasonal metabolic changes.
  4. Of course, YMMV.
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