Interesting article, great question. I think it is a bit of a "virtuous cycle."

I have always thought of heat tolerance as a skill. My dad worked shoveling coal in a foundry when he was a young man and the heat never bothered him at all after that, so he apparently got permanently conditioned. This obviously was very physically strenuous. My mother, who was the most sedentary human being who ever sat of the face of the earth, but was quite thin, always wilted in the heat (and tended to carry on and complain about it). They were both of Polish descent, so theoretically they were both evolved to be cold-adapted rather than heat-adapted.

Every other year or so I go to New Mexico in June and dance about six hours a day for a week or so (it's also a significant change in altitude), no air conditioning in the studios (typically). I have to concentrate on really pounding the electrolytes for the first couple of days (Gatorade tastes like an elixir of life!), and it's amazing how much I can drink during the day and not pee. I'm not currently a runner but when I was I ran in hot weather in Montana and West Virginia.

I do wonder now how much of it is the exercise that adapts us to heat, and whether sitting in a sauna, say, is heat "training" in the same way that people now "train" doing CT etc. The article seems to be saying that we have to reacclimatize to hot weather every year and it takes a couple of weeks. That doesn't seem quite right to me.

I moved to Santa Fe 10 months ago and it seems to have taken me this long to adapt to the heat/dryness. I have learned to drink more before being active & that's helped a lot.

Contrasting this with my experience living in South Carolina, on the coast in high summer. I seemed to adapt much faster, perhaps due to the high humidity.

My husband, on the other hand adapted right away to the high desert, but gets sick every time we visit Iowa in the hot, humid summer.

We are both very active, so I think there's something else going on.

I wish I knew the answer to this. I'm a sweater and it sucks. I will eventually adapt a little to hot weather, but not completely. It doesn't seem to matter what my physical activity level is at either, as I've been anywhere from sedentary to very active at different periods of my life and always had the same problem.