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I find that on long desert treks, I experience noticeable heat adaptation (less sweating) after 5 to 7 days.
There are a couple parts to heat adaptation in a dry climate and less sweating isn’t one of them. Heat adaptation in a dry climate means your body sweats more and sooner in response to an increase in core temperature.
There are multiple sources, but from this one (https://www.sciencedirect.com/science/article/pii/B9780444640741000318):
A critical physiologic change that occurs during heat acclimation is the “training” of sweat glands to produce a greater amount of sweat. This leads to a progressive increase in whole-body sweat rate for a given work intensity or core temperature
If someone says they sweat less after 5-7 days in the desert and they consider this as being “heat adapted”, then I think they are sweating less because they have become progressively dehydrated during the trip and have total lower water volume. From the same paper as above:
As water availability in the body decreases, so does the ability to sweat and increase skin blood flow during heat stress (Horstman and Horvath, 1972). Dehydration increases the core temperature at which the onset threshold for skin vasodilation occurs and decreases sweat rate for a given body core temperature
Increased sweating is a heat acclimation (HA) response (and a good one at that), but it doesn’t get after the core physiological process by which HA occurs. The real benefit of HA is that you become physiological more capable as you adapt to heat. So, for a given amount of effort, your heart rate stays low, your skin capillaries promote better blood flow, thermoregulatory processes aren’t stressed, and thus, you sweat less for a given amount of effort on i.e., day 7 vs. day 1.
Increased sweating comes into play as an HA mechanism when you’re exertion level is high enough (think, closer to your AeT) to stress thermoregulation in order to maintain core temp.
At lower heart rates, HA results in a few really important changes that can actually reduce the need to sweat: more efficient cellular fluid balance, higher plasma volume, and more water retention.
Check out some of the work by Julien Periard and colleagues, they’ve built a pretty good picture of HA across the spectrum of athletic performance.
Getting this back to water treatment options, I ordered the tiny Ultralight model Ryan recommended and then returned it when I realized it only treats in 1L increments. I’d prefer the ability to treat in .5L quantities so I ordered the Ultra instead. Doing so also gives me the option to treat in a Smartwater or similar bottle without carrying a scoop or pot (one more thing to keep track of).
The Ultra is rated for 50L (Ultralight is rated for 20L) and weighs about 2.5 ounces more. This weight is offset by the need to carry extra recharge capacity that if need if I was using the Ultralight in the manner I’d prefer on a trip of more than a couple nights and it doesn’t require a scoop.
YMMV
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