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I've been under the impression my entire life, even after taking a moderate amount of chemistry and biology in college, that reducing sodium intake and being in better overall shape would cause someone to sweat less, or begin sweating less easily.

However, upon some internet research I discovered this is not the case. According to many sources, people who are in shape sweat much more even while doing normal daily activities than they would if they weren't in shape.

Because sodium controls the water flow into and out of cells, I always thought reducing your daily sodium intake would make your ambient sweating decrease. There is no good source even mentioning this.

For people who sweat too much, everyone recommends obvious things like stronger deodorant or light/cool clothes. At the other end of the spectrum, some medical procedure is recommended like Botox or shock treatments.

Can a person control one's ambient sweating level? And by "ambient sweating" here I mean how easily a given activity makes someone begin sweating, not necessarily how much someone may sweat during exercise.

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TLDR:

Fitness is not a good predictor of overall levels of sweat, but high-fitness individuals have better thermal regulation (likely through either cooling off faster/more efficiently) than low-fitness individuals (who will remain with significantly higher body temperatures for longer periods of time).

Dietary sodium intake does more directly correlate with overall levels of sweat, with higher sodium diets sweating more than lower sodium diets.


Details:

I think this a great question/example where someone from outside the field would have a hard time looking up the term, and therefore a harder time finding the correct research. I even had to go back to my textbook to make sure I had it right, as it is outside my expertise.

Generally, the research measurement to consider is "Whole Body Sweat[ing] Rate" (WBSR) though you may be interested in "Regional Sweat[ing] Rate" (RSR) [note there are some publications that go with or without the -ing]. A good quick comparison between the two can be found here.

There are some interesting differences when this is studied, but one can imagine a few factors and combinations coming into play. For all our comparisons, let us consider we have two populations, where one is generally "high fitness" and the other "low fitness," and then we could subdivide based on sodium intake or blood levels, as well as sex or anything else of interest.

One could imagine that if you just ask both groups to climb stairs or run a treadmill for "x" amount of time, and you allow the participant to set the pace, the fitness group will likely go further/produce more work, and the good old 2nd law of thermodynamics would have us considering that more energy was therefore needed (perhaps falsely assuming similar mass), and likely that more waste heat was accordingly generated. This could then be mitigated/exasperated through differences in work or cooling efficiency, etc. Luckily, it is an extensively researched issue.

To restate the question at hand, we want to know, with all else being equal as possible, what are the differences in WBSR when comparing fitness and/or dietary sodium intake.

Now one comparison, that you didn't actually ask, is worth pointing out first: there is generally no difference between fit and unfit groups when you compare the sodium concentration in the sweat. This is worth considering because we want to be sure your two questions aren't confounding, and evidence suggests they are not. Sodium levels are mostly determined by diet and genetics, and there are high/low sodium individuals in both fitness groups. One could imagine that high-fitness individuals are biased towards low-sodium diets, but at least within the US, it's not hard to find high-sodium consumers who are also relatively high-fitness.

Next, we want to look at the fitness question directly (teenagers being a fair choice for reduction in other unrelated comorbidities, but may not be extendable to adults) where we see the trend that differences in WBSR are not normally found, but differences in internal temperature usually are.

In other words, although the sweat levels were similar, unfit individuals had a greater increase in body temperature, which is not compensated for by an increase in sweat over the same period of time. Further, this dysregulation may persist longer than most of the other immediate improvements caused by starting exercise in inactive individuals, which may point to actual fat reduction being necessary for improvement (which occurs over a longer term).

One complication I noted in most studies, was not continuing to monitor WBSR until both groups returned to baseline, as it could very well be that given enough time for the increased body temperature of unfit groups to normalize, they sweated more in total, just by virtue of the larger window. That said, I suspect this difference is slight, but I think it could hint at the common perception that fit individuals sweat less. Fit induvial certainly become less hot and/or cool more quickly, and generally have better thermal regulation.

Moving to sodium, which can be a little trickier to measure accurately, the paradigm is more straightforward: the more sodium you have in your blood/diet, the higher concentration of sodium in your sweat. When you directly change people's diet, and then measure their sodium and sweat amounts, you do find that sodium levels and sweat positively correlate. Thus, if you have a low-sodium diet, we would expect you to sweat less than if you were otherwise eating a high-sodium diet, and this effect happens fairly quickly (as expected by blood sodium levels).

Ambient sweating is harder to nail down, and generally, I suspect attempts at sweat control should be done under medical supervision. An easy, and very ill-advised way to control one's sweat, for example, would be to simply be dehydrated. More commonly, individuals slather themselves with chemical antiperspirants. This, while not as obviously harmful as dehydration, can also lead to significant stress on the body, and again, should be considered under medical supervision. If you are worried about the level of sweat you produce, it's important to rule out causes completely unrelated to fitness and sodium intake, which again, would point you back to your primary care provider.

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    I'm not sure I follow re. your TL:DR summary. Fit individuals sweat more in similar situations than the unfit. The improvement in thermoregulation involves increased vasodilation in skin in order to sweat more. (Also, fit individuals are more likely to suffer from hyperthermia than unfit ones due to the fact that they generate more heat - 2nd law stands.) This, however, doesn't address diet, serum sodium, control of sweating or the composition of sweat. Commented May 8 at 2:53
  • @anongoodnurse do you have a source for that? I found about ~4 references that did not find a significant difference but can keep looking.
    – Atl LED
    Commented May 8 at 10:22
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  • And there's this one that seems like it may have kicked off the method and mechanism for the finding. I'm going to have to read more then on the measurements used to make sure I'm looking at apples to apples, because a lot of the papers I found showed the trend but weren't significant.
    – Atl LED
    Commented May 8 at 19:27
  • In the exercise thermoregulation literature, it's oft repeated. I'm not sure that it affects your answer, though, which is about reducing sweating. It was just something in the TL;DR: that I didn't follow. Commented May 8 at 20:04

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