I've heard similar things about athletic training while mildly overheated. Training while overheated promotes adaptations to increase the cooling efficiency of your body. These adaptations include increasing heart rate, circulation, blood plasma volume, etc. This, in turn, promotes improvements to endurance, ability to perform in both normal and overheated states, etc.

The body is certainly capable of acclimatizing the high body temps. I don't have the source, but I remember reading that Israeli tank soldiers (who were fighting from inside a tank in the desert) had body temperature of 101 - 102F and were completely fine. At that body temperature most people would start to feel pretty awful.

That's intriguing. Where might one be able to read more about that?

I can't remember what journal I was reading, but I was able to quickly find a fluff article with some decent information [1] that happens to mention an on-point cycling study [2].

For me, this involves training outside in the Texas summer heat (100F+) with plain water. When I race, it is usually earlier in the day (80-90F) and I sometimes use ice socks in my jersey and ice water in my bottles to keep my core temperature down.

[1] http://blog.airiarunning.com/heat-training-does-it-boost-run...

[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2963322/

That would corroborate long distance runners from Kenya and Ethiopia.

What doesn't kill you makes you stronger... I guess?

No, or at least definitely not as a blanket statement. There are many kinds of damage where the effect is highly or completely cumulative. In particularly one should be extremely careful of any such claims that involve the nervous system (including the brain) and immune system, neither of which can heal in the same way that many other parts of our body can. With regards to the brain, concussions are a good current example of something showing strong evidence of very dangerous cumulative effects at least somewhat independent of each incidents severity, and with potential for significant delays between events and progression of symptoms. With the immune system a key part of its training and ability to fight off disease could be somewhat summarized as "what doesn't kill you makes you stronger", and we use this with vaccines. Exposure to a varied environment looks to be particularly important as a child. But in some cases later repeated exposure to allergens can result in the development of immune hypersensitivity, which can be deadly serious. The danger can be increased because the sensitization event(s) often does not result in any significant problems itself, rather it is only on exposure following sensitization that the symptoms appear. Of course while these have all been internal systems examples, even exercise can be very dangerous if done inappropriately, without sufficient prep/stretching/maintenance/rest, etc. It's possible someone to hurt themselves in ways that will take a long time to recover from and they certainly won't be stronger at the end of it, rather they'll have to start again from a reduced base.

Ultimately our biological systems are simply too complex to get boiled down to any one universal bit of "common sense". I think it's laudable to try to improve ourselves, and it's true that in some cases that may require challenging oneself and/or cycles damage/repair as in working the muscles, but risks should be appreciated and approached with deliberation too. I'm certainly not trying to rag on you or anything like that, I'm just wary of this sort of thinking without caveat because I've seen it amongst my own circle used to justify some "medicines" and "treatments" and "training programs" that turned out to be quite harmful.

This is increasingly sounding true with research that show periods of starvation are probably beneficial for health. Of course there has been a lot of discussion about fasting, but there is also data that show parental stress during child-bearing probably leads to increased health among progeny.

I'd characterize it as, there are things that can make you stronger that people don't want to do, e.g. fasting, undergoing stress, temperature variation, intensive exercise. There are things that make you stronger that people already like to do (eating food, moving, etc), but we haven't made the effort as a society to automate those things out of our lives.