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.