saw someone quote this in a running chat:
10:28 < Mattias> Found it! "While all running improves blood volume, a 2012 study from New Zealand documented significantly increased plasma volume and performance following training in the heat, with the stipulation that mild dehydration (no more than 2 percent) must be allowed to occur to reap the full benefit. Runner's World's Alex Hutchinson noted in his blog, Sweat Science, that studies like this illustrate "the importance of allowing your body to undergo training-induced stresses, rather than making heroic efforts to cushion your body from discomfort...[Leaving] the water bottle at home may be a good call.""
nice.