In desert races, drinking too little was the commoner failure
Of 174 runners tested after an 80 km desert stage, 30 finished with blood sodium above the normal range and 11 below it. The high ones had lost 4.7 kg on average.
What the study found
In desert ultras, high blood sodium beat low by nearly three to one
Underdrinking is the commoner failure in real heat. Of 174 runners tested after an 80 kilometre desert stage, 30 finished above the normal range for blood sodium and 11 below it. The high ones were the dehydrated ones, down 4.7 kilograms on average; the low ones had mostly overdrunk.
The caveat · The 4.7 kg average hides wide spread
Five desert stage races, which is what extreme heat does across six days, not what a hot afternoon does. Only 174 of the 266 runners enrolled gave a post-race sample. The link between hot races and low blood sodium is not statistically reliable, its interval running from 0.9 to 25.9; the link with high blood sodium is much firmer. And the 4.7 kilogram average carries a standard deviation twice its own size.