Athletes who cramped had the same blood salts as those who did not
In 210 Ironman triathletes and 88 marathon finishers, the ones who cramped matched everyone else on sodium, potassium and how much weight they had lost.
What the study found
Athletes who cramp and athletes who do not test the same
This is where the electrolyte theory of cramp should show up. In 210 Ironman triathletes, serum electrolytes and weight change did not differ between the 43 who cramped and the 166 who did not; in 88 marathon finishers, the 20 crampers matched the rest on sodium, potassium and hydration.
The caveat · Measured in blood, not in the muscle
One real exception sits inside the largest of these studies: among American footballers alone, sweat measurements did carry a weak signal, and in the other ten sports none did. Beyond that, cramping was self-reported throughout, and serum electrolytes were sampled before and after a race rather than inside the muscle at the moment a cramp struck, which is where the theory actually locates the problem. Forty-three crampers is enough to rule out a dominant effect, not a small one.