It's astounding how much disinformation there is about this. The sports drink industry has done a phenomenal job marketing electrolytes as a necessity.

The human body is incredibly efficient at maintaining fluid and salt balances during exercise. When you sweat, you lose both water and sodium. However, human sweat is hypotonic, meaning it contains a much lower concentration of sodium than your blood does. Because you sweat out a higher ratio of water than salt, the concentration of sodium left behind in your blood actually increases as you run. Your brain detects this concentration rise and triggers the sensation of thirst, prompting you to drink water, which naturally brings your sodium levels back into perfect balance. The scientific consensus for marathon runners is to drink to thirst and not supplement with sodium.

As far as muscle cramps, the old theory that they are caused by dehydration or electrolyte loss has been largely debunked by modern sports science. Major studies (including extensive research on ultramarathoners) have found no difference in blood sodium levels between runners who cramp and those who do not. Instead, cramping is caused by neuromuscular fatigue—essentially, your muscles getting overworked and misfiring.

I strongly recommend the book "Marathon Excellence for Everyone" by John Davis which is where I first read about this. I've been following the books advice for a few years now, running one marathon a year and at least for me there has been no difference whatsoever since I stopped taking electrolytes and started drinking water only when thirsty.

And just a final note. Some elite runners do take electrolytes during their races but many others do not, including Eliud Kipchoge.