Robert Sallis has seen it all. As a medical director for the Ironman World Championships in Kona, Hawaii, he’s spent 20 years watching athletes in every manner of distress get wheeled into the medical tent. He’s seen hyponatremia, or overhydration, a handful of times. He’s seen hundreds, maybe thousands, of dehydration cases. Sallis has even seen athletes show symptoms of both at the same time: they’ve dropped weight over the course of the race, signaling dehydration, but their blood sodium levels are dangerously low, a sign of hyponatremia.