Last August, midway through a tempo run in 94-degree heat, my hamstrings started cramping so violently I had to sit down on someone’s lawn. I’d been hydrating all morning. I’d drunk nearly a gallon of water before noon. And yet there I was, folded up on a stranger’s grass, wondering why my body was betraying me despite doing everything “right.” That was the day I stopped trusting plain water. What I didn’t know then — and what took me three weeks of research, bloodwork conversations with a sports nutritionist, and some fairly embarrassing trial and error to figure out —…