Modern sodium intake is highly variable, and most performance-focused diets are sodium-deficient. Sodium drives fluid balance, blood volume, and cognitive performance; potassium opposes it; magnesium is the cofactor.
Dose: Typical: 1-2g sodium + 200-500mg potassium per day from electrolyte mix, more in heat or training.
Caveats: Low-carb diets need more sodium, not less. Watch potassium if on ACE inhibitors or other RAAS-blocking meds.