Water alone isn't enough on long runs

You sweat for two reasons: cooling and effort. The longer or hotter the run, the more you lose — and what comes out of your sweat glands isn't pure water. It's water with a meaningful amount of sodium, plus smaller amounts of potassium, magnesium, and calcium.

For short, cool runs, you can ignore most of this. Your body keeps its electrolyte balance steady on its own and a few sips of water afterward covers it. The math changes once you cross roughly an hour of continuous effort, or anytime the temperature and humidity are working against you.

Two things go wrong if you only replace water. First, your stomach can absorb fluid only so fast, and water without sodium doesn't get pulled into your bloodstream efficiently — so you can drink and drink and still feel dehydrated. Second, and more dangerously, drinking large volumes of plain water during a long effort can dilute your blood sodium to the point of hyponatremia. Symptoms range from headache and nausea to confusion and, in serious cases, hospitalization.

Endurance nutrition guidance generally lands around 300–600 mg of sodium per hour during prolonged exercise, adjusted up for heavy sweaters and hot conditions. That can come from a sports drink, salt tabs, electrolyte mixes, or even salty real food on ultra-long efforts.

What to do with this: for runs under an hour in mild weather, water is fine. Past that, or in heat, build sodium into your plan. Drink to thirst, not to a fixed target — and if you ever feel worse the more you drink, stop and add salt before more water.

Source: Utah State University Extension — Hydration Guide for Endurance Runners