Hydration during running is about more than simply drinking plenty of water. Doctors recommend replacing roughly 80% of the fluids lost through sweat rather than trying to replace every drop. However, this is only a general guideline—hydration needs vary based on sweat rate, pace, fitness, temperature and humidity. Losing more than about 2% of body weight through dehydration can affect performance, while excessive water intake can dangerously lower blood sodium, causing exercise-associated hyponatraemia. Runners can estimate their sweat rate by weighing themselves before and after a run while tracking fluid intake. For longer or hotter runs, electrolytes—especially sodium—may also be important. The goal is personalised hydration, not simply drinking more.
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