Too much water during a marathon can dangerously lower blood sodium

Doctors warn marathon runners that drinking more water than the body needs can cause exercise-associated hyponatraemia, a drop in blood sodium that can mimic dehydration. Dr Judhajit Sengupta of Narayana RN Tagore Hospital, Kolkata, explained that excess fluid during prolonged exercise dilutes sodium in the blood, so hydration during long runs needs balance rather than drinking at every checkpoint.