Doctor warns overhydrating during marathons can dangerously lower sodium

Drinking more water than needed during a marathon can cause exercise-associated hyponatraemia, a drop in blood sodium, warns Dr Judhajit Sengupta of Narayana RN Tagore Hospital, Kolkata. He says even ORS and sports drinks carry less sodium than blood. The hormone arginine vasopressin makes kidneys retain water during exercise, so excess intake can dilute sodium. Runners should hydrate in balance, not at every checkpoint.