yea, this is totally true. You dilute your electrolytes and get hyponatremia. Very common in marathons now - here's a paper on it: https://www.nejm.org/doi/full/10.1056/nejmoa043901

"On univariate analyses, hyponatremia was associated with substantial weight gain, consumption of more than 3 liters of fluids during the race, consumption of fluids every mile"