"endurance athletes such as marathon runners often ingest salt along with water and/or sports drinks containing salt. explain the physiological mechanisms by which this would improve hydration."

Carrying watery fluids during the time of marathon run, it is termed as crucial because of retention of salts in the body where finally it will attain homeostasis. Electrolytes like sodium, potassium, chloride, salts, the water they are going to be environment out of the body by sweating during the time of running. The blood PH decreases because of water content and more carbon dioxide goes ahead and produces carbonic acid and hydrogen ions and this decreases blood PH which results in metabolic acidosis.

Hmmm i'm not sure about this one