(covers information from several alternate timelines)
An electrolyte is a substance that contains free ions which behave in an electrically conductive medium.
In an alternate timeline in 2366, when the crew of the USS Enterprise-C were beamed aboard the USS Enterprise-D for medical attention, Dr. Beverly Crusher asked medical assistant Martinez to run a full electrolyte report. (TNG: "Yesterday's Enterprise")
In 2367, the Enterprise-D was apparently drawn into a wormhole near a T-Tauri type star and the entire crew, save for Lieutenant Commander Data, were knocked unconscious. According to Data, the crew was out for thirty seconds. Dr. Beverly Crusher started to become suspicious and later became convinced that the crew was out for an entire day when she had Nurse Alyssa Ogawa scan Ensign Locklin for her electrolyte concentration, which was found to have deviated. (TNG: "Clues")
In 2372, Neelix told Captain Kathryn Janeway that she was missing her meals and that it could do serious damage to her electrolytes. After viewing the food Neelix prepared in the USS Voyager's mess hall, she remarked that it was bound to increase her electrolyte levels. (VOY: "Persistence of Vision")
Later that year, when Lieutenant Tom Paris was admitted to sickbay, The Doctor found that his electrolytes were breaking down when he became allergic to the water in his coffee. Paris was later discovered to be undergoing the process of hyper-evolution, brought on after he broke the transwarp threshold. (VOY: "Threshold")