A classic 20-ounce bottle of lemon-lime Gatorade contains nearly one-eighth teaspoons of salt and 80 milligrams of potassium — electrolytes "to help replace what you sweat out," the product's ads say.

In the past few years, a slew of ionized waters and beverages boasting electrolyte-enhanced formulas have hit the market, promising us that well feel better and more energized if we drink them. Theyre ...