Hyaluronic acid, also called hyaluronan, is a substance that is naturally present in the human body. It is found in the highest concentrations in fluids in the eyes and joints. The body makes hyaluronic acid naturally with the main function to keep tissues cushioned and well-lubricated. As people age, the amount of hyaluronic acid decreases in the body. To compensate low levels, hyaluronic acid supplements are taken. Hyaluronic acid has a variety of uses. Besides taking it as a supplement, it is also used in topical serums, eye drops and injections.