Essential Fatty acids. USN fish oil is a natural supplement filled with Omega 3 fatty acids. Fatty acids are the basis of fats and oils necessary for overall health. These specific fatty acids are termed "essential" because your body cannot manufacture them by itself. EFA's must come from food or supplemented sources. They are also essential because they are a component of every living cell in the body, and are necessary for rebuilding existing cells and the production of new cells.