Either they have a high EPA content and come mainly from anchovies and/or sardines, or they contain a significant amount of DHA and are generally derived from tuna oil.

There is now a growing demand for concentrated oils with high levels of polyunsaturated fatty acids (PUFA), particularly for the supply of concentrated oil capsules, which reduce catches for an equivalent amount of active product. This is a chemical concentration process that can be supplemented by a second enzymatic process.

To obtain oils with a high polyunsaturated fatty acid content, the oils are first concentrated by a process that transforms triglycerides (TG) into ethyl esters (EE), involving the use of a solvent such as ethanol. Ethyl esters are an artificial chemical form that does not occur in nature.

An additional step is possible to return the fatty acids to their natural form (TG). This new concentration is achieved using enzymes that convert the fatty acids back to the triglyceride form, which is more stable and resistant to oxidation.