Unlike saturated and monounsaturated fatty acids which can be synthesized by all mammals, including humans, the omega-3 PUFA cannot be easily synthesized in the body and must be provided through the diet.

The unique feature of marine oils, such as seal oil, relates to their high content of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and, to a lesser extent, docosapentaenoic acid (DPA). These PUFA are formed in unicellular phytoplankton and multicellular sea algae and eventually pass through the food web and become incorporated into the body of fish and higher marine species. The high content of omega-3 fatty acids in marine lipids is suggested to be a consequence of cold temperature adaptation in which omega-3 PUFA remain liquid and oppose any tendency to crystallize.