Fish oils come from fatty or oily fish, such as trout, mackerel, tuna, herring, sardines, and salmon. They contain omega-3 fatty acids, and many contain vitamins A and D. Omega-3 fatty acids are fats commonly present in plants and marine life. Two types are plentiful in oily fish: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Alpha-linoleic acid (ALA), meanwhile, mainly occurs in plant-based foods, such as flaxseed. Omega-3 is present throughout the body, especially in the brain, retina, and sperm cells. The body cannot produce omega-3 on its own, however, so people need to obtain it from dietary sources.