Essential polyunsaturated fatty acids (PUFAs), vital to the normal functioning of the body, belong to several different chemical families. The fat in fish contains so called omega-3 PUFAs (also known as n-3 PUFAs), which differ from omega-6 PUFAs found in vegetable oils. Omega-3 and omega-6 PUFAs have different roles in body metabolism. Fish oils contain about 50 different fatty acids, and are the main source of the omega-3 essential PUFAs eicosapentaenoic acid (EPA) and docosahexanoic acid (DHA). EPA and DHA levels are very low (or absent) in domesticated land animals. The habitat in which the fish grow has a major influence on their fatty acid composition. The fish obtain omega-3 PUFAs by eating plankton, which in turn contain more omega-3 PUFAs the colder the sea water (cold water fish also have a higher fat content). Cold water fish such as salmon, tuna and mackerel have the highest levels of omega-3 PUFAs. Farmed fish fed commercial fish food contain lower levels of omega-3 PUFAs.