Taking cod liver oil as a source of vitamin D first became popular in 19th Century England. Fish oil has been studied for heart health since it was found that Greenland Inuit people may have a lower risk of heart disease despite eating a high-fat diet.

The main essential fatty acids in the human diet are omega-3 and omega-6 fatty acids. Foods that provide omega-3 fatty acids include fish oil and certain plant and nut oils, while omega-6 fatty acids can be found in palm, soybean, rapeseed, and sunflower oils. Omega-9 fatty acids, which are not considered essential, can be found in animal fat and olive oil.

Fish oil contains two omega-3 fatty acids called docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Some nuts, seeds, and vegetable oils contain alpha-linolenic acid (ALA), which may be converted to DHA and EPA in the body.

In the United States, percentages of DHA and EPA have been found to be lower compared to other nations with lower heart disease rates, such as Japan. High levels of omega-6 fatty acids have been linked to an increased risk of some conditions such as heart disease and depression.

Omega-3 fatty acids are thought to provide a wide range of health benefits Due to these potential health benefits, fish oil, which is rich in omega-3 fatty acids, has become a popular supplement.