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, including a lower risk of coronary heart disease and improvement in cholesterol. There have also been promising results from studies looking at omega-3 for cancer, depression, and attention-deficit hyperactivity disorder (ADHD). Due to these potential health benefits, fish oil, which is rich in omega-3 fatty acids, has become a popular supplement.
There is evidence that recommended amounts of DHA and EPA, taken as fish or fish oil supplements, may lower triglycerides and reduce the risk of heart attack, abnormal heartbeat, and stroke in people who have heart disorders. DHA and EPA may also benefit people who have hardening of the arteries or high blood pressure. Similar effects have been found for ALA, but more evidence is needed to support its potential benefits.
High doses may have harmful effects, such as increased bleeding risk, higher levels of low-density lipoprotein (LDL, or “bad”) cholesterol, blood sugar control problems, and a “fishy” odor. In some at-risk populations, such as people who have had a heart transplant, omega-3 fatty acids may affect the heart rate. Omega-3 fatty acids should be used only under medical care in people who have heart disease. Some fish such as swordfish, king mackerel, tilefish, and albacore tuna may carry a higher risk of mercury poisoning, though fish oil has not been found to carry a significant risk.