There are two different types of fish oils – those that come from the liver of “lean” fish such as cod, and those that are extracted from the flesh of fatty fish such as mackerel, herring, or salmon. Fish oils have a high content of fatty acids, with 20 or more carbon chains that are either predominantly monounsatured, or polyunsaturated with five or six double bonds belonging to the n-3 family. Fish liver oils also contain large amounts of vitamins A and D and are available as oil or in capsules as a dietary supplement of these vitamins. Fish body oils are usually purified and supplements to not generally contain the vitamins.
Many years ago it was observed that Greenland Inuit had a low incidence of coronary heart disease despite their high fat and cholesterol intakes. Most of the fat was from marine foods, with large quantities of the long-chain n-3 fatty acids elcosapentanoate (EPA) and docosahexanoate (DHA) and research has now demonstrated that these fatty acids affect blood clotting mechanisms so that blood clots that might block coronary arteries narrowed by atherosclerosis are less likely to form. These effects are seen with purified fish oils as well as when fatty fish are consumed. Shark, swordfish, and marlin may be contaminated with mercury and should not be eaten by pregnant women, women who intend to become pregnant, and infants and children under 16. Purified fish oils represent a way of obtaining the n-3 fatty acids, but care should be taken to keep to recommended doses. As with plant foods, the foods themselves may contain other, as yet unknown, substances that could also have important effects.
Research showed the evidence that consumption of the types of long-chain fatty acids found in fish oils benefits heart health.
There has been much recent interest in the idea that supplements of long-chain n-3 fatty acids may improve behavior and reading and spelling ability in some children, specifically those with a condition called developmental coordination disorder. Although there is evidence that these fatty acids are important in pre-natal brain development and for post-natal development in premature infants more research is needed to clarify the possible cognitive benefits in older children and in the elderly.
Some studies have suggested that consumption of long-chain n-3 polyunsaturated fatty acids may help protect against diabetes (incidence is lower in populations with high fatty fish intakes), and may lower blood pressure in patients with hypertension. Fish oil supplements (fish liver oils as well as fish body oils) have been shown in some studies to reduce pain and inflammation in rheumatoid arthritis, although more research is needed here. Trails of fish oils in the treatment of skin diseases such as atopic (allergic) eczema and psoriasis have shown mixed results. Fish oils do not seem to have much effect on eczema, but they have been shown to have modest beneficial effects on psoriasis, especially when taken in conjunction with other treatments, as they may alleviate some of the side-effects of drug treatments. If you are receiving medical treatment for any of these disorders it is advisable to see your doctor before consuming fish oil supplements.