Omega 3 - ALA - EPA - DHA

Important Omega 3 fatty acids include: alpha-linoleic acid (ALA), eicosapentanic acid (EPA) and docosahexanic acid (DHA).
Fish oil is a modern form of what was earlier called cod liver oil. Whereas cod liver oil, with its many qualities, contained large amounts of vitamins A and D as well as triglycerides, vitamins A and D are often extracted from fish oil so that overdosage of vitamin A due to fish oil is not possible. Modern fish oil is often prepared so the essential fatty acids can more easily be absorbed.
Omega 3 fatty acids benefit the brain and nervous system, sense of sight and the immune defence system. They activate over a 100 different genes and, not least, are beneficial to the heart.
About 20% of the brain's fat is of the Omega 3-type; and it looks as if there is a statistical connection between high intake of omega 3 fatty acids and high intelligence and prolonged life span.
A supplement of Omega 3 fatty acids is used in the treatment of a long line of diseases. A daily supplement of Omega 3 fatty acids reduces the blood level of triglycerides and increases the level of the "good" HDL cholesterol. People with an overreacting immune system (such as allergy) often can benefit from a supplement of Omega 3 fatty acids. This is because deficiency in Omega-3 fatty acids leads to a low level of the hormone-like substance, prostaglandin E3.
Flaxseed Oil
It may be of importance whether one's source of Omega-3 fatty acids is animal or vegetable. Many people, especially sufferers from allergy may have difficulty processing, for example alpha-linoleic acid from flaxseed to DHA. Therefore flaxseed oil is considered a less optimal source of Omega 3 fatty acids.
In case of deficiency in important enzymes - such as delta-6-desaturase - the Omega-3 fatty acids cannot be converted to prostaglandins.
Omega 3:6 ratio
It is common in western countries that people's intake of Omega 6 and Omega 3 fat is disproportionate. Far too much Omega 6 is consumed compared to Omega 3. It is thought that the Omega 3:6 ratio is 1:15. It is not known precisely where it should be, but a guess is that it should be 1:3 - maybe lower.
Also see "Essential fatty acids - Linolic acid".