I noticed a question by B. Harvey further down, quote:
"I am full of questions today. I take Flax seed oil and use only olive oil in cooking. I was wondering if a cod liver oil supplement was necessary, usefull etc. Anyone have any ideas?"
Harvey, first of all you should not use olive oil for cooking, especially frying. Extra virgin or cold pressed olive oil contains far too many solids, great if used in salads, but when heated they burn and release cancerous and harmful substances. The so-called light olive oil has been refined and was probably hexane extracted, so they are not a good alternative either. Olive oil is only rich in Omega-9, which is nice but won't give you other essential oils so it has to be substituted with other oils.

The only fats safe for frying are Ghee (clarified butter), some of the topical fats such as coconut, palm, and cocoa butter and animal fats such as lard, goose fat etc. Although some vegetable oils and grapessed oil might be ok for frying, IMO they have been overly processed or refined.

Avoid fats high in Omega-6 or polyunsaturated fats because they tend to become oxidized or rancid when subjected to heat, oxygen and moisture as in cooking and processing. Polyunsaturated from vegetable oils derived mostly from soy, as well as from corn, safflower and canola.

Although Sesame Oil contains 42% oleic acid, 15% saturated fat, and 43% omega-6 linoleic acid it can be used for frying because it contains unique antioxidants that are not destroyed by heat. However, again the high percentage of omega-6 militates against exclusive use.

Now back to your question "if a cod liver oil supplement was necessary, useful?" The answer is yes, cod liver oil supplements are useful because Flax Seed oil does not provide all the fatty acids of Omega-3. Most of you think that Omega-3 is one product when in fact it is made up of a number of fatty acids. Flax Seed Oil is rich in alpha-linolenic acid, the most common omega-3 fatty acid. Alpha-linolenic acid is also found in cold water fish, in plants like spinach, kale, romaine lettuce and evening primrose oils. In addition to alpha-linolenic acid, there are two other important omega-3 fatty acids known as EPA and DHA. These fatty acids are found in fish and fish oils, mainly salmon, tuna, and sardines and of course are abundant in cod liver oil. The body has a limited ability to make EPA and DHA from alpha-linolenic acid. However, this process may be further hindered because of excess linoleic (omega-6) fatty acid in many diets. An enzyme called delta-6 desaturase is required to change linoleic (omega-6) into arachidonic acid and to change linolenic (omega-3) into EPA and DHA. Linoleic and alpha-linolenic acids compete against each other for this enzyme. Because there is so much more linoleic (omega-6) acid than linolenic acid (omega-3) in many diets, the enzyme stays busy converting linoleic acid to arachidonic acid. Therefore, the body may not be able to make enough EPA and DHA to maintain good health; and that may be a factor in many of the health problems.

To round of the subject I should also like to mention another important long-chain fatty acid, gamma-linolenic acid (GLA) which is found in evening primrose, borage and black currant oils. Some people can make this very-long-chain fatty acid from EFA's, but others, particularly those whose ancestors ate a lot of fish, lack enzymes to produce them. These people must obtain them from animal foods such as organ meats, egg yolks, butter, fish oils or again, our friend cod liver oil. The most important very-long-chain fatty acids are dihomo-gamma-linolenic acid (DGLA), arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). All of these except DHA are used in the production of prostaglandins, localized tissue hormones that direct many processes in the cells. In addition, AA and DHA play important roles in the function of the nervous system.