Fish oils comprise a complex mixture of fatty acid moieties, mostly straight chain with an even number of carbon atoms. The fatty acids, usually present as their glycerides, are either saturated or mono- or polyunsaturated. Unlike vegetable oils and fats from terrestrial animals, which contain mainly fatty acids having a maximum of eighteen carbons and two or three double bonds, fish and marine mammal oils contain substantial amounts of fatty acids having twenty or twenty-two carbons and four, five or six double bonds, Stansby, "Fish Oils", Avi Publishing Company, Inc. (1967). Among the fatty acid moieties unique to fish oils are the following n-3 compounds: 18:4, 20:4, 20:5, 22:4, 22:5, and 22:6. The n-3 designation means that the first double bond begins at the third carbon counting from the methyl end of the chain. In the number: number designation, the first number indicates chain length and the second number indicates how many double bonds are present. For example, 18:4 indicates a straight chain fatty acid having eighteen carbon atoms and four methylene-interrupted double bonds.
In addition to fatty acid glycerides, fish oils contain numerous other substances such as cholesterol, cholesterol esters, wax esters, hydrocarbons like squalene, pigments like chlorophyll and astaxanthin, amines, and phospholipids, as well as products of autoxidation and the heating of proteinaceous materials. Many of these substances contribute to the unpleasant oder and flavor of fish oils. For instance, cod liver oil, as sold in drugstores, has a notoriously strong smell and taste. The offending substances cannot be removed readily by traditional processing techniques without damaging or destroying the polyunsaturated components of the oil.
Up until World War II, the nutritionally important components of cod liver oil were Vitamin A and Vitamin D, but now these substances are produced synthetically. More recently, it has been observed that Greenland Eskimos, whose food intake comprises mainly fish and marine animals, exhibit unusually low incidences of cardiovascular diseases, and a number of chronic degenerative diseases such as arthritis, diabetes and ulcerative colitis. Fish and marine oils are now recognized to be of value because they contain substantial quantities of polyunsaturated fatty acids, important dietary factors beneficial in reducing the development of atherosclerotic lesions, Dyerberg et al, "Nutritional Evaluation of Long-chain Fatty Acids in Fish Oil", pages 245-261, Academic Press, London (1982). Eicosapentaenoic acid (EPA or 20:5 n-3) and docosahexaenoic acid (DHA or 22:6 n-3) in particular, and other polyunsaturated fatty acids having their double bonds in the cis-configuration appear most beneficial.
Commercially available fish oils, such as cod liver oil, are not suitable for prolonged use as a nutritional supplement or as a medicament for the prevention or treatment of disease. The high concentrations of Vitamins A and D and also the toxic products of autoxidation, post-death metabolism and processing render them highly unpalatable and, more importantly, unwholesome. Extended use of fish oils in the diet would require removal of toxic as well as unpalatable components.