From medieval times to the early 20th Century in Europe fish was considered second to meat as a food and considered to be food for the poor. The situation was different in countries bordering the Mediterranean where fish was more highly regarded. Mosaics in the Bardo museum show large catches of fish being brought ashore, much of the catch was dried and salted to preserve it and this dried fish was regarded as healthy and is still popular. The medicinal properties of fish oil were recognised both in Egypt and Mesopotamia where it was used in ointments and where fish bile was used to treat diseases of the eye.

Fish liver oil was used by fisherman in Europe, Africa and America to treat a variety of ailments but these benefits were not widely recognised. It was not until the early part of the 20th C, following the discovery of vitamins, that benefits of fish liver oils were recognised. Specifically their ability to prevent the deficiencies of Vitamin A and E. In Britain during the 1939-45 Second World War food was rationed and the availability of
these vitamins from butter was limited, to overcome these shortages the UK government provided free cod liver oil to mothers and children. During this period meat was rationed and fish was not as plentiful however supplies of fish were available from fishing. As a consequence there was an increase in fish consumption. The health of the population was found to be better during the war than before or after it; this was attributed by some nutritionists to the drop in saturated fat consumption and the increase in the unsaturated
fats from fish.

The first significant observation that fish consumption could have medicinal benefits, other than that from the vitamins it gave, was provided by the British biochemist Hugh Sinclair in the 1970s. He had worked on identifying the essential nature of some unsaturated fatty acids (EFAs) for human health and after a period working as a medical officer in Labrador he observed that Eskimos had a low incidence of heart disease and a low mortality rate despite consuming a high fat diet. He and others then speculated that the higher polyunsaturated fatty acids (PUPAs) found in the oily fish that was consumed by the Eskimos could play a role in keeping them healthy. In a paper published in 1990 Sinclair suggested that the deficiencies in PUPAs could be linked to heart disease, atherosclerosis, hypertension, cancers, digestive disorders, diabetes and cancer.

Over the subsequent years many of these speculations have been examined and been largely substantiated. Many other pathological conditions have since been found to be alleviated by PUPAs present in fish oils. The mechanisms by which the benefits of fish oil consumption are produced have not been fully elucidated but the majority of the benefits are known to be due to three PUPAs found in the oil. These have been given the shorthand acronyms EPA (eicosapentaenoic acid), DHA(doocosahexaenoic acid) and AA (arachidonic acid) of the omega-3 series. Each of these PUPAs have been found to have different properties although they may combine to produce better effects. Oils from different fish species will contain varying ratios of these free EFAs. Cold water fish in general have higher levels of the omega-6 series than the omega-3 series and in warm water fish the levels of the omega-3 series are higher.

PUPAs act as major structural components of the cell membranes. They are responsible for the flexibility of these membranes and for controlling the pores that allow the passage of nutrients and hormones into and out of the cell. Dietary deficiencies of PUFAs can impair cell function and this can result in wide range of disorders and illnesses.

A major advance in the understanding of one of the physiological roles of PUFAs was made by British and Swedish scientists who demonstrated that in the body PUPAs were transformed into eicosanoids; hormone like substances that were involved in a range of physiological effects. The most significant role for the eicosaoids is that they are responsible for controlling the inflammatory response. PUPAs of the omega-6 series are present in seed oils, such as sunflower seeds, and can be transformed in the body to pro-inflammatory eicosanoids. Those of the omega-3 series have antiinflammatory
properties. An imbalance in the ratios of these two types of eicosanoids are responsible for a wide range of disorders.