CHAPTER 23 

 Fish-Liver Oils 



Introduction 



The belief in the health-giving quality of fish-liver oils has been recognized 

 for several hundred years. In fact, it is impossible to determine the exact date of 

 the first use of these oils for this purpose. In the Middle Ages it is reported that 

 fish livers were suggested as a treatment for rickets. As early as 1657 it was 

 believed that "night blindness" was the result of a dietary deficiency which could 

 be relieved by taking certain fish-liver oils. The discovery that they contain 

 vitamins A and D has given a scientific basis for the early belief. This discovery 

 also led to the idea that many kinds of fish livers might also be a source of these 

 necessary dietary factors. Investigations of fish -liver oils have been continuously 

 carried out during the past quarter of a century, with the result that vitamin oils 

 and concentrates are now prepared from a large variety of fish livers. 



In recent years it was found that vitamin D can be prepared by exposing 

 certain substances— ergosterol and 7-dehydrocholesterol— to ultraviolet radiation. 

 This has resulted in the elimination of fish-liver oils for their vitamin D content; 

 but since they are the best source of naturally occurring vitamin A, in high con- 

 centrations, they are now much more sought after than previously. Not only have 

 liver oils of a great number of known species of fish been assayed for vitamin A, 

 but a large variety of sharks have been included in this search. Recently assays 

 have been made of the liver oils of seals, both hair- and fur-bearing species. 



Though vitamins A and D are both contained in most fish-liver oils, the 

 market prices are based entirely upon the vitamin A content as expressed in 

 U.S. P. units. In discussing the fish-liver oils here vitamin A ^vill be given the 

 major consideration for the above reason. To simplify the discussion the classifica- 

 tion devised by Butler (1948) wall be used. The fish livers are divided into three 

 classes: (1) high oil content— low vitamin A potency; (2) low oil content— high 

 vitamin A potency; (3) high oil content— high vitamin A potency. 



Examples of the first of these classes are the cod, grayfish, and haddock, which 

 have livers containing from 60 to 75 per cent oil by weight, and vitamin A potency 

 ranging from 500 to 20,000 U.S.P. units per g. The second class is composed of 

 tuna, halibut, sablefish, and others which have livers containing from 4 to 28 per 

 cent oil by weight and vitamin A potency ranging from 25,000 to 600,000 U.S.P. 

 units. All those livers which cannot be included in either of the first two groups are 

 placed in a third group which is again broken down into two classes : those having 

 an oil content of 40 to 75 per cent by weight and a vitamin A potency of 20,000 

 to 200,000 U.S.P. units, and those having from 25 to 75 per cent oil and to 

 300,000 U.S.P. units of vitamin A per g. 



The liver oil is contained in the protein of the livers and in some instances is 



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