The Fish Foundation

Fish Oils

Micronutrients Omega-3 Dietary Sources Fats & Oils Macronutrients Balance

 

Effects of Processing

 

Scientists find the terms "fats" and "oils" to be confusing, and to avoid the confusion and needless repetition, use the term "lipid" instead. The differences between fats and oils lies only in whether or not they are liquid at normal room temperature. Thus changing the temperature of storage will change an oil into a fat or vice-versa.  Palm oil for example is a liquid in the warm countries like Malaysia where it is produced, yet when it arrives in the UK, the temperature is much lower, and the "oil"  sets solid and is in practice a "fat". Use of the term lipid gets rid of this problem, and  in most of what follows,  the word lipid is used rather than fats/oils.

If you are  not familiar with the chemistry and biochemistry of lipids, a lay persons guide can be found on the Fats/Oils page.

 

The lipid content of seafoods is primarily in the form of   triglycerides, or triacylglycerols, and is the area of most interest nutritionally, since seafoods are the only major source of certain long chain polyunsaturated fatty acids.

   Variation in Lipid Levels

    The level of lipid in fish flesh varies widely, not only between different species, but also within the same species depending on season, feeding grounds, water salinity and other factors. Demersel fish in general do not have a great deal of lipid in their flesh. Lipid stores in these fish are to be found in the liver, in the lining of the peritoneum, and/or immediately under the skin. Pelagic fish on the other hand, tend to store their lipid in head and muscle tissue. Some types, such as halibut, store some lipid in the liver, and some in muscle. In general, fish do not feed when they are spawning, in spite of the higher nutritional demands of spawning. Their nutrient supply is thus provided by body stores, and as a result, the level of lipid falls steadily as spawning progresses. Thus as salmon head upstream into freshwater rivers during the early months of the year, to their spawning grounds, flesh lipid content can be as much as 13%. As they journey upriver they do not feed, so that by the time of spawning, around November, the lipid level may be as low as 5%. After spawning, the lipid level continues to fall, and by the time the fish dies, it may be below 1%. Herring show a more seasonal variation in lipid level. From an overwinter low of 3-4% in April, the level rises within a few weeks to as much as 20%. Feeding begins to diminish as the autumn spawning season approaches, so the lipid level begins to drop to around 10-15%. During winter, lower sea temperatures and a scarcity of food causes the level to continue falling. When lipid is drawn from muscle reserves, it is replaced by water, so that the total mass of the fish remains much the same. The muscle water content obviously rises, and this makes the flesh weak and accounts for the poor eating quality of spawning fish.

Fish Liver Oils

    The high liver lipid content (which can exceed 50% of wet weight) of species in the family Gadidae, such as cod, saithe/coley or haddock, is exploited to produce cod liver oil, a rich source of the omega-3 long chain polyunsaturates as well as vitamins A and D (see Micronutrients page for more detail). Fish liver oils are also from time to time produced from ling, shark, huss, halibut and tuna. The livers are removed at the time of evisceration of the fish, and processed to separate the oil. This can be by simple steam cooking to obtain the highest quality medicinal oil, with various other techniques used to extract the residual oil which, being of poorer quality, is used for veterinary or industrial purposes. Though cod liver oil is still a popular product, the level of production now is around 20,000 tonnes /year, down considerably from the 70-80,000 tonnes in the first half of the century.

Omega-3 Fatty Acids.

Biochemical Structure

Polyunsaturated fatty acids are characterised by the presence of two or more double bonds (see Fats/Oils page for explanation). The  fate of polyunsaturates  when they are eaten depends on the exact chemical configuration, since mammals in general lack the means to alter the configuration of this part of the polyunsaturate molecule. Thus no matter what changes may be brought about in the biochemical structure of the molecule the part which determines whether it is an omega-3 polyunsaturate or an omega-6 polyunsaturate remains unaltered. Polyunsaturates can be elongated, desaturated, shortened, or converted to other bio-active molecules such as prostaglandins or leukotrienes, but the configuration of the methyl end of the molecule remains unchanged.

Long-chain Omega-3 Polyunsaturates

Fish and seafoods from cold waters characteristically and uniquely contain significant quantities of long chain omega-3 polyunsaturates  (see Balance page for further detail). Though there is some evidence that fish can elongate and desaturate the shorter chain omega-3 polyunsaturates, current opinion is that most of the long chain omega-3 polyunsaturates are formed in the microscopic algae, plankton and planktonic crustacea at the bottom of the marine food chain. They are then passed up the food chain into the higher fish, and of course ultimately to humans. There are three significant members of the omega-3 group, all with 20 or more carbon atoms, and all with five or more double bonds.

 

Omega-3 Contents

The pattern of individual polyunsaturates in fish can be a characteristic of the species, though in practice, the potential variations which can occur make it difficult to draw conclusions based on this alone. The geographical location of the feeding grounds, water temperature, water salinity, stage of breeding cycle, and the season of the year are all factors which can and do complicate this issue etc. The table below  gives typical values for the three major omega-3 polyunsaturated fatty acids and total lipid content in a number of major fish species.

Table 1. Omega-3 Polyunsaturate Content (g/100g edible portion) of Selected Fish and Shellfish

Type of Fish

(raw unless specified)

Lipid Content

20:5 w-3

22:5 w-3

22:6 w-3

Total w-3

Huss/Dogfish

9.7

0.8*

0.2

1.4

2.5

Skate

0.4

0.0

0.0

0.1

0.1

Oyster

1.3

0.2

tr

0.2

0.5

Squid

1.7

0.1

tr

0.3

0.5

Lobster

1.6

0.2

tr

0.1

0.3

Crab

5.5

0.5

0.1

0.5

1.2

Saithe/Coley

1.0

0.0

0.0

0.2

0.2

Mussel

1.8

0.3

tr

0.1

0.5

Shrimp(boiled)

2.4

0.4

tr

0.3

0.8

Herring

13.2

0.8

0.1

1.0

2.0

Mackerel

16.1

0.7

0.1

1.1

2.0

Sprat

9.9

0.9

0.1

1.3

2.4

Pilchard/Sardine

9.2

0.9

0.1

1.1

2.2

Tuna

4.6

0.3

0.1

1.1

1.6

Salmon

11.0

0.5

0.4

1.3

2.3

Trout(Rainbow)

5.2

0.2

0.1

0.8

1.2

Cod Liver Oil

100.0

9.0

1.0

9.0

20.0

* shown in source as 20:4 w-6, but assumed to be a printers error

tr=trace amounts only; n= data not available

source: Holland, B., Brown, J., & Buss, D.H., 1993. Fish and Fish Products; the third supplement to McCance & Widdowson’s The Composition of Foods (5th Edition), HMSO, London.

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