Omega3 PUFA in Fish and Shellfish

The PUFA of many fish lipids are dominated by two members of the omega-3 (n-3) family, C20:5 n-3 (EPA) and C22:6 n-3 (DHA). They are so named because the first of several double bonds occurs three carbon atoms away from the terminal end of the carbon chain.

All fish and shellfish contain some omega-3, but the amount can vary, as their relative concentrations are species specific (Table 2). Generally, the fattier fishes contain more omega-3 fatty acids than the leaner fishes. The amount of omega-3 fatty acids in farmraised products can also vary greatly, depending on the diet of the fishes or shellfish. Many companies now recognize this fact and provide a source of omega-3 fatty acids in their fish diets. Omega-3 fatty acids can be destroyed by heat, air, and light, so the less processing, heat, air exposure, and storage time the better for preserving omega-3 in fish. Freezing and normal cooking cause minimal omega-3 losses, whereas deep frying and conditions leading to oxidation (rancidity) can destroy some omega-3 fatty acids.

The beneficial effects of eating fish for human health have been well documented. Research has shown that EPA and DHA are beneficial in protecting against cardiovascular and other diseases (Table 3). Studies examining the effects of fish consumption on serum lipids indicate a reduction in triacylglycerol and VLDL-cholesterol levels, a factor that may be protective for some individuals. Research also indicates that EPA in particular reduces platelet aggregation, which may help vessels injured by plaque formation. Fish oils also appear to help stabilize the heart rhythm, a factor that may be important in people recovering from heart attacks.

Table 2 Selected fish and shellfish grouped by their omega-3 fatty-acid content

Low-level group

Medium-level group

High-level group

(<0.5g per 100g)

(0.5-1 g per 100 g)

(>1g per 100g)

Finfish

Carp

Bass

Anchovy

Catfish

Bluefish

Herring

Cod, Haddock,

Halibut

Mackerel

Pollock

Grouper

Pike

Sablefish

Most flatfishes

Red Snapper

Salmon (most

species)

Perch

Swordfish

Tuna (bluefin)

Snapper

Trout

Whitefish

Tilapia

Whiting

Shellfish

Most crustaceans

Clams

Most molluscs

Oysters

Reproduced with permission from Arino A, Beltran JA, and Roncales P (2003) Dietary importance of fish and shellfish. In: Caballero B, Trugo L, and Finglas P (eds.) Encyclopedia of Food Sciences and Nutrition, 2nd edn. Oxford: Elsevier Science Ltd. pp. 2471-2478.

Reproduced with permission from Arino A, Beltran JA, and Roncales P (2003) Dietary importance of fish and shellfish. In: Caballero B, Trugo L, and Finglas P (eds.) Encyclopedia of Food Sciences and Nutrition, 2nd edn. Oxford: Elsevier Science Ltd. pp. 2471-2478.

Table 3 Summary of the beneficial effects of eating fish for cardiovascular and other diseases

Cardiovascular disease

Protects against heart disease

Prolongs the lives of people after a heart attack

Protects against sudden cardiac arrest caused by arrhythmia

Protects against stroke (thrombosis)

Lowers blood lipids such as triacylglycerols and VLDL-cholesterol Lowers blood pressure

Other diseases

Protects against age-related macular degeneration Alleviates autoimmune diseases such as rheumatoid arthritis Protects against certain types of cancer Mitigates inflammation reactions and asthma

VLDL, very low-density lipoprotein.

Reproduced with permission from Arino A, Beltran JA, and Roncales P (2003) Dietary importance of fish and shellfish. In: Caballero B, Trugo L, and Finglas P (eds.) Encyclopedia of Food Sciences and Nutrition, 2nd edn. Oxford: Elsevier Science Ltd. pp. 2471-2478.

The major PUFA in the adult mammalian brain is DHA. It is among the materials required for development of the fetal brain and central nervous system and for retinal growth in late pregnancy. Brain growth uses 70% of the fetal energy, and 80-90% of cognitive function is determined before birth. However, the placenta depletes the mother of DHA, a situation that is exacerbated by multiple pregnancies. Dietary enhancement or fortification with marine products before and during pregnancy, rather than after the child is born, would be of great benefit to the child and mother. Furthermore, the food sources that are rich in DHA are also rich in zinc, iodine, and vitamin A, so it may be possible to provide several dietary supplements at one time. Deficiencies of the latter micronutrients are established causes of mental retardation and blindness.

The typical Western diet has a ratio of omega-6 to omega-3 essential fatty acids of between 15:1 and 20:1. Several sources of information suggest that a very high omega-6 to omega-3 ratio may promote many diseases, including cardiovascular disease, cancer, and inflammatory and autoimmune diseases. Fish provides an adequate intake of these omega-3 fats, thus improving the omega-6 to omega-3 fatty-acid ratio. Most experts do not advise the routine use of fish-oil supplements: they favor eating fish and shellfish regularly in the context of a healthy diet and a regular pattern of physical activity. Whereas some research shows benefits of fish-oil supplements, research has also shown that people with weakened immune systems should avoid large doses of fish oil. The final conclusion as to whether it is possible to substitute fish consumption with fish oils or omega-3 fatty-acid supplements, and gain the same reduction in mortality from CHD, awaits more studies. However, the protective role of fish consumption is unquestioned.

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