One of the two fats identified as being essential for humans to consume is alpha-linolenic acid (ALA or 18:3n-3) which, due to the position of its first double bond, is classified as an omega-3 essential fatty acid (n-3 EFA). Although mammals have the ability to introduce double bonds into most positions of the fatty acid chain in fat metabolism, therefore creating a variety of unsaturated metabolites, they lack the capacity to insert double bonds at the n-3 and n-6 position. Consequently, linoleic acid (LA) and alpha-linolenic acid (ALA), which already have the double bond at the n-3 or n-6 position, respectively, are considered essential and must be consumed in the diet. When the EFAs are consumed in this precursor state they follow a pathway of further elongation and desaturation via the action of delta-6- and delta-5-desaturase until they form the 'active' fatty acids: eicosapentaenoic acid (20:5 n-3) (EPA) and docosahexaenoic acid (DHA) (22:6 n-3).
Fish oils, also known as marine oils, are rapidly absorbed from the gastrointestinal tract and compete with arachidonic acid (AA) for incorporation into phospholipids, particularly of platelets, erythrocytes, neutrophils, monocytes and liver cells (Simopoulos 1999). When stimulated, the cell membranes release polyunsaturated fatty acids (PUFAs), which are then converted into 20-carbon eicosanoids, which have important and extensive physiological effects. The most active of these metabolites are prostaglandins, prostacyclins and thromboxanes, which affect blood chemistry, muscle contraction, immune function and inflammation. Dietary fats not used in this way are stored in adipose tissue and ultimately oxidised to produce energy. The FA cell membrane profile of different tissues will have varying ratios of EPA and DHA, but generally DHA is considered the major component of phospholipids in the retina, brain, male reproductive tissue, and myocardium (Groff & Gropper 2004).
Supplements based on fish liver oils, such as cod and halibut, contain EPA and DHA together with high levels of vitamins A and D. As such, they have additional actions and safety issues besides those found with traditional marine lipid supplements. This review will focus on the research surrounding those fish oils that are not liver extractions.
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