Fish oil is beneficial to the health because of the long-chain trienoic, pentaenoic and hexaenoic polyunsaturated fatty acids (PUFA) that it contains. In particular, the omega 3 fatty acids eicosapentanoic acid (EPA, C20:5 (n-3)) and docosahexanoic acid (DHA, C22:6 (n-3)) are known to increase production of series 3 eicosanoids which are anti-inflammatory, lower blood pressure and inhibit platelet aggregation. For these metabolic pathways to function, the cyclooxygenase (COX) enzyme is required to oxidise the fatty acids to their active autocrine forms. Oxidation of these fatty acids in vivo is therefore desirable and facilitated by the double bond structures of the fatty acids. However, this reaction is tightly controlled by enzymes and cellular conditions and only proceeds through tight metabolic regulation. Uncontrolled auto-oxidation (lipid peroxidation) of these fatty acids is also possible ex vivo, resulting in the formation of hydroperoxides, which if ingested can lead to disease formation.
Initiation of lipid peroxidation occurs when free radicals abstract hydrogen from PUFA to form fatty acid (carbon centred) radicals. Initiation is slow in the presence of air because activated forms of oxygen, such as singlet oxygen or the superoxide anion, are required. The resulting carbon centred radicals can in turn react with molecular oxygen to form peroxyl (peroxy) radicals. Reaction of the this type are rapid and likely at the surface of the oil where oxygen is present in the air. Propagation occurs when the peroxyl radicals abstract hydrogen from another unsaturated fatty acid to become a hydroperoxide molecule (termination), while at the same time propagating the reaction by the creation of a new carbon centred radical. This carbon centred radical in turn reacts with oxygen to form a new peroxyl radical. Antioxidants (AH) are able to prevent free radical chain reaction of unsaturated fatty acids through hydrogen donation at either the initiation or propagation steps.
The abstraction of hydrogen in the propagation step occurs to the weakest bound hydrogens within the fatty acids of the oil. Fatty acids with a higher number of double bonds are thus more prone to lipid peroxidation because of their relative structural frailty. Estimates suggest that the relative lipid peroxidation rates of oleic (OA, C18:1 (n-9)), linoleic (LA, C18:2 (n-6)) and α-linolenic acid (ALA, C18:3 (n-3) are around 1:12:25. Therefore ALA with three double-bonds is almost twice as unstable as LA with two double bonds, which is an order of magnitude less stable than OA with one double bond. The long-chain fatty acids in fish oil such as EPA and DHA with 5 and 6 double bonds, respectively, are much less stable that ALA and so undergo lipid peroxidation more readily. Experiments suggest that this rate may be around 4 and 12 times faster than LA and ALA, respectively.
Ultra violet light causes the creation of singlet oxygen from molecular oxygen and so accelerates the generation of lipid peroxides. Likewise, heat can accelerate lipid peroxidation because of the increased kinetic energy possessed by the molecules. Care should therefore be taken to protect delicate unsaturated oils from heat and light. Storing the oil in the refrigerator at 4ºC will decreases the initiation and propagation steps and limit the formation of hydroperoxides due to the lower levels of kinetic energy within the molecular structure. The use of opaque capsules and containers for the storage of unsaturated oils is also advised were possible to decrease exposure to light. Some fish oils are encapsulated with plant antioxidants in order to prevent initiation and propagation steps, while some oils come with added vitamin E for the same reason. Limiting exposure to oxygen in the air by use of capsules or thin necked bottles may also be advantageous.
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