Omega in your Body
Oxidation of lipids
'Lipid oxidation' is a term explaining different types of reactions, having both positive and negative implications on the human body. In the body lipid oxidation is important for several physiological reactions, for instance when utilizing fatty acids for the production of energy through β-oxidation. Oxidation is also involved in the production of signalling
substances called eicosanoids. These are formed from the omega-3 fatty acid eicosapentaenoic acid (EPA) and the omega-6 fatty acid arachidonic acid (AA) by the action of specific enzyme systems. Lipid oxidation could also refer to uncontrolled oxidative degradation of lipids initiated by free radicals stealing electrons, which is the first step in the formation of several cytotoxic and mutagenic substances in the body. Uncontrolled oxidative damage also affect food products, influencing the overall quality.
Fatty acids and oxidation – affected by the number of double bonds
Fatty acids are long aliphatic chains consisting of carbon and hydrogens. The carbon chain vary in length, degree of unsaturation, and structure. In foods, fatty acids are mainly found in lipid complexes called triglycerides (read more in "Digestion of lipids"). Some fatty acids are saturated, while other have different degrees of unsaturation. However, when talking about lipid oxidation it is only the polyunsaturated fatty acids which are of interest. Polyunsaturated fatty acids contain two or more double bonds, and it is these double bonds which are prone to oxidation. Consequently, the risk of oxidation increases with the number of double bonds present in the fatty acid. For instance, EPA (C20:5) having five double bonds, is more prone to oxidation than linolenic acid (C18:3), having only three double bonds.
Oxidation in food products - sensoric and nutritional changes
Due to oxidation, edible oils containing unsaturated fatty acids are of major concern in the food industry. Degradation of unsaturated fatty acids by oxidation is directly related to economic, nutritional, flavor, safety and storage problems. There are two major oxidation reactions which can occur in food stuff containing lipids; auto-oxidation and photo-oxidation, of which auto-oxidation is the most common. Auto-oxidation occurs in the presence of oxygen and is described as the auto-catalytic generation of free radicals. It is initiated when a hydrogen atom is abstracted in the presence of initiators such as light, heat, metals or oxygen, forming a lipid radical, which reacts with oxygen making a lipid peroxide radical. These peroxide radicals reacts with a second lipid, yielding a lipid radical and a hydroxyperoxide. The reaction may be staggered by antioxidants producing a combination of radical species to give non-radical and non-propagating species. Photo-oxidation occurs when norma triplet oxygen are converted to singlet oxygen by the exposure of UV radiation. The singlet oxygen interacts with polyunsaturated fatty acids to form hydroxyperoxide which initiate the auto-oxidation reaction .
The lipid oxidation process leads to the formation of several components causing off-flavours and reduced nutritional quality. Among these compounds are the free radicals known to be ‘hydrogen thief’s’, steeling hydrogen from other molecules. This will initiate the auto-catalytic oxidation reaction described above, leading to the formation of primary oxidation products such as hydroxyperoxides . The hydroxyperoxides will be decomposed into secondary oxidation products with bad smell and taste, also influencing the appearance of food . The secondary oxidation products such as reactive aldehydes, alcohols and ketones have also been suggested to have negative health implications due to their cytotoxic, mutagenic and neurotoxic action [2-5]. Lipid oxidation may also severely change the nutritional quality of foods by impairing vitamins and polyunsaturated fatty acids.
Dietary PUFAs are susceptible to oxidation both during processing and storage. The oxidative reactions are dependent of the environment. First of all, the fatty acid composition will affect the rate of oxidation, as an increase in the available double bonds in PUFAs also means that there are more sites where the oxidation reaction can occur. In general there are also several other pro-oxidants in foods, such as oxygen and metal-ions. High temperature are also a factor that may initiate lipid oxidation. Hence, special precautions are taken for products containing PUFAs to maintain the nutritional quality and prolong shelf life. One approach is to avoide environmental pro-oxidants like light, high temperature and oxygen. Another approach is to remove oxidative products and pro-oxidants through refining of oil products (read more in ‘Fish oil and health’). It is also possible to delay oxidation by adding antioxidants that are being oxidized themselves.
Oxidation in the body (in vivo)
When eating foods the oxidation continues into the gastrointestinal tract. Previous studies have shown that there are pro-oxidant present in the stomach, like oxygen, metal ions (e.g Fe2+ and Cu2+), reactive nitrogen, sulphite and nitrite species. This, combined with a low pH, free fatty acids from the action of the gastric lipase, and the presence of oxygen makes the stomach a potential good oxidative environment . Thus, it is likely that oxidation of food lipids continues also inside the body. Certain bile salts have been shown to be good pro-oxidants. This, combined with the emulsification of lipids in the small intestine, increasing the lipid droplet surface, suggests that there is a potential of initiating oxidation also within the small intestine .
As mentioned above, oxidation is a natural process when the body is producing energy from fatty acids, or signalling molecules such as the eicosanoids. Since the travelling of free radicals in the body could lead to potential harm, the human cells have developed multiple protection mechanisms against the damaging effects of oxidation. For instance, the presence of antioxidants that inhibit the accumulation of free radicals, and specific enzyme systems which breaks down the lipid peroxides into oxygen and water, both being harmless molecules. However, the protective systems of the human body is limited. An imbalance between reactive oxygen species and the organism’s capacity to neutralize and eliminate the free radicals may lead to accumulation of oxidative damage, commonly called oxidative stress, which is well known to be potential harmful. Oxidative stress amplify the oxidative reaction by repressing proteins included in the oxidative defence, and by depleting cellular storage of antioxidants like vitamin E and carotenoids . This is the reason why it is so important with daily intake of foods containing antioxidants, especially for atlets during the restitution phase. Polyphenols from olive, such as hydroxytyrosol, are very active and well-documented antioxidants that scavenge reactive oxygen and nitrogen species in the body .
Keep eating fish and fish oils
Oxidized lipids have previously been suggested to be involved in the pathology and development of chronic diseases [1, 2, 10-14], and some scepticism have been expressed concerning increased intake of polyunsaturated fatty acids. Based on this concern, the Norwegian Committee of Food safety (VKM) evaluated the positive and negative health effects of omega-3 fatty acids in food supplements and fortified food  by employing the European Food Safety Authoriy (EFSA)-guideline for risk-benefit assessment of foods . The Norwegian Health Authorities concluded that it is safe to consume the essential marine omega-3 fatty acids, EPA and DHA, by intake of fatty fish or fish oils.
Written by Dr. Kristi Ekrann Aarak and Dr. Linda Saga, BioActive Foods
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