286 OLIVE OILS AND HEALTH Dietary sources of omega-3 polyunsaturated fats (ω-3) include fish and seafood, algae, krill (Euphausia superba¸a small crustacean living in the open ocean), and certain plant foods. The main dietary source of EPA and DHA is fish, especially oily fish (also known as “fatty fish”, containing 8-10% fat), such as tuna, salmon, sardines, mackerel, horse mackerel, and anchovies, which contain EPA and DHA in their flesh. In contrast, lean or “white” fish (with up to 2% fat), such as cod, sole, hake, sea bream, or monkfish, store EPA and DHA almost exclusively in their livers, which are the usual source of “fish oils”. In each species, the EPA and DHA content varies depending on seasonality, whether the fish is wild or farmed, and the cooking method. Because oily fish have a higher fat content and seas are increasingly polluted with fat-soluble toxins (e.g. dioxins, mercury, lead, furans, arsenic, etc.), these species tend to accumulate more contaminants. For this reason, it is recommended to consume small oily fish species, which contain less fat and therefore fewer toxins, and to alternate among different species, as each tends to accumulate different contaminants preferentially. ALA (alpha-linolenic acid) is found in plant-based foods, especially walnuts, rapeseed, soya, and flax seeds, and in oils and oil-based products such as margarines. In the United States, Northern Europe, Japan, and Brazil, soya is the main source of ALA, whereas in Mediterranean countries it mainly comes from walnuts and, to a lesser extent, olive oil. In olive oil, alpha-linolenic acid (C18:3—ω-3 polyunsaturated—) typically makes up 1.5% or less of the total fatty acids. As previously described, the predominant fatty acid in olive oil is oleic acid (C18:1 —ω-9 monounsaturated—), representing 55% to 83% of total fatty acids, followed by linoleic acid (C18:2 —ω-6 polyunsaturated—): 3.5% to 21%, palmitic acid (C16:0 -saturated-): 7.5% to 20%, alpha-linolenic acid (C18:3 —ω-3 polyunsaturated—): up to 1.5%, and trace amounts of other fatty acids. The exact composition varies depending on the species of olive used, the geographical origin (such as soil composition and climate), and the method of extraction or processing. EPA and DHA, two omega-3 polyunsaturated fatty acids, play an important role in the development of the central and visual nervous systems, particularly during intrauterine life and the early years of childhood. They are also involved in the regulation of blood pressure, kidney function, blood clotting, and inflammatory and immune responses. Within cells, they can be incorporated into membrane lipids, where they fulfil a structural function, and can also give rise to various signalling molecules. In this regard, omega-3 fatty acids (ω-3), like omega-6 (ω-6), are precursors of different eicosanoids. In general, eicosanoids derived from omega-6, particularly arachidonic acid, promote inflammatory responses, whereas those derived from omega-3 are weak promoters of inflammation and are even considered anti-inflammatory. This is because they reduce the production of pro-inflammatory eicosanoids by inhibiting arachidonic acid metabolism and by regulating the expression of inflammation-related genes. Omega-3 fats are considered healthy, together with monounsaturated fats, and are therefore recommended as part of a regular diet. It is advised to consume fish or seafood at least three times per week, with two of those servings coming from oily fish. The role of ω-3 polyunsaturated lipids in health has been extensively studied since the 1970s, when it was observed that Inuit populations living in Arctic regions of Greenland, Canada, Alaska, and Siberia had low cardiovascular mortality despite consuming a diet rich in protein and fat, especially ω-3 polyunsaturated fats (approximately 5-15 g/day).
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