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  1. Fat or oil is an important part of a diet. It is primarily made up of building blocks called fatty acids (FAs). Depending on the presence of double bonds between carbon atoms in its molecule FAs can be either saturated or unsaturated. In turn unsaturated FAs may be categorised into monounsaturated (MUFAs) having one double bond available and polyunsaturated fatty acids (PUFAs) with more than one double bond.[1] Unsaturated fatty acids are often referred to as omega fatty acids due to their chemical classification into omega-3, omega-6 and omega-9 FAs. The number stands for the position of first double bond in FA molecule which in turn determines its biochemical properties. While most of the FAs are present in the human body or may be produced in it, there are two fatty acids that can not be derived from other FAs and must be consumed from dietary sources to maintain good health condition. These FAs are called Essential Fatty acids (EFA) and include alpha-linolenic (omega-3) and linoleic (omega-6) FAs.[1][2] Docosahexaenoic acid may also become essential in some conditions but normally it can be synthesized from alpha-linoleic acid.[1]  Omega FAs have many crucial roles within the body. They reduce inflammation and possess protective properties against cardiovascular disease, are essential for pregnant woman for foetal growth and brain development.[3] They are also important for these processes in infants, who obtain them from breast milk.[4] Omega-3 FAs are also involved in the formation of cell walls and help to improve circulation and oxygen uptake.[5][6]   Other benefits include the improvement of mental health and brain function.[5][7] Scientists continue to argue on the role and potency of omega FAs to fight cancer but there is no consensus yet.[6][8][9] Omega-3 FAs have been part of the human diet throughout evolution. In early times, it is thought that the ratio of omega-3 to omega-6 FAs in our diets was approximately 1:1.[10] In Western diets the ratio of omega-3 to omega-6 FAs has dramatically shifted and is now thought to be from 1:15 to 1:20, thus there is a large disparity between modern diet and our inbuilt genetic patterns.[11][12] The reason for this change over time is thought to come from the industrial revolution and development of modern agricultural and aquacultural methods including changes in animal and fish feeds. The good example is the lower omega-3 to omega-6 ratio in cultured fish species than in wild ones. Generally omega-3 content decreased in multiples in regularly consumed food products, including eggs, fish and animal meat.[2][13] It is thought that this large shift in the ratio of omega-3 to omega-6 consumed in the Western diet is a contributory factor in the increasing incidence of atherosclerosis, coronary heart disease, hypertension, obesity, collagen vascular diseases and cancer. Scientists explain this by different biochemical properties of omega-3 and -6 FAs. While the first ones contribute to anti-inflammatory and suppressive effects for many diseases, the latter have proinflammatory and prothrombotic properties. And thus omega-3 to omega-6 ratio in the diet is crucial.[2][10][11]  According to EFSA “250 mg daily intake of long chain omega-3 FAs for adults may reduce the risk of heart disease” and this amount of eicosapentaenoic acid plus docosahexaenoic acid is stated as an adequate intake level.[14] However there is no any recommendations on the omega-3/6 ratio which is surprising if to take into account the recent scientific evidence noted above.[3]   The primary sources of omega-3 FAs are vegetable oils (ALA) and marine fish oils (DHA and EPA), although minor quantities of omega-3 FAs are also found in meat, egg yolk, nuts, seeds, green leafy vegetables and some fruits.[2][15] Flaxseed oil contains 53-62% omega-3 FAs, while canola oil, walnut oil, wheat germ oil and soybean contain 7-11%.[15][16] The richest fish sources of omega-3 FAs are salmon, mackerel, herring and halibut. Raw salmon and mackerel contain 1.2% and 2.5% of omega-3 FAs, respectively.[15] Fresh fish is thought to be higher in omega-3 FAs than processed varieties as processing (especially with heating) may denature omega-3 FAs and convert them to harmful substances.[2] The changes in agriculture and food processing techniques over the last fifty years have resulted in an imbalance in the ratio of omega-3 to omega-6 FA intake. This may have a negative impact on our health, but could still be redressed by increasing consumption of foods high in omega-3 FAs. Unfortunately farmed fish and animals are not that beneficial as wild ones due to a lower omega-3 FAs content. It is also worth noting that processing decreases the contents of PUFAs in food and should preferably be reduced.   [1] Tvrzicka, E., Kremmyda, L., Stankova, B., & Zak, A. (2011). Fatty acids as biocompounds: their role in human metabolism, health and disease – a review. part 1: classification, dietary sources and biological functions. Biomedical Papers, 155(2), 117-130. http://dx.doi.org/10.5507/bp.2011.038  [2] Das, U. (2006). Essential Fatty Acids - A Review. Current Pharmaceutical Biotechnology, 7(6), 467-482. http://dx.doi.org/10.2174/138920106779116856 [3] Scientific Opinion on Dietary Reference Values for fats, including saturated fatty acids, polyunsaturated fatty acids, monounsaturated fatty acids,transfatty acids, and cholesterol. (2010). EFSA Journal, 8(3), 1461. http://dx.doi.org/10.2903/j.efsa.2010.1461 [4] Muhlhausler, B., Gibson, R., & Makrides, M. (2010). Effect of long-chain polyunsaturated fatty acid supplementation during pregnancy or lactation on infant and child body composition: a systematic review. American Journal Of Clinical Nutrition, 92(4), 857-863. http://dx.doi.org/10.3945/ajcn.2010.29495 [5] Wysoczański, T., Sokoła-Wysoczańska, E., Pękala, J., Lochyński, S., Czyż, K., & Bodkowski, R. et al. (2016). Omega-3 Fatty Acids and their Role in Central Nervous System - A Review. Current Medicinal Chemistry, 23(8), 816-831. http://dx.doi.org/10.2174/0929867323666160122114439 [6] Kremmyda, L., Tvrzicka, E., Stankova, B., & Zak, A. (2011). Fatty acids as biocompounds: their role in human metabolism, health and disease – a review. part 2: fatty acid physiological roles and applications in human health and disease. Biomedical Papers, 155(3), 195-218. http://dx.doi.org/10.5507/bp.2011.052 [7] Lin, P., Chiu, C., Huang, S., & Su, K. (2012). A Meta-Analytic Review of Polyunsaturated Fatty Acid Compositions in Dementia. The Journal Of Clinical Psychiatry, 73(09), 1245-1254. http://dx.doi.org/10.4088/jcp.11r07546 [8] Jing, K., Wu, T., & Lim, K. (2013). Omega-3 Polyunsaturated Fatty Acids and Cancer. Anti-Cancer Agents In Medicinal Chemistry, 13(8), 1162-1177. http://dx.doi.org/10.2174/18715206113139990319 [9] Fabian, C., Kimler, B., & Hursting, S. (2015). Omega-3 fatty acids for breast cancer prevention and survivorship. Breast Cancer Research, 17(1). http://dx.doi.org/10.1186/s13058-015-0571-6 [10] Simopoulos, A. (2008). The Importance of the Omega-6/Omega-3 Fatty Acid Ratio in Cardiovascular Disease and Other Chronic Diseases. Experimental Biology And Medicine, 233(6), 674-688. http://dx.doi.org/10.3181/0711-mr-311 [11] Simopoulos, A. (2016). An Increase in the Omega-6/Omega-3 Fatty Acid Ratio Increases the Risk for Obesity. Nutrients, 8(3), 128. http://dx.doi.org/10.3390/nu8030128 [12] Simopoulos, A. (2006). Evolutionary aspects of diet, the omega-6/omega-3 ratio and genetic variation: nutritional implications for chronic diseases. Biomedicine & Pharmacotherapy, 60(9), 502-507. http://dx.doi.org/10.1016/j.biopha.2006.07.080 [13] van Vliet, T., & Katan, M. (1990). Lower ratio of n-3 to n-6 fatty acids in cultured than in wild fish. Trends In Food Science & Technology, 1, 51. http://dx.doi.org/10.1016/0924-2244(90)90038-z [14] EFSA. (2010). EFSA sets European dietary reference values for nutrient intakes | European Food Safety Authority. Efsa.europa.eu. Retrieved 10 October 2017, from https://www.efsa.europa.eu/en/press/news/nda100326 [15] Kris-Etherton, P., Taylor, D., Yu-Poth, S., Huth, P., Moriarty, K., & Fishell, V. et al. (2000). Polyunsaturated fatty acids in the food chain in the United States. Am J Clin Nutr, 71(1), 179S-88S. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/10617969 [16] Hunter, J. (1990). n-3 fatty acids from vegetable oils. Am J Clin Nutr, 51(5), 809-14. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/1970702