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Fatty acids are long hydrocarbon chains with a methyl group at one end (the omega or n- end) and an acid group at the other. Unsaturated fatty acids are hydrocarbon chains containing at least one carbon-carbon double bond; monounsaturated fatty acids (MUFAs) contain one double bond, polyunsaturated fatty acids (PUFAs) contain many double bonds. Fat slows down the digestion of foods, thus contributing to meal satiety. Dietary fat aids in the absorption of the fat-soluble vitamins. Specific fatty acids have important structural, biochemical, and regulatory functions that are required for optimal tissue function, growth, and repair.
Fat is the most energy dense of all the macronutrients with 1g providing 9kcal. However, the constituent parts of fat, fatty acids, are required by the body for many other functions than as simply an energy source and there is an increasing awareness of the potential health benefits of specific types of fatty acids. Fat is found in most food groups and foods containing fat generally provide a range of different fatty acids, both saturated and unsaturated fatty acids. In humans, the essential fatty acids are the omega 3 polyunsaturated fatty acid α-linolenic acid(ALA) and the omega 6 polyunsaturated fatty acid linoleic acid (LA). Docosahexaenoic acid (DHA) is an omega-3 (n-3) long-chain (LC) polyunsaturated fatty acid (PUFA) that is a primary structural component of the human brain, testes and retina. It is derived from LA and arachidonic acid (AA)
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Linoleic acid and α-linolenic acid are substrates for the synthesis of physiological regulators called eicosanoids. Eicosanoids include prostaglandins, prostacyclins, thromboxanes, and leukotrienes. For each, there are two or three separate series. These are potent mediators of many biochemical processes and play key roles in the regulation of blood clots, blood pressure, blood lipid levels, immune function, inflammation, pain and fever, and reproduction. The omega-6 eicosanoids (derived from LA) are generally pro-inflammatory. Consuming large amount of linoleic acid increases the quantity of arachidonic acid(AA) in cell membranes. Upon activation, in a variety of cells, arachidonic acid is converted to eicosanoids of the 2 series and leukotrienes of the 4 series. However, dietary α- linolenic acid is converted to Eicosapentaenoic acid (EPA) in the membrane, and when cells are activated, EPA is converted to eicosanoids of the 3 series and leukotrienes of the 5 series.
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Sources of dietary unsaturated fatty acids : |
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A good dietary source useful for increasing n3 fatty acid consumption is a greater consumption of vegetable derived n-3 fatty acids. There are only three major commercial sources that have significant amounts of ALA: Linum usitatissimum, linseed or flaxseed oil (53% ALA); Brassica spp. Canola or rapeseed oil (9% ALA) and Glycine max, soybean oil (7% ALA) [1]. Perilla oil, Perilla frutescens, also contains a high percentage in ALA (approx. 60%) although its consumption is restricted to Asia, In plants, ALA is found in leaves, mainly in glycolipids, and as tri acyl glycerol (TAG) in certain seed oils (rapeseed, flaxseed, perilla seed, chia seed), beans (soybeans, navy beans) and nuts (walnuts).
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Etiology of primary and secondary essential fatty acid deficiency in human beings |
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Chronic malnutrition, especially in children |
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Long-term fat-free total parental nutrition (TPN) |
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Various fat malabsorption conditions |
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Sjogren-Larsson syndrome |
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Acrodermatitis enteropathica |
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End-stage liver disease |
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Clinical manifestations of essential fatty acid deficiency in human beings : |
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Dermatitis; dry, scaly skin; impetigo; eczema; generalized erythema |
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Coarse, sparse hair |
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Increased frequency of stools |
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Decreased growth rate |
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Cellular hyperproliferation in skin, alimentary tract, and urinary tract |
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Possible immune impairment |
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Slow wound repair |
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Slower early learning behaviour |
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Mental retardation in human genetic defects involving desaturases and elongases |
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Neurological Development and Function : |
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DHA and arachidonic acid selectively accumulate during fetal and infant brain development and are found concentrated in the brain and retina. Fetal accretion of these lipids occurs during the last trimester of gestation, and a deficiency can result in visual impairment and brain dysfunction. Although docosahexaenoic acid,( DHA) and arachidonic acid can be synthesized from dietary precursors, the efficiency of this conversion in young infants (particularly preterm infants) may not be sufficient to meet their high need. [2,3,4] The phospholipids in brain have a very high DHA content and it is clear that DHA is critical for adequate brain development, however it is still not fully understood which unique feature makes DHA so essential for brain functioning. Because DHA levels are reduced in some neurodegenerative diseases such as Alzheimer?s disease (AD) [5,6], there has been an increased emphasis on studying the impact of DHA on mitigating the course of this disease.
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A deficiency of either n-6 or n-3 essential fatty acids is associated with a reduced immune function. Amongst the fatty acids, the n-3 fatty acids possess the most potent immunomodulatory activities, and amongst the n-3 fatty acids, those from fish oil (EPA and DHA) are more potent than a-linolenic acid. In particular, it has been demonstrated that feeding DHA and eicosapentaenoic acid (EPA) decreases the rate of infections and improves outcome (i.e., reduced complications, length of stay, hospital cost) in immunosuppressed patients.
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DHA is the most abundant PUFA in the central nervous system, being particularly concentrated in synaptic plasma membranes [7,8] and in photoreceptor cells [9]. It is critical that the correct acquisition of n-3 and n-6 fatty acids occurs during embryogenesis and early postnatal stages of development. In humans this process takes place during the last trimester and first 6-10 months after birth [10,11]. The rapid accumulation of DHA is directly related to its crucial need for nor mal neurological and visual development [12]. n-3 deficiency is associated with biochemical changes in brain and retina including decreased DHA, altered enzymatic activities depressed learning ability, and disturbed behavior and water intake [13,14] optimal neurological and visual development [12].
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Cardiovascular disease and dietary plasma lipid levels : |
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Cardiovascular disease (CVD) in general and coronary heart disease (CHD) in particular, is the leading cause of death in industrialized countries. The relationship between dietary n3 fatty acids, particularly eicosapentaenoic acid( EPA) and docosahexaenoic acid,( DHA), and risk of developing CVD began to emerge in the late 1970s [15,16]. Some of the potential mechanisms for the cardioprotective effect of n3 fatty acids include antiarrhythmic, anti-inflammatory, hypotensive, and hypotriglyceridemic effects [17]. Studies using relatively high doses of fish oil indicated that the most consistent effect on plasma markers of cardiovascular disease risk is a decrease in triacylglycerol (TAG) levels [17]. This effect is dose-dependent and is influenced by the baseline plasma TAG level of the subjects. Both fish and vegetable oil consumption increases EPA and DPA n-3 content in serum, platelet, and RBC. Studies in humans demonstrate that ALA is rapidly incorporated into lipoproteins within 3 h after ALA consumption and consequently plasma ALA, EPA and DPA concentrations are elevated after consumption of ALA-enriched sources.
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ALA - antiarrhythmic properties : |
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The antiarrhythmic properties of fish oils were first shown in rats, non-human primates and dogs. The mechanism for the antiarrhythmic effects probably involves the modification ion channels currents by the incorporation of these polyunsaturated fatty acids into the cardio myocytes membrane phospholipid [18-20].
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ALA - anti-iflammatory properties : |
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Inflammation is the cell physiological response to exposure to certain substances released, predominantly, by activated leukocytes. This response usually consists in the reddening and swelling of the targeted area and it is mediated by at least two different groups of biomolecules, the n6 eicosanoids and the cytokines. The n-6 eicosanoids are biosynthetically derived from ARA. One of the most common pharmacological approaches to treat inflammation is to inhibit the biosynthesis of n-6 eicosanoids. This could be achieved by providing COX1/2 inhibitors, e.g. aspirin, or by increasing n3 fatty acid content, particularly EPA and DHA [21]. In turn, n-3 enriched diets can also suppress production of cytokines.
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Polyunsaturated fatty acids have important structural, biochemical, and regulatory functions that are required for optimal tissue function, growth, and repair. They especially DHA plays a central role in the normal development and functioning of the brain and central nervous system and retina apart from its effect on the heart, plasma lipid levels and anti-inflammatory action.
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1.
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Lands WEM. Fish, omega3 and human health. Urbana, Illinois: AOCS Press;2005.
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2.
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Bourre JM, Francois M, Youyou A, Dumont O, Piciotti M, Pascal G, et al. The effects of dietary alpha-linolenic acid on the composition of nerve membranes, enzymatic activity, amplitude of electrophysiological parameters, resistance to poisons and performance of learning tasks in rats. J Nutr 1989;119:1880-92.
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3.
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Wainwright PE,Huang YS, Bulman-Fleming B,MillsDE, Redden P,McCutcheon D. The role of n3 essential fatty acids in brain and behavioral development: a cross-fostering study in the mouse. Lipids 1991;26:37-45.
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4.
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Yamamoto N, Hashimoto A, Takemoto Y, Okuyama H, Nomura M, Kitajima R, et al. Effect of the dietary alpha-linolenate/linoleate balance on lipid compositions and learning ability of rats. II. Discrimination process, extinction process, and glycolipid compositions. J Lipid Res 1988;29: 1013-21.
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5.
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Corrigan FM, Horrobin DF, Skinner ER, Besson JA, Cooper MB. Abnormal content of n6 and n3 long-chain unsaturated fatty acids in the phosphoglycerides and cholesterol esters of parahippocampal cortex from Alzheimerfis disease patients and its relationship to acetyl CoA content. Int J Biochem Cell Biol 1998;30:197-207.
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6.
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Prasad MR, Lovell MA, Yatin M, Dhillon H, Markesbery WR. Regional membrane phospholipid alterations in Alzheimerfis disease. Neurochem Res 1998;23:81-8.
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7.
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Foot M, Cruz TF, Clandinin MT. Influence of dietary fat on the lipid composition of rat brain synaptosomal and microsomal membranes. Biochem J 1982;208:631-40.
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8.
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Breckenridge WC, Gombos G, Morgan IG. The lipid composition of adult rat brain synaptosomal plasma membranes. Biochim Biophys Acta 1972;266:695-707.
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9.
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Anderson RE, Benolken RM, Dudley PA, Landis DJ, Wheeler TG. Proceedings: polyunsaturated fatty acids of photoreceptor membranes. Exp Eye Res 1974;18:205-13.
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10.
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Clandinin MT, Chappell JE, Leong S, Heim T, Swyer PR, Chance GW. Intrauterine fatty acid accretion rates in human brain: implications for fatty acid requirements. Early Hum Dev 1980;4:121-9.
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11.
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Clandinin MT, Chappell JE, Leong S, Heim T, Swyer PR, Chance GW. Extrauterine fatty acid accretion in infant brain: implications for fatty acid requirements. Early Hum Dev 1980;4:131-8.
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Carlson SE, NeuringerM. Polyunsaturated fatty acid status and neurodevelopment: a summary and critical analysis of the literature. Lipids 1999;34:171-8.
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13.
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Reisbick S, Neuringer M, Connor WE, Barstad L. Postnatal deficiency of omega3 fatty acids in monkeys: fluid intake and urine concentration. Physiol Behav 1992;51:473-9.
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14.
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Reisbick S, Neuringer M, Hasnain R, Connor WE. Home cage behavior of rhesus monkeys with long-term deficiency of omega3 fatty acids. PhysiolBehav 1994;55:231-9.
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15.
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Bang HO, Dyerberg J, Sinclair HM. The composition of the Eskimo food in north western Greenland. Am J Clin Nutr 1980;33:2657-61.
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16.
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Dyerberg J, Bang HO, Stoffersen E, Moncada S, Vane JR. Eicosapentaenoic acid and prevention of thrombosis and atherosclerosis? Lancet 1978;2:117-9.
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17.
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Balk EM, Lichtenstein AH, Chung M, Kupelnick B, Chew P, Lau J. Effects of omega3 fatty acids on serum markers of cardiovascular disease risk: a systematic review. Atherosclerosis 2006;189:19-30.
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18.
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Kang JX, Xiao YF, Leaf A. Free, long-chain, polyunsaturated fatty acids reduce membrane electrical excitability in neonatal rat cardiac myocytes. Proc Natl Acad Sci USA 1995;92:3997-4001.
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19.
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Kang JX, Leaf A. Evidence that free polyunsaturated fatty acids modify Na+channels by directly binding to the channel proteins. Proc Natl Acad Sci USA 1996;93:3542-6.
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20.
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Leaf A, Xiao YF, Kang JX. Interactions of n3 fatty acids with ion channels in excitable tissues. Prostag Leukotr Essent Fatty Acids 2002;67:113-20.
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21.
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James MJ, Gibson RA, Cleland LG. Dietary polyunsaturated fatty acids and inflammatory mediator production. Am J Clin Nutr 2000;71:343S-8S.
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