A brief Overview of Omega-3 and Omega-6 Fatty Acids
The following is a overview of omega-3 and omega-6 fatty acids from a number of papers published by NUTRIMENTHE researchers (references 1-5 and those therein)
Fatty acids are aliphatic compounds comprising a carboxyl group and a hydrocarbon chain of varying length and degree of saturation. Saturated fats have no double bonds in the hydrocarbon chain, monounsaturated fats have one double bond, polyunsaturated fats (PUFAs), have two or more. Omega-3 and omega-6 fatty acids are classified as polyunsaturated fatty acids (PUFAs). They are derived from alpha linolenic (ALA) and linoleic acid (LA) respectively. Humans are unable to insert double bonds into either of these fatty acids, thus they must be obtained from the diet ie., they are ‘essential’ fatty acids.
Sources of LA include plant oils such as corn, safflower, soybean and sunflower. Sources of ALA include nuts, green plants and plant oils, such as rapeseed and flaxseed. In a typical western diet, up to 20% of dietary fat is comprised of PUFA, the most abundant of which is LA, up to 95% of PUFA intake. Fatty acids are stored between three main ‘compartments’, short-term storage (eg in blood plasma), medium-term storage (eg in the membranes of red blood cells) and long-term storage (eg adipose tissue).
In humans, LA and ALA can be converted to fatty acids with longer chain lengths and a higher degree of unsaturation (addition of more double bonds) by a series of alternating desaturation and chain elongation steps.
The enzymes involved in these steps are Delta 5 desaturase (D5D), Delta 6 desaturase (D6D), and elongase. D5D and D6D are encoded by the genes FADS1 and FADS2. A FADS3 gene also exists but the function of the protein product of this gene has yet to be fully characterised. The D5D and D6D enzymes are found in the majority of h
uman tissues with the highest activities in the liver and significant activity also in adipose tissue, brain, heart and lung.
LA and ALA are converted to other PUFAs and also to the LC-PUFAs, arachidonic acid (AA) and eicosapentaenoic acid (EPA) respectively. EPA is further converted to docosahexaenoic acid (DHA). All three can be obtained from the diet. AA is found in high quantities in meat, eggs and offal. EPA and DHA from oily fish such as salmon, mackerel, herring and tuna. Dietary sources of these fatty acids may be of importance as in adults (and newborn’s), conversion of ALA and LA to LC-PUFAs is rather low and especially for the conversion of EPA to DHA. It might be assumed therefore that LC-PUFAs should be obtained, pre-formed, from the diet, although this is currently a matter of debate.
AA, EPA and DHA comprise 10-15% of the content of all structural lipids in cell membranes. AA and EPA are both processed further to other biologically active substances, eicosanoids, including, prostaglandins and leukotrienes which have a role in inflammation and the regulation of immunity. In general, AA-derived eicosanoids have pro-inflammatory effects whereas EPA derived eicosanoids are rather less inflammatory. The enzymes involved in the synthesis of PUFAs favour the production of omega-6 fatty acids, including AA. Furthermore it has been suggested that there is a link between high dietary intake of omega-6 PUFAs and inflammatory disease.
DHA is the most abundant omega-3 fatty acid in the mammalian brain, incorporated into the nervous tissues during the pre- and post- natal period of rapid neural growth. DHA levels have been shown to affect visual, cognitive and motor functions in animal and human studies thus levels of fatty acids in humans become significant when considering the development of cognitive abilities. In a series of publications, the ALSPAC study team has documented that maternal fish intake during pregnancy has a positive effect on communication skills and behavioural development in children (6). The implication is that the fatty acid content in fish may underlie this relationship (NUTRIMENTHE unpublished).
Fatty acid levels in humans may not only be effected by dietary intake but also in variation of the genes encoding the D5D and D6D enzymes, FADS1 and FADS2. A number of studies have reported that genetic variation in FADS1 and FADS2 affect the synthesis of AA and contribute to the variability of fatty acid levels in the short, medium and long-term storage compartments. However, DHA levels were shown not to be associated with FADS variation reflecting the need for this fatty acid to be obtained, predominantly, via the diet. However, DHA synthesis is higher during pregnancy compared to men and non-pregnant women, which may indicate an increase need for this fatty acid during pregnancy. In the future, NUTRIMENTHE expects to learn more about how genetic variation impacts on how fatty acids are processed during pregnancy and childhood and to link the studies to mental performance measurements in children.
Mammalian pathways of omega-3 and omega-6 fatty acid synthesis
References
1: Glaser et al., 2011. Genetic variation in polyunsaturated fatty acid metabolism and its potential relevance of human development and health. Maternal and Child Nutrition. 7 (suppl 2), 27-40.
2: Glaser et al., 2010 Role of FADS1 and FADS2 polymorphisms in polyunsaturated fatty acid metabolism. Metabolism Clinical and Experimental. 59(7): 993-999.
3: Lattka et al., 2010. Genetic variants of the FADS1 FADS2 gene cluster as related to fatty acid metabolism. Curr. Opin. Lipidol. 21: 64-36.
4: Lattka et al., 2010 Do FADS genotypes enhance our knowledge about fatty acid related phenotypes? Clinical Nutrition. 29: 277-287.
5: Koletzko et al., 2009. Does dietary DHA improve neural function in children? Observations in phenylketonuria. Prostaglandins, Leukotrienes and Essential Fatty Acids. 81(2-3): 159-164.
6: Hibbeln et al., 2007. Maternal seafood consumption in pregnancy and neurodevelopmental outcomes in childhood (ALSPAC study): an observational cohort study. Lancet 369: 578-585.
July 2011
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