The Significance of Alpha-Linolenic Acid for Humans

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The omega 3 polyunsaturated fatty acids have had a major impact on thinking in medicine in the last twenty-five years. The parent fatty acid in the omega 3 fatty acid family is alpha-linolenic acid (ALA) which is an essential fatty acid found in high concentrations in certain plant oils, such as flaxseed oil, walnut oil and canola oil. Several longer chain or derived omega 3 fatty acids are formed in the body from ALA, including eicosapentaenoic acid, EPA, decosapentaenoic acid and docosahexaenoic acid, DHA. Dietary sources of these longer-chain fatty acids are fish, fish oils and from other marine organisms. DHA is specifically localised in the retina and the brain in humans and other mammals where it has a primary role via effects on membrane order (fluidity), the activity of membrane-bound enzymes, ion channels and signal transduction.
   For many years, ALA has been considered to be important only as a precursor of these longer chain compounds, however it is now recognaised that ALA is metabolished by other pathways which may be quantiatively more important than the formation of DHA. For example, ALA is extensively β-oxidised in mammals including humans, especially on diets rich in polyunsaturated fatty acids. ALA appears to play an important role in the nervous system (carbon recycling) for the synthesis of saturated and monounsaturated fatty acids and cholesterol, with estimates of the proportion of ALA being matabolised via this pathway being several fold greater than that to DHA. Furthermore, recent studies have indicated that ALA is deposited in the skin of small mammals and may also play a role in fur. Since ALA is extensively metabolised via these pathways (β-oxidation, carbon recycling and deposition in skin/fur) and is subject to competitive inhibition from linoleic acid in metabolism via the delta-6 desaturase, it is not surprising tha dietary DHA is substantially more effctive than an equivalent dose of dietary ALA in maintaining neural DHA levels. Despite the limited efficiency of DHA formation from ALA, it is clear that certain tissues possess the capacity to synthesise DHA at rates greater than other tissues. The brain, retina and reproductive tissue may be able to synthesis sufficient DHA to meet the DHA demands of these tissues, although in infants at the peak of brain growth and DHA deposition, a dietary supply of DHA (as in milk) may be important to maintain the brain DHA level. In plants, ALA is a substrate for lipoxygenase enzymes leading to the poduction of bioactive hydroxy compounds. This information might provide a clue to other routes of ALA metabolism in mammals. It is concluded that it may no longer be appropriate to consider the function for ALA to be solely a precursor of EPA and DHA.