ALA, DHA, EPA and Lipoproteins

Eicosapentanoic acid (EPA, C20:5 (n-3)) and docosahexanoic acid (DHA, C22:6 (n-3)), long chain fatty acids from fish, are incorporated into lipoprotein phospholipids membranes where they are thought to provide cardioprotective effects. Alpha linolenic acid (ALA, C18:3 (n-3)) is an essential fatty acid found in certain plant foods that can be converted in humans to both EPA and DHA. Because the conversion of ALA to EPA is not efficient, the cardioprotective effects of ALA are not as well established as those of EPA and DHA. Despite this some studies have compared the effects of ALA with that of EPA and DHA. From this research it appears that ALA has its own physiological effects that are distinct from fish derived long chain fatty acids, suggesting that the benefits from ALA supplementation are not derived solely through conversion to EPA and DHA.

Researchers1 have used a parallel study design to investigate the effects of ALA, EPA and DHA on the lipoproteins of 74 healthy men and women aged 19 to 43 years. Participants were assigned randomly to receive either 4.4 grams of ALA, 2.2 grams of EPA or 2.3 grams of DHA per day in the form of a specially prepared margarine. All participants had an increase in the n-3 content of their lipoproteins relative to their specific treatment. Supplementation with ALA increased the EPA content of the lipoprotein membranes by 36%, but had no effect on DHA. However, supplementation with EPA did increase DHA content of the lipoproteins by 24%. Conversely, supplementation with DHA resulted in a 249% increase in EPA content of the lipoproteins. Supplementation with all of the n-3 fatty acids caused a decrease in triglyceride levels with DHA having the greatest effect.

These data support previous findings that suggest that different n-3 fatty acids have different physiological effects. In humans, ALA is converted to EPA via a series of elongation and desaturation enzymes. Subsequently, EPA is converted to DHA by being twice elongated to a C24:5 fatty acid, followed by a desaturation at position 6 to produce tetracosahexanoic acid (THA, C24:6 (n-3)). β-oxidation of THA then reduces the chain length to produce DHA. Research has shown that the conversion of ALA to EPA is only around 5%. In addition, supplemental EPA is much more readily converted to DHA than supplementation ALA, which supports the absence of DHA in lipoproteins of those individuals supplemented with ALA in this study. Because it appears that the physiological functions of ALA, DHA and EPA may be distinct but overlapping, it makes sense to ensure that all three fatty acids are present in the diet.

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1Egert, S., Kannenberg, F., Somoza, V., Erbersdobler, H. F. and Wahrburg, U. 2009. Dietary α-linolenic acid, EPA, and DHA have differential effects on LDL fatty acid composition but similar effects on serum lipid profiles in normolipidemic humans. Journal of Nutrition. 861-868

About Robert Barrington

Robert Barrington is a writer, nutritionist, lecturer and philosopher.
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