What impacts the conversion of alpha-linolenic acid to DHA and EPA?

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In the body, ALA can function as a precursor for the production of the long-chain omega-3 fatty acids EPA and DHA. However, these conversions are limited; they have been estimated to occur at rates of <8% and <4% for EPA and DHA, respectively.[1] There are many factors that can impact the efficiency of this conversion, such as sex, genetic polymorphisms, disease states, and habitual fatty acid consumption.[2][1]

For example, premenopausal women can convert ALA to EPA and DHA at a significantly greater rate due to higher levels of estrogen, which can upregulate the expression of the enzymes required for the conversion.[1][3] This likely relates to the importance of DHA during fetal development and lactation. Another contributing factor may be the level of omega-6 fatty acids consumed.

Both ALA and alpha-linoleic acid (an essential omega-6 fatty acid) compete for the same enzymes that convert them into longer chain fatty acids. Restricting alpha-linoleic acid while increasing ALA intake has been shown to increase levels of EPA and sometimes DHA in clinical trials, although the effect is small.[4]

Lastly, the dose of ALA is important. For example, a clinical trial found that 30 grams daily of ground flaxseed was sufficient to raise levels of EPA in the blood, but 10 grams daily was not.[5]

References
1.^Yuan Q, Xie F, Huang W, Hu M, Yan Q, Chen Z, Zheng Y, Liu LThe review of alpha-linolenic acid: Sources, metabolism, and pharmacology.Phytother Res.(2022-Jan)
2.^Adam Ameur, Stefan Enroth, Asa Johansson, Ghazal Zaboli, Wilmar Igl, Anna C V Johansson, Manuel A Rivas, Mark J Daly, Gerd Schmitz, Andrew A Hicks, Thomas Meitinger, Lars Feuk, Cornelia van Duijn, Ben Oostra, Peter P Pramstaller, Igor Rudan, Alan F Wright, James F Wilson, Harry Campbell, Ulf GyllenstenGenetic adaptation of fatty-acid metabolism: a human-specific haplotype increasing the biosynthesis of long-chain omega-3 and omega-6 fatty acidsAm J Hum Genet.(2012 May 4)