Essential fatty acids (EFAs) are fatty acids your body needs but cannot synthesize: you must ingest them. There are two types of EFAs: omega−3 (n−3) and omega−6 (n−6).
Alpha-linolenic acid (ALA) is an omega−3 fatty acid that your body can convert to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)
Linoleic acid (LA) is an omega−6 fatty acid that your body can convert to arachidonic acid (AA).
For these conversions, LA and ALA need the same enzyme: Δ6-desaturase.
Kim et al. Food Chem Toxicol. 2014 May
Because LA and ALA compete for the same enzyme, higher LA intakes might result in less ALA being converted to EPA and DHA. In a 1993 randomized controlled trial, 8 participants took ALA (7 g/day) and LA (either 21 or 50 g/day) for 18 days. The lower-LA group had a higher proportion of EPA in their platelet phosphatidylcholine, plasma phosphatidylcholine, and plasma phosphatidylethanolamine.
There’s also evidence that conversion of ALA to DHA is greater in women than men. Women’s greater capacity for DHA synthesis may exist to satisfy the DHA needs of fetuses and neonates — particularly the needs of their developing brains.
EPA and DHA are involved in various biological processes, including brain function and several metabolic signaling pathways. Since conversion of ALA to EPA and DHA is inefficient (<8% of ALA becomes EPA, and <4% of ALA becomes DHA), taking EPA and DHA directly is more effective at raising your levels of these two n−3 EFAs.
EPA and DHA are present in the fat of fish and other seafoods, so eating those is an option. When it comes to supplements, the most common is fish oil, but vegans now have access to algae-derived EPA+DHA supplements.