Linoleic acid and alpha-linolenic acid run through the same enzymes in the liver, and whichever is more abundant occupies the machine.Omega-6 (linoleic acid, LA) and omega-3 (alpha-linolenic acid, ) enter the liver ER and share the same enzymes: delta-6-desaturase → elongase → delta-5-desaturase. Not two parallel tracks — they fight for the same workstation; whoever is more abundant occupies the machine.
2 · Delta-6-desaturase is the bottleneck
Delta-6-desaturase is the first and rate-limiting step, and because modern diets carry far more linoleic acid than alpha-linolenic acid, the latter has a harder time getting converted.The first and rate-limiting step is delta-6-desaturase. Modern diets carry far more LA than ALA: by Blasbalg 2011's estimate for the US food supply, the LA:ALA ratio rose from about 6.4 to 10 over the 20th century. With more LA competing for the enzyme, ALA has a harder time getting converted. Enzyme activity also varies with FADS1/FADS2 genes, insulin, and total energy — large individual differences.
3 · ALA conversion is tiny
becomes at about 5-10% and often at under 1%, so most plant omega-3 you eat never reaches the long-chain products. → is about 5-10% (slightly higher in women via estrogen); ALA → is often under 1%. A considerable share of ALA is simply burned for energy. Relying on flaxseed or walnuts for EPA/DHA is unreliable — the vast majority of plant omega-3 you eat never reaches the long-chain products.
4 · Direct EPA/DHA bypasses the fight
Fish or algae oil provides / directly, bypassing the entire conversion line — at which point the omega-6:omega-3 ratio barely matters.The ratio has logic (substrate competition), but absolute amount matters more: if both are low, a pretty ratio is meaningless; eat enough / directly and the ratio stops being the lever.