α-Linolenic acid can only be obtained by humans through their diets because the absence of the required 12- and 15-desaturase enzymes makes "de novo" synthesis from stearic acid impossible. Eicosapentaenoic acid (EPA; 20:5, "n"−3) and docosahexaenoic acid (DHA; 22:6, "n"−3) are readily available from fish and algae oil and play a vital role in many metabolic processes. These can also be synthesized by humans from dietary α-linolenic acid: ALA → stearidonic acid → eicosatetraeonic acid → eicosapentaenoic acid → docosapentaenoic acid → 9,12,15,18,21-tetracosapentaenoic acid → 6,9,12,15,18,21-yetracosahexaenoic acid → docosahexaenoic acid, but with an efficiency of only a few percent.