Fig. 2.

Scheme illustrating PUFA biosynthesis, metabolism and function. PUFA can be provided directly by the diet or can be synthesized from their respective essential dietary precursors, α-linolenic acid (ALA, 18:3n-3) and linoleic acid (LA, 18:2n-6). For example, DHA (22:6w3) is composed of 22 carbon atoms and six double bonds, and its upstream metabolic precursor is EPA (20:5w3). Because the first double bond, as counted from the methyl terminus, is at position 3, they belong to the omega-3 group. Two key enzymes, the Δ6 and Δ5 desaturases, catalyse the desaturation and elongation of ALA and LA in the endoplasmic reticulum and in one terminal cycle of β-oxidation in the peroxisome