The fatty acids that cannot be synthesised by the body and, therefore, should be supplied in the diet are known as essential fatty acids (EFA). Chemically, they are polyunsaturated fatty acids (PUFA), namely linoelic acid (18: 2; 9, 12) and linolenic acid (18: 3; 9, 12, 15). Arachidonic acid (20: 4; 5, 8, 11, 14) becomes essential. Linolenic and linoleic acids are essential since humans lack the enzyme that can introduce double bonds beyond carbons 9 and 10.