Human beings, during synthesis of fatty acids cannot introduce double bonds beyond the position:
Higher animals have d4, d5, d6, and d9 desaturases but cannot inse new double bonds beyond the 9 position of fatty acids. Thus, the essential fatty acids linoleic (6) and -linolenic (3) must be obtained from the diet. Ref: Harper 28thedition, chapter 23.
4
7
9
11