<prompt> Human beings, during synthesis of fatty acids cannot introduce double bonds beyond the position:47911  <response>: 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. 