Linoleic acid and alpha-linolenic acid must be supplied in the diet because humans cannot synthesize them. The biochemical basis of this essentiality is that human desaturases cannot introduce double bonds:
- A Beyond the delta-6 position
- B Beyond the delta-4 position
- C Between the delta-9 carbon and the methyl (omega) end ✓
- D In any unsaturated fatty acid
Correct answer: C. Between the delta-9 carbon and the methyl (omega) end
Explanation
Humans possess delta-4, delta-5, delta-6 and delta-9 desaturases but lack delta-12 and delta-15 desaturases, so we cannot create double bonds closer than seven carbons from the methyl end. Linoleic acid (18:2, delta-9,12) and alpha-linolenic acid (18:3, delta-9,12,15) therefore cannot be made de novo. Elongation and desaturation of dietary linoleic acid produce arachidonic acid, the precursor of eicosanoids, which is why linoleic acid is the truly indispensable dietary fatty acid.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.