An essential fatty acid is an unsaturated fatty acid that is essential for growth and health but cannot be synthesised by the body — it must be eaten. Humans lack the enzymes to insert double bonds at the omega-3 and omega-6 positions, so linoleic acid (omega-6) and alpha-linolenic acid (omega-3) are dietary essentials: they build cell membranes, make eicosanoid signalling molecules, and support brain and eye development. Deficiency — rare, but seen in fat-free intravenous feeding — produces scaly skin, poor growth, and impaired immunity. The longer-chain derivatives EPA and DHA, made inefficiently from alpha-linolenic acid, are best obtained directly from oily fish or algae. The essential fatty acids are the reason nutrition insists on some fat in the diet — not all fats are optional, and these two are the non-negotiable minimum.