The second essential fatty acid, alpha linolenic acid, is the major polyunsaturated fatty acid of plants. It contains 18 carbon atoms and three double bonds and belongs to the omega-3 family of fatty acids, in which the first double bond occurs at the third carbon of the fatty acid. Alpha linolenic acid is converted to a very long fatty acid, EICOSAPENTANEONIC ACID (EPA), which forms prostaglandins (PGE₃) and thromboxane (TXA₃) that counterbalance the effects of proinflammatory products derived from arachidonic acid by reducing the tendency to clot, reducing pain and inflammation. Dietary alpha linolenic acid will tend to inhibit arachidonic acid conversion to inflammatory agents, and thus further limit inflammation. Alpha linolenic acid occurs in high levels in BORAGE oil and flaxseed oil, while EPA occurs in high levels in fish and shellfish.

If the diet lacks adequate amounts of either linoleic or alpha linolenic acid, deficiency symptoms will develop that include scaly skin, hair loss and slow wound healing. Omega-3 fatty acid deficiency leads to impaired brain and retinal development in experimental animals and possibly in premature births.

Polyunsaturated seed oils and fish oils supply the essential fatty acids in the typical U.S. diet. There is no RECOMMENDED DIETARY ALLOWANCE for linoleic acid or alpha linolenic acid. However, the amount of linoleic acid and related omega-6 fatty acids needed to prevent deficiency in adults is estimated to be I to 3% of calories, equivalent to approximately 6 g/day. Linoleic acid in typical American diets ranges from 5 to 10% of calories due to a high consumption of vegetable oils. It has been proposed that alpha linolenic acid represent 0.25 to 0.54% of daily calories for optimal health. Large amounts of polyunsaturated fatty acids may increase the requirement for antioxidants like VITAMIN E to prevent their chemical degradation.