Omega-3s are a shorthand way to describe a class of polyunsaturated fatty acids that is generally more highly unsaturated than others in foods. They are distinguished from all other unsaturates by the location of their first double bond (where they are missing a pair of hydrogens). This makes it impossible for humans to make omega-3s from scratch or from other fatty acids, so we must obtain them—or at least one of them—from foods.

The “3” simply says (to chemists) that the first double bond occurs 3 carbons away from the non-acid or “omega” end of the fatty acid.

There are 3 main omega-3s in foods: alpha-linolenic acid, EPA (eicosapentaenoic acid), and DHA (docosahexaenoic acid). Only alpha-linolenic acid is found in plants. EPA and DHA are found almost exclusively in fish and shellfish or fish oils.


Terrestrial plants have only alpha-linolenic acid. This omega-3 has some, but not all, of the health benefits associated with fish oil omega-3s. It can also be converted in the body to EPA and very small amounts of DHA. Humans perform this conversion very poorly, less than 5%, so relying only on alpha-linolenic acid for all omega-3s risks a shortfall in EPA and especially, DHA. This topic is discussed in detail on the web page “Are All Omega-3s the Same?”

The fish oil fatty acids EPA and DHA are also called “long-chain” omega-3s. This is because they have at least 20 carbons in their chain. Long-chain omega-3s have the most critical functions in health and in some cases, cannot be replaced by alpha-linolenic acid. This is especially true for DHA, a key component of brain cells, the retina of the eye, and certain other tissues. DHA is incorporated into brain during fetal development and the first 2 years of life.


The many benefits of Omega-3 EPA and DHA in fish. In the early 1970s when Danish physicans observed that Greenland Eskimos had an exceptionally low incidence of heart disease and arthritis despite the fact that they consumed a high fat diet. It was soon discovered that two of the fats they consumed in large quantities, EPA and DHA , were actually highly beneficial. Recent research has found that EPA and DHA play a role in the prevention of atherrosclerosis, heart attack, depression and cancer.”>For strict vegetarians, supplements produced from microorganisms or marine algae provide EPA and DHA. Marine algae vary widely in their content of total omega-3s ranging from 17% to nearly 50%. The content of alpha-linolenic acid compared with EPA also varies widely (3% to 24%). Marine algae are not commonly available in the U.S.

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