The two major classes of polyunsaturated fatty acids (PUFAs) are the omega-3 and omega-6 fatty acids. Like all fatty acids, PUFAs consist of long chains of carbon atoms with a carboxyl group at one end of the chain and a methyl group at the other. PUFAs are distinguished from saturated and monounsaturated fatty acids by the presence of two or more double bonds between carbons within the fatty acid chain.

Omega-3 fatty acids (omega-3s) have a carbon–carbon double bond located three carbons from the methyl end of the chain. Omega-3s, sometimes referred to as “n-3s,” are present in certain foods such as flaxseed and fish, as well as dietary supplements such as fish oil. Several different omega-3s exist, but the majority of scientific research focuses on three: alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). ALA contains 18 carbon atoms, whereas EPA and DHA are considered “long-chain” (LC) omega-3s because EPA contains 20 carbons and DHA contains 22.

PUFAs are frequently designated by their number of carbon atoms and double bonds. ALA, for example, is known as C18:3n-3 because it has 18 carbons and 3 double bonds and is an n-3, or omega-3, fatty acid. Similarly, EPA is known as C20:5n-3 and DHA as C22:6n-3. Omega-6 fatty acids (omega-6s) have a carbon–carbon double bond that is six carbons away from the methyl end of the fatty acid chain. Linoleic acid (C18:2n-6) and arachidonic acid (C20:4n-6) are two of the major omega-6s.

The human body can only form carbon–carbon double bonds after the 9th carbon from the methyl end of a fatty acid. Therefore, ALA and linoleic acid are considered essential fatty acids, meaning that they must be obtained from the diet. ALA can be converted into EPA and then to DHA, but the conversion (which occurs primarily in the liver) is very limited, with reported rates of less than 15%. Therefore, consuming EPA and DHA directly from foods and/or dietary supplements is the only practical way to increase levels of these fatty acids in the body.

ALA is present in plant oils, such as flaxseed, soybean, and canola oils. DHA and EPA are present in fish, fish oils, and krill oils, but they are originally synthesized by microalgae, not by the fish. When fish consume phytoplankton that consumed microalgae, they accumulate the omega-3s in their tissues.

After ingestion, dietary lipids are hydrolyzed in the intestinal lumen. The hydrolysis products—monoglycerides and free fatty acids—are then incorporated into bile-salt– containing micelles and absorbed into enterocytes, largely by passive diffusion. The process is efficient, with an absorption rate of about 95%, which is similar to that of other ingested fats. Within intestinal cells, free fatty acids are primarily incorporated into chylomicrons and enter the circulation via the lymphatic system. Once in the bloodstream, lipoprotein particles circulate within the body, delivering lipids to various organs for subsequent oxidation, metabolism, or storage in adipose tissue