Omega-3 polyunsaturated fatty acids (n-3 PUFAs) are distinguished from other fatty acids in that they contain more than two cis double bonds beginning at the third carbon from the omega (methyl) end (Calder and Yaqoob, 2009) (Fig. 1.1).
Eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) are considered long-chain n-3 PUFAs and can be derived from the shorter chain n-3 PUFA, α-linolenic acid (ALA, 18:3n-3). Animals do not have the ability to synthesize ALA. As a result, ALA is considered an essential fatty acid. However, ALA can be further desaturated and elongated to EPA and ultimately DHA. In humans, these conversion processes are extremely inefficient (Calder and Yaqoob, 2009). This is due, in part, to competition between ALA and linoleic acid (LA, 18:2n-6) to be converted to EPA and arachidonic acid (AA, 20:4n-6), respectively, by a Δ6 desaturase enzyme. In the Western diet, the amount of LA consumed far exceeds ALA, so conversion of ALA to EPA is limited (Burdge and Calder, 2006). Therefore, EPA and DHA are almost exclusively taken in through diet.
ALA is plant-derived and can be found in leafy green vegetables, pea plants, and a wide variety of seed oils (Williams and Burdge, 2006). In contrast, EPA and DHA are largely derived from marine sources such as tuna, herring, mackerel, anchovies, sardines, salmon, krill, and microalgae (Rimm et al, 2018; Monroig et al., 2013).
The omega-3 index is defined as the erythrocyte content of EPA and DHA and is represented as a percentage of total fatty acids. The omega-3 index is a biomarker for n-3 PUFA consumption and is inversely correlated with cardiovascular health - a low omega-3 index is a cardiovascular disease (CVD) risk factor (von Schacky, 2014). Using previous epidemiological and prospective cohort studies examining the association between n-3 PUFAs and CVD risk, Harris and von Schacky determined that an omega-3 index of =8% is cardioprotective (Harris and von Schacky, 2004).
