Jose M. Lorenzo - Food Lipids: Sources, Health Implications, and Future Trends

n -3 Long-chain polyunsaturated fatty acids; Eicosapentaenoic acid; Docosahexaenoic acid; Conjugated linoleic acids; Biological functions; Human health

Chapter outline

The n -3 long-chain polyunsaturated fatty acids ( n -3 LC-PUFA) have been the focus of numerous studies since the 1970s due to their vital role in many metabolic processes. The most biologically important n -3 LC-PUFA appear to be eicosapentaenoic acid (C20:5 n -3, EPA) and docosahexaenoic acid (C22:6 n -3, DHA); however, significant roles for docosapentaenoic acid (C22:5 n -3, DPA) have also been well documented ().

Fig. 13.1 Synthesis of n -3 LC-PUFA. No permission required.

The synthesis of EPA might be reduced by linoleic acid (C18:2 n -6, LA) because of the competition between ALA and LA for the rate-limiting enzyme 6-desaturase transforming LA into -linolenic acid (C18:3 n -6, GLA) while blocking ALA into C18:4 n -3. Hence, the conversion of ALA into the n -3 LC-PUFA, particularly EPA and DPA, might be changed by increasing ALA or C18:4 n -3; however, when certain levels of ALA have been exceeded, a decrease in DHA was observed ().

The rates of elongation and desaturation of ALA to EPA and DHA in the body, and whether the synthesized quantities of these fatty acids are sufficient to meet physiological demands, are highly debatable. Previous research using stable isotope tracer techniques with single oral administration of deuterated of 13C-U labeled ALA reported low rates of conversion of dietary ALA to EPA and DHA and found that most of the labeled ALA is -oxidized or lost. Only minimal amounts are converted to n -3 LC-PUFA ( assessed the rate of synthesis-secretion of several esterified n -3 LC-PUFA from circulating unesterified ALA providing new estimates of whole-body synthesis-secretion rates in human subjects.

In recent decades, it was revealed that the activity of the desaturase enzymes and the contents of LC-PUFA (both n -6 and n -3) in blood and tissues have been markedly impacted by single-nucleotide polymorphism (SNP) in the fatty acids desaturase (FADS) gene clusters () showed that minor alleles were associated with high levels of LC-PUFA synthesis, whereas the major genotypes were associated with low levels of LC-PUFA synthesis. Therefore, the supply of preformed LC-PUFA may be particularly important in these populations to achieve similar plasma and tissue concentrations, as well as related biological effects.

In the body, n -3 LC-PUFA are transported in the bloodstream esterified into triacylglycerols (TAG), phospholipids (PL), and cholesteryl esters as components of lipoproteins or noncovalently bound to albumin in the nonesterified form ().

According to ).

Table 13.1

Adapted from Norwegian food compositional table (2018) and McCance and Widdowsons The Composition of Foods Integrated Dataset (2015). London: Public Health England. Both sources are available from http://www.fao.org/infoods/infoods/tables-and-databases/europe/en/.

The demand for fish and seafood as a source of n -3 LC-PUFA has been constantly increasing with the global increase of the population. The aquaculture has filled this demand with over half of all fish and seafood now being farmed. Since the fish, similarly to animals and humans, produce low amounts of n -3 LC-PUFA, these fatty acids are almost entirely derived from their diet, and hence the only way to ensure farmed fish contain high levels of EPA and DHA is to include these fatty acids in the feed as fish oil or fish meal ().