- Linoleic acid (LA), an omega-6 fatty acid, and α-linolenic acid (ALA), an omega-3 fatty acid, are considered essential fatty acids (EFA) because they cannot be synthesized by humans. (More information)
- The long-chain omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), can be synthesized from ALA, but due to low conversion efficiency, it is recommended to obtain EPA and DHA from additional sources. (More information)
- LA, arachidonic acid (AA), and DHA are the most common polyunsaturated fatty acids (PUFA) accumulating in tissues. (More information)
- Both omega-6 and omega-3 fatty acids are important structural components of cell membranes, serve as precursors to bioactive lipid mediators, and provide a source of energy. Long-chain omega-3 PUFA in particular exert anti-inflammatory effects and it is recommended to increase their presence in the diet. (More information)
- Both dietary intake and endogenousmetabolism influence whole-body status of EFA. Genetic polymorphisms in fatty acid synthesizing enzymes can have a significant impact on fatty acid levels in the body. (More information)
- DHA is important for visual and neurological development. Feeding infants formula enriched with DHA and AA appears to have no significant effect on cognitive development and a very modest effect on visual acuity. (More information)
- Replacing saturated fat in the diet with a mixture of PUFA (both omega-6 and omega-3) is associated with beneficial effects on the cardiovascular system. (More information)
- A large body of scientific research suggests that higher dietary omega-3 fatty acid intakes are associated with reductions in cardiovascular diseaserisk. Thus, the American Heart Association recommends that all adults eat fish, particularly oily fish, at least twice weekly. (More information)
- The results of prospective cohort studies and randomized controlled trials indicate that fish and fish oil consumption decreases the risk of coronary heart disease (CHD) mortality, including fatal myocardial infarction (heart attack) and sudden cardiac death. (More information)
- Low DHA status may be a risk factor for Alzheimer's disease and other types of dementia, but it is not yet known whether DHA supplementation can help prevent or treat such cognitive disorders. (More information)
- Increasing EPA and DHA intake may be beneficial in individuals with type 2 diabetes, especially those with elevated serumtriglycerides. (More information)
- Randomized controlled trials have found that fish oil supplementation reduces the requirement for anti-inflammatory medication in patients with rheumatoid arthritis. (More information)
- Although some data suggest that omega-3 fatty acid supplementation may be a beneficial adjunct in the therapy of depression, bipolar disorder, and schizophrenia, great heterogeneity between trials prevents conclusive determination of therapeutic efficacy. (More information)
Omega-6 and omega-3 fatty acids are polyunsaturated fatty acids (PUFA), meaning they contain more than one cis double bond (1). In all omega-6 fatty acids, the first double bond is located between the sixth and seventh carbon atom from the methyl end of the fatty acid (n-6). Similarly, in all omega-3 fatty acids, the first double bond is located between the third and fourth carbon atom counting from the methyl end of the fatty acid (n-3). Scientific abbreviations for fatty acids tell the reader something about their chemical structure. One scientific abbreviation for α-linolenic acid (ALA) is 18:3n-3. The first part (18:3) tells the reader that ALA is an 18-carbon fatty acid with three double bonds, while the second part (n-3) tells the reader that the first double bond is in the n-3 position, which defines it as an omega-3 fatty acid (Figures 1a and 1b). Double bonds introduce kinks in the hydrocarbon chain that influence the structure and physical properties of the fatty acid molecule (Figure 1c).
Although humans and other mammals can synthesizesaturated fatty acids and some monounsaturated fatty acids from carbon groups in carbohydrates and proteins, they lack the enzymes necessary to insert a cis double bond at the n-6 or the n-3 position of a fatty acid (1). Consequently, omega-6 and omega-3 fatty acids are essential nutrients. The parent fatty acid of the omega-6 series is linoleic acid (LA; 18:2n-6), and the parent fatty acid of the omega-3 series is ALA (Table 1 and Figure 2). Humans can synthesize long-chain (20 carbons or more) omega-6 fatty acids, such as dihomo-γ-linolenic acid (DGLA; 20:3n-6) and arachidonic acid (AA; 20:4n-6), from LA and long-chain omega-3 fatty acids, such as eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3), from ALA (see Metabolism and Bioavailability).
|Omega-6 Fatty Acids||Omega-3 Fatty Acids|
|Linoleic acid||LA||18:2n-6||α-Linolenic acid||ALA||18:3n-3|
|γ-Linolenic acid||GLA||18:3n-6||Stearadonic acid||SDA||18:4n-3|
|Dihomo-γ-linolenic acid||DGLA||20:3n-6||Eicosatetraienoic acid||ETA||20:4n-3|
|Arachidonic acid||AA||20:4n-6||Eicosapentaenoic acid||EPA||20:5n-3|
|Adrenic acid||22:4n-6||Docosapentaenoic acid||DPA (n-3)||22:5n-3|
|Tetracosatetraenoic acid||24:4n-6||Tetracosapentaenoic acid||24:5n-3|
|Tetracosapentaienoic acid||24:5n-6||Tetracosahexaenoic acid||24:6n-3|
|Docosapentaenoic acid||DPA (n-6)||22:5n-6||Docosahexaenoic acid||DHA||22:6n-3|
Metabolism and Bioavailability
Prior to absorption in the small intestine, fatty acids must be hydrolyzed from dietary fats (triglycerides and phospholipids) by pancreaticenzymes(2). Bile salts must also be present in the small intestine to allow for the incorporation of fatty acids and other fat digestion products into mixed micelles. Fat absorption from mixed micelles occurs throughout the small intestine and is 85-95% efficient under normal conditions.
Blood concentrations of fatty acids reflect both dietary intake and biological processes (3). Humans can synthesize longer omega-6 and omega-3 fatty acids from the essential fatty acids LA and ALA, respectively, through a series of desaturation (addition of a double bond) and elongation (addition of two carbon atoms) reactions (Figure 3) (4, 5). LA and ALA compete for the same elongase and desaturase enzymes in the synthesis of longer polyunsaturated fatty acids, such as AA and EPA.
The capacity to generate DHA from ALA is higher in women than men. Studies of ALA metabolism in healthy young men indicate that approximately 8% of dietary ALA is converted to EPA and 0-4% is converted to DHA (6). In healthy young women, approximately 21% of dietary ALA is converted to EPA and 9% is converted to DHA (7). The better conversion efficiency of young women compared to men appears to be related to the effects of estrogen(8, 9). Although ALA is considered the essential omega-3 fatty acid because it cannot be synthesized by humans, evidence that human conversion of EPA and, particularly, DHA is relatively inefficient suggests that EPA and DHA may be considered conditionally essential nutrients.
In addition to gender differences, genetic variability in enzymes involved in fatty acid metabolism influences one's ability to generate long-chain polyunsaturated fatty acids (LC-PUFA). Two key enzymes in fatty acid metabolism are delta-6 desaturase (FADS2) and delta-5 desaturase (FADS1) (see Figure 3 above) (10). Two common haplotypes (a cluster of polymorphisms) in the FADS genes differ dramatically in their ability to generate LC-PUFA: haplotype D is associated with increased FADS activity (both FADS1 and FADS2) and is more efficient in converting fatty acid precursors (LA and ALA) to LC-PUFA (EPA, GLA, DHA, and AA) (11). These FADS polymorphisms are relatively common in the population and may explain up to 30% of the variability in blood levels of omega-3 and omega-6 fatty acids among individuals (3).
Finally, DHA is retroconverted to EPA at a low basal rate and following supplementation (see Figure 3 above) (12). After supplementing omnivores (n=8) and vegetarians (n=12) for six weeks with an EPA-free preparation of DHA (1.62 g/day), both EPA and DHA levels increased in serum and platelet phospholipids (13). Based on the measured changes, the estimated percent retroconversion of DHA to EPA was 7.4-11.4% (based on serum phospholipid data) and 12.3-13.8% (based on the platelet phospholipid data), with no significant difference between omnivores and vegetarians. Due to this nontrivial retroconversion efficiency, DHA supplementation represents an alternative to fish oil to increase blood and tissue levels of EPA, DPA, and DHA (5) (see Supplements).
Membrane structure and function
Omega-6 and omega-3 PUFA are important structural components of cell membranes. When incorporated into phospholipids, they affect cell membrane properties, such as fluidity, flexibility, permeability, and the activity of membrane-bound enzymes(14). In addition to endogenousmetabolism, dietary consumption of fatty acids can modify the composition and molecular structure of cellular membranes. Thus, increasing omega-3 fatty acid intake increases the omega-3 content of red blood cells, immune cells (15), atherosclerotic plaques (16), cardiac tissue (17), and other cell types throughout the body.
DHA is selectively incorporated into retinal cell membranes and postsynaptic neuronal cell membranes, suggesting it plays important roles in vision and nervous system function.
DHA is found at very high concentrations in the cell membranes of the retina; the retina conserves and recycles DHA even when omega-3 fatty acid intake is low (18). Animal studies indicate that DHA is required for the normal development and function of the retina. Moreover, these studies suggest that there is a critical period during retinal development when inadequate DHA will result in permanent abnormalities in retinal function. Research indicates that DHA plays an important role in the regeneration of the visual pigment rhodopsin, which plays a critical role in the visual transduction system that converts light hitting the retina to visual images in the brain (19).
The phospholipids of the brain’s gray matter contain high proportions of DHA and AA, suggesting they are important to central nervous system function (20). Brain DHA content may be particularly important, since animal studies have shown that depletion of DHA in the brain can result in learning deficits. It is not clear how DHA affects brain function, but changes in DHA content of neuronal cell membranes could alter the function of ion channels or membrane-associated receptors, as well as the availability of neurotransmitters(21).
Synthesis of lipid mediators
Eicosanoids are potent chemical messengers that play critical roles in immune and inflammatory responses. The term 'eicosanoid' encompasses numerous bioactive lipid mediators that are derived from 20-carbon LC-PUFA. Following stimulation by hormones, cytokines, and other stimuli, DGLA, AA, and EPA are released from cell membranes and become substrates for eicosanoid production (Figure 4). Eicosanoid synthesis relies primarily on three families of enzymes: cyclooxygenases (COX), lipoxygenases (LOX), and cytochrome p450 mono-oxygenases (P450s) (22). From 20-carbon lipid precursors, COX enzymes produce prostaglandins, prostacyclins, and thromboxanes (collectively known as prostanoids); LOX produces leukotrienes and hydroxy fatty acids; and P450s produce hydroxyeicosatetraenoic acids ("HETEs") and epoxides (Figure 5).
Physiological responses to AA-derived eicosanoids differ from responses to EPA-derived eicosanoids. In general, eicosanoids derived from EPA are less potent inducers of inflammation, blood vessel constriction, and coagulation than eicosanoids derived from AA (23). Nonetheless, it is an oversimplification to label all AA-derived eicosanoids as pro-inflammatory. AA-derived prostaglandins induce inflammation but also inhibit pro-inflammatory leukotrienes and cytokines and induce anti-inflammatory lipoxins, thereby modulating the intensity and duration of the inflammatory response via negative feedback (see Figure 5 above) (16).
A separate class of PUFA-derived bioactive lipids, specialized pro-resolving mediators (SPMs), has been recently identified (reviewed in 24). These molecules function as local mediators of the resolution phase of inflammation, actively turning off the inflammatory response. SPMs are derived from both omega-6 and omega-3 PUFA (see Figure 5 above) (25). The S-series of SPMs results from the LOX-mediated oxygenation of EPA and DHA, giving rise to S-resolvins, S-protectins, and S-maresins. A second class of SPMs, the R-series, is generated from the aspirin-dependent acetylation of COX-2 and subsequent generation of aspirin-triggered SPMs from AA, EPA, and DHA. It appears that these mediators may explain many of the anti-inflammatory actions of omega-3 fatty acids that have been described (15, 26).
Isoprostanes are prostaglandin-like compounds that are formed by non-enzymatic, free radical-induced oxidation of any PUFA with three or more double bonds (see Figure 5 above) (22). Because they are produced upon exposure to free radicals, isoprostanes are often used as markers for oxidative stress. In contrast to prostanoids, isoprostanes are synthesized from esterified PUFA precursors and remain bound to the membrane phospholipid until cleaved by PLA2 and released into circulation. In addition to being used as markers of oxidative stress, isoprostanes may also function as inflammatory mediators, exerting both pro- and anti-inflammatory effects (22).
Regulation of gene expression
The results of cell culture and animal studies indicate that omega-6 and omega-3 fatty acids can modulate the expression of a number of genes, including those involved with fatty acid metabolism and inflammation(27, 28). Omega-6 and omega-3 fatty acids regulate gene expression by interacting with specific transcription factors, such as peroxisome proliferator-activated receptors (PPARs) (29). In many cases, PUFA act like hydrophobichormones (e.g., steroid hormones) to control gene expression and bind directly to receptors like PPARs. These ligand-activated receptors then bind to the promoters of genes and function to increase/decrease transcription.
In other cases, PUFA regulate the abundance of transcription factors inside the cell's nucleus(30). Two examples include NFκB and SREBP-1. NFκB is a transcription factor involved in regulating the expression of multiple genes involved in inflammation. Omega-3 PUFA suppress NFκB nuclear content, thus inhibiting the production of inflammatory eicosanoids and cytokines. SREBP-1 is a major transcription factor controlling fatty acid synthesis, both de novolipogenesis and PUFA synthesis. Dietary PUFA can suppress SREBP-1, which decreases the expression of enzymes involved in fatty acid synthesis and PUFA synthesis. In this way, dietary PUFA function as feedback inhibitors of all fatty acid synthesis.
Essential fatty acid deficiency
Clinical signs of essential fatty acid deficiency include a dry scaly rash, decreased growth in infants and children, increased susceptibility to infection, and poor wound healing (31). Omega-3, omega-6, and omega-9 fatty acids compete for the same desaturase enzymes. The desaturase enzymes show preference for the different series of fatty acids in the following order: omega-3 > omega-6 > omega-9. Consequently, synthesis of the omega-9 fatty acid eicosatrienoic acid (20:3n-9, mead acid, or 5,8,11-eicosatrienoic acid) increases only when dietary intakes of omega-3 and omega-6 fatty acids are very low; therefore, mead acid is one marker of essential fatty acid deficiency (32). A plasma eicosatrienoic acid:arachidonic acid (triene:tetraene) ratio greater than 0.2 is generally considered indicative of essential fatty acid deficiency (31, 33). In patients who were given total parenteral nutrition containing fat-free, glucose-amino acid mixtures, biochemical signs of essential fatty acid deficiency developed in as little as 7 to 10 days (34). In these cases, the continuous glucose infusion resulted in high circulating insulin levels, which inhibited the release of essential fatty acids stored in adipose tissue. When glucose-free amino acid solutions were used, parenteral nutrition up to 14 days did not result in biochemical signs of essential fatty acid deficiency. Essential fatty acid deficiency has also been found to occur in patients with chronic fat malabsorption(35) and in patients with cystic fibrosis(36). Recently, it has been proposed that essential fatty acid deficiency may play a role in the pathology of protein-energy malnutrition (32).
Omega-3 fatty acid deficiency
At least one case of isolated omega-3 fatty acid deficiency has been reported. A young girl who received intravenouslipid emulsions with very little ALA developed visual problems and sensory neuropathy; these conditions were resolved when she was administered an emulsion containing more ALA (37). Isolated omega-3 fatty acid deficiency does not result in increased plasma triene:tetraene ratios, and skin atrophy and dermatitis are absent (1). Plasma DHA concentrations decrease when omega-3 fatty acid intake is insufficient, but no accepted plasma omega-3 fatty acid or eicosanoid concentrations indicative of impaired health status have been defined (1). Studies in rodents have revealed significant impairment of n-3 PUFA deficiency on learning and memory (38, 39) prompting research in humans to assess the impact of omega-3 PUFA on cognitive development and cognitive decline (see Visual and neurological development and Alzheimer's disease).
The omega-3 index is defined as the amount of EPA plus DHA in red blood cell (RBC) membranes expressed as the percent of total RBC membrane fatty acids(40). The EPA + DHA content of RBCs correlates with that of cardiac muscle cells (41, 42), and several observational studies indicate that a lower omega-3 index is associated with an increased risk of coronary heart disease (CHD) mortality (43). It is therefore proposed that the omega-3 index be used as a biomarker for cardiovascular disease risk, with proposed zones being high risk, <4%; intermediate risk, 4-8%; and low risk, >8% (44).
Supplementation with EPA + DHA from fish oil capsules for approximately five months dose-dependently increased the omega-3 index in 115 healthy, young adults (20-45 years of age), validating the use of the omega-3 index as a biomarker of EPA + DHA intake (45). Before the omega-3 index can be used in routine clinical evaluation, however, clinical reference values in the population must be established (46). Additionally, fatty acid metabolism may be altered in certain disease states, potentially making the omega-3 index less relevant for some cardiovascular conditions (5).
Visual and neurological development
The last trimester of pregnancy and first six months of postnatal life are critical periods for the accumulation of DHA in the brain and retina(47). Human milk contains a mixture of saturated fatty acids (~46%), monounsaturated fatty acids (~41%), omega-6 PUFA (~12%), and omega-3 PUFA (~1.3%) (48). Although human milk contains DHA in addition to ALA and EPA, ALA was the only omega-3 fatty acid present in conventional infant formulas until the year 2001.
Although infants can synthesize DHA from ALA, they generally cannot synthesize enough to prevent declines in plasma and cellular DHA concentrations without additional dietary intake. Therefore, it was proposed that infant formulas be supplemented with enough DHA to bring plasma and cellular DHA levels of formula-fed infants up to those of breast-fed infants (49). Although formulas enriched with DHA raise plasma and red blood cell DHA concentrations in preterm and term infants, the results of randomized controlled trials (RCTs) examining measures of visual acuity and neurological development in infants fed formulas with or without added DHA have been mixed (50, 51). A 2012 meta-analysis of RCTs (12 trials, 1,902 infants) testing LC-PUFA supplemented versus unsupplemented formula, started within one month of birth, found no effect of LC-PUFA supplementation on infant cognition assessed at approximately one year of age (52). A lack of effect was observed regardless of the dose of LC-PUFA or the prematurity status of the infant. With respect to visual acuity, a 2013 meta-analysis of RCTs (19 trials, 1,949 infants) found a beneficial effect of LC-PUFA-supplemented formula, started within one month of birth, on infant visual acuity up to 12 months of age (53). Notably, two different types of visual acuity assessment were evaluated in the meta-analysis. Visual acuity assessed by using the visually evoked potential (VEP) (10 trials, 852 infants) showed a significant positive effect of LC-PUFA supplemented formula at 2, 4, and 12 months of age. When assessed by the behavioral method (BM) (12 trials, 1,095 infants), a significant benefit of LC-PUFA-supplemented formula on visual acuity was found only at the age of two months. No moderating effects of dose or prematurity status were observed.
Maternal supplementation (placental transfer and breast milk)
The effect of maternal omega-3 LC-PUFAsupplementation on early childhood cognitive and visual development was evaluated in a 2013 systematic review and meta-analysis(54). Included in this assessment were 11 RCTs (a total of 5,272 participants) that supplemented maternal diet with omega-3 LC-PUFA during pregnancy or during pregnancy and lactation. Visual outcomes (eight trials) could not be evaluated in the meta-analysis due to variability in assessments; overall, four of six trials had null findings and the remaining two trials had very high rates of attrition. Cognitive outcomes (nine trials) included the Developmental Standard Score (DSS; in infants, toddlers, and preschoolers) or Intelligence Quotient (IQ; in children) and other aspects of neurodevelopment, such as language, behavior, and motor function. No differences were found between DHA and control groups for cognition measured with standardized psychometric scales in infants (<12 months), toddlers (12-24 months), and school aged children (5-12 years); preschool children (2-5 years) in the DHA treatment group had a 3.92 point increase in DSS compared to controls. The authors note that many of the trials of LC-PUFA supplementation in pregnancy had methodological weaknesses (e.g., high rates of attrition, small sample sizes, high risk of bias, multiple comparisons) limiting the confidence and interpretation of the pooled results.
Although epidemiological investigations have demonstrated that higher intakes of omega-3 LC-PUFA from fish and seafood during pregnancy are associated with improved developmental outcomes in offspring (54), trial evidence does not conclusively support or refute this relationship. At present, the potential benefits associated with obtaining long-chain omega-3 fatty acids through moderate consumption of fish (e.g., 1-2 servings weekly) during pregnancy and lactation outweigh any risks of contaminant exposure, though fish with high levels of methylmercury should be avoided (55). For information about contaminants in fish and guidelines for fish consumption by women of childbearing age, see Contaminants in fish.
Gestation and pregnancy
The results of randomized controlled trials (RCTs) during pregnancy suggest that omega-3 fatty acid supplementation does not decrease the incidence of gestational diabetes, pregnancy-induced hypertension, or preeclampsia(56-58) but may result in modest increases in length of gestation, especially in women with low omega-3 fatty acid consumption. A 2006 meta-analysis of six randomized controlled trials in women with low-risk pregnancies found that omega-3 PUFA supplementation during pregnancy resulted in an increased length of pregnancy by 1.6 days (59). A 2007 meta-analysis of randomized controlled trials in women with high-risk pregnancies found that supplementation with long-chain PUFA did not affect pregnancy duration or the incidence of premature births but decreased the incidence of early premature births (<34 weeks of gestation; 2 trials, N=291; Relative Risk [RR]: 0.39 (95% CI: 0.18-0.84) (60).
Because maternal dietary intake of LC-PUFA determines the DHA status of the newborn, several expert panels in the US recommend that pregnant and lactating women consume at least 200 mg DHA per day, close to the amount recommended for adults in general (250 mg/day) (47, 61). The European Food and Safety Authority (EFSA) recommends that pregnant and lactating women consume an additional 100-200 mg of preformed DHA on top of the 250 mg/day EPA plus DHA recommended for healthy adults (62).
Omega-6 fatty acids: linoleic acid
LA is the most abundant dietary PUFA and accounts for approximately 90% of dietary omega-6 PUFA intake (63). Taking into consideration the results from RCTs and observationalcohort studies, a 2009 American Heart Association scientific advisory concluded that obtaining at least 5-10% of total caloric intake from omega-6 PUFA is associated with a reduced risk of coronary heart disease (CHD) relative to lower intakes (64, 65). A pooled analysis of 11 cohort studies, encompassing 344,696 individuals followed for 4 to 10 years, found that replacing 5% of energy from saturated fatty acids (SFAs) with PUFA was associated with a 13% lower risk of coronary events (95% CI: 0.77, 0.97) and a 26% lower risk of coronary deaths (95% CI: 0.61, 0.89) (66). A 2012 meta-analysis of seven RCTs corroborated this beneficial effect, with an estimated 10% reduction in CHD risk (RR: 0.90, 95% CI: 0.83-0.97) for each 5% energy increase in PUFA consumption (67).
In controlled feeding trials, replacing dietary SFA with PUFA consistently lowers serum total and LDLcholesterol concentrations (68, 69). In fact, LA has been shown to be the most potent fatty acid for lowering serum total and LDL cholesterol when substituted for dietary SFA (70). The mechanisms by which linoleic acid lowers blood cholesterol include (1) the upregulation of LDL receptor and redistribution of LDL-C from plasma to tissue, (2) increased bile acid production and cholesterol catabolism, and (3) decreased conversion of VLDL to LDL (71).
Although dietary LA lowers blood cholesterol levels, supplementation with concentrated sources of LA may have adverse cardiovascular effects in individuals with preexisting CHD (see Disease Treatment).
Omega-3 fatty acids: α-linolenic acid
Several prospective cohort studies have examined the relationship between dietary ALA intake and cardiovascular disease (CVD). A 2012 meta-analysis of observational studies evaluated the risk of incident CVD related to dietary consumption or biomarkers of ALA (72). The analysis included 27 studies, 251,049 individuals and 15,327 CVD events (fatal coronary heart disease [CHD], nonfatal CHD, total CHD, and stroke). Overall, the pooled analysis found a moderately lower risk of CVD with higher ALA exposure (Relative Risk [RR]: 0.86; 95% CI: 0.77, 0.97).
Unlike LA, the cardioprotective effects of higher ALA intakes do not appear to be related to changes in serumlipid profiles. A meta-analysis of 14 randomized controlled trials concluded that ALA supplementation had no effect on total cholesterol, LDL cholesterol, or triglyceride levels (73). However, several controlled clinical trials have found that increasing ALA intake decreased serum concentrations of C-reactive protein (CRP), a marker of inflammation that is strongly associated with the risk of cardiovascular events, such as MI and stroke (74-76).
Long-chain omega-3 fatty acids: eicosapentaenoic acid and docosahexaenoic acid
Evidence is accumulating that increasing intakes of long-chain omega-3 fatty acids (EPA and DHA) can decrease the risk of cardiovascular disease by (1) preventing arrhythmias that can lead to sudden cardiac death, (2) decreasing the risk of thrombosis (a clot) that can lead to myocardial infarction (MI) or stroke, (3) decreasing serumtriglyceride levels, (4) slowing the growth of atherosclerotic plaque, (5) improving vascular endothelial function, (6) lowering blood pressure slightly, and (7) decreasing inflammation(77).
In spite of these possible biological effects, clinical trials have not shown a significant effect of long-chain omega-3 supplementation on major cardiovascular events. A 2006 systematic review and meta-analysis of randomized controlled trials and prospective cohort studies concluded that long-chain omega-3 fatty acids do not significantly reduce the risk of total mortality or cardiovascular events (78). Likewise, a 2012 meta-analysis of secondary prevention trials (20 RCTs, including 68,680 patients) found no significant effect of omega-3 supplements (~1.5 g/day of EPA + DHA for a median of 2 years) on all-cause mortality, cardiac death, sudden death, myocardial infarction, or stroke (79). The same lack of effect was observed in a 2012 systematic review and meta-analysis of RCTs investigating the impact of omega-3 supplementation on inflammatorybiomarkers in both healthy and ill individuals (80).
Although supplementation trials have not demonstrated a clear clinical benefit of omega-3 supplements, a recent multicenter, prospective, observational cohort study found a strong relationship between plasmaphospholipid omega-3 PUFA levels (a biomarker of omega-3 status) and cardiovascular mortality (81). The Cardiovascular Health Study (CHS) related circulating levels of total and individual LC-PUFA (EPA, DPA, and DHA) to risk of total and CVD-specific mortality in 2,692 older (≥65 years) US adults. Higher levels of individual and combined total omega-3 PUFA in plasma phospholipids were associated with lower total mortality (HR for total omega-3: 0.73, 95% CI: 0.61-0.86). Looking more closely at cause-specific mortality, the observed reduction in risk was attributed mainly to fewer arrhythmic cardiac deaths (HR: 0.52, 95% CI: 0.31-0.86) and specifically with higher circulating DHA content (45% lower risk). Only EPA was associated with nonfatal MI (28% lower risk), while DPA was most strongly associated with stroke death (47% lower risk).
Coronary heart disease: A 2012 meta-analysis of 17 cohort studies with 315,812 participants and an average follow-up of 15.9 years calculated the pooled effect of fish consumption on coronary heart disease (CHD) mortality (82). Low (1 serving/week) or moderate fish consumption (2-4 servings/week) had a significant beneficial effect on the prevention of CHD mortality. Specifically, compared with the lowest fish consumption (<1 serving/month or 1-3 servings/month), consumption of 1 serving of fish per week and 2-4 servings/week was associated with a 16% (RR: 0.84, 95% CI: 0.75, 0.95) and 21% (RR: 0.79, 95% CI: 0.67,0.92) lower risk of fatal CHD, respectively.
Overall, among the various CVD outcomes (Figure 6), findings from prospective cohort studies and RCTs consistently indicate that consumption of fish or fish oil significantly reduces CHD mortality, including fatal myocardial infarction and sudden cardiac death (77, 83, 84). There is little evidence that these effects differ by sex, age, or race/ethnicity (77).
Sudden cardiac death: Sudden cardiac death (SCD) is the result of a fatal ventriculararrhythmia, which usually occurs in people with CHD. Studies in cell culture indicate that long-chain omega-3 fatty acids decrease the excitability of cardiac muscle cells (myocytes) by modulating ion channel conductance (85). The results of epidemiological studies suggest that regular fish consumption is inversely associated with the risk of SCD. A 2011 systematic review and meta-analysis of eight prospective cohort studies evaluated the impact of consuming <250 mg versus ≥250 mg omega-3 PUFA per day on various CHD outcomes (86). Consumption of ≥250 mg omega-3 PUFA per day was associated with a significant, 35% reduction in the risk of SCD (RR: 0.65; 95% CI: 0.54, 0.79).
A meta-analysis of nine randomized controlled trials found no significant effect of omega-3 supplements on SCD or ventricular arrhythmias in patients with previous MI compared to those taking placebo(87). Notably, although the pooled analysis reported no significant effect, the included trials reported either a protective effect (six trials) or null effect (three trials), with no harmful outcomes reported.
Stroke:Ischemic strokes are the result of insufficient blood flow to an area of the brain, which may occur when an artery supplying the brain becomes occluded by a clot. Hemorrhagic strokes occur when a blood vessel ruptures and bleeds into the brain. In the United States, 87% of strokes are ischemic strokes (88). A 2012 meta-analysis of 16 prospective studies, encompassing 402,127 individuals for a mean of 12.8 years, found that increased fish intake was associated with a decreased risk of ischemic stroke, but not hemorrhagic stroke (89). According to this analysis, consuming fish even once per week may significantly reduce the risk of ischemic stroke. In a separate dose-response meta-analysis of these prospective studies, a 3-servings/week increase in fish consumption was associated with a 6% decreased risk of total stroke (95% CI: 0.89-0.99) (90). Again, the association remained significant only for ischemic stroke (RR: 0.90, 95% CI: 0.84-0.97).
Although the protective effect of fish intake could be attributed to many things (e.g., the displacement of red meat, a marker of an overall healthier lifestyle and dietary pattern, nutrient interactions (91, 92)), its high content of omega-3 PUFA may be a major contributing factor. A meta-analysis of eight prospective studies that assessed the association between omega-3 PUFA intake on stroke risk found evidence of a nonlinear relationship between LC-PUFA intake and stroke risk, with only moderate intakes of 200-400 mg/day omega-3 PUFA associated with significantly reduced risk of total stroke (93). Additionally, when analyzed by stroke type, the risk for ischemic stoke was lower in the highest versus lowest category of long-chain omega-3 PUFA intake (RR: 0.82, 95% CI: 0.71-0.94).
Another 2012 systematic review and meta-analysis assessed both prospective cohort studies and RCTs that investigated fish consumption or omega-3 supplementation on cerebrovascular disease (any fatal or non-fatal ischemic stroke, hemorrhagic stroke, cerebrovascular accident, or transient ischemic attack) (91). From 26 prospective cohort studies, the pooled relative risk (RR) for cerebrovascular disease for 2-4 versus ≤1 serving of fish per week was 0.94 (95% CI: 0.90-0.98); for >5 servings versus ≤1 serving per week, the RR was 0.88 (95% CI: 0.81-0.96). No significant association was found between long-chain omega-3 biomarkers and risk of cerebrovascular disease. From the 12 RCTs analyzed, 10 of which recruited subjects with preexisting cardiovascular disease at baseline, no significant effect of omega-3 supplementation (mean dose of 1.8 g/day for a mean duration of 3 years) on cerebrovascular disease outcomes was observed. The same lack of effect of omega-3 supplementation on total stroke risk was observed in a second meta-analysis of RCTs (nine trials) (79).
Serum triglycerides: A meta-analysis of 17 prospective studies found hypertriglyceridemia (serumtriglycerides >200 mg/dL) to be an independent risk factor for cardiovascular disease(94). Numerous controlled clinical trials have demonstrated that increasing intakes of EPA and DHA significantly lower serum triglyceride concentrations (95). The triglyceride-lowering effects of EPA and DHA increase with dose (96), but clinically meaningful reductions in serum triglyceride concentrations have been demonstrated at doses of 2 g/day of EPA + DHA (97). In its recommendations regarding omega-3 fatty acids and cardiovascular disease (see Intake Recommendations), the American Heart Association indicates that an EPA + DHA supplement may be useful in patients with hypertriglyceridemia (23).
A 2011 meta-analysis of RCTs compared the effect of EPA alone (10 trials), DHA alone (17 trials), or EPA versus DHA (6 trials) on serum lipids (98). Although both EPA and DHA reduce triglyceride levels, they have different effects on LDL and HDL levels. DHA raises LDL and HDL, whereas EPA has no significant effect. Importantly, DHA may increase LDL via increased conversion of VLDL to LDL and by producing larger, more buoyant LDL particles (3).
Summary: omega-3 and omega-6 PUFA and cardiovascular disease prevention
The results of observational studies and randomized controlled trials suggest that replacing dietary SFA with omega-6 and omega-3 PUFA (from both plant and marine sources) lowers LDL cholesterol and decreases cardiovascular diseaserisk. Additionally, the results of epidemiological studies provide consistent evidence that increasing dietary omega-3 fatty intake is associated with significant reductions in cardiovascular disease risk through mechanisms other than lowering LDL cholesterol. In particular, increasing fish consumption to at least two servings of oily fish per week has been associated with significant reductions in fatal myocardial infarction and sudden cardiac death (77). This amount would provide about 400-500 mg/day of EPA + DHA (23).
Alzheimer's disease is the most common cause of dementia in older adults. Alzheimer's disease is characterized by the formation of amyloid plaque in the brain and nerve cell degeneration. Disease symptoms, including memory loss and confusion, worsen over time (99). Some epidemiological studies have associated high intake of fish with lower risks of impaired cognitive function (100), dementia (101), and Alzheimer's disease (101, 102). Proposed mechanisms for a protective effect of long-chain omega-3 fatty acids in the brain and vascular system include (1) the mitigation of inflammation, (2) improved cerebral blood flow, and (3) reduced amyloid aggregation (103).
A 2009 systematic review reported on the association between eating fish (as a source of long-chain omega-3 fatty acids) or taking an omega-3 supplement and the risk of cognitive decline or Alzheimer's disease (103). Out of 11 observational studies, three reported a significant benefit of omega-3 fatty acids on cognitive decline; four of eight observational studies reported positive findings on incident Alzheimer's disease or dementia. The four small clinical trials reviewed showed no evidence for prevention or treatment of any form of dementia (103).
DHA, the major omega-3 fatty acid in the brain, appears to be protective against Alzheimer's disease (104). Observational studies have found that lower DHA status is associated with increased risk of Alzheimer's disease (105-107), as well as other types of dementia (106). The relationship between DHA status and cognitive decline may be dependent on apolipoprotein E (APOE) genotype. Of three common APOE alleles (epsilon 2 [ε2], ε3, and ε4), the presence of the APOE ε4 (E4) allele is associated with increased risk and earlier onset of Alzheimer's disease (108). A protective effect of consumption of fatty fish on the risk for dementia and AD may not apply to carriers of the E4 allele (109, 110). EPA + DHA supplementation did not increase plasma levels of these omega-3 PUFA to the same extent in E4 carriers compared to non-carriers (111), and [13C]DHA tracer studies indicate that DHA metabolism differs in E4 carriers, with greater oxidation and lower plasma levels in E4 positive versus negative individuals (112).
Overall, the data favor a role for diets rich in long-chain omega-3 fatty acids in slowing cognitive decline but not for supplementation in the prevention or treatment of any type of dementia. The efficacy of omega-3 supplementation may depend on the underlying pathology of AD (i.e., the involvement of a vascular issue) (103) or the presence of the APOE4 allele (110, 111). Additionally, consistency in outcome measures and diagnostic criteria, and longer duration trials may be necessary to see a consistent effect.
Coronary heart disease
Dietary intervention trials
Omega-6 fatty acids (linoleic acid): In a reanalysis of the Sydney Heart Health Study (SHHS), a single-blind, RCT in 458 men (ages 30-59 years) with a recent coronary event, the replacement of dietary saturated fat with omega-6 linoleic acid led to higher rates of death from all-causes, cardiovascular disease, and coronary heart disease (CHD) compared to controls (113). Furthermore, a meta-analysis that included the SHHS and two other secondary prevention trials revealed an increased risk of mortality when saturated fat is replaced with concentrated sources of linoleic acid. There are some important limitations and considerations with the SHHS to keep in mind: (1) LA intake went from 6% to 15% of total energy for the study participants; in US adults, the average intake of LA is approximately 7% of total energy (114); (2) there may have been displacement of monounsaturated fatty acids and other PUFA in addition to saturated fats in the experimental intervention; and (3) the experimental formulation of safflower oil margarine may have provided trans fat. Regardless of these issues, substituting dietary saturated fat with mixed PUFA (both omega-6 and omega-3) rather than linoleic acid alone reduces CVD risk (113) and is recommended for both the primary and secondary prevention of CVD (65, 67).
Omega-3 fatty acids: In the Diet and Reinfarction Trial (DART), total mortality and fatal MI decreased by 29% in male MI survivors advised to increase their weekly intake of oily fish to 200-400 g (7-14 oz)—an amount estimated to provide an additional 500-800 mg/day of long-chain omega-3 fatty acids (EPA + DHA) (115). The Diet and Reinfarction Trial 2 (DART-2) administered similar dietary advice but to a different cohort of high-risk individuals: those with stable angina (116). In this case, advice to eat oily fish or fish oil did not affect all-cause mortality but was associated with an increased risk of sudden cardiac death. This increased risk was confined to the use of fish oil capsules rather than dietary fish intake. Though the results of the DART trials seem to contradict each other, there are important differences that offer explanations, namely the timing of the intervention (shortly after first MI in the first trial) and the stage of CHD (early versus stable) in the study population. These trials suggest that fish oil may reduce mortality during recovery from MI but perhaps not during later stages of the disease.
The Alpha Omega Trial tested if low doses of EPA + DHA (400 mg per day), ALA (2 g per day), or both in margarines reduced the risk of cardiovascular (CV) events among 4,837 patients (78% male; mean age, 69 years) who had a MI in the previous 10 years (117). After 40 months, none of the omega-3 PUFA treatments significantly affected the rate of major CV events compared to placebo. Notably, medication use in the study population was high: antithrombotic agents (97.5%), antihypertensive drugs (89.7%), and statins (85%).
In the largest randomized controlled trial of supplemental omega-3 fatty acids to date, the GISSI-Prevenzione Trial, CHD patients who received supplements providing 850 mg/day of EPA + DHA for 3.5 years had a risk of sudden death that was 45% lower than those who did not take supplements; supplement users also experienced a 20% lower risk of death from all causes compared to non-supplement users (118). Interestingly, it took only three months of supplementation to demonstrate a significant decrease in total mortality and four months to demonstrate a significant decrease in sudden death (119).
The results of a meta-analysis that pooled the findings of 29 randomized controlled trials of dietary or supplementary omega-3 fatty acids indicated that omega-3 fatty acids were not associated with a statistically significant reduction in all-cause mortality or risk of restenosis in high-risk cardiovascular patients (120). Heterogeneity in trial size and follow-up time limited the analysis, and the authors note that the probability of benefit from omega-3 fatty acids still remains high for both endpoints.
The results of randomized controlled trials in individuals with documented CHD suggest a beneficial effect of dietary and supplemental omega-3 fatty acids. Based on the results of these trials, the American Heart Association recommends that individuals with documented CHD consume approximately 1 g/day of EPA + DHA, preferably by consumption of oily fish (see Intake Recommendations) (121).
Cardiovascular disease are the leading causes of death in individuals with diabetes mellitus (DM). The dyslipidemia typically associated with diabetes is characterized by a combination of hypertriglyceridemia (serum triglycerides >200 mg/dL), low HDL-C, and abnormal LDL composition (122).
A 2009 meta-analysis of 23 randomized controlled trials (RCTs), including 1,075 individuals with type 2 diabetes, found that omega-3 fatty acid supplementation (mean dose, 3.5 g/day) lowered serum triglyceride levels by 0.45 mmol/L, lowered VLDL-C by 0.07 mmol/L, but raised LDL-C by 0.11 mmol/L (123). No significant changes in total cholesterol, HDL-C, HbA1c, fasting glucose, fasting insulin, or body weight were observed.
Since 2009, the results of two additional trials of omega-3 supplementation in diabetic patients have been published. The Alpha Omega Trial evaluated the effect of low-dose supplementation with omega-3 fatty acids on ventriculararrhythmias and fatal CHD in stable, post-MI patients (117). While the main analysis found no effect of supplementation, a secondary analysis of 1,014 diabetic participants found that low-dose supplementation of combined omega-3 fatty acids (~400 mg EPA + DHA and 2 g ALA per day in an experimental margarine spread for 40 months) resulted in fewer ventricular arrhythmia-related events (HR 0.16, 95% CI 0.04-0.69) compared to placebo margarine (124). In the ORIGIN trial, 1 g/day of omega-3 fatty acids (465 mg EPA + 375 mg DHA per day for 6 years) had no effect on rates of major vascular events, all-cause mortality, cardiovascular mortality, or death from arrhythmia in 12,536 dysglycemic patients at high risk for cardiovascular events compared to placebo (125)
Perhaps at no time in human history has the human diet changed so dramatically and rapidly than in the past 75 years in developed countries. It is estimated that foods supplying 72% of the dietary calories consumed presently in western diets would not have been found in hunter-gatherer diets . Changes in food type (quality) and quantity found in the modern western diet (MWD) have been largely driven by technological changes in food production and processing to provide high calorie and taste appealing (high sugars, refined grains and oils) foods to large urban populations . Evidence is accumulating that many of these changes have led to detrimental increases in obesity and gene-diet interactions that are responsible for an elevation in localized and systemic inflammation; this inflammation then contributes to a wide range of human diseases including cardiovascular disease, diabetes, cancer, asthma, allergies, chronic joint disease, skin and digestive disorders, dementia and Alzheimer’s disease [2,3,4,5,6,7,8,9]. For example, three decades of research show that high intakes of refined carbohydrates, added sugars and a radical change in the nature of ingested fats, and animal-source foods have dramatically escalated obesity in the developed and now developing world . With regard to fats, animal husbandry has led to production of beef with profoundly abnormal interstitial fat (called “marbling”) and widespread n-3 deficiency.
As challenging as obesity and gene-diet interactions have been for overall populations of developed countries such as the US, they manifest themselves in a particularly negative way for certain populations and ethnic groups [10,11,12,13,14,15]. Several lines of evidence now indicate that a disproportionate burden of preventable disease, death, and disability exists in certain racial and ethnic minority populations, especially African Americans. This review examines the current state of our knowledge regarding the relationship between the intake of polyunsaturated fatty acids (PUFAs) from the MWD and the genetics of PUFA biosynthesis and metabolism in distinct human populations. Specifically, this review discusses how the combination of dramatic increases in levels of certain dietary PUFAs together with diet-gene interactions within PUFA pathways may be driving chronic diseases and health disparities.
2.1. A Dramatic Change in the PUFA Content in Our Diet
For the purposes of this review, we will discuss the metabolism, genetics and biology of the most abundant n-6 or n-3 18 carbon (C18) PUFAs and the n-6 or n-3 long chain (LC; 20–24 carbon) PUFAs. Two C18 PUFAs, linoleic acid (LA, 18:2, n-6) and α-linolenic acid (ALA, 18:3, n-3) are considered essential fatty acids because they cannot be synthesized by mammals including humans and thus must be obtained from the diet (Figure 1). LA is found in vegetable oil products (soybean, corn, palm, and canola oils as well as margarine and shortenings), and is by far the most abundant PUFA in today’s MWD, contributing more than 90% of ingested PUFAs and 7%–8% of food energy consumed . ALA is found in green plants, nuts and botanical oils, such as flax seed oil, and represents ~1% of food energy.
Figure 1. LCPUFA Biosynthetic Pathways. LCPUFA are derived from C18PUFAs (LA and ALA obtained from the mammalian diet) by alternate desaturation (red/orange enzymes) and elongation (blue enzymes) steps. These enzymes utilize both n-6 and n-3 substrates. n-3 LCPUFAs undergo further metabolism through a β-oxidation step (green box) to the generate DHA.
Figure 1. LCPUFA Biosynthetic Pathways. LCPUFA are derived from C18PUFAs (LA and ALA obtained from the mammalian diet) by alternate desaturation (red/orange enzymes) and elongation (blue enzymes) steps. These enzymes utilize both n-6 and n-3 substrates. n-3 LCPUFAs undergo further metabolism through a β-oxidation step (green box) to the generate DHA.
Following the 1961 American Heart Association Central Committee Advisory Statement to replace saturated fat with PUFAs in the diet, food production companies began replacing the saturated fatty acids in processed foods with unsaturated fatty acid oils, especially soybean oil . As a result, vegetable oils, shortening and margarine were recommended as replacements for animal fats such as butter, cream, and cheese . These changes led to a marked (two to three-fold) increase in dietary LA, an estimated 40% reduction in total n-3 LCPUFAs levels, and a large shift in the ratio of dietary n-6/n-3 C18 PUFAs consumed from ~5:1 to >10:1 [17,18].
2.2. A Debate about the Health Impact of n-6 PUFAs
Recently, there has been intense debate in the scientific community over the impact of the recommendation to replace saturated fatty acids with n-6 PUFAs and the health effects of raising dietary n-6 PUFA levels in general. In favor of recommendations to increase dietary n-6 PUFAs, several randomized controlled trials and population cohort studies have shown benefits of n-6 PUFAs when measuring cardiovascular disease biomarkers such as serum lipids and lipoproteins [19,20,21]. Based on these studies, the American Heart Association, once again in 2009, recommended human diets should include high levels of n-6 PUFAs that comprise at least 5%–10% of the energy intake .
On the other side of the argument, Ramsden and colleagues recently re-examined studies utilized to support this recommendation and found that many of the oils used in the aforementioned clinical trials were mixtures of n-6 and n-3 PUFAs. Their data suggest that only substituting n-6 PUFAs for saturated and trans fatty acids actually trended toward increased risk of death from all causes [23,24]. This same group also recently reexamined the Sydney Diet Heart Study (458 men aged 30–59 with a recent coronary event), which replaced dietary saturated fatty acids with a high LA-containing diet utilizing, recently recovered data . As expected from the previous studies examining CVD biomarkers, the LA intervention group had lower levels of total cholesterol. However, unexpectedly this group had higher rates of death than controls (all cause 17.6% vs. 11.8%, hazard ratio 1.62 (95% confidence interval 1.00–2.64), p = 0.05; cardiovascular disease 17.2% vs. 11.0%, 1.70 (1.03–2.80), p = 0.04; coronary heart disease 16.3% vs. 10.1%, 1.74 (1.04–2.92), p = 0.04) once again raising the question of whether n-6 PUFAs may in some cases increase coronary heart disease.
It also has been recognized for more than a half a century that LA can be converted in humans to arachidonic acid (ARA) and this ARA can then be metabolized via cyclooxygenase and lipoxygenase pathways into eicosanoids such as prostaglandins, thromboxanes and leukotrienes. In general, eicosanoids have been shown to act as local hormones to promote acute and chronic inflammation in numerous human diseases including cardiovascular disease (encompassing atherosclerosis leading to heart disease and stroke) asthma, and arthritis . Eicosanoids also have potent impact on bronchoconstriction, vascular permeability, platelet aggregation, and leukocyte recruitment, all postulated to contribute to several human diseases . More recently, epidemiological, clinical and animal studies have provided evidence that ARA metabolism to eicosanoids is an important mechanism by which dietary fats impact carcinogenesis. Additionally, blocking the cyclooxygenase pathway mediating ARA metabolism with non-steroidal, anti-inflammatory drugs (NSAID) have been reported to have beneficial effects in reducing the risk of developing breast, colon, lung, and prostate cancer .
However, some metabolites of ARA are clearly not pro-inflammatory including the epoxyeicosanoids, while epoxyeicosanoid metabolites are deleterious. The pleiotropic properties of arachidonate metabolites as well as the inflammation resolving properties of certain metabolites of EPA and DHA make it difficult to completely understand the role of PUFA metabolites in inflammation [29,30,31]. Clarifying these concepts requires knowing which metabolites are made at local tissue sites at concentrations that regulate the cellular response we term inflammation.
Another critical, but much less discussed factor, in this debate is the potential interaction between ancestral genetic background and dietary PUFA environments, especially with regard to ARA synthesis and eicosanoid production. The scientific inquiry to date almost exclusively has examined the impact of dietary PUFAs found in the MWD in European or European American populations. However, recent studies discussed in detail below suggest that certain racial and ethnic groups (and especially those of African ancestry) have much higher frequencies of genetic variants in key genes that enhance their capacity to synthesize ARA and potentially eicosanoids. Many of these are the same variants that have been associated with CVD biomarkers and disease endpoints. Consequently, diet-gene interactions are likely to have a significant impact with regard to whether dramatically increasing dietary n-6 C18 PUFA, such as LA abundant in the MWD, is beneficial, slightly detrimental or a major risk factor for a given racial or ethnic group. Innis has  recently proposed that similar ancestral considerations must be taken into account with PUFAs in human breast milk and points out that that several recent studies suggest interactions between breastfeeding and genotype, child cognitive development, and risk of allergic disease.
2.3. Biosynthesis of LCPUFAs
As mentioned above, the primary n-6 LCPUFA, ARA can be synthesized from LA utilizing three (two desaturation and one elongation) enzymatic steps (Figure 1) . The n-3 LCPUFAs, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) can also be synthesized from dietary ALA through seven (three desaturation, three elongation and one β-oxidation) enzymatic steps. The first three enzymatic steps that form ARA and EPA from LA and ALA, respectively, are thought to be carried out by the same three enzymes. EPA is then further converted to DHA utilizing additional biosynthetic steps (two elongation, one desaturation and one β-oxidation). Recent studies suggest that the efficiency of each of these steps in individuals is highly impacted by variants in the genes that encode for these enzymatic steps .
In addition to these biosynthetic pathways, LCPUFAs can also be obtained directly from the diet. Dietary ARA is sourced primarily from organ meats, eggs, poultry, and certain fish, whereas dietary EPA and DHA are found primarily in seafood . Daily dietary intakes of ARA can range from as low as 50 mg to greater than 500 mg per day [35,36]. Consuming a meal of oily fish, such as salmon, albacore tuna, or mackerel provides roughly 500 mg to 2 g of n-3 LCPUFAs . However, average n-3 LCPUFA consumption does not exceed 100 mg per day in the MWD. It is also well recognized that providing EPA and DHA as fish oil supplements enhances circulating and tissue levels of the n-3 LCPUFAs [38,39,40].
Given the shared enzymatic steps involved in the processing of LA and ALA, it is generally accepted that these n-6 and n-3 PUFAs and their metabolic intermediates compete with each other in the liver and other tissues as substrates for synthesis enzymatic reactions [41,42]. Specifically, LA and ALA are converted to gamma (γ)-linolenic acid (GLA, 18:3, n-6) and stearidonic acid (SDA, 18:4, n-3), respectively, by the enzyme encoded for by the gene fatty acid desaturase 2 (FADS2; chromosome 11q12.2). This has long been thought to be a rate-limiting step in LCPUFA biosynthesis [41,43,44]. Subsequently, GLA is elongated to dihomo-γ-linolenic acid (DGLA, 20:3, n-6) and SDA to eicosatetraenoic acid (ETA, 20:4, n