Effect of EPA and DHA on TAG levels

Raised fasting and postprandial TAG concentrations are now widely recognised as markers of cardiovascular risk⁽Reference Cullen^20, Reference Stalenhoef and de Graaf²¹⁾. There is strong evidence from epidemiological and intervention studies that EPA+DHA consumption decreases TAG levels⁽Reference Eslick, Howe and Smith²²⁾, thus improving cardiovascular health, and this appears to be dose-dependent⁽Reference Mattar and Obeid²³⁾. When administered individually for 6 weeks or more, both EPA and DHA decrease TAG levels in normolipidaemic⁽Reference Grimsgaard, Bonaa and Hansen^24, Reference Egert, Kannenberg and Somoza²⁵⁾ and hyperlipidaemic subjects⁽Reference Mori, Burke and Puddey²⁶⁾ from 15 to 30%. Interventions of ⩽4 weeks are less consistent. One study showed that 3 weeks of supplementation with EPA or fish oil, but not DHA reduced TAG levels in healthy human subjects⁽Reference Rambjor, Walen and Windsor²⁷⁾. More recently, Buckley et al. ⁽Reference Buckley, Shewring and Turner²⁸⁾ showed that 4 weeks of supplementation with DHA significantly reduced TAG levels in normolipidaemic human subjects by 22%, while EPA decreased TAG levels by 15% without reaching significance. In another 4-week intervention in healthy human subjects, both EPA and DHA reduced postprandial TAG without affecting fasting TAG levels⁽Reference Park and Harris²⁹⁾. However, when given for a sufficient period, EPA and DHA seem to reduce triglyceridaemia with no apparent differential effect ⁽Reference Grimsgaard, Bonaa and Hansen^24–Reference Grimsgaard, Bonaa and Hansen^26, Reference Buckley, Shewring and Turner^28–Reference Nestel, Shige and Pomeroy³²⁾ (Table 1).

Table 1. Differential effect of EPA and DHA supplementation on plasma fasting TAG levels in human subjects.

EE, ethyl ester; NS, non-significant; T2D: type 2 diabetes; ↓, decrease; =, no change.

Effect of EPA and DHA on lipoprotein profiles

Fish oils generally have no effect on total cholesterol but their influence on LDL and HDL cholesterol is variable, depending on the dose, form and population. Meta-analysis of EPA+DHA supplementation studies showed a very slight increase in LDL (n14 009) and HDL (n15 106) cholesterol levels, but these were clinically insignificant⁽Reference Eslick, Howe and Smith²²⁾. The majority of studies investigating the effect of algal DHA (that also contains docosapentaenoic acid, 22:5n-3) reported a moderate but significant increase in both HDL and LDL levels⁽Reference Holub^33–Reference Nelson, Schmidt and Bartolini³⁷⁾. Few studies have reported the differential effect of purified EPA and DHA from fish oils on plasma LDL and HDL cholesterol. Relatively high doses of DHA (2–4 g/d; 6–7 weeks) increased HDL levels by 4–13% in normolipidaemics, whereas similar doses of EPA had no effect⁽Reference Grimsgaard, Bonaa and Hansen^24, Reference Egert, Kannenberg and Somoza²⁵⁾. However, DHA but not EPA (3·7 and 3·8 g/d, respectively, 6 weeks) increased total LDL by 8% in hyperlipidaemic subjects, while no significant effect was observed on total HDL levels⁽Reference Mori, Burke and Puddey²⁶⁾. Our recent research observed that neither EPA nor DHA (3 g/d, 6 weeks) affected TAG, HDL or LDL cholesterol levels in normolipidaemic young men (SC Cottin, TAB Sanders and WL Hall, unpublished results).

Beyond cholesterol levels, LDL and HDL subfractions have emerged as candidate markers of cardiovascular risk. LDL particle size correlates negatively with TAG levels and positively with HDL levels⁽Reference Kondo, Muranaka and Ohta³⁸⁾. Larger HDL (HDL-2) carry more cholesterol and are more protective than their counterpart (HDL-3)⁽Reference Morgan, Carey and Lincoff³⁹⁾. In general, dietary fish oil increases HDL-2 levels⁽Reference Chan, Watts and Nguyen^40, Reference Wilkinson, Leach and Ah-Sing⁴¹⁾, sometimes without a significant change of HDL level⁽Reference Agren, Hanninen and Julkunen⁴²⁾, and also decrease small dense LDL levels⁽Reference Wilkinson, Leach and Ah-Sing^41, Reference Griffin, Sanders and Davies⁴³⁾. When given individually, DHA but not EPA increased both LDL and HDL particle size in hyperlipidaemic and healthy human subjects⁽Reference Mori, Burke and Puddey^26, Reference Rambjor, Walen and Windsor²⁷⁾, although EPA alone also increased HDL-2:HDL-3 in hypercholesterolaemic subjects⁽Reference Nozaki, Matsuzawa and Hirano⁴⁴⁾. High doses of DHA alone (3 g/d, 45 d) also increased LDL particle size in hypertriglyceridaemic men⁽Reference Kelley, Siegel and Vemuri³⁵⁾, whereas low doses of DHA alone (0·7 g/d) increased LDL by 7% and LDL:apoB ratio by 3·1% in middle-aged women and men⁽Reference Liao, Liou and Shieh⁴⁵⁾, suggesting an increase in LDL size.

Hypertriglyceridaemia is a result of overproduction and/or decreased catabolism of TAG-rich lipoproteins, including VLDL and chylomicrons. There is growing evidence that EPA and DHA exert their TAG lowering effects by reducing VLDL TAG release from the liver and by increasing TAG clearance from chylomicrons and VLDL particles⁽Reference Harris, Miller and Tighe⁴⁶⁾, as well as altering VLDL concentration and particle size⁽Reference Burdge, Powell and Dadd⁴⁷⁾. The potential molecular mechanisms have been comprehensively reviewed by Harris et al. ⁽Reference Harris, Miller and Tighe⁴⁶⁾ and notably involves the modulation of transcription factors activity, including SREBP and PPAR. SREBP-1c controls enzymes responsible for fatty acid and TAG synthesis, while SREBP-2 modulates enzymes involved in cholesterol synthesis. In animal and in vitro models, both EPA and DHA were reported to down-regulate SREBP-1c activity, and this was associated with a decrease in lipogenic enzymes expression⁽Reference Caputo, Zirpoli and Torino^19, Reference Howell Iii, Deng and Yellaturu^48, Reference Kajikawa, Harada and Kawashima⁴⁹⁾. In addition, both EPA and DHA are PPAR ligands and EPA and DHA stimulate β-oxidation of fatty acids through PPARα-dependent mechanisms in rats⁽Reference Harris and Bulchandani⁵⁰⁾, thus contributing to the decrease in TAG release by the liver. Lipoprotein lipase, located in capillary endothelium, hydrolyses circulating TAG in TAG-rich lipoprotein, generating NEFA. EPA and DHA (4 g/d, 4 weeks) were equally as effective in accelerating chylomicron TAG clearance by stimulating lipoprotein lipase activity in healthy human subjects⁽Reference Park and Harris²⁹⁾; possibly via PPARγ-dependent mechanisms⁽Reference Chambrier, Bastard and Rieusset⁵¹⁾.

3-Hydroxy-3-methylglutaryl (HMG)-CoA reductase is a key enzyme in cholesterol synthesis and is inhibited by both EPA and DHA in hepatocytes, probably through SREBP-2 dependent mechanisms⁽Reference Arai, Kim and Chiba^52, Reference Le Jossic-Corcos, Gonthier and Zaghini⁵³⁾. Although HMG-CoA reductase inhibitors (statins) are well known for their hypocholesterolemic effect, this has not been a consistent outcome of fish oils consumption. However, a common mechanism for statins and EPA/DHA may be related to increasing LDL and HDL particle size⁽Reference Sone, Takahashi and Shimano^54–Reference Ikewaki, Terao and Ozasa⁵⁶⁾; in fact n-3 LCP and statins may exert a synergistic beneficial effect on lipid levels⁽Reference Nordoy⁵⁷⁾, and it can be postulated that EPA and DHA modulate particle size by a mechanism analogous to that of HMG-CoA reductase inhibitors⁽Reference Das⁵⁸⁾.

In summary, both DHA and EPA reduce fasting plasma TAG concentrations with no apparent differential effect, probably by inhibiting VLDL-TAG release and increasing TAG clearance. DHA appears to increase HDL and LDL particle size through the regulation of cholesterol synthesis and lipid transfer between lipoprotein.

Blood pressure

Hypertension is a strong predictor of cardiovascular risk, and there is convincing evidence that reducing blood pressure (BP) decreases the risk of total mortality, cardiovascular mortality and stroke⁽Reference Turnbull, Neal and Ninomiya^59, Reference Law, Morris and Wald⁶⁰⁾. Numerous epidemiological and intervention studies have demonstrated a hypotensive role of fish oils⁽Reference Mori⁶¹⁾. In a meta-analysis of thirty-one placebo-controlled trials, Morris and co-workers showed that fish oils reduced BP with a dose-dependent effect (systolic BP/diastolic BP: −0·66/−0·35 mm Hg/g n-3 fatty acids), and so is of potential benefit to patients with hypertension, atherosclerosis or hypercholesterolaemia⁽Reference Morris, Sacks and Rosner⁶²⁾. A more recent meta-analysis of thirty-six intervention trials confirmed the hypotensive role of fish oils on both systolic BP and diastolic BP, especially in elderly and hypertensive patients, although the clinical effect of doses lower than 0·5 g/d, equivalent to one portion of oily fish a week, could not be established⁽Reference Geleijnse, Giltay and Grobbee⁶³⁾. Few human studies have investigated the separate effects of EPA and DHA on BP; these have generally been assessed by seated office measurements, with no significant lowering effects in hypertensive, dyslipidaemic and healthy human subjects⁽Reference Woodman, Mori and Burke^31, Reference Nestel, Shige and Pomeroy^32, Reference Grimsgaard, Bonaa and Hansen⁶⁴⁾. However, low doses of DHA alone (from algal sources) were shown to decrease diastolic BP in healthy subjects⁽Reference Theobald, Goodall and Sattar⁶⁵⁾. Ambulatory BP, where monitors are worn and take readings at regular intervals over 24 h, considered to be an estimate of the true mean BP level⁽Reference Pickering, Shimbo and Haas⁶⁶⁾, is more sensitive than the conventional office BP in predicting cardiovascular events⁽Reference Ohkubo, Imai and Tsuji^67, Reference Staessen, Thijs and Fagard⁶⁸⁾. Mori and co-workers investigated the effect of 6-weeks supplementation with EPA or DHA (4 g/d) on ambulatory BP and showed that DHA but not EPA decreased both 24 h and daytime systolic and diastolic ambulatory BP in mildly hyperlipidaemic males⁽Reference Mori, Bao and Burke⁶⁹⁾.

Heart rate

A high heart rate (HR) has been long associated with cardiovascular morbidity and mortality in epidemiological studies. It is positively correlated with hypertension and has only recently emerged as an independent cardiovascular risk factor to be targeted to reduce cardiovascular events, especially in high-risk populations⁽Reference Perret-Guillaume, Joly and Benetos⁷⁰⁾. A meta-analysis including thirty randomised controlled trials showed that fish oil intake reduces HR, especially in populations with a high-baseline HR and when consumed for a longer intervention period⁽Reference Mozaffarian, Geelen and Brouwer⁷¹⁾. This effect appears to be mediated by DHA rather than EPA: DHA alone (2·8 g/d) decreased HR by 7% in postmenopausal women⁽Reference Stark and Holub⁷²⁾, and DHA but not EPA decreased HR by 3·5 beats per minute (bpm) and 2·2 bpm, in hyperlipidaemic males⁽Reference Mori, Bao and Burke⁶⁹⁾ and healthy males⁽Reference Grimsgaard, Bonaa and Hansen⁶⁴⁾, respectively. In contrast, Woodman and co-workers showed no significant effect of neither EPA nor DHA on HR in healthy males for similar dosage and treatment duration⁽Reference Woodman, Mori and Burke³¹⁾.

HR variability (HRV) is also a strong predictor of CVD, including sudden cardiac death, arrhythmic CHD and atrial fibrillation. Fish oils have shown anti-arrhythmic properties in animal studies⁽Reference McLennan⁷³⁾, and several clinical and epidemiological studies have reported an association between an increase (improvement) of HRV and n-3 LCP blood cell levels and/or fish oil consumption⁽Reference Christensen, Skou and Fog^74–Reference Christensen, Christensen and Dyerberg⁷⁶⁾. However, fish oils fail to improve HRV in several other human interventions. For example, n-3 LCP supplementation did not increase HRV in haemodialysis patients⁽Reference Svensson, Schmidt and Jorgensen⁷⁷⁾, and failed to increase HRV calculated from 10 min recordings⁽Reference Geelen, Zock and Swenne⁷⁸⁾ or 24 h Holter recordings⁽Reference Dyerberg, Eskesen and Andersen⁷⁹⁾ in healthy men. Nevertheless, the authors of the latter study noted that subjects presented with a particularly high baseline HRV, and subanalysis showed a significant improvement of HRV for subjects with lower baseline values⁽Reference Dyerberg, Eskesen and Andersen⁷⁹⁾. Inconsistency in the results of intervention trials might be due to variability of design, treatment duration, sample size or duration of HRV measurement; a prospective observational study (n 4263) reported that fish oil consumption, recorded over a year, correlated with an improvement of HRV, especially in older people⁽Reference Mozaffarian, Stein and Prineas⁸⁰⁾.

As previously mentioned, the incorporation of EPA and DHA into the cell membrane influences its organisation, fluidity and permeability, as well as the activity of transmembrane proteins, including receptors, enzymes and ion channels. Both EPA and DHA were shown to modulate K, Na and Ca channel activities in myocardial cells, regulating myocyte electrical excitability and contractility⁽Reference Xiao, Gomez and Morgan^81–Reference Xiao, Sigg and Leaf⁸³⁾. These effects, observed in a concentration-dependent manner, are thought to be mediated by the effect of EPA and DHA on membrane fluidity⁽Reference Xiao, Sigg and Leaf⁸³⁾, although other mechanisms, such as direct binding of n-3 LCP to the channel could be involved⁽Reference Kang and Leaf⁸⁴⁾. Furthermore, there is growing evidence from animal studies that DHA, compared to EPA, is preferentially incorporated into the myocardial cell membrane⁽Reference McLennan⁷³⁾. Collectively, these findings help to explain the anti-arrythmic and HR-lowering effects observed with DHA but not EPA in human subjects⁽Reference Mori, Bao and Burke⁶⁹⁾. In addition, incorporation of DHA into the membrane of cardiomyocytes influences the beta adrenergic system to a greater extent than EPA⁽Reference Grynberg, Fournier and Sergiel⁸⁵⁾, potentially an important mechanism in the hypotensive and anti-arrhythmic effects of DHA. DHA incorporation into the membrane of endothelial cells stimulates ATP release from the endothelium, increasing vasodilation by stimulating nitric oxide (NO) release⁽Reference Hashimoto, Shinozuka and Gamoh⁸⁶⁾. The induction of NO release, together with the decrease in noradrenaline levels, is likely to be responsible for the BP-lowering effect of DHA⁽Reference Hashimoto, Shinozuka and Gamoh⁸⁶⁾.

DHA, but not EPA, seems to have lowering effects on BP and HR, very probably mediated by the increased fluidity in the membrane cardiomyocytes, potentially improving channel activity and beta adrenergic signalling. More studies are necessary to confirm this differential effect and understand the mechanisms involved.

Endothelial function

Animal studies demonstrated that EF could be modulated by feeding EPA and DHA⁽Reference Mark and Sanders^89–Reference Shimokawa and Vanhoutte⁹¹⁾. An observational study reported that plasma and erythrocyte DHA levels were positively associated with FMD in young smokers and young adults at greater metabolic risk⁽Reference Leeson, Mann and Kattenhorn⁹²⁾. Recent findings suggest that fish oil consumption can improve EF in human subjects, particularly in those with a high risk of CVD (Table 2). Supplementation with n-3 LCP for periods ranging from 2 weeks up to 8 months improved endothelium-dependent vasodilation, prevented vasoconstriction or augmented exercise-induced blood flow at doses≥0·5 g/d⁽Reference Chin, Gust and Nestel^93–Reference Wright, O'Prey and McHenry¹⁰⁷⁾.

Table 2. Effect of fish oils on endothelial function in human randomised controlled trials.

T2D, type 2 diabetes; PAD, peripheral arterial disease; CAD, coronary artery disease; FMD, flow-mediated dilation; RH, reactive hyperaemia; BA, brachial artery; EF, endothelial function; ED/EA, endothelial dysfunction/endothelial damage; NOx, nitrates/nitrites; vWF, von Willebrand factor; VCAM, vascular cell adhesion molecule; ICAM, intercellular adhesion molecule; sTM, soluble thrombomodulin; FBF, forearm blood flow; Ach, acetylcholine; SNP, sodium nitroprusside (NO donor); L-NAME, nitro-L-arginine-methyl ester (NOS inhibitor); GTN, glyceryl trinitrate (NO donor); AT-II, angiotensin II; NAd, noradrenaline (norepinephrine); ADMA, asymmetrical dimethylarginine; L-NMMA, N _G-monomethyl-L-arginine.

↑, increase; ↓, decrease; =, unchanged.

The comparative effects of EPA and DHA on EF have been seldom investigated in human subjects (Table 2). Supplementation with EPA alone (1·8 g/d; 3 months) increased endothelium-dependent forearm blood flow response in untreated hypertriglyceridaemic males⁽Reference Okumura, Fujioka and Morimoto¹⁰⁸⁾, whereas DHA alone (1·2 g/d; 6 weeks) improved endothelium-dependent FMD in hyperlipidaemic children receiving nutritional counselling⁽Reference Engler, Engler and Malloy¹⁰⁹⁾. Supplementation with low doses of algal DHA did not affect salbutamol-induced changes in digital volume pulse reflection index (a measure of endothelium-dependent vasodilation), but more extensively validated techniques such as FMD are required to confirm this⁽Reference Theobald, Goodall and Sattar⁶⁵⁾. When the vasodilatory effects of high doses of EPA and DHA (4 g/d, 6 weeks) were compared in overweight mildly hyperlipidaemic males, DHA, but not EPA, decreased vasoconstrictive responses to noradrenaline and increased vasodilatory responses to acetylcholine⁽Reference Mori, Watts and Burke⁹⁷⁾. However, DHA (but not EPA) also increased vasodilation in response to the co-infusion of acetylcholine and N _G-monomethyl-L-arginine citrate (an NOS inhibitor), as well as sodium nitroprusside (a NO donor), suggesting that the vasodilatory effects of DHA were mainly mediated through endothelium-independent mechanisms⁽Reference Mori, Watts and Burke⁹⁷⁾. In healthy volunteers, fish oil concentrate, but not EPA alone, increased urinary excretion of NO metabolites (nitrates/nitrites), suggesting that EPA is unlikely to be responsible for the enhancement of NO production⁽Reference Harris, Rambjor and Windsor¹¹⁰⁾. In summary, there are too few data to conclude whether EPA and DHA have differing effects on endothelium-dependent vasodilation, but early indications are that DHA might be more effective in improving EF.

Arterial compliance

Little is known about the influence of dietary n-3 LCP on arterial stiffness, although it has been observed that Japanese populations with higher intakes of n-3 LCP have reduced arterial stiffness⁽Reference Hamazaki, Urakaze and Sawazaki¹¹¹⁾, and the results of two randomised controlled trials indicate a beneficial effect⁽Reference McVeigh, Brennan and Cohn^112, Reference Wang, Ma and Song¹¹³⁾. Even less is known about the individual effects of either EPA or DHA. Three-month supplementation with low doses of DHA (0·7 g/d) in healthy subjects had no effect on indices of arterial stiffness using digital volume pulse, suggesting that either larger doses are necessary for measurable changes to occur during that time period, that fish oil is more effective in subjects at greater cardiovascular risk (our group also found no effects of 3 g/d EPA or DHA for 6 weeks in young healthy males (SC Cottin, TAB Sanders and WL Hall, unpublished results)) or lastly, that EPA rather than DHA is the active constituent of fish oil in relation to arterial stiffness⁽Reference Theobald, Goodall and Sattar⁶⁵⁾. In support of this, EPA supplementation (1·8 g/d; 3 months) improved pulse wave velocity and cardio-ankle vascular index in obese Japanese subjects, the latter measure being a novel index of arterial stiffness that is less influenced by BP than pulse wave velocity⁽Reference Satoh, Shimatsu and Kotani¹¹⁴⁾. The same dose of EPA consumed for 12 months by hyperlipidaemic patients also prevented the increase of pulse wave velocity due to ageing, even after adjustment for gender, age and BP change⁽Reference Tomiyama, Takazawa and Osa¹¹⁵⁾. However, not all evidence supports the theory that EPA is the sole active constituent of fish oil in relation to arterial stiffness; the only trial to compare the individual effects of EPA and DHA showed that 7 weeks supplementation with EPA increased systemic arterial compliance in dyslipidaemics by 36% and DHA increased it by 27%, with no significant differences in the size of the effect between the two groups⁽Reference Nestel, Shige and Pomeroy³²⁾.

Microvascular dysfunction, as observed in hypertensive and insulin-resistant states, is characterised by capillary rarefaction in skin and muscle⁽Reference Jonk, Houben and de Jongh^116, Reference Antonios, Singer and Markandu¹¹⁷⁾. Fish oil supplementation increased capillary density in ventricles⁽Reference Mitasikova, Smidova and Macsaliova¹¹⁸⁾ and skin (cheek pouch)⁽Reference Conde, Cyrino and Bottino¹¹⁹⁾ in hypertensive rats and hamsters, respectively, suggesting there might be beneficial effects on human microvasculature. Videomicroscopic techniques (capillaroscopy) have been developed and used in human subjects in order to look at microcirculation in the skin and oral mucosa (tongue), which are readily accessible for microscopic measurements (see comprehensive review⁽Reference Awan, Wester and Kvernebo¹²⁰⁾). Capillaries appear either parallel (loops) or perpendicular (dot or comma shaped) to the skin, and can be analysed in terms of shape (tortuousness and diameter) and number (density). Capillaroscopy also gives the possibility to assess the velocity of the erythrocytes by video or laser Doppler. However, to date, the effect of fish oil on this outcome has not been investigated. In our recent trial, neither EPA nor DHA changed finger capillary density (Capiscope; KK Technologies) in healthy young men (SC Cottin, TAB Sanders and WL Hall, unpublished results), but further research is needed in sub-populations with impaired microvascular function to ascertain whether fish oils or individual n-3 LCP have protective effects.

The expression of endothelial NOS is vital to EF and therefore a major factor in atherogenesis. Cell membranes are organised in microdomains, called lipid rafts, that co-localise transmembrane proteins involved in intracellular signalling pathways. When incorporated into the membrane, EPA and DHA can alter this organisation, thus modulating signalling in various types of cells⁽Reference Yaqoob¹²¹⁾. In endothelial cells, both EPA and DHA were shown to alter the organisation of caveolae, a particular subset of lipid rafts, and displace endothelial NOS from caveolae, a necessary step in the activation of endothelial NOS⁽Reference Li, Zhang and Wang^122, Reference Li, Zhang and Wang¹²³⁾. This could potentially lead to an increase in NO release by the endothelium and explain the putative beneficial effects of EPA and DHA on vasodilation observed to date (Table 2). EPA and DHA probably also influence the production of the two other main vasodilators produced by the endothelium: prostacyclin and endothelium-dependent hyperpolarising factor. EPA, as a direct substrate for COX, may be converted to 3-series prostacyclin, analogue to the 2-series prostacyclin derived from AA. Both EPA and DHA increased 3-series prostacyclin production by endothelial cells to the same extent without affecting 2-series prostacyclin levels, suggesting a retroconversion from DHA to EPA⁽Reference Hishinuma, Yamazaki and Mizugaki¹²⁴⁾. In addition, EPA was shown to increase acetylcholine-induced endothelium-dependent hyperpolarising factor-mediated vasodilation in diabetic rats⁽Reference Matsumoto, Nakayama and Ishida¹²⁵⁾. Endothelium-independent vasodilatory effects are mediated by the modulation of Ca²⁺ signalling in smooth muscle cells, but the mechanisms, especially with respect to the type of Ca channels involved, remain uncertain⁽Reference Engler and Engler^126–Reference Engler, Ma and Engler¹²⁹⁾.

AA, EPA and DHA are also substrates for the CYP450 enzymes that act as monooxygenases, catalysing hydroxylation, epoxidation or allylic oxidation. CYP450-dependent derivatives include epoxyeicosatrienoic acids and hydroxyeicosatetraenoic acids, potentially important factors in the regulation of vasodilation and vasoconstriction, and modulate renal, vascular and cardiac function. CYP450 enzymes are highly regioselective and stereospecific, and several isoforms prefer n-3 LCP as substrates rather than n-6 LCP. Investigation into the physiological roles of CYP450-dependent EPA and DHA metabolites is at an early stage, but recent data suggest that CYP450-dependent mediators derived from EPA and DHA contribute to the vasodilatory and cardioprotective effects of fish oils⁽Reference Conquer and Holub¹³⁰⁾. Interestingly, different CYP450 isoforms have a different affinity, regioselectivity and stereospecificity for EPA and DHA (see comprehensive review⁽Reference Konkel and Schunck¹⁵⁾), leading to various sets of mediators that will exert varying effects on the vasculature.

In addition, EPA and DHA modulate EF through anti-inflammatory effects. When endothelial cells undergo inflammatory activation, they increase the expression of adhesion molecules, allowing the migration of leucocytes through the endothelium, an important process in the pathophysiology of atherosclerosis⁽Reference Ross¹³¹⁾. General patterns that emerge from in vitro experimental literature indicate that DHA has a greater effect than EPA in reducing endothelial inflammation. DHA tends to inhibit markers of EF, such as inflammatory cell adhesion molecules and monocyte chemoattractant protein-1 gene and protein expression, and the adhesion of leucocytes to the endothelium, whereas EPA either up-regulated gene expression of monocyte chemoattractant protein-1 or was a weaker inhibitor of cell adhesion molecules than DHA⁽Reference Shaw, Hall and Jeffs^132–Reference Goua, Mulgrew and Frank¹³⁷⁾. The effect of DHA on vascular cell adhesion molecule-1 is likely to be mediated by the inhibition of the mobilisation of the nuclear transcription factor, NF-κB⁽Reference Weber, Erl and Pietsch¹³⁶⁾, which regulates the expression of numerous cytokines and other adhesion molecules.

Fish oils generally improve EF and arterial compliance in subjects at high cardiovascular risk. However, the effects of fish oils in healthy human subjects and the mechanisms (endothelium dependent and independent) by which EPA and/or DHA improve vascular function are yet to be fully established.