Marine n-3 polyunsaturated fatty acids and coronary heart disease

Thrombosis Research (2005) 115, 163 — 170

intl.elsevierhealth.com/journals/thre

REVIEW

Marine n-3 polyunsaturated fatty acids and coronary heart disease Part I. Background, epidemiology, animal data, effects on risk factors and safety Erik Berg Schmidta,*, Harald Arnesenb, Raffaele de Caterinac, Lars Hvilsted Rasmussena, Steen Dalby Kristensend a

˚rhus, Denmark Department of Preventive Cardiology, Aalborg Sygehus, 2rhus University Hospitals, A Department of Cardiology, Ullevaal University Hospital, Oslo, Norway c Institute of Cardiology and Center of Excellence on Aging, bG. d’AnnunzioQ University, Chieti, Italy d Department of Cardiology, Skejby University Hospital, Skejby, Denmark b

Received 27 August 2004; accepted 13 September 2004 Available online 12 October 2004

KEYWORDS Polyunsaturated fatty acids; Fish consumption; Coronary heart disease

Abstract In this paper, we will give an overview of the background for the possible effects of long-chain marine n-3 (synonomously called omega-3) polyunsaturated fatty acids (PUFA) in coronary heart disease (CHD) with focus on recent findings. In a forthcoming paper [Schmidt EB, Arnesen H, Christensen JH, Rasmussen LH, Kristensen SD, De Caterina R, Marine n-3 polyunsaturated fatty acids and coronary heart disease: Part II. Clinical trials and recommendations. Thromb Res (in press)], we will focus on the clinical trial data, current recommendations and suggest trials to further study the role of marine n-3 PUFA in the prevention and treatment of CHD. D 2004 Elsevier Ltd. All rights reserved.

Contents n-3 PUFA—notes on biochemistry and metabolism Epidemiology . . . . . . . . . . . . . . . . . . . . . Marine n-3 PUFA and atherosclerosis in humans . n-3 fatty acids, atherosclerosis and thrombosis in

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* Corresponding author. Tel.: +45 993231158. E-mail address: [email protected] (E.B. Schmidt). 0049-3848/$ - see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.thromres.2004.09.006

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E.B. Schmidt et al.

Experimental data on antiarrhythmic effects of n-3 PUFA Marine n-3 PUFA and risk factors for CHD . . . . . . . . . Marine n-3 PUFA: types and safety . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . .

n-3 PUFA—notes on biochemistry and metabolism n-3 PUFA belong to an essential fatty acid family characterized by their first double bond at carbon atom number 3, counted from the methyl end of the carbon chain constituting the backbone of fatty acids. n-3 PUFA are chemically and biologically distinct from n-6 PUFA, where the first double bond is at carbon atom number 6 [2]. There are two subgroups of n-3 PUFA. One, alinolenic acid, is derived from plant oils (examples are canola oil, rapeseed oil, and linseed oil) and is composed of 18 carbon atoms with three double bonds (nomenclature; 18:3). The other group of n3 PUFA is derived from seafood, and the major marine n-3 PUFA are eicosapentaenoic acid (EPA; 20:5) and docosahexaenoic acid (DHA; 22:6). In humans, a-linolenic acid can to a limited extent be elongated and desaturated to EPA and DHA. Otherwise, these PUFA are only acquired from seafood. The content of marine n-3 PUFA varies between fish species [3—5] being high in fatty fish like mackerel and salmon and low in lean fish such as flounder and cod (Table 1). The content of n-3 PUFA in seafood varies considerably in relation to location and the season of capture. Values given in Table 1 should therefore be viewed in this light. The intake of marine n-3 PUFA differs considerably between populations being very high in traditionally living Eskimos (10—14 g/day), low in most Western populations (b0.2 g/day), and intermediate in areas like Japan and Norway (1—3 g/day).

Table 1 The approximate content of n-3 PUFA in seafood (depends on season, place of capture, etc.) Seafood

g/n-3 PUFA/100 g

Mackerel Herring Salmon Trout Tuna Halibut Shrimp Cod and flounder

1.8—5.3 1.2—3.1 1.0—2.0 0.5—1.6 0.5—1.6 0.5—1.0 0.2—0.4 0.2

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Epidemiology There is some evidence from epidemiological data that intake of marine n-3 PUFA is associated with a reduced risk of coronary heart disease (CHD). This was originally found in Greenland Eskimos [6], and later a low occurrence of CHD was also reported in other populations with diets rich in seafood in Alaska [7], Japan [8], and China [9]. In Western populations, with an average intake of n3 PUFA well below 1 g/day, an inverse correlation between CHD and fish consumption was reported by some, but not by other investigators [2,5]. Results also differ among the most recent large studies. Thus, in the Nurses’ Health Study comprising 84,688 female nurses followed for 16 years, there was a higher risk of CHD in subjects consuming fish less than monthly compared to those with a higher intake [10]. The effect was more pronounced on total and fatal CHD than on nonfatal myocardial infarction (MI). In a subgroup of diabetic women (n=5.103) free from CHD at baseline, fish consumption was associated with a reduced risk of both fatal CHD and nonfatal MI with some evidence for a dose-dependent effect [11]. In another major trial, The US Physicians’ Health Study of 22,071 male physicians, sudden cardiac death (SCD) but not nonfatal MI, was reduced in men with at least weekly consumption of fish [12,13]. In contrast, in the Health Professional Follow-up Study of 43,671 men, fish consumption did not protect against CHD [14], but, interestingly, the risk of ischemic stroke was higher in men not consuming fish without any correlation with the number of fish servings [15]. In the EURAMIC case-control study from eight European countries and Israel, adipose levels of fatty acids were determined in 639 cases with a first MI and 700 controls [16]. The mean proportion of DHA in adipose tissue (a marker for longterm dietary intake of marine n-3 PUFA) was similar in cases and controls, but adipose levels of a-linolenic acid were lower in cases. Siscovick et al. [17] assessed whether dietary intake of marine n-3 PUFA was associated with a reduced risk of SCD in a population-based case-control study including 334 case patients with SCD and 493 control cases. Spouses of case patients and

Marine n-3 polyunsaturated fatty acids and coronary heart disease control subjects were interviewed to quantify dietary n-3 PUFA intake during the month prior to the event. Eating one fatty fish meal weekly was associated with a 50% risk reduction in SCD [17]. In a prospective, population-based study, the Cardiovascular Health Study, from the US, 5201 men and women above 65 years free of CHD at inclusion were followed for 9 years [18—20]. Consumption of tuna or other broiled or baked fish at least three times per week reduced the risk of arrhythmic CHD by 58% compared to people eating fish less than once per month [18]. Fish sandwiches or fried fish, on the other hand, offered no protection against CHD. A subgroup analysis [19] found that baseline levels of EPA and DHA in plasma phospholipids were lower in those who died from CHD during followup (n=54) compared with controls (n=179). Finally, tuna or other broiled or baked fish was inversely related with the incidence of atrial fibrillation during 12 years of follow-up with a 28% lower risk with intake one to four times per week compared to less than monthly consumption, while fried fish or fish sandwiches had no such effect [20]. In an Italian case-control study, patients with a nonfatal MI (n=507) had a lower intake of n-3 PUFA than controls admitted to hospital for conditions unrelated to CHD [21]. Also, in a group of 415 patients with various expressions of CHD, there was an inverse relation between the content of EPA and DHA in serum cholesterol esters and the risk of coronary death and all-cause mortality, respectively, during follow-up for 5 years [22]. Finally, in a Danish study of 7389 subjects followed for 11 years, no effect of fish consumption on fatal CHD could be demonstrated [23]. Recently, a meta-analysis was published on fish consumption and CHD mortality from 13 cohort studies including a total of 222,364 individuals with an average of 11.8 years of follow-up [24]. Fish consumption was inversely related with fatal CHD in a dosedependent way, where each 20 g/day increase in fish intake was associated to a 7% lower risk of fatal CHD. In summary, there is some evidence from epidemiological studies that intake of marine n-3 PUFA is protective against CHD, more convincingly on fatal CHD and SCD than on nonfatal MI. Explanations for the inconsistency of results include the study of different populations for various periods of time, the lack of differentiation between lean and fatty fish [18,25] and perhaps also between EPA and DHA, and the use of different methods for the assessment of fish consumption and clinical endpoints.

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Marine n-3 PUFA and atherosclerosis in humans Results from autopsy studies in Greenland have been variable [26], while advanced atherosclerosis was less at necropsy in Alaskan Eskimos compared to other Alaskans eating less seafood [7]. Furthermore, pulse wave velocity of the aorta was slower (suggesting less atherosclerosis) and carotid intima media thickness was less in subjects from Japan living in fishing villages compared to people living in farming villages (and eating less fish) [8,27]. Furthermore, case-control studies have suggested an inverse correlation between coronary atherosclerosis and the content of n-3 PUFA in various tissues [28,29]. Considering the importance of atherosclerotic plaque composition for future clinical events [30], the effect of n-3 PUFA on plaque characteristics is likely to be very important, but has only been scarcely studied. Rapp et al. [31] back in 1991 showed that n-3 PUFA were incorporated into human atherosclerotic plaques in a heterogenous group of 11 patients with various atherosclerotic diseases fed an extremely high dose (N20 g/day) of EPA+DHA. In a recent study [32], 188 patients scheduled for carotid endarterectomy were allocated to treatment with 1.4 g of n-3 PUFA per day, n-6 PUFA or a control oil (a mixture of fat resembling the average diet in the UK) given for a median period of 6 weeks (range: 7—189 days). The main findings of the study were that EPA and DHA were indeed incorporated into carotid plaques in patients randomized to n-3 PUFA, and those patients had less infiltration with macrophages (but not T-lymphocytes) and a thicker fibrous cap than controls. These findings suggest that dietary n-3 PUFA may change the composition of atherosclerotic plaques making them less prone to rupture. Further studies on this issue are urgently warranted. Two clinical trials have evaluated the effect of n-3 PUFA on regression/progression of atherosclerosis in patients with documented CHD. In the first study, treatment with 6 g n-3 PUFA per day for 2 years to 31 patients with CHD did not induce beneficial effects on coronary atherosclerosis compared with a group of 28 control patients [33]. However, in a larger trial [34], 223 patients with N20% diameter coronary stenosis in at least one coronary artery or balloon angioplasty planned or performed in one such vessel in the previous 6 months were randomized in a double-blind design to supplementation for 2 years with 2 g n-3 PUFA/ day or placebo capsules (with a fatty acid composition resembling that of the average Euro-

166 pean diet). The primary endpoint was changes in the coronary angiogram visually assessed by a blinded expert panel, while a secondary endpoint included evaluation of coronary angiograms by quantitative coronary angiography. There was a significant borderline improvement in coronary anatomy in patients given n-3 PUFA, mainly due to some regression of atherosclerosis rather than a reduction in the progression of atherosclerosis. There were seven cardiovascular events in the placebo group compared to two in the n-3 group ( p=0.10), but the study was not powered to assess clinical outcome. In the same patients, treatment with n-3 PUFA was not associated with any improvement in the carotid intima-media thickness [35].

n-3 fatty acids, atherosclerosis and thrombosis in animals The majority of animal studies shows that fish oil feeding decreases atherosclerosis [2,5,36]. There have, however, been variable results perhaps according to differences in species, dosing, and type of endpoints. Thus, in mouse models of atherosclerosis, some decrease in atherosclerosis by n-3 PUFA compared with n-6 PUFA was found in the LDLreceptor null-mice, but not in the apo E-null mice (R. De Caterina, personal communication). n-3 PUFA may also reduce thrombogenesis and thereby decrease the risk of thrombotic complications after plaque rupture/fissure. Thus, Hornstra et al. [37] have shown an antithrombotic effect of n3 PUFA in rats, in particular when n-3 PUFA were given in combination with a diet low in saturated fat. Cod liver oil reduced platelet deposition onto carotid arteries subjected to deep intimal injury by balloon angioplasty and reduced injury-related vasoconstriction in pigs [38]. Moreover, when blood obtained from cod liver oil treated pigs perfused normal pig aortas, platelet deposition onto the aortas was lower than when blood from control pigs was used. These results indicate a beneficial effect of n-3 PUFA on the platelet—vessel wall interactions. In another study, baboons were fed a very high dose of fish oil for 22 weeks [39]. Femoral arterio-venous Dacron shunts were surgically implanted, and thrombotic responses of blood to graft segments were studied. Later on, endarterectomies were performed on carotid arteries. In this model, vascular thrombus formation and lesion development were inhibited by n-3 PUFA. Also of interest there have been reports that fish oil feeding reduces infarct size in animals [2].

E.B. Schmidt et al.

Experimental data on antiarrhythmic effects of n-3 PUFA McLennan et al. [40,41] in a series of studies in rats and monkeys have convincingly shown that dietary n-3 PUFA reduce the incidence and severity of malignant ventricular tachyarrhythmias after coronary artery ligation and during reperfusion. In a canine model, infusion of n-3 PUFA also protected the animals from ventricular fibrillation [42]. Finally, in a series of studies, Leaf et al. [43] have shown that both EPA and DHA prevented arrhythmias through effects on sodium, potassium and calcium channels. Also, changes in eicosanoid formation, a modulating effect of n-3 PUFA on hreceptors and specific actions of DHA in the central nervous system may be of importance [44]. This may involve a common pathway, namely a change in the (patho)physiological function of cell membranes induced by alterations in their lipid composition, as seen after an increased intake of marine n-3 PUFA. Furthermore, n-3 PUFA decrease the heart rate [45—47] and increase heart rate variability [44], indicating a beneficial alteration in cardiac autonomic balance—which may relate to the experimental findings discussed above—and explain a decreased risk of SCD by consumption of n-3 PUFA.

Marine n-3 PUFA and risk factors for CHD The effects of n-3 PUFA on risk factors for CHD have been extensively investigated. While dietary n-3 PUFA in practical doses in contrast to common public belief have minimal effect on the concentration of LDL cholesterol, they may slightly increase antiatherogenic HDL2 cholesterol, substantially decrease fasting and postprandial plasma triglycerides [2,48,49], and possibly reduce the number of atherogenic small dense LDL particles [50]. n-3 PUFA reduce platelet reactivity and may also slightly decrease blood pressure [2,5,46]. Most studies have indicated no effect of n-3 PUFA on plasma fibrinogen and coagulability [51,52], while PAI-1 levels may increase after intake of n-3 PUFA, suggesting that fibrinolysis may be adversely affected by n-3 PUFA [2]. Other reported effects of n-3 PUFA with potential relevance for the prevention of atherosclerosis and thrombosis by modifying risk factors for CHD include a reduction in leucocyte activation and a possible beneficial effect on blood rheology [2,5]. Recently, markers of endothelial function, and inflammatory markers, including C-reactive protein measured with a

Marine n-3 polyunsaturated fatty acids and coronary heart disease Table 2

Major beneficial effects of n-3 PUFA in CHD

Triglycerides A HDL2-cholesterol z Platelet reactivity A Monocyte reactivity A Neutrophil reactivity A Blood pressure A Improvement of endothelial function Antiarrhythmic properties z=increase; A=decrease.

sensitive assay have gained much attention as risk predictors for CHD. Most data suggest a beneficial effect of n-3 PUFA on endothelial function [53—57], but this is not an unequivocal finding [58]. Finally, there is little indication that n-3 PUFA reduce CRP levels [59,60], while measures of leucocyte reactivity commonly are reduced by n-3 PUFA [2]. The biochemical effects of n-3 PUFA are generally achieved after daily doses between 2 and 5 g, but are dose-dependent with the most favourable effects usually seen with high doses of n-3 PUFA [2,5]. One exception from this might be the effects on circulating levels of adhesion molecules, where it has been reported that while low doses of n-3 PUFA may beneficially affect adhesion molecules, the contrary may be seen with high doses of n-3 PUFA [54,55]. The latter negative effect has been linked to the prooxidative potential of the highly unsaturated n-3 PUFA, and is probably most relevant in individuals under oxidative stress. It is, however, puzzling, that clinical effects may be seen with an intake of marine n-3 PUFA at a level well below those exerting effects on traditional risk factors for CHD (summarized in Table 2). The concept of assessing the individual risk of CHD as the sum of risk factors, instead of focusing on single risk factors, is very important, and has been increasingly stressed in the updated international guidelines on prevention of CHD [61,62]. As mentioned above, n-3 PUFA induce several beneficial changes in risk factors and although the individual effects are less than what can be obtained, as an example, by antiplatelet agents and lipid-lowering drugs, these multiple beneficial effects in our view make n-3 PUFA attractive in the prevention of CHD.

Marine n-3 PUFA: types and safety Lean fish provides low amounts of n-3 PUFA, but contain little saturated fat and cholesterol and can be recommended for this reason. Fatty fish have a lower content of saturated fat than many servings

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containing meat and may for this reason, independent of the likely beneficial effects of n-3 PUFA, be a better food source. The concept of substituting fish for other food sources and not supplementing fish oils to the habitual diet must be emphasized. However, contamination of fish by mercury or other toxic compounds [63—65] may reduce the health benefit of fish consumption. Eating a variety of fish may also minimize any potential risk of adverse effects. During industrial processing, it is possible to remove toxic substances from fish oil concentrates eliminating such problems with properly processed fish oils. If fish oils are considered, products of high quality with declared amounts of EPA and DHA, and with antioxidants added, should be chosen. All PUFA (including n-3 PUFA) are susceptible to peroxidation. This may be important, because of the believed pivotal role of LDL oxidation in atherogenesis. Epidemiological data do, however, not suggest increased atherosclerosis in fish consumers, and whether an increased intake of n-3 PUFA leads to a clinically relevant enhanced in vivo oxidation of LDL is debated [60,66,67]. Still, oxidation of n-3 PUFA in fish oil concentrates should be minimized before their intake, which can be achieved by the addition of small doses of antioxidants (e.g. vitamin E), proper storage and encapsulation. There has been concern about an increased risk of bleeding, especially after consumption of large doses of fish oil concentrates, but there is little clinical evidence in support of this even in patients treated with aspirin or anticoagulants [68—70]. It has been claimed that n-3 PUFA may deteriorate glycaemic control in patients with diabetes mellitus, but recent data have discarded this hypothesis [71,72]. While immune responses can be modulated by n-3 PUFA, there is no evidence that intake of n-3 PUFA is associated with an increased risk of serious infections or cancer [2,73].

Conclusions Data from epidemiological studies, animal and laboratory experiments and from studies on the effects of marine n-3 PUFA on risk factors for CHD lend support for a potential role for these PUFA in the prophylaxis and treatment of CHD. However, only results from controlled clinical trials with hard endpoints can establish whether n-3 PUFA reduce the risk of CHD. In a forthcoming paper [1], we will review such data and conclude on the use of marine n-3 PUFA in the prevention and treatment of CHD.

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