Effects of Seal Oil and Tuna-Fish Oil on Platelet Parameters and Plasma Lipid Levels in Healthy Subjects

Neil J. Mann • Stella L. O’Connell •

Kylie M. Baldwin • Indu Singh • Barbara J. Meyer

Abstract Fish are a rich source of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), two long-chain polyunsaturated n-3 fatty acids (LC n-3 PUFA) with cardiovascular benefits. A related but less-investigated LC n-3 PUFA, docosapentaenoic acid (DPA), is more common in seal oil and pasture-fed red meats. This study compared indicators of platelet function and plasma lipids in healthy volunteers given supplements containing these different fatty acids (FA) for 14 days. Subjects, randomised into three groups of ten, consumed capsules of tuna oil (210 mg EPA, 30 mg DPA, 810 mg DHA), seal oil (340 mg EPA, 230 mg DPA, 450 mg DHA) or placebo (sunola) oil. Supplementary LC n-3 PUFA levels were approximately 1 g/day in both fish and seal oil groups. Baseline dietary FA and other nutrient intakes were similar in all groups.

Both fish and seal oil elevated platelet DHA levels (P\0.01). Seal oil also raised platelet DPA and EPA levels (P\0.01), and decreased p-selectin (P = 0.01), a platelet activation marker negatively associated with DPA (P = 0.03) and EPA (P\0.01) but not DHA. Plasma

triacylglycerol decreased (P = 0.03) and HDL-cholesterol levels increased (P = 0.01) with seal oil only. Hence, seal oil may be more efficient than fish oil at promoting healthy

plasma lipid profiles and lowering thrombotic risk, possibly due to its high DPA as well as EPA content.

Keywords: • Docosapentaenoic acid • Omega-3 polyunsaturated fatty acids • Cardiovascular disease • Triacylglycerols • Platelet activation • P-selectin

Abbreviations

AA Arachidonic acid

ADP Adenosine diphosphate

ANOVA Analysis of variance

ATP Adenosine triphosphate

BMI Body mass index

cAMP Cyclic adenosine monophosphate

CRP C-reactive protein

CVD Cardiovascular disease

DHA Docosahexaenoic acid

DPA Docosapentaenoic acid

EPA Eicosapentaenoic acid

FA Fatty acid(s)

HDL High density lipoprotein

LC n-3 PUFA Long-chain omega-3 polyunsaturated fatty acids

LDL Low density lipoprotein

MPV Mean platelet volume

MUFA Monounsaturated fatty acids

SD Standard deviation

Greek Symbols

X Ohm (electrical resistance)

Introduction

Cardiovascular diseases (CVD) are the leading causes of death in most westernised countries [1], the primary cause being atherosclerosis [2], a multi-factorial condition which

becomes critical when it affects the supply of blood to the heart or brain [3]. One critical indicator of CVD risk appears to be the ratio between low-density lipoprotein (LDL)-cholesterol and high-density lipoprotein (HDL)-cholesterol [4], as atherosclerosis develops when distinct morphological forms of LDL invade the inner cellular lining of arteries and become oxidised [5, 6], promoting endothelial cell dysfunction. Subsequently, trapped mono-cytes and necrotic debris can rupture and trigger platelet

activation and aggregation to induce an occlusive thrombus [6].

In general, if circulating platelets encounter endothelial damage, agonists such as collagen, adenosine triphosphate (ATP), adenosine diphosphate (ADP) and thrombin pro-

mote the activation of platelets and their adherence to exposed subendothelium, leading to thrombus formation [7]. Once activated, the platelets undergo shape change,

exposing the cell adhesion molecule (CAM) p-selectin on their surface, initiating aggregation and releasing procoagulants. Mechanisms for controlling platelet activation are poorly understood, but there are indications that this may be linked to prostacyclin and nitrous oxide increasing intracellular levels of cyclic adenosine monophosphate

(cAMP) [8, 9], as cAMP controls intracellular levels of calcium ions important for aggregation and adhesion; if levels of cAMP are increased, platelet activation and

aggregation are reduced [9].

Epidemiological studies indicate that high intakes of long chain ([18 carbons) omega-3 polyunsaturated fatty acids (LC n-3 PUFA) are associated with decreased morbidity and mortality from CVD [10]. These LC n-3 PUFA at doses of around 3 g/day or more become incorporated in cell membrane phospholipids and then act to reduce inflammatory factors [10] and decrease hepatic triacylglycerols [11]. However, free LC n-3 PUFA in extracellular fluids appear to affect cardiac arrhythmias at doses of only 1 g/day [10]. In general intake of fish or use of fish oil supplements lowers CVD risk factors [12–14], apparently through the actions of eicosapentaenoic acid, C20:5n-3 (EPA), and docosahexaenoic acid, C22:6n-3 (DHA).

Both EPA and DHA have been shown to lower plasma triacylglycerol levels, an independent risk factor for CVD [15], as well as increase levels of HDL-cholesterol [16]. Supplementary EPA has successfully suppressed the expression of platelet activation markers and the release of platelet-derived microparticles capable of generating

thrombin (a potent platelet aggregator), thereby retarding further platelet activation [17, 18]. There is evidence that EPA and DHA also reduce platelet aggregation [19].

Red meat contains relatively small amounts of EPA and DHA, but is rich in another LC n-3 PUFA: docosapentaenoic acid, C22:5n-3 (DPA), which is only present in small amounts in a few oily fish [20]. Reported intakes of DPA in countries such as France [21], Japan [22] and Denmark [23] are generally similar to Australian values [24]. Since Australians consume around six times as much meat as fish and seafood [25], this food contributes significantly (up to 43%) to the total LC n-3 PUFA intake of many Australians, and DPA may make up almost one-third of the LC n-3 PUFA in the average adult Australian diet [24]. As such, further investigation of its effects on cardiovascular and general health is required.

The similarity in molecular structure of DPA to EPA and DHA makes it feasible that DPA has similar beneficial effects to DHA and EPA on cardiovascular parameters. All

three of these LC n-3 PUFA are significantly and inversely related to carotid intimal-medial thickness [22], and DPA, like EPA and DHA, has been shown to have an anti-aggregatory effect on platelets [26, 27]. A large prospective study in healthy males found that serum levels of EPA were not associated with reduced risk of acute coronary events, whereas subjects with the highest levels of combined DHA and DPA were at reduced risk compared to those with the lowest levels [28]. Supplementary seal oil, relatively rich (compared with fish oil) in DPA, has been found to induce an increase in the LC n-3 PUFA status of serum phospholipids in normal men, with the suggestion of a favourable shift in the balance of plasma pro-coagulant and anticoagulant activity [29]. LC n-3 PUFA have also shown anti-inflammatory effects in many experimental models as well as in clinical conditions of inflammation [10], and evidence for the specific benefits of seal oil in such cases has been mainly positive [30, 31], although not always significant [32].

Commercial quantities of pure food-grade DPA are not currently available for use in a clinical trial. Seal oil (which also contains EPA and some DHA) is the richest commercially-available source of DPA but, to our knowledge, no report of its effect on platelet activation has previously been published. This study examined the effects of seal oil on blood lipids and platelet factors compared to those of tuna-fish oil (which contains minimal DPA, 40% less EPA and double the DHA of seal oil) in healthy volunteers. A placebo oil containing no LC n-3 PUFA provided a control in this randomised double-blind parallel intervention study.

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