Fish Oil vs. Seal Oil as Omega 3 Essential Fatty Acids

Fish Oil Versus Seal Oil as Sources of n-3 Polyunsaturated Fatty Acids (n-3 PUFA)

Dr. Tony Nakhla - Memorial University of Newfoundland

Dr. Cosmas Ho - Terra Nova Fisheries Company Limited

1998

In the 1970's, it was reported that Greenland Eskimos exhibited low incidences of heart disease and cancer in comparison to populations of the Western world (Bang and Dyerberg, 1972; Bang et al., 1975). These findings were somewhat surprising considering the high-fat Eskimo diet, which consists mostly of fats of marine mammalian origin, such as seal and whale (Bang et al., 1976; Bang et al., 1980). Since those early studies, which emphasized the importance of dietary intake of n-3 PUFA. These include beneficial effects in relation to coronary heart disease (Kromhout et al., 1985; Norell et al., 1986), atherosclerosis (Weiner et al., 1986; David et al., 1987), arthritis (Robinson et al., 1991), hypertension (Norris et al., 1986; Singer and Hueve, 1991) and thrombosis (Goodnight et al., 1986).

More recently, clinical trials have demonstrated that supplementation of diet with a source of n-3 PUFA (modest levels of seal oil, cod liver oil and whale oil) can have beneficial effects associated with cardiovascular and thrombotic diseases (Osterud et al., 1995). Further, a clinical study in the United States concluded that there exists an inverse relationship between fish consumption and death from coronary heart disease, particularly non-sudden death from myocardial infarction (Daviglus et al., 1997).

It is believed that these beneficial effects are contingent on the ratio of n-3 fatty acids in the diet. For instance, a study in Israel has suggested that diets high in n-6 PUFA may have some long-term side effects, including hyperinsulinemia, atherosclerosis, and tumorigenesis (Yam et al., 1996). Accordingly, Health and Welfare Canada has recommended in 1990 that the ratios of n-6 to n-3 fatty acids be reduced from that of the typical Western diet of roughly 20:1 (Simopoulos, 1991) to within the range of 5:1 and 7:1. Incorporating more n-3 PUFA into teh diet, while reducing the intake of n-6 PUFA is the obvious course to regulating this ratio.

Oils from a variety of origins are known to be good sources of n-3 PUFA, including oils from fish and seal, although fish oils have been examined more extensively in terms of their efforts on health, largely because of their availability. As compared to fish oils, marine mammal oils, such as seal oils have similar n-3 PUFA compositions, including eicosapentaenoic acid (EPA) and docosapentaenoic acid (DPA) (Ackman and Ratnayake, 1989; Grampone, 1992; Christensen et al., 1994; Shahidi et al., 1994). Although the role of DPA is not fully understood, a recent study (Kanayasu-Toyoda et al., 1996) has suggested that the anti-atherogenic effects that have been associated with DPA, may be induced by DPA. In that study, DPA was demonstrated to be a potent stimulator of endothelial cell migration, a process that is important in teh repair of blood vessels. Further, it has been demonstrated that the DPA concentration in platelets is inversely associated with coronary artery disease in women (Hodgson et al., 1993).

In addition to the differences in the fatty acid profiles that exist between the oils from fish and marine mammals, seal oils are believed to have other advantages over fish oils as n-3 PUFA sources. For instance, it is well known that n-3 PUFA are highly susceptible to natural oxidation processes, primarily auto-oxidation, in the absence of anti-oxidants (Labuza, 1971; Frankel, 1980). The major products of auto-oxidation are lipid hydroperoxides which function as chemically toxic substances that are believed to lead to a variety of deteriorative processes in humans, including cancer (Frankel, 1980). One potential advantage of seal oils involves a report that described an enhanced resistance to oxidative changes as compared to those observed in fish oils (Shahidi et al., 1994). Further remarkable similarities were observed in terms of the PUFA content of seal oil assessed in 1987 and 1995, to which, as far as can be ascertained, no exogenous antioxidant was added.

Preliminary experiments in our laboratories, conducted over a range of temperatures, have indicated lesser amounts of oxidation products in seal oils than in fish, as determined by the thiobaritoric acid reactive substances (TBARS) assay and p-anisidine values (p-A.V.). This is particularly interesting in view of the high level of concern among the edible oil industry about the oxidation products resulting from endogenous n-3 PUFA metabolism.

Another apparent and potentially significant difference between fish and seal oils is the positional distributions of the n-3 PUFA in the triacylglycerols. In seal oils, n-3 PUFA are mainly esterified at the sn-1,3 position (Brockerhoff et al., 1966; Brockerhoff et al., 1968). These structural differences may influence the uptake and bioavailability of the n-3 PUFA due to the stereospecificity of pancreatic and lipase in the gastrointestinal tract, which both hydrolyse primary ester bonds in the triacylglycerols (Chen et al., 1987; Christensen et al., 1992). The n-3 PUFA are therefore mainly absorbed as sn-2 monoglycerols when fish oil is ingested, but as free fatty acids when the seal oil is consumed, which may affect the digestion and absorption of n-3 PUFA. This may, in turn, result in increased levels of n-3 PUFA in teh erythrocyte membrane and neuronal tissue.

In addition, it has been demonstrated that the metabolism of chylomicrons is affected by the intramolecular structure of the triglyceride in the dietary oils rich in n-3 PUFA (Christensen et al., 1995). This study found, when comparing rates of clearance of chylomicrons following a single injection of either fish or seal oil, that seal oil chylomicrons were cleared faster than fish chylomicrons, suggesting a potential enhanced clearance of circulating cholesterol following ingestion of seal oil.

There appear to be considerable beneficial biological effects associated with the appropriated dietary intake of n-3 PUFA. Accordingly, the recommended daily intake of n-3 PUFA for adult humans can be as high as 1.5g for males and 1.1g for females in the twenty-five to forty-nine age group (Health and Welfare Canada, 1990). Seal oils may have plausible advantages over fish oils as n-3 PUFA supplements which include a favourable n-3 PUFA composition, enhanced resistance to oxidation and a distribution that may allow for augmented digestion and absorption, and perhaps a more rapid clearance of circulating cholesterol.