Accumulation of dietary fish fatty acids in the body fat reserves of some carnivorous fur-bearing animals

Rouvinen, K., Mäkelä, J.,Kiiskinen, T. & Nummela, S. 1992. Accumulation ofdietary fish fatty acids in the body fat reserves of some carnivorous fur-bearing animals. Agric. Sei. Finl. I: 483-489. (Agric. Res. Centre of Finland, Fur Farming Res. Sta., SF-69100 Kannus, Finland, Finnish Fur Breeders’ Association, P.0.80x 5, SF-01601 Vantaa, Finland, Agric. Res. Centre of Finland, Inst. Anim. Prod., SF-31600 Jokioinen, Finland and Agric. Res. Centre ofFinland, Centr. Lab., SF-31600 Jokioinen, Finland.)


Introduction
In monogastric animals, dietary fat has a strong influence on the fatty acid composition of the tissues and organs. Feeding vegetable oils and fish oils to mink and blue foxes has been shown to increase the levels of linoleic and omega-3 fatty acids, respectively, in the fat depots and the liver of the animals (Rouvinen and Kiiskinen 1989, Skrede and Gulbrandsen 1985, Skrede 1984, Ahman 1965. In blue and silver foxes, feeding an abundance of fish fat is known to cause prominent accumulation of the typical fish fatty acids, i.e. cetoleic (C 22: Icol 1), eicosapentaenoic (EPA, C20:5c03) and docosahexaenoic (DHA,C22:6c03) acids, in their subcutaneous fat, liver tissue and heart muscle (Rouvinen 1991, Rouvinen 1992). In the rat, feeding fish oil or high erucic acid (C22;1c09) rapeseed oil has been shown to cause lipid infiltration, cell destruction, local inflammatory reactions and fibrous scar tissue growth in the heart muscle (Beare-Rogers 1977, Kjnsella 1987. Accumulation of these long-chained fatty acids in body tissues is apparently due to their impaired oxidation. The 20 or 22 carbon atomchained fatty acids should first be shortened in the peroxisomes to 16 or 18 carbon atom-fatty acids before they can be metabolized by the mitochondrial p-oxidation (Opstvedt 1984).
The present paper reports the effects of feeding lard or fish oil supplemented diets on the body fat composition of some farm-raised carnivorous furbearing animals, mink, polecat and the raccoon dog.

Material and methods
Mink (Mustela vison), polecats (Mustela putorius) and raccoon dogs (Nyctereutes procyonoides) were fed diets based on slaughterhouse offal, fish offal and cereals supplemented either with lard (LA) or fish oil (FO), at 5% in the diet. The animals used were all bora during the spring of 1988, and were raised according to normal fur farming practices at the Veikkola Research Farm of the Finnish Fur Breeders' Association, Kirkkonummi. The trial lasted from July until pelting during the autumn of 1988. Composition of the experimental diets is presented in Table 1 where p is the general mean, Sj is the species effect (/ = 1-3), Dj is the dietary effect (j = 1-2), SD.j represents the species diet interaction and eijk is the error term. Differentiation among the mean values was done by Duncan's multiple-range test. There were no statistical differences (p>0.05) between sexes in the fatty acid composition of the tissue samples taken from the raccoon dogs, therefore the means given represent pooled data from both sexes.

Results
Chemical composition of the experimental diets was similar in lard (LA) and fish oil (FO) supplemented groups (Table 2). Dietary fat content was high, 27-30% in DM, which accounted for approximately 50% of the metabolizable energy in both dietary groups.
Dietary fatty acid composition showed a great difference between the lard and fish oil supplemented diets (Table 3). The lard diet contained more stearic (C18;0) and oleic (C 18:1 co 9) acids, but in the fish oil diet the content of cetoleic (C 22: Icol 1), eicosapentaenoic (C20:5c03) and do- cosahexaenoic (C22;6c03) acids was considerably higher. Body weights of the animals and the weights of the liver and the heart did not differ between the dietary treatments. The body weights for the mink, polecats and the raccoon dogs were on average 2249 g, 2055 g, and 8709 g. The average weights of the liver and (heart) were 48.6 g, (10.6 g), 57.9 g, (8.5 g), and 208.7 g (32.7 g) for the mink, polecat and the raccoon dog, respectively. There were no species diet interactions.
Clear species differences were found in the liver fat content and body fat composition of the animals (Table 4). Besides higher fat content in the mink livers the variation in the fat content was considerably higher for this species. Dietary background of the animals did not affect the liver fat content.
The fatty acid composition of the tissue samples strongly reflected the fatty acid profile of the dietary fats in all species and in all fat and organ samples studied (Table 4). Furthermore, interesting  4. Liver fat content and the content of cetoleic, eicosapentaenoic and docosahexaenoic acids in the liver tissue, heart muscle and subcutaneous fat of the mink, polecat and the raccoon dog fed two different diets, LA = lard, FO = fish oil. differences between the individual fish fatty acids in their affinity to certain fat depots in different species were observed. In the FO diet, cetoleic acid accumulated especially in the heart muscle and the subcutaneous fat, while its levels in the liver fat were comparatively low. The highest levels ofcetoleic acid were found in the heart and subcutaneous fat of the raccoon dogs. Moreover, in all species studied the accumulation of EPA and DHA was more prominent in the liver and heart tissue than in the subcutaneous fat. The highest levels of EPA were found in the tissues of the raccoon dogs and polecats fed the FO diet. In addition, the polecats and raccoon dogs fed the FO diet had significantly higher levels of DHA in their livers than did the mink or the corresponding animals receiving the LA diet. The DHA levels in heart were the highest in polecats fed the FO diet.

Discussion
Fatty acid profiles of the lard and fish oil supplemented diets differed greatly reflecting the composition of the supplemental fat. Both feed mixtures contained, however, a considerable amount of fish offal, which contributed to omega-3 fatty acids in both diets.
The accumulation of the polyunsaturated omega--3 fatty acids was prominent in the fish oil diet forall fur animal species included in this study. There were also significant species differences, polecats and raccoon dogs having remarkably higher contents of these fatty acids in their tissues and especially in the liver than the mink. These results are in agreement with earlier studies with mink and blue fox (Rouvinen and Kiiskinen 1989), and with blue and silver foxes (Rouvinen 1991, Rouvinen 1992. The blue foxes fed a fish oil supplemented diet tended to concentrate more omega-3 fatty acids in their livers than the mink (ROUVINEN and KIIS-KINEN 1989), while in silver fox livers the levels of these fatty acids were even higher than in blue foxes when the animals were fed the same diet (ROU- VINEN 1991, Rouvinen 1992). In the earlier study (Rouvinen 1991), the fat accumulation pattern differed between the fish oil and saturated fat diets. In the fish oil diet, the fat was present in the liver in small droplets, which was considered to be unphysiological. In addition, the degenerative changes observed were more numerous and severe in this dietary group (ROUVINEN 1991).
It is interesting to note that the levels of DHA in the liver and heart tissues were very high also for the LA diet. This may suggest that even very low amounts of dietary DHA will accumulate in these tissues. In addition, significant species differences could be observed with the lowest amount of DHA found in the liver of the mink, whereas in the heart the lowest amount was found in the raccoon dog. Moreover, despite equal amounts of DHA and EPA within both diets, the accumulation of DHA was in nearly all cases much more severe than ofEPA. The 22 carbon chain of DHA could simply be more difficult to shorten to 16 or 18 carbons compared to EPA molecule with only 20 carbon atoms.
In the rat, dietary erucic acid and its omega-11 isomer, cetoleic acid, have been shown to cause lipid infiltrationand tissue degeneration in the heart muscle (Beare-Rogers 1977, Kinsella 1987).
Several cases of unexplained cardiac failure have recently been reported in Finnish silver foxes (SMEDS 1992). In the 1980's, during the years of intensive fox production, the dietary composition of the wet fiir animal feed has changed considerably. Meat and slaughter offal based ingredients have been replaced by fish offal and industrial fish dueto their more affordable price. Moreover, the use of fish oil as a cheap energy supplement has increased. It is thus reasonable to believe that there may be a connection between the accumulation of the longchained marine fatty acids, especially cetoleic acid, in the silver fox heart tissue and the pathological condition observed. Support to this hypothesis may also be found from a recent work on blue fox vixens, where long-term fish feeding prior to breeding and suckling period was shown to increase pup mortality (ROUVINEN and NIEMELÄ 1992).
The key enzymes of the peroxisomal and mitochondrial [3-oxidation are the fatty acyl-CoA oxidase (FAO) and carnitine palmitoyl transferase (CPT), respectively. Their activities reflect the capacities of the corresponding oxidation pathways (Moves et al. 1991). The peroxisomal (3-oxidation is known to be induced when the diet contains high fat levels or fatty acids which are poor substrates for mitochondria (MOVES et al. 1991). In species, such as seals and salmon, which normally encounter polyunsaturated fatty acids in their diets, the mitochondria are, however, better capable to oxidize long-chained unsaturated fatty acids (Moves et al. 1991). In the rat, feeding marine oils is known to induce the peroxisomal (3-oxidation. This is considered to be the response of an omnivore to specific fatty acids which are not normally obtained in the diet. It is essentially a detoxification response (Moves et al. 1991).
It is very likely that the differences in the accumulation of the long-chained marine fatty acids found in the present study are caused by species differences in the efficiency of the peroxisomal (3oxidation. This is apparently a consequence of adaptation to certain food sources during the evolutionary development of the different fur animal species (Nelson and Ackman 1988). Unlike the foxes (Dekker 1983, Fay andStephenson 1989), raccoon dog (Mäkelä and Kiiskinen 1978) and polecat (Fox 1988), the mink seems to be well capable of utilizing the long-chained polyunsaturated fatty acids due to its adaptation to a semiaquatic habitat (Kyne et al. 1989, Tolonen 1982. Therefore, the other farm-raised carnivorous furbearers should not be fed fatty fish or fish oil in excess, since the 20 and 22 carbon-atom fatty acids are more readily accumulated in the tissues and organs of these species. The present results suggest that more emphasis should be placed on a species specific formulation of the diets for the farm-raised fur animals. Minkeillä ja siniketuilla rehun kasviöljylisäyksen tiedetään lisäävän linolihapon määrää jakalaöljytäydennyksen puolestaan omega-3 rasvahappojen pitoisuutta elimistössä. Sini-ja hopeaketuilla kalarasvaruokinnan on todettu aiheuttavan tyypillisten kalarasvahappojen, kuten ketoleeni-, eikosapentaeeni-(EPA) ja dokosaheksaeenihappojen (DHA) kertymistä nahanalaisrasvakudokseen, maksaan ja sydänlihakseen. Rehun kalarasvatäydennyksen tai erukahappopitoisen rypsiöljyruokinnan tiedetään myös aiheuttavan rotalla rappeutumamuutoksia sydänlihaksessa.