172
Y. Olsen
believed to have had a ratio 003 to 006 in the upper part of this range (0.5-1)
(Skjervold, 1992; Weber, 1989), whereas the typical Western diet of today contains a much higher fraction of 006 fatty acids «0.1, see Fig. 8.2). The change in
dietary lipid composition for humans is a consequence of the increasing population and our industrial agricultural practices, and this change has been rapid on an
evolutionary time scale (Skjervold, 1992). Some have suggested that this change
in diet is a primary cause of typical Western life-style diseases, be it cardiovascular diseases or inflammatory and allergic disorders (Leaf, 1993; Weber, 1989).
The proposed mechanism of prostaglandin synthesis and the potential metabolic impacts may easily explain why many marine animals, be it marine larvae or
Atlantic salmon, cannot grow efficiently on food rich in 006 fatty acids or if the
003/006 ratio is low. There are no obvious reasons why the long-chain polyunsaturated AA, or its 22C analogue (22:5006), which may be common in membrane
phospholipids (Stubbs, 1992), should not work satisfactorily as a membrane component. It is likely that food too rich in 006 fatty acids may yield an unacceptable,
even fatal, prostaglandin composition for marine species.
8.2.3.5. General Considerations and Concluding Remarks
The general considerations on EFA requirements of marine species are probably
also valid for freshwater species, but there may be major systematic differences at
the species level. Freshwater fish have during their evolution consumed more
EFAs originating from terrestrial sources, with a higher fraction of 006 fatty acids
than the food of marine species at the same trophic level. This is probably reflected both in the normal fatty acid composition of freshwater fish and their
actual requirements for specific EFA. Literature indeed suggests that AA is more
abundant in freshwater than in marine food webs, whereas EPA is less abundant
(Ahlgren et aI., 1994; Henderson and Tocher, 1987). DHA seems to be equally
important in both systems. There is no reason to believe that the quantitative
physiological requirements of PUFA of freshwater fish species adapted to cold
water are different from those of cold-water marine fish species. The metabolic
flexibility to elongate and desaturate short EFA may, however, be systematically
higher in freshwater fish. If so, freshwater species may be less dependent on high
levels of DHA in the diet, because DHA may partly be synthesized from 18:3003.
Experience in mariculture reveals a fundamental difference in DHA metabolism among species of zooplankton and fish. Whereas the metabolism of fatty
acids tends to vary more or less similarly in all species, there are two distinct
patterns of DHA metabolism. Some species exhibit a DHA-conservative metabolism, characterized by a lower catabolism of DHA than of all other fatty acids
during starvation, resulting in increasing percentage DHA of total fatty acids.
Other species tend to catabolize DHA much faster than all the other fatty acids,
yielding a reduced percentage of DHA. It is well known that DHA is the most
important fatty acid in the metabolic, short-term adaptation to low temperature for
many species living in a fluctuating environment (Williams and Hazel, 1992;
Olsen and Skjervold, 1991; Farkas et aI., 1980). I suggest that the different
Y. Olsen
believed to have had a ratio 003 to 006 in the upper part of this range (0.5-1)
(Skjervold, 1992; Weber, 1989), whereas the typical Western diet of today contains a much higher fraction of 006 fatty acids «0.1, see Fig. 8.2). The change in
dietary lipid composition for humans is a consequence of the increasing population and our industrial agricultural practices, and this change has been rapid on an
evolutionary time scale (Skjervold, 1992). Some have suggested that this change
in diet is a primary cause of typical Western life-style diseases, be it cardiovascular diseases or inflammatory and allergic disorders (Leaf, 1993; Weber, 1989).
The proposed mechanism of prostaglandin synthesis and the potential metabolic impacts may easily explain why many marine animals, be it marine larvae or
Atlantic salmon, cannot grow efficiently on food rich in 006 fatty acids or if the
003/006 ratio is low. There are no obvious reasons why the long-chain polyunsaturated AA, or its 22C analogue (22:5006), which may be common in membrane
phospholipids (Stubbs, 1992), should not work satisfactorily as a membrane component. It is likely that food too rich in 006 fatty acids may yield an unacceptable,
even fatal, prostaglandin composition for marine species.
8.2.3.5. General Considerations and Concluding Remarks
The general considerations on EFA requirements of marine species are probably
also valid for freshwater species, but there may be major systematic differences at
the species level. Freshwater fish have during their evolution consumed more
EFAs originating from terrestrial sources, with a higher fraction of 006 fatty acids
than the food of marine species at the same trophic level. This is probably reflected both in the normal fatty acid composition of freshwater fish and their
actual requirements for specific EFA. Literature indeed suggests that AA is more
abundant in freshwater than in marine food webs, whereas EPA is less abundant
(Ahlgren et aI., 1994; Henderson and Tocher, 1987). DHA seems to be equally
important in both systems. There is no reason to believe that the quantitative
physiological requirements of PUFA of freshwater fish species adapted to cold
water are different from those of cold-water marine fish species. The metabolic
flexibility to elongate and desaturate short EFA may, however, be systematically
higher in freshwater fish. If so, freshwater species may be less dependent on high
levels of DHA in the diet, because DHA may partly be synthesized from 18:3003.
Experience in mariculture reveals a fundamental difference in DHA metabolism among species of zooplankton and fish. Whereas the metabolism of fatty
acids tends to vary more or less similarly in all species, there are two distinct
patterns of DHA metabolism. Some species exhibit a DHA-conservative metabolism, characterized by a lower catabolism of DHA than of all other fatty acids
during starvation, resulting in increasing percentage DHA of total fatty acids.
Other species tend to catabolize DHA much faster than all the other fatty acids,
yielding a reduced percentage of DHA. It is well known that DHA is the most
important fatty acid in the metabolic, short-term adaptation to low temperature for
many species living in a fluctuating environment (Williams and Hazel, 1992;
Olsen and Skjervold, 1991; Farkas et aI., 1980). I suggest that the different