166
Y. Olsen
efforts to scale down the analytical methods for determination of lipid and fatty
acids.
Most laboratories now use quantitative methods in their determination of fatty
acids (see Parrish, this volume). By adding an internal fatty acid standard before
extraction of the sample (normally 19:0 or 21 :0), it is possible to derive quantitative values for individual fatty acids after careful calibration, establishment of
response curves, and the use of on-column injection of the material (see Rainuzzo,
1993). The total weight offatty acids in the sample can then be estimated as the
sum of the individual fatty acids, which is the basis for the estimation of percentage fatty acid distribution. Absolute fatty acid content (e.g., milligrams fatty acid
per gram dry material) and relative fatty acid distribution (i.e., percentage of a
given fatty acid of total fatty acids) are complementary assessment criteria for
nutritional value. The quantitative content of given EFA is obviously important
during evaluations of the EFA requirements of given species. The percentage
distribution is important because different EFAs tend to compete with each other
in many enzymatic reactions. The relative proportions of the competing EFAs of
the diet or the ratio between specific EFAs may then be more important than their
absolute quantitative contents.
Quantitative determination of fatty acids was not common some 10-15 years
ago. Most values reported were in terms of percentage content of the total fatty
acids, primarily because the quantitative method was little known. Unfortunately.
the relative values in the older literature cannot easily be converted to quantitative
values. because the absolute sum of fatty acids, which forms the 100% value, is
normally not known. It is important to realize that the quantitative sum of fatty
acids is not equivalent to the quantitative content of lipids. which was indeed
frequently measured in earlier studies. Approximate estimates of the absolute
fatty acid contents based on percentage fatty acids and quantitative lipid content.
thus assuming that lipids constitute the 100% level for fatty acids, will overestimate fatty acid values to some degree. The fraction of fatty acids to total lipids is
variable, dependent on the material analyzed.
Algae show normally relatively low fractions of fatty acids to lipids (e.g .. 3253%) (Reitan et aI., I 994b). Animals, however, tend to exhibit higher fractions, in
particular fat animals with high content of TAGs. For example. the fatty acid
fraction of total lipids found for relatively fat Atlantic salmon (s. Safar) was
higher than that for the leaner B. plicatilis (Fig. 8.4). This apparent discrepancy is.
however. part of the same general pattern of variation between lipid content and
the fractioin of fatty acids, suggesting that this is representative for animals that
store TAGs, including freshwater animals (Fig. 8.4).
The international cooperation in aquaculture research that developed gradually
through the 1980s exposed the need for intercalibration of methods for lipid and
fatty acid determination. This was necessary for comparison of results derived
from different laboratories and therefore also for the progress of the research. The
most recent calibration exercise has revealed that our methodological capabilities
have indeed improved during the past decade. This exercise, which was run by the
International Council for the Exploration of the Sea, involved a standardized
Y. Olsen
efforts to scale down the analytical methods for determination of lipid and fatty
acids.
Most laboratories now use quantitative methods in their determination of fatty
acids (see Parrish, this volume). By adding an internal fatty acid standard before
extraction of the sample (normally 19:0 or 21 :0), it is possible to derive quantitative values for individual fatty acids after careful calibration, establishment of
response curves, and the use of on-column injection of the material (see Rainuzzo,
1993). The total weight offatty acids in the sample can then be estimated as the
sum of the individual fatty acids, which is the basis for the estimation of percentage fatty acid distribution. Absolute fatty acid content (e.g., milligrams fatty acid
per gram dry material) and relative fatty acid distribution (i.e., percentage of a
given fatty acid of total fatty acids) are complementary assessment criteria for
nutritional value. The quantitative content of given EFA is obviously important
during evaluations of the EFA requirements of given species. The percentage
distribution is important because different EFAs tend to compete with each other
in many enzymatic reactions. The relative proportions of the competing EFAs of
the diet or the ratio between specific EFAs may then be more important than their
absolute quantitative contents.
Quantitative determination of fatty acids was not common some 10-15 years
ago. Most values reported were in terms of percentage content of the total fatty
acids, primarily because the quantitative method was little known. Unfortunately.
the relative values in the older literature cannot easily be converted to quantitative
values. because the absolute sum of fatty acids, which forms the 100% value, is
normally not known. It is important to realize that the quantitative sum of fatty
acids is not equivalent to the quantitative content of lipids. which was indeed
frequently measured in earlier studies. Approximate estimates of the absolute
fatty acid contents based on percentage fatty acids and quantitative lipid content.
thus assuming that lipids constitute the 100% level for fatty acids, will overestimate fatty acid values to some degree. The fraction of fatty acids to total lipids is
variable, dependent on the material analyzed.
Algae show normally relatively low fractions of fatty acids to lipids (e.g .. 3253%) (Reitan et aI., I 994b). Animals, however, tend to exhibit higher fractions, in
particular fat animals with high content of TAGs. For example. the fatty acid
fraction of total lipids found for relatively fat Atlantic salmon (s. Safar) was
higher than that for the leaner B. plicatilis (Fig. 8.4). This apparent discrepancy is.
however. part of the same general pattern of variation between lipid content and
the fractioin of fatty acids, suggesting that this is representative for animals that
store TAGs, including freshwater animals (Fig. 8.4).
The international cooperation in aquaculture research that developed gradually
through the 1980s exposed the need for intercalibration of methods for lipid and
fatty acid determination. This was necessary for comparison of results derived
from different laboratories and therefore also for the progress of the research. The
most recent calibration exercise has revealed that our methodological capabilities
have indeed improved during the past decade. This exercise, which was run by the
International Council for the Exploration of the Sea, involved a standardized