FISH NUTRITION
443
The implications of the concept of a relatively stable 2-monoglyceride structure could be important in dietary considerations. Thus,
in the provision of fatty acids and especially essential fatty acids, it is
possible that fish may prefer to absorb, or alternatively to utilize
metabolically, a 2-monoglyceride structure. Indications of such a
situation may be implicit in the work of Nicolaides and Woodall
(1962) mentioned earlier in which depigmentation in young salmon
reared on a fat-free diet was prevented by incorporating trilinolein
but not linolenic acid into the diet. Linolenic acid, however, promoted
growth. This result could indicate that certain cells, e.g. those engaged
in melanin formation, may prefer 2-monolinolein to free linolenic acid
for their normal functioning.
More importantly much of the work referred to earlier on essential
fatty acids in fish shows clearly that free acids are incorporated into
tissue lipids and give growth responses. For these reasons there would
appear to be no reason why fatty acids administered in the diet of fish
should not be readily incorporated into tissue triglycerides and
phospholipids. Nonetheless in the natural state fish do absorb much
of their dietary polyenoic acids from triglycerides as 2-monoglycerides.
5. Fatty alcohols
Long chain fatty alcohols are derived biochemically by enzymic
reduction of long chain fatty acids. They are now known to be the
immediate precursors of two classes of lipids common in certain marine
fish, the wax esters and glyceryl ethers (Fig. 6).
The occurrence of wax esters in marine organisms has been reviewed
by Nevenzel (1970). Certain copepods such as Calanus contain high
proportions of wax esters so that these compounds must figure largely
in the natural diet of certain fishes such as the clupeoids. The intestinal
hydrolysis of wax esters to fatty acid and fatty alcohol and their
absorption is well documented in the rat (Hansen and Mead, 1965).
It is now known that anchovies, herring, sardines and salmon, all
predators on wax-rich animals, possess active wax ester lipases that
cleave the wax esters rapidly leaving no trace of wax esters in serum
lipoproteins (Benson and Lee, 1972). In certain fish such as elasmobranchs, where wax esters may constitute a considerable proportion
of the dietary lipid (Lewis, 1969), oxidation of fatty alcohol to fatty
acid is known to occur readily (Malins and Sargent, 1971 ; Sargent et al.,
1971) so that waxes, where available, should provide useful dietary
supplements. At the same time, many wax esters from marine sources
consist largely of one or two fatty alcohols, e.g. palmityl or oleyl
alcohol, together with one or two fatty acids such as palmitoleic and
443
The implications of the concept of a relatively stable 2-monoglyceride structure could be important in dietary considerations. Thus,
in the provision of fatty acids and especially essential fatty acids, it is
possible that fish may prefer to absorb, or alternatively to utilize
metabolically, a 2-monoglyceride structure. Indications of such a
situation may be implicit in the work of Nicolaides and Woodall
(1962) mentioned earlier in which depigmentation in young salmon
reared on a fat-free diet was prevented by incorporating trilinolein
but not linolenic acid into the diet. Linolenic acid, however, promoted
growth. This result could indicate that certain cells, e.g. those engaged
in melanin formation, may prefer 2-monolinolein to free linolenic acid
for their normal functioning.
More importantly much of the work referred to earlier on essential
fatty acids in fish shows clearly that free acids are incorporated into
tissue lipids and give growth responses. For these reasons there would
appear to be no reason why fatty acids administered in the diet of fish
should not be readily incorporated into tissue triglycerides and
phospholipids. Nonetheless in the natural state fish do absorb much
of their dietary polyenoic acids from triglycerides as 2-monoglycerides.
5. Fatty alcohols
Long chain fatty alcohols are derived biochemically by enzymic
reduction of long chain fatty acids. They are now known to be the
immediate precursors of two classes of lipids common in certain marine
fish, the wax esters and glyceryl ethers (Fig. 6).
The occurrence of wax esters in marine organisms has been reviewed
by Nevenzel (1970). Certain copepods such as Calanus contain high
proportions of wax esters so that these compounds must figure largely
in the natural diet of certain fishes such as the clupeoids. The intestinal
hydrolysis of wax esters to fatty acid and fatty alcohol and their
absorption is well documented in the rat (Hansen and Mead, 1965).
It is now known that anchovies, herring, sardines and salmon, all
predators on wax-rich animals, possess active wax ester lipases that
cleave the wax esters rapidly leaving no trace of wax esters in serum
lipoproteins (Benson and Lee, 1972). In certain fish such as elasmobranchs, where wax esters may constitute a considerable proportion
of the dietary lipid (Lewis, 1969), oxidation of fatty alcohol to fatty
acid is known to occur readily (Malins and Sargent, 1971 ; Sargent et al.,
1971) so that waxes, where available, should provide useful dietary
supplements. At the same time, many wax esters from marine sources
consist largely of one or two fatty alcohols, e.g. palmityl or oleyl
alcohol, together with one or two fatty acids such as palmitoleic and