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Departments of Animal Sciences, Ohio Agricultural Research and Development Center, Wooster, OH 44691 * Food Science and Nutrition, The Ohio State University, Columbus, OH 43210
2To whom correspondence should be addressed.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: • conjugated linoleic acid • trans-vaccenic acid • desaturation • rumenic acid • mice
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, Ip et al. 1991
). Some
studies have implicated the cis9,
trans11 isomer as physiologically most
active (Belury 1995
). However, more recent studies have
shown the trans10,
cis12 isomer specifically influences fat
metabolism and body composition (Park et al. 1999b
). The most important dietary source of CLA is
milk fat, but it can be found also in other fat from ruminants
(Chin et al. 1992
, Parodi, 1994
and 1997
). Although the human dietary requirement for CLA, (if any)
is unknown, extrapolation from animal studies suggests that human
dietary intake may be less than adequate for optimal physiologic
responses (Chin et al. 1992
, Ip et al. 1991
).
On the basis of observations by Holman and Mahfouz (1981)
and Pollard et al. (1980)
, Parodi (1994)
proposed that cis9,
trans11 18:2 (rumenic acid, a specific
isomer of CLA; Kramer et al. 1998
) could be produced
endogenously from trans11 octadecenoic acid
by 
-9 desaturase (EC 1.14.99.5). Mahfouz et al. (1980)
, and Pollard et al. (1980)
described
desaturation of trans monoenes to cis,
trans 18:2 derivatives by -9 desaturase.
Trans11 18:1 (trans-vaccenic
acid, TVA) is the predominant trans monoene in ruminant
fats; it is formed by incomplete biohydrogenation of dietary fatty
acids in the rumen (Noble et al. 1974
).
Trans fatty acids in milk normally comprise ~2% of
total fatty acids, but this can be increased to 4–10% of total fatty
acids by increasing dietary unsaturated oils in the cow’s diet.
Trans fatty acids in margarine and vegetable oils are
formed by chemical hydrogenation; usually
trans9 predominates in these (Dutton 1979
). Intake of trans fatty acids was reviewed
by Emken (1995)
; although estimates range widely, he
concluded that the total trans intake in the U.S.
approximates 7–8% of total fatty acid intake. Intake of TVA is more
difficult to estimate; Wolff (1995)
reported intakes in
the European Economic Community of 1.3–1.8 g/d of
trans-18:1 from ruminant fats, except for Spain and
Portugal, which were estimated at 0.8 g/d. Therefore, on the basis of
typical daily consumption of ruminant fats, daily intake of TVA by the
Western population probably exceeds 1 g/d.
It is important to determine the extent to which dietary TVA may
contribute to the body’s supply of CLA. Thus, we investigated whether
TVA is a quantitatively important precursor of rumenic acid. Mice were
chosen as the animal model because of the simplicity of working with
total tissues and the high cost of pure TVA. The main objectives of
these studies were to quantify the conversion of dietary TVA to rumenic
acid in the whole animal and to determine whether conversion of TVA to
CLA is influenced by metabolic modifiers, such as clofibrate, which has
been reported to induce -9 desaturase (Diczfalusy et al. 1995
), and increased dietary polyunsaturated fatty acids
(PUFA), which reportedly inhibit desaturation (Ntambi 1995
). A secondary objective was to examine the effects of TVA
on food intake.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). Animals and diets.
Female C57BL/6 mice (6–7 wk old) were obtained from Harlan Sprague
Dawley (Indianapolis, IN). The mice were housed individually in plastic
cages with wire mesh bottoms in a room with a 12-h light:dark cycle.
Mice were fed and weighed daily at the end of the light cycle; the dark
cycle began at 0800 h. Upon arrival, mice were conditioned to a
powdered purified diet for 4 d, then randomly assigned to dietary
treatments. The purified diets, prepared in the laboratory, were based
on NRC requirements for mice (NRC 1995
; Table 1
), according to guidelines presented by Reeves et al. (1993)
. All experimental fatty acids were of >97% purity,
obtained from Nu-Chek-Prep, Elysian, MN. Samples of diet taken during
the experiment were composited for analysis.
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Eight mice (body weight 16.1 ± 0.51 g, mean ± SEM) were allotted randomly to two treatments with four replicates. Mice were given free access to water and diets containing either 0 or 1% TVA for 2 wk. Two additional mice were used to determine initial body composition and content of TVA and CLA.
Experiment 1.
Mice (n = 30; body weight 16.1 ± 0.61 g)
were allotted randomly to six treatments with five replicates per
treatment as follows: two levels of TVA (0 or 1%) and three effectors
of -9 desaturase activity (none, increased or decreased) in a 2
x 3 factorial arrangement of treatments. The effectors included
no treatment (control), supplemental clofibrate (0.5%), a commonly
used agent to induce desaturase activity (Diczfalusy et al. 1995
), or additional PUFA (as 10% corn oil) reported to
inhibit desaturase activity (Ntambi 1995
). Diets without
TVA contained 1% stearic acid to maintain fatty acid level without
increasing unsaturated fatty acids. Mice were given free access to
water and the experimental diets for 2 wk. Five mice were used to
determine initial body composition and content of TVA and CLA.
Experiment 2.
Mice (n = 40; body weight 17.3 ± 0.49 g)
were allotted randomly to 10 replicates of four treatments in a
completely randomized design. Mice were given free access to water and
purified diets containing 1% stearic acid, TVA, elaidic acid or CLA
for 2 wk. The CLA was described by the supplier as 9, 11 and 10,
12 cis and trans isomers. Material from
this supplier has been reported independently (Sehat et al. 1998
) to be 29% c, t/t, c-10,12–18:2 and
29.5% c, t/t, c-9, 11–18:2, with the remainder being
many other isomers. Six mice were used to determine initial body
composition and content of stearic, trans-vaccenic,
elaidic or conjugated linoleic acids.
Measures.
Body weights and food offered and refused were measured daily. Feces were collected nonquantitatively from the cage bottoms daily and pooled by treatment except for Experiment 2. All mice were killed by CO2 asphyxiation early in the dark cycle on d 14 (except those for initial body composition), 2–4 h after feeding. Immediately after asphyxiation, the abdomen was opened and contents of the stomach, intestines and cecum were removed to obtain empty carcass weight and to avoid carcass contamination with unabsorbed fatty acids. The carcass was freeze dried, chopped and ground in a blender with liquid nitrogen, and the dry weight was determined.
Dry matter and nitrogen determination.
Dry matter (DM) content of the carcass, diets and feces was determined
after drying in a 100°C oven for 24 h. Total N in carcasses was
determined by the Kjeldahl method (AOAC 1980
); crude
protein content of carcasses is reported as the percentage of N x 6.25.
Fatty acid determination.
Fatty acid contents (mg/g DM) and profiles (g/100 g of total fatty acid
methyl esters) of carcasses were determined according to the sodium
methoxide procedure for preparing methyl esters described by
Christie (1982)
. Dried, ground carcass (250 mg; ~50 mg
of lipid), 2 mL of internal standard (triheptadecanoin; 2.0 g
17:0/L benzene), and 2 mL diethyl ether were heated for 2 h at
70°C in a 70-mL sealed screw-capped tube. After cooling, 200
µL each of methyl acetate and 1 mol/L sodium methoxide
in methanol were added. After agitation, the tubes were allowed to
stand at room temperature for 5 min; then 300 µL of
saturated oxalic acid in diethyl ether was added to stop the reaction.
The tubes were agitated, centrifuged for 5 min at 2000 x g to precipitate sodium oxalate, and the methyl esters
in solvent were transferred to 2-mL vials and sealed. Fatty acids were
quantified by gas-liquid chromatography (GC; Hewlett Packard 5890A
Gas Chromatograph, Santa Clarita, CA). Conditions were as follows:
injector temperature 250°C, flame ionization detector 250°C,
nitrogen carrier gas at 0.5 mL/min, detector make up gas
(N2) at 20 mL/min, injector split ratio 1:100. The initial
column temperature was 160°C, programmed at 3°C/min to a final
temperature of 180°C. The column was 30 m x 0.32 mm fused
silica, coated with SP2340 (Supelco, Bellefonte, PA). The detector
signal was collected by a Hewlett-Packard 7673A controller, and
analyzed and summarized by Hewlett-Packard 3365 Chemstation
software.
Fatty acids in the diets and feces were methylated by the acetyl
chloride procedure (Sukhija and Palmquist 1988
) and
quantified by GC as above.
Isomers were identified by authentic standards of each fatty acid (Nu-Chek-Prep), processed at the same time as unknowns. The retention time of elaidic acid was 15.4–15.6 min; the TVA retention time was 15.7–15.8 min, and the CLA retention time was 21.7–23 min. Recoveries of elaidic acid, TVA and CLA were 97–100%. Samples were analyzed in duplicate; <5% difference occurred between duplicate analyses.
Lipid extraction.
For each treatment group in Experiment 2, three carcasses were chosen randomly and pooled. An aliquot of 10 g of pooled carcass was homogenized with 120 mL of Radin solvent (hexane/isopropanol, 3:2, v/v) with a Polytron homogenizer (Brinkman Instruments, Westbury, NY) at 27,000 rpm for a total of 5 min (30 s on, 30 s off) at room temperature. The homogenate was filtered through Whatman #1 paper (Whatman, Clifton, NJ) into 70-mL screw-capped tubes. Organic and aqueous phases were separated by adding excess water to the filtrate; the sample was vortexed, then centrifuged for 5 min at 2000 x g. The hexane layer was adjusted to 100 mL with additional hexane.
Separation of lipid classes.
Lipid classes were separated on a 500-mg silicic acid column (#309250, Alltech Associates, Deerfield, IL), and aliquots were collected in eight fractions by serial solvent development. Fractions were identified with authentic standards (Nu-Chek-Prep), and purity was verified by spotting aliquots on 20 x 20 cm silica gel TLC plates (Whatman). Plates were developed with petroleum ether/diethyl ether/acetic acid (90:10:1). The tubes containing triacylglycerides and phospholipids were methylated by the acetyl chloride procedure described previously, and fatty acid profiles were determined by GC.
Calculations.
The net gain of CLA represents the difference between mean CLA in the carcass of groups of mice fed TVA and mean CLA in the carcass of groups of mice not fed TVA (treatment CLA in carcass - control CLA in carcass). The conversion of dietary TVA represents the net gain in the amount of CLA in the carcass of treatment groups as a percentage of TVA consumed {[carcass CLA (treatment - control)/dietary TVA] x 100}. The conversion of stored TVA is reported as the net gain in the amount of CLA found in the carcass of the treatment group as a percentage of TVA equivalents in the tissues (TVA equivalents = net gain of TVA + CLA). Dietary conversion depends on the total amount of fat in the body as well as proportion of TVA desaturated, whereas stored conversion is independent of the amount of body fat. Because groups were the experimental unit, no statistical confidence for conversion within experiments is possible.
Statistical analysis.
Data were analyzed by ANOVA using the general linear models procedure
of SAS (1988)
. Treatment differences were analyzed for main effects
(diet and modifiers) and interactions (diet x modifiers) for
Experiment 1. For Experiment 2, treatment means separation was
established by Student-Newman-Keuls test (Steel et al. 1997
).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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One mouse in the treatment group was lost by accident. Total food and
fatty acid intakes were not affected by treatment (P < 0.05; Table 2
); by design, intake of TVA was higher (P < 0.0001) for
the experimental group. Total fat and TVA in feces was higher than
those in control when TVA was fed (2.71 vs. 1.17% fat; 57.8 vs. 1.2%
of fecal fatty acids, respectively). CLA accounted for 0.3% of the
fecal fat in mice fed TVA, and 2.1% in controls. The net gain of CLA
in the carcass of mice fed TVA was 38.8 mg (Table 2)
, or 12.3% of the
net TVA consumed during the 14-d feeding period. Because the amount of
TVA available in the tissues for desaturation is dependent upon
absorption and oxidation, we also computed conversion as a percentage
of retained TVA equivalents (see Materials and Methods). On this basis,
48.8% of TVA in tissues was found as CLA.
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Food intake was lower (P < 0.05) in mice fed 10% corn
oil with 1% TVA, compared with those fed 4% corn oil without TVA
(Table 3
). Total fatty acid intake was higher when 10% corn oil was fed, and
1% TVA increased intake of this fatty acid, as planned.
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), and it was decreased by clofibrate (P < 0.05). The
proportion of TVA in fecal fat was increased by its inclusion in the
diet (P < 0.0001), and decreased by feeding clofibrate
or 10% corn oil. The proportion of CLA in fecal fat was <1% and was
not influenced by treatment (P > 0.05).
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. Effects of TVA (P < 0.001),
metabolic modifier (P < 0.001) and the interaction of
these (P < 0.001) were significant for carcass
contents of TVA and CLA for mice fed both TVA and clofibrate. Both TVA
and CLA in the carcass were higher when TVA was fed. Both clofibrate
and 10% corn oil decreased the amounts of TVA and CLA in the carcass,
compared with feeding no modifier of -9 desaturase activity. The
amount of TVA in the carcass of mice fed clofibrate was lower than that
in mice fed control or 10% corn oil diets. Net bioconversion of dietary TVA to CLA was 12.0, 7.5 and 5.1% for mice fed no modifier of desaturase activity, clofibrate and increased PUFA, respectively. Conversion based on TVA equivalents available for desaturation was 52.6, 55.5 and 37.0%, respectively. Thus, clofibrate did not increase desaturation of stored TVA, but higher PUFA intake decreased desaturation by 30%.
Supplementing with clofibrate increased 16:1, whereas TVA, 18:2 and CLA
were decreased in total carcass fatty acids (P < 0.05;
Table 5
). Feeding 10% corn oil decreased 16:0, 16:1 and cis 18:1,
and increased 18:2 and 18:3 (P < 0.05). PUFA tended to
be decreased by clofibrate compared with feeding no supplement.
Inclusion of TVA increased proportions of TVA and CLA in all
treatments.
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Including 1% CLA in the diet decreased food and fatty acid intakes
(P < 0.05, Table 6
). Weight gain of the group fed CLA was lowest (P < 0.05, Table 7
), and reflects a lower accumulation of body fat (P < 0.05). The amount of protein in the body was not influenced by any
dietary manipulations. Unsaturated isomers accumulated in tissues
according to the isomer fed; only TVA was desaturated to CLA. The
amount of elaidic acid found in tissues of mice fed elaidic acid
approximated the TVA equivalents in mice fed TVA, suggesting that these
two trans monoenes were incorporated similarly. The amount
of CLA found in mice fed TVA was higher than that in mice fed CLA
(P < 0.05) but did not differ as a proportion of the
total carcass fatty acids. Retention of dietary CLA was 9.5% of the
amount consumed during the 14-d feeding period. Net conversion of
dietary TVA was 10%; that of TVA equivalents stored was 51%.
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Fatty acid profiles were not analyzed statistically because only one
analysis was made on pooled carcasses from each dietary group. Fatty
acid profiles of triacylglycerol and phospholipids (Table 8
) are typical of these lipid classes (Patton et al. 1982
). Isomeric 18-carbon fatty acids in the triacylglycerol
fractions reflected dietary treatments. Elaidic, but not
trans-vaccenic, acid was found in the phospholipid fraction.
Arachidonic and docosahexaenoic acids were lower in phospholipids of
mice fed elaidic acid and CLA. Of particular interest, CLA was found in
phospholipids only in the group fed CLA. The small amount of CLA found
in triacylglycerol of mice fed stearic and elaidic acids was carried
over from preexperimental diets (amounts not different from amounts
found in preliminary slaughter groups); a small amount could have been
obtained also by coprophagy (Chin et al. 1994b
).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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), an important factor determining digestibility of fatty
acids. It is apparent that stearic acid, fed as the free acid, was not
well absorbed even when it was fed with corn oil. Increasing the amount
of PUFA in the diet decreased the amount of TVA in feces, as expected
from synergistic effects of unsaturated fatty acids on saturated fat
digestibility (Wiseman 1984
). Feeding clofibrate also
decreased TVA and total fat in feces; presumably this was caused by
increased degradation of cholesterol in liver (Reddy and Mannaerts,1994
), which would increase bile acid output and
fatty acid digestibility.
Fatty acid profiles of the carcass were influenced by all treatments.
Modest decreases in saturated fatty acids when the corn oil content of
the diet was increased to 10% reflect the low amount of saturates in
the oil. The proportion of 16:1 increased 20–40%, and cis
18:1 tended to increase when clofibrate was fed, which would be
expected if -9 desaturase activity were increased. Interestingly,
proportions of TVA, 18:2 and other PUFA, including CLA, tended to be
decreased by clofibrate; this was shown previously to occur in liver
microsomal phosphatidylcholine (Nakagawa et al. 1986
). Thus, the failure to increase CLA content of the carcass
by feeding clofibrate was caused by preferential degradation of CLA, as
well as increased degradation of fat in general, resulting in lower fat
content of the body.
The mean conversion of TVA to CLA in the three experiments (3, 5 and 10
mice per treatment group) was 11.4 ± 1.25% of the dietary TVA or
50.8 ± 1.91% of the stored TVA. Thus, bioconversion of TVA
clearly has the potential to increase CLA in tissues, as suggested by
Parodi (1994)
. Emken et al. (1986)
reported that they found no evidence of desaturation of TVA in young
men given a single (7–8 g) dietary bolus of deuterium-labeled TVA.
Possibly the pool was not labeled sufficiently to detect desaturation.
The report of Salminen et al. (1998)
suggests that TVA
is desaturated to CLA; however, the study provided no quantitative
estimate of desaturation.
CLA produced from TVA desaturation was found only in triacylglycerols,
whereas when CLA was fed, it was found in both triacylglycerol and
phospholipid classes. This suggests that desaturation of TVA occurred
in the adipose tissue, consistent with the complete repression of the
hepatic stearoyl-CoA desaturase gene when unsaturated fatty acids
are fed (Ntambi 1995
).
A question arises whether endogenously synthesized CLA in adipose
tissue is biologically available. Park et al. (1999a)
showed that CLA in adipose tissue of mice is mobilized, and thus
available to other tissues. In our short study, mobilization was
apparently insufficient to detect accumulation of endogenously
synthesized CLA in tissue phospholipids (Table 8)
. Further, to the
extent that it occurs (Konrad et al. 1998
), de novo
fatty acid synthesis takes place predominately in liver of humans
(Patel et al. 1975
, Shrago and Spennetta, 1976
); presumably -9 desaturase activity does so also.
Although the genes for stearoyl-CoA desaturase have been identified
in many human tissues (Tocher et al. 1998
), quantitative
expression of activity in tissues is less certain. Liver CLA also is
mobilized and is available for redistribution (Park et al. 1999a
).
We investigated whether desaturation of TVA could be modified by
providing agents in the diet known to influence desaturase activity
(Diczfalusy et al. 1995
, Ntambi 1995
).
Adding 0.5% clofibrate to the diet did not increase desaturation.
Although total CLA in the tissues was decreased, it was not different
from nonsupplemented controls when expressed as a proportion of total
fatty acids. Total body fat content of mice fed clofibrate was
decreased compared with controls (2.2 vs. 2.9 g, P
< 0.0005). The significant effects caused by feeding clofibrate
document that it was biologically active; it is likely that clofibrate
induced the fatty acid degrading (oxidation) as well as desaturating
activity of the peroxisomes (Katoh et al. 1987
),
resulting in lower fatty acid accumulation in the body. Further,
clofibrate preferentially decreases PUFA in diacyl
glycerophosphatidylcholine (Nakagawa et al. 1986
). It
was possible to decrease desaturation of TVA to CLA by 30% by
increasing the polyunsaturated fat content of the diet (10 vs. 4% corn
oil).
Implications for human diets.
Daily intake of CLA by the general population is uncertain.
Salminen et al. (1998)
reported a mean intake of 310
mg/d by 80 persons consuming a diet high in dairy fat.
Ritzenhaler et al. (1998)
reported that females consumed
52 mg/d, and males 136 mg/d from 3-d food records; food-frequency
questionnaires suggested that intake was 60% higher. Intake of TVA has
been estimated at 1.26 (Emken 1995
) and 1.3 -1.8 g/d
(Wolff 1995
). Thus, if retention of CLA and desaturation
of TVA in humans were similar to our quantitative estimates in mice,
TVA intake would increase current estimates of CLA available for
tissues in the general population by 6–10 fold; further, the specific
rumenic acid isomer would be increased.
Numerous effects of dietary CLA on food intake, body weight gain and
body composition have been reported. Increasing CLA in the milk of the
dam increased pup growth, which extended beyond weaning for those
continuing to consume CLA (Chin et al. 1994a
). Improved
feed efficiency was reported as well. Conversely, West et al. (1998)
reported that CLA fed to mice (1–1.2% of diet) from 5
to 11 wk of age reduced food intake, growth rate and body fat content,
and increased the metabolic rate. Park et al. (1997)
observed lower body fat and increased lean body mass with no change in
feed intake when 0.5% dietary CLA was fed to 5-wk-old mice for 32 d. Dietary CLA reduced backfat thickness in pigs with no
(Dunshea et al. 1998
) or variable (Cook et al. 1998
) effects on feed intake. In this study, CLA decreased food
intake and body fat content without changing protein content. In all of
these studies, a mixture of CLA isomers, containing mainly
cis/trans 9, 11 and 10, 12 isomers, was fed.
Trans monoenes have been documented to decrease fatty acid
synthesis (Loor and Herbein 1998
, Teter et al. 1990
); more recently, the trans10
monoene has been implicated specifically (Griinari et al. 1998
). McGuire et al. (1998)
found that
trans9 and
trans10 monoenes were correlated with
reduced milk fatty acid synthesis in lactating women, whereas the
trans11 monoene was not correlated. A
mixture of CLA isomers also decreased de novo fatty acid synthesis
(Loor and Herbein 1998
); recently, direct evidence has
shown that the trans10,
cis12 isomer of CLA is responsible for
decreased fat synthesis, decreased food intake and changes in body
composition (Baumgard et al. 1999
; Park et al. 1999b
)
Food intake and body composition were not changed by desaturation of
TVA to the cis9,
trans11 isomer of CLA, suggesting that this
isomer does not influence fat synthesis, consistent with results of
Baumgard et al. (1999)
and Park et al. (1999b)
. Further research with pure isomers will be required to
determine whether the natural rumenic acid isomer
(cis9, trans11)
is responsible for other biological effects of CLA reported extensively
in the literature (Belury 1995
, Ha et al. 1990
, Ip et al. 1991
).
The efficient conversion of TVA to rumenic acid by desaturation has implications for the role of TVA in health and for proposed regulations for labeling foods for trans fatty acid content. Our data clearly show that trans11 18:1 is metabolized differently from the trans9 monoene and suggest that this fact should be taken into account when decisions are made regarding the trans fatty acid content of foods.
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FOOTNOTES |
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3 Abbreviations used: CLA, conjugated linoleic acid; DM, dry matter; GC, gas chromatography; PUFA polyunsaturated fatty acids; TVA, trans-vaccenic acid.
Manuscript received April 15, 1999. Initial review completed May 28, 1999. Revision accepted October 7, 1999.
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