204 R
.J.B. Bessa et al. Livestock Production Science 63 2000 201 –211
lipogenesis and increased lipolysis in adipose tissue developed for the large scale synthesis of CLA
Park et al., 1997. Furthermore, West et al. 1998 which may allow the production of CLA rich meat
concluded that in mice fed high-fat and low-fat CLA and eggs through feeding regimes containing these
supplemented diets, CLA reduces body fat by de- synthesised CLA’s. In monogastrics, the inclusion of
creasing feed intake, by increasing metabolic rate CLA in diets is particularly promising because its
and by decreasing the night-time respiratory quot- products have low CLA content and hydrogenation is
ient. This lipid catabolic response is in agreement minimal in the gastrointestinal tract. Kramer et al.
with observations by Belury et al. 1997, suggesting 1998b introduced 1 of a CLA commercial mix
that CLA acts as a typical hepatic peroxisome into pig diets which resulted in the enrichment of
proliferator. Besides the effects on lipid metabolism, meat lipids with the isomers present in the mix.
the proposed reduction of PGE synthesis in skeletal
2
muscles induced by CLA Cook et al., 1993 may result in reduced proteolysis Rodemann and Gold-
5. Rumen biohydrogenation and CLA synthesis
berg, 1982. Rumenic acid was initially identified by Kepler et
al. 1966 as being an intermediate agent in the
4. The dietary sources of CLA biohydrogenation of linoleic acid by the Butyrivibrio
fibrisolvens rumen bacteria. The cis-12, trans-11 CLA is present in a great variety of feeds,
octadecenoate isomerase which catalyses the trans- although usually in residual quantities Forgerty et
formation of linoleic acid into rumenic acid needs al., 1988; Chin et al., 1992; Lin et al., 1995. CLA
the free COOH radical Kepler et al., 1970. This concentration is higher in ruminant products, par-
implies a prior lipolysis of galactolipids, phos- ticularly rumenic acid Parodi, 1977 and 1994. The
pholipids and triglycerides of the diet before isomeri- presence of the conjugated double bond system in
sation takes place. The initial isomerisation is fol- milk was first described by Booth et al. 1935. Riel
lowed by the saturation of cis-9 double bond through 1963 studied and reviewed the presence of total
the reductase characterised by Hughes et al. 1982 conjugated dienes in milk fat and observed that they
resulting in trans-vaccenic acid trans-11 C18: 1, can vary between 2 and 28 mg g, showing a marked
the major trans isomer of ruminant tissues. seasonal fluctuation related to the feed cycle. How-
This metabolic pathway Fig. 2 is the best known ever, common milk CLA concentrations are between
and it is thought to be quantitatively the most 3 and 6 mg g Kelly et al., 1998a. Consequently,
expressive Harfoot and Hazlewood, 1988. The cheese and other dairy products are also excellent
diversity of
octadecenoic Table
1 and
oc- sources of CLA Ha et al., 1989; Shantha et al.,
tadecadienoic isomers in the reticulo-rumen and fat 1992; Werner et al., 1992; Lin et al., 1995. In some
produced by ruminants reflects the high biological cheeses, an enrichment of CLA levels is possible
diversity of the reticulo-rumen ecosystem. Scarce during the technological process Shantha et al.,
information is available concerning metabolic path- 1992; Werner et al., 1992. Ruminant meat is also an
ways and implicated microorganisms for the most important source of CLA. In the case of mutton, 14.9
part of these isomers, and especially for methylene mg g concentrations were found in intramuscular
interrupted octadecadienoic isomers. Butyrivibrio lipids in Australia Forgerty et al., 1988, 5.6 mg g
fibrisolvens and most bacteria capable of bio- in the USA Chin et al., 1992 and 12.0 mg g in
hydrogenation are unable to hydrogenate mono- Germany Fritsche and Steinhart, 1998. In one of
enoic acids. In the reticulo-rumen only three bacteri- our studies we determined a 5.1 mg g CLA con-
al strains which were able to hydrogenate trans-11 centration in the total fatty acids of the Longissimus
C18:1 and cis-9 C18:1, thus producing stearic acid dorsi muscle in lambs fed with dehydrated lucerne
Harfoot and Hazlewood, 1988. The concentration Bessa et al., 1998.
of trans C18:1 isomers in the rumen is affected by Several methods such as alkaline isomerisation of
several factors such as the concentration of unsatu- vegetable oils Kramer et al., 1998b have been
rated fatty acids Bessa, unpublished.; Bateman and
R .J.B. Bessa et al. Livestock Production Science 63 2000 201 –211
205
that the accumulation of trans octadecenoic acids in the reticulo-rumen ecosystem may also be useful as a
response to environmental stress such as low pH, presence of ionophores and high concentrations of
fatty acids.
In monogastrics, gut microorganisms also produce CLA Chin et al., 1994b, although to a lesser extent
because of the reduced size and the anatomical position of fermentative compartments.
As shown by Pollard et al. 1980 and Holman and Mahfouz 1981 in mice, rumenic acid may also
be synthesised by the desaturation of trans-vaccenic acid trans-11 C18:1 resulting from the action of
hepatic microssomal D9 desaturases. This mecha- nism of CLA formation may also be observed in
ruminants, where trans-
11 C18:1 availability is greater than in monogastric animals. The abomasal
infusion of trans-11 C18:1 and trans-12 C18:1 in
Fig. 2. Pathway of linoleic acid biohydrogenation in the rumen
dairy cows induced an increase in rumenic acid
Harfoot and Hazlewood, 1988.
concentration in the milk as well as that of cis-9, Jenkins, 1998; rumen pH Kalscheur et al., 1997;
trans-12 C18:2 which was previously non existent and ionophores Fellner et al., 1997; Sauer et al.,
Corl et al., 1998. Bauman’s team at Cornell, Ithaca, 1998. Therefore, the reticulo-rumen micropopula-
is presently studying this hypothesis Bauman et al., tion may be extremely sensitive to these factors.
1998. It is not likely that trans octadecenoic acid In other biological systems, it was proven that
availability limits this pathway in ruminants, because certain bacterial strains have direct octadecenoate
the concentration of trans octadecenoic acids in the cis –trans isomerisation mechanisms in the cellular
milk and fat depots is always much higher 3 to 12 membrane, thus reducing membrane fluidity as a
times than CLA. The possibility of humans syn- defence mechanism against lipophylic and toxic
thesizing CLA from trans octadecenoic acids in the stress stimuli Keweloh and Heipieper, 1996. If it is
same way as ruminants Parodi, 1994; Bauman et al., accepted that trans fatty acids may play a protective
1998 may bring new meaning to the role of trans role for bacteria which are subject to different
octadecenoic acids, especially that of trans-11 sources of stress as proposed by Guckert et al.
C18:1, which is present mainly in ruminant fat. 1986 and Sikkema et al. 1995, it is conceivable
Importance was given to trans octadecaenoic
Table 1
a
Proportion of cis and trans octadecenoic isomers in cow milk fat butter adapted from Wolff et al., 1995 trans isomers
of total trans isomers cis isomers
of total cis isomers trans-8
1.5 cis-8
0.5 trans-9
13.6 cis-9
94.0 trans-10
6.4 cis-10
0.2 trans-11
64.4 cis-11
4.2 trans-12
2.4 cis-12
0.1 trans-13
2.3 cis-13
0.5 trans-14
3.6 cis-14
0.1 trans-15
2.3 trans-16
3.0
a
There are seasonal variations.
206 R
.J.B. Bessa et al. Livestock Production Science 63 2000 201 –211
isomers in this paper because they are found in large tadecaenoic isomers with precision and observed that
quantities in ruminant tissues and because positive the milk fat content is negatively correlated with
correlations between CLA and trans C18:1 isomer only one minor isomer trans-10 C18:1. These
concentrations were observed in milk Jiang et al., results were confirmed by Newbold et al. 1998
1996; Newbold et al., 1998, lamb muscle and who showed that trans-10 C18:1 concentration is not
adipose tissue Fig. 3 Bessa et al., 1998. There- correlated with that of CLA or trans-11 C18:1.
fore, the formation of CLA rich products implies a Bauman et al. 1998 go even further in suggesting
parallel enrichment in trans C18:1 isomers. that the negative effects of CLA in milk and body
The trans C18:1 CLA ratio may vary widely. In fat synthesis could be due to isomers which have a
reticulo-ruminal contents and bacteria, Bessa un- trans-10 bond.
published findings found values that ranged from 25 Trans acids may have negative effects not only on
to 32. In growing lambs, we found values of 8.7 in animal productivity but also on human health Wahle
perirenal fat, 7.8 in subcutaneous fat and 3.4 in and James, 1993. There is considerable concern and
Longissimus dorsi muscle lipids Bessa et al., 1998. controversy regarding the latter, particularly in plas-
In cow milk, Jiang et al. 1996 determined a value ma
lipoprotein concentrations
and composition
of 4.0 for trans-11 C18:1 CLA ratio. These differ- which are related to an increase in cardiovascular
ences may be partially explained by the possible diseases Judd et al., 1994; Precht and Molkentin,
preferential incorporation
of CLA
into phos-
1995; Ackman, 1997; ASCN AIN Task Force on pholipids, preferential secretion of CLA in milk or
trans Fatty Acids, 1996. Further studies need to be perhaps by differences in D9 desaturase activity in
carried out regarding the specific biological effects of several tissues.
each isomer and the confirmation that trans-11 The association between CLA and trans-oc-
C18:1 may be a quantitatively relevant precursor for tadecenoic acids may limit the interest of increasing
rumenic acid. their concentration in ruminant products. Trans-oc-
tadecenoic isomers have been blamed for the de- crease of fat in cow milk Gaynor et al., 1994;
6. Manipulation of rumen biohydrogenation as
