R .J.B. Bessa et al. Livestock Production Science 63 2000 201 –211
203
rumenic acid. Contrary to what is observed with hamsters Nicolosi et al., 1997 fed atherogenic
mixed conjugated octadienoic acids, Lee et al. diets.
1998 demonstrated that rumenic acid alone does Ha et al. 1990 claimed that CLA was effective
not inhibit the expression of stearoyl-CoA desaturase preventing peroxide formation from unsaturated fatty
mRNA. acids, although nothing in the structure of CLA
Like all conjugated dienes, CLA absorbs UV light suggest that it can possess such antioxidant activity.
at 234nm. This allows it to be detected by UV These authors hypothesised that an oxidised deriva-
spectrophotometry Riel, 1963. This property has tive of CLA formed by the introduction of a b-
been extensively used to study the peroxidation of hydroxy acrolein moiety across the conjugated doub-
unsaturated fatty acids in biological systems because le-bound system is the active antioxidant species
of the characteristic formation of conjugated diene rather than CLA itself. In later revaluations, van den
fatty acid hydroperoxides Logani and Davies, Berg et al. 1995 and Banni et al. 1998 concluded
1980. Conjugated unsaturation in both types of that CLA has only a minor effect preventing linoleic
compounds causes difficulties in interpreting peroxi- acid oxidation, that was attributed to a competition
dation data measured by this method Banni et al., effect, due to the higher susceptibility of CLA to
1998, particularly when CLA content is high. oxidation. Some works even suggest that CLA can
be a pro-oxidant agent Schonberg and Krokan, 1995; Chen et al., 1997.
3. Biological activities of conjugated CLA participates in the modulation of the immune
octadecadienoic acids system, enhancing the mitogen-induced lymphocyte
blastogenesis, lymphocyte cytotoxic activity and The role of CLA in animal metabolism is currently
macrophage killing ability Michal et al., 1992; under strong investigation. The already known ef-
Chew et al., 1997; Wong et al., 1997. CLA reduces fects can be very diverse, and are not fully integrated
catabolic effects in skeletal muscles in birds, rats and into a comprehensive framework of knowledge. The
mice after immune stimulation, without compromis- interest of the scientific community was triggered by
ing the efficacy of the immune response Cook et al., the accidental discovery of anticarcinogenic activity
1993; Miller et al., 1994. The proposed mechanism in fried beef extracts Pariza and Hargraves, 1985 in
to explain this action involves the reduction of which Ha et al. 1987 later isolated CLA isomers as
prostaglandin E PGE synthesis Cook et al.,
2 2
being responsible for the anticarcinogenic action. 1993. The inhibitory effects of CLA on PGE were
2
Since then, the inhibitory action of CLA on several confirmed in several tissues and experimental con-
chemical induced carcinogenesis models was demon- ditions Liu and Belury, 1997; Sugano et al., 1997;
strated, including epidermal Ha et al., 1987; Belury Li and Watkins, 1998; Liu and Belury, 1998. The
et al., 1996, mammary Ip et al., 1991, 1994b; modulation of the eicosanoid synthesis by CLA
Thompson et al., 1997 and gastrointestinal car- probably includes the elongation and desaturation
cinomas Ha et al., 1990; Liew et al., 1995. of CLA resulting in the synthesis of conjugated
Physiological concentrations of CLA inhibits prolif- eicosatrienoic
and eicosatetraenoic
metabolites eration of several human cancer cell lines in vitro
Banni et al., 1996. These metabolites were found in Shultz et al., 1992a,b; Schonberg and Krokan,
phospholipids of the ovine hepatocyte Banni et al., 1995. Anticarcinogenic effects of CLA appear to be
1996 and in the liver Sebedio et al., 1997 and dose dependent, from 0.1 to 1 in the diet Ip et al.,
mammary gland Thompson et al., 1997 of rats fed 1991, 1994b. When extrapolated to humans, this
CLA supplemented diets. range of concentrations is only slightly higher than
Experiments with pigs Dugan et al., 1997 and the estimated consumption of CLA in the USA
mice Park et al., 1997 fed a CLA supplemented population Ip et al., 1994a.
diet, suggest that CLA can act as a fat to lean Dietary CLA supplementation reduces the plasma
repartitioning agent. A similar effect was observed concentration of low density lipoproteins and inhibits
by Chin et al. 1994a in rats. This effect on fat to atherosclerosis onset in rabbits Lee et al., 1994 and
lean repartition apparently results from the reduced
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
