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 Posted: Sat Nov 29, 2008 8:51 am Post subject: Wild animals protected from PUFAs by biohydrogenation |
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Biohydrogenation
Biohydrogenation of unsaturated fatty acids is the second major
transformation that
dietary lipids can undergo in the rumen. It requires a free fatty acid
to proceed and as a
consequence rates are always less than those of hydrolysis, and
factors that affect
hydrolysis also impact biohydrogenation (Figure 1). Most
biohydrogenation (>80%)
occurs in association with the fine food particles and this has been
attributed to
extracellular enzymes of bacteria either associated with the feed or
free in suspension
(Harfoot and Hazlewood, 1997). The major substrates are linoleic and
linolenic acids
(Table 1) and the rate of rumen biohydrogenation of fatty acids is
typically faster with
increasing unsaturation. For most diets linoleic acid and linolenic
acid are hydrogenated
to the extent of 70-95% and 85-100%, respectively (Doreau and Ferlay,
1994; Beam et
al., 2000). This extensive level of hydrogenation is reduced when
diets high in
concentrates are fed, which can be attributed to inhibition of
lipolysis at the low pH that
is typically observed on these diets (Van Nevel and Demeyer, 1995;
1996b).
Hydrogenation is also adversely affected when excessive unprotected
lipid is present in
the diet. How fat interferes with microbial fermentation is not clear,
but is believed to
result from either the coating of feed particles or a direct toxic
effect on the rumen
microorganisms (Jenkins, 1993).
Fish oil contains two longer chain fatty acids, eicosapentaenoic acid
(EPA; 20:5) and
docosahexaenoic acid (DHA; 22:6), that are often included in rumen-
protected lipid
supplements designed to improve reproductive performance. The degree
to which EPA
and DHA are biohydrogenated in the rumen is still not well understood.
Studies
conducted in vitro suggest that there is little biohydrogenation of
these two omega-3
fatty acids (Ashes et al., 1992; Gulati et al., 1999). However, in
vivo studies involving
dietary supplements of fish oil indicate that much of the EPA and DHA
is
biohydrogenated, although to a lesser extent than typically observed
for linoleic and
linolenic acids (Chilliard et al., 2000; Wachira et al., 2000; Scollan
et al., 2001).
Classical pathways of biohydrogenation were established using pure
cultures of
rumen organisms and these are presented in Figure 2. The rumen
bacteria involved in
biohydrogenation have been classified into two groups, A and B, based
on their
metabolic pathways (Kemp and Lander, 1984). To obtain complete
biohydrogenation of
polyunsaturated fatty acids (PUFA), bacteria from both groups are
generally required (Figure 2). Although Group A contains many bacteria
that can hydrogenate PUFA to
trans 18:1 fatty acids, only a few species characterized as Group B
can hydrogenate a
trans 18:1 fatty acid to stearic acid (Harfoot and Hazlewood, 1997).
This feature of
biohydrogenation explains why increased feeding of PUFA simultaneously
causes an
increase in the rumen concentration of monounsaturated fatty acids and
a decrease in
the concentration of saturated fatty acids (Noble et al., 1974;
Fellner et al., 1995).
The initial step in rumen biohydrogenation typically involves an
isomerization of the
cis-12 double bond to a trans-11 configuration resulting in a
conjugated di- or trienoic
fatty acid (Figure 2). Next is a reduction of the cis-9 double bond
resulting in a trans-11
fatty acid. The final step is a further hydrogenation of the trans-11
double bond
producing stearic acid (linoleic and linolenic acid pathways) or
trans-15 18:1 (linolenic
acid pathway). It is also possible to differentially affect steps in
the biohydrogenation
process. For example, dietary supplements of fish oil appear to
inhibit the last step of
biohydrogenation because they increase rumen outflow of trans 18:1
fatty acids and
reduce the outflow of stearic acid (Wachira et al., 2000; Shingfield
et al., 2003). Thus,
supplements containing fish oils must primarily affect group B
bacteria. Likewise, diets
that cause a low rumen pH and the feeding of ionophores inhibit the
final step in
biohydrogenation resulting in an accumulation of trans 18:1 fatty
acids. However, the
extent of the inhibition is much lower than their inhibition of
hydrolysis (Van Nevel and
Demeyer, 1995, 1996b).
Two key biohydrogenation intermediates are trans-11 18:1 (vaccenic
acid; VA)
formed from linoleic and linolenic acids and cis-9, trans-11
conjugated linoleic acid (CLA) formed in the biohydrogenation of
linoleic acid. These intermediates are present
in appreciable quantities in ruminant fat at a ratio of about 3:1, but
in the rumen cis-9,
trans-11 CLA is only a transitory intermediate and instead it is VA
that accumulates. The
difference is because most of the cis-9, trans-11 CLA found in
ruminant fat originates in
the mammary gland and adipose tissue from endogenous synthesis
involving the
enzyme delta-9 desaturase with rumen-derived VA as the substrate (see
review by
Bauman et al., 2003). This discovery is of special importance in
considerations of
“designing foods” because cis-9, trans-11 CLA is among the most potent
naturally
occurring anti-carcinogens (see discussion in Lock and Bauman, 2003).
See the pictures and read more in the full text:
http://www.ansci.cornell.edu/bauman/cla/conference_proceedings/articles/2003_cnc_bauman_et_al.pdf
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Dietary Fats Are Modified in the Rumen by Bacteria
The ruminal microbes will convert unsaturated fats to saturated fats
by replacing the double bonds with single bonds between the carbons
(called biohydrogenation). Some scientists have speculated that this
act of biohydrogenation by bacteria is an attempt to protect the
bacteria, as unsaturated fats can be toxic especially to fiber
digesters. The majority of the consumed unsaturated essential fatty
acids, C18:2 and C18:3, are converted by the bacteria to C18:0.
Whereas approximately 20 g of C18:0, 110 g of C18:1, 280 g of C18:2,
and 40 g of C18:3 are consumed daily by cows fed typical totally mixed
rations, approximately 370 g of C18:0, 25 g of C18:1, 40 g of C18:2,
and 4 g of C18:3 leave the rumen daily because of biohydrogenation.
Several intermediate forms of fatty acids, called trans fatty acids,
also are formed during biohydrogenation. Some of the trans fatty
acids, such as the trans-10, cis-12 conjugated linoleic acid (CLA) and
the trans-10 C18:1, can influence the cow’s metabolism, such as
depressing milk fat synthesis. This intervention by ruminal bacteria
to change essential fatty acids in the diet to other fatty acids has
made the study of dietary fat effects on reproduction quite
challenging.
SOURCE: http://www.extension.org/pages/Lipids_and_Longevity/print |
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