Biomedical Engineering Reference
In-Depth Information
between long- and short-day exposed animals,
although the mean body weight was somewhat
higher under long days (Clarke et al.
2003
) . In a
study, the serum leptin level decreased over
16 weeks under short days and increased under
long days, roughly paralleling the changes in
body weight of the rams (Lincoln et al.
2001
) . In
seasonal breeding mammals, there are fl uctuations
in leptin gene expression. In the Djungarian ham-
ster, adipose tissue leptin gene expression was
greatly reduced during winter or during exposure
to a short photoperiod (Klingenspor et al.
1996
) .
The decrease observed in leptin expression with
decreasing photoperiod is attributed to an adap-
tive behaviour to decrease energy expenditure.
which are then fermented by microorganisms to
produce short-chain volatile fatty acids, namely,
acetate, propionate and butyrate (Hocquette and
Bauchart
1999
). These fatty acids get absorbed in
the rumen by simple diffusion. Acetate, the major
constituent of VFAs, is not greatly metabolised by
the liver and is distributed to other tissues to be
used as an energy source and substrate for lipo-
genesis in the adipocytes. However, 80-90% of
propionate and butyrate are removed by liver for
energy requirements. A rise in the level of the
volatile fatty acids in peripheral plasma occurs
within 2-4 h of feeding concentrate to the animal
(Blum et al.
2000
) . Triacylglycerols are also
hydrolysed by rumen lipases, but most of the
absorption of medium- and long-chain fatty acids
occurs in the jejunum after interaction with bile
and pancreatic lipase. Plasma concentration of
FFA in ruminants increases before the morning
feeding and decreases rapidly after the feeding
(Marie et al.
2001
) . Rumination occurs mainly
during times of rest, and the time spent in rumi-
nating depends on the type of diet and range from
almost negligible for diets high in grain to about
10 h per day for diets high in forage.
9
Lipid Metabolism
Fatty acids are an important source of energy in
body of many organisms. Triglycerides (triacylg-
lycerols or triacylglycerides) are glycerides of
the glycerol esterified with three fatty acids. The
breakdown of fat stored in fat cells is referred as
lipolysis that occurs under the influence of
lipases. Triglycerides are broken down into glyc-
erol and free or nonesterified fatty acids (FFA or
NEFA) by lipases with the help of bile salts. The
hormones adrenaline, noradrenaline, glucagon
and adrenocorticotrophin help in inducing lipoly-
sis (Voet and Voet
2002
) . Triglycerides yield
more than twice as much energy for the same
mass as carbohydrates or proteins. All cell mem-
branes are made of phospholipids, each of which
contains two fatty acids, proteins and cholesterol.
Fatty acids are also commonly used for protein
modification, and all steroid hormones are derived
from fatty acids. The metabolism of fatty acids,
therefore, consists of catabolic processes which
generate energy and primary metabolites from
fatty acids (Heidemann
2002
) .
9.2
Daily Rhythms of Free Fatty
Acids (FFA) and Glycerol
Regular daily fluctuations of plasma concentra-
tions of FFA have been observed in rats (Dallman
et al.
1999
), sheep (Marie et al.
2001
) and cattle
(Blum et al.
2000
) . No signi fi cant daily rhythm of
FFA levels has been observed in dairy cows with
free access to feed (Bitman et al.
1990
) . The daily
changes in FFA concentrations occurring during
voluntary feeding behaviour have been attributed
to the time intervals elapsed between eating due
to lipolysis during fasting increases the plasma
FFA levels. Rapid changes in the lipolysis/
lipogenesis balance have been observed in rats
(Escobar et al.
1998
; Dallman et al.
1999
) , sheep
(Marie et al.
2001
) and cattle (Blum et al.
2000
)
with restricted feeding. Other factors that are
likely to contribute to maintenance of the daily
rhythm of lipid metabolism are time and fre-
quency of feeding. Extended fasting in rats aboli-
9.1
Lipid Metabolism in Ruminants
Cattle and buffaloes differ from monogastric
animals in that dietary carbohydrates are
degraded in rumen to hexoses and pentoses,
