acids in the mammary gland. Unprotected
fats lead to only slight increases in poly-
unsaturated fatty acids in milk fat, but may
lead to appreciable increases in oleic acid
because of intestinal and mammary
desaturation of stearic acid produced by
ruminal biohydrogenation of dietary
unsaturated fatty acids (Table 5.2). The
bovine mammary gland readily incorporates
unsaturated fatty acids presented to it
(LaCount et al., 1994). Producing milk with
more monounsaturated and polyunsaturated
fatty acids depends on the development of
practical strategies to protect dietary
unsaturated fatty acids from hydrogenation
by rumen microbes. Currently, formalde-
hyde treatment of protein–fat mixtures is the
best methodology for rumen protection
(Doreau and Chilliard, 1997), but regulatory
approval may limit its application in many
countries.
LipolysisMobilization of fatty acids from adipose
tissue triacylglycerols (lipolysis) occurs
during times of negative energy balance or
in response to stresses. The reaction
proceeds by the sequential release of fatty
acids from the glycerol backbone. The fatty
acids released increase the size of the intra-
cellular free fatty acid pool and, in the
absence of stimuli to re-esterify those fatty
acids, they diffuse from the cell into the
blood. The free fatty acids are adsorbed
quickly to binding domains on serum
albumin, and circulate to various tissues as
a fatty acid–albumin complex. Physiological
states characterized by high rates of lipo-
lysis, such as early lactation in dairy cows
and sows (McNamara, 1991), often are also
characterized by relatively lower concen-
trations of albumin in the blood. Hence,
the ratio of free fatty acids to albumin in
blood increases, which favours greater
uptake of the free fatty acids by tissues of
the body because more fatty acids occupy
lower affinity binding sites on the albumin
molecule. Furthermore, the increased ratio
of fatty acids to albumin increases the size
of the tissue free fatty acid pool, which in
turn increases re-esterification of fatty
acids in adipose tissue and thus provides
feedback regulation on lipolysis (Metz and
van den Bergh, 1977).
The initial step in lipolysis is catalysed
by hormone-sensitive triacylglycerol lipase.
This enzyme is activated by binding of
hormones that stimulate formation of cAMP
by adenyl cyclase. In mammals, the primary108 J.K. Drackley
Table 5.1.Typical profiles of major fatty acids
found in lipids from subcutaneous adipose tissue or
longissimus muscle from cattle, sheep and pigs (g
kg^1 ). (Adapted from Rule et al., 1995.)
Fatty acid Cattle Sheep Pigs
Adipose tissue
14:0 40 40 10
16:0 280 260 240
18:0 110 160 130
18:1 430 410 440
18:2 30 30 120
Muscle
14:0 40 30 10
16:0 270 250 250
18:0 130 110 110
18:1 380 460 490
18:2 80 60 70
Table 5.2.Fatty acid composition of milk fat from
cows fed a basal low-fat diet or the basal diet
supplemented with tallow. (Adapted from Palmquist
et al., 1993.)Diet (g kg^1 of methyl esters)
Fatty acid Basal Basal + tallow4:0 33 35
6:0 27 23 a
8:0 18 13 a
10:0 40 26 a
12:0 46 29 a
14:0 130 103 a
14:1 15 13
15:0 13 10 a
16:0 299 284
16:1 17 18
17:0 6 8 a
18:0 90 104
18:1 172 233 a
18:2 22 16 a
18:3 6 9 a
aDifferent from basal diet, P< 0.05.