BLBS102-c26 BLBS102-Simpson March 21, 2012 13:51 Trim: 276mm X 219mm Printer Name: Yet to Come
26 Equid Milk: Chemistry, Biochemistry and Processing 511Table 26.9.Monounsaturated and Polyunsaturated Fatty Acids
(Percent of Total Fatty Acids±Standard Deviations) in the Milk
Fat of Some Ruminants and Non-RuminantsMUFAs PUFAs CLANon-ruminantsEquine 20.70 36.80 0.09
Asinine 15.30 16.00 –
Porcine 51.80 12.40 0.23
Human 33.20 12.50 0.39RuminantsCaprine 26.90 2.58 0.65
Bovine 23.20 2.42 1.01
Ovine 23.00 3.85 1.08Data from Jahreis et al. 1999, Salimei et al. 2004.
MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty acids; CLA,
conjugated linoleic acid.3-hydroxybutanoic acid, which is synthesised by bacteria in the
rumen (Pikul and Wojtowski 2008). Caprylic acid, C ́ 8:0,isvery
high in equid milk compared to the level in human and bovine
milk (Table 26.10). Levels of middle chain-length FAs, espe-
cially C10:0and C12:0, are high in equid milk (20–35% of all FAs
contain<16 C) and in all non-ruminant herbivores, suggesting
that they arise from the use of glucose as the principal precursor
for fatty acid synthesis (Palmquist 2006). Mammaliande novo
fatty acid synthesis requires a carbon source (acetyl-CoA) and
reducing equivalents in the form of NADPH+H+.Inrumi-
nants, acetate andβ-hydroxybutyrate are the primary sources
of carbon while glucose and acetate are the primary sources
of reducing equivalents; in non-ruminants, for example equid
species, glucose is the primary source of both carbon and re-
ducing equivalents and also supplies some of the glycerol for
milk TGs (Dils 1986). When horses were infused with either
glucose or acetate and palmitate, C12:0and C14:0were formed
exclusively from acetate, as in ruminants, and C16:0was formed,
partly from acetate and partly from palmitate; unlike ruminants,
44% of C18:0and 7% of C18:1are formed from acetate in the horse
(Palmquist 2006). If this is so, acetate and 3-hydroxybutyrate are
presumably produced by bacterial fermentation in the lower in-
testine of the horse and why 3-hydroxybutyrate is not converted
to butanoic acid, as in ruminants, is unclear. Fatty acids from
C6:0to C16:0are released from the fatty acid synthesis complex
by acyl-specific thioesterases; presumably, the middle chain-
length-specific thioesterases are particularly active in equids;
investigation of this possibility is warranted.
Equine milk-fat contains a relatively high level of C16:1
(2–10%, w/w) and C18:1, reflecting high-9 desaturase activity.
Equid milk fats contain a very high level ofn-3-octadecatrienoic
acid (linolenic acid), which reflects the high level of PUFAs in
the diet and the lack of biohydrogenation, as occurs with rumi-
nants. In the rumen, extensive hydrogenation of double bondsoccurs and most fatty acids taken up from the intestinal tract
are saturated. The large intestine of equids shows significant
differences in the relative rate of transport of volatile fatty acids
compared to ruminants andde novosynthesis of C18:0fatty acids
occurs with a further high proportion of C6:0to C14:0carbon fatty
acids and some C16:0arising from products of their large bowel
fermentation.
Equine and asinine milks have similar fatty acid profiles al-
though the former has a higher content of monosaturated fatty
acids (Tables 26.9 and 26.10). Both equine and asinine milk have
characteristic low levels of stearic acid, and oleic acid is excep-
tionally low in asinine milk. Asinine and zebra milk fat contain a
high level of PUFAs, although considerably lower than in equine
milk. The well-balanced ratio ofn-6:n-3 of 1.17:1 in asinine milk
compared to 3.14:1 in equine milk makes it an interesting prod-
uct for human nutrition.n-6 andn-3 fatty acids are essential in
human metabolism as components of membrane phospholipids,
precursors of eicosanoids, ligands for membrane receptors and
transcription factors that regulate gene expression. The impor-
tance ofn-6 C18:2, linoleic acid (LA), has been known for many
years but the significance ofn-3 C18:3,α-linolenic (ALA) was not
recognised until the late 1980s and has since been identified as a
key component in the diet for the prevention of atopic dermatitis
(Horrobin 2000). LA and ALA are not inter-convertible but are
the parent acids of then-6 andn-3 series of long chain (LC)
polyunsaturated fatty acids, respectively (e.g.,n-6 C20:4, arachi-
donic acid (AA);n-3 C20:5, eicosapentaenoic acid (EPA) andn-3
C22:6, docosahexaenoic acid (DHA)) which are components of
cellular membranes and precursors of other essential metabolites
such as prostaglandins and prostacyclins (Cuthbertson 1999, In-
nis 2007). DHA and AA are now recognised as being crucial for
normal neurological development (Carlson 2001). Humans have
evolved on a diet with a ratio ofn-6 ton-3 fatty acids of approx-
imately 1:1 but Western diets nowadays have a ratio of 15:1 to