Great Lakes Dairy Sheep Symposium - the Department of Animal ...

Figure 1. Nutritional and non-nutritional factors affecting milk fat content in ruminants.
Nutritional Factors
fiber in the diet
unsaturated
fatty acids
specific feeds
feeding strategy
ionophores
Milk fat
De novo fatty acids are synthesized from acetate (C2) and β-hydroxybutryate (C4) and
include the short and medium chain (4 to 14 carbons) and a portion of the 16 carbon fatty acids.
This is different compared to monogastric animals, which primarily use glucose as the carbon
source for milk fat synthesis. Acetate and β-hydroxybutryate are extracted from the blood by the
mammary gland and it is estimated that acetate contributes about 90% and β-hydroxybutryate
contributing the remainder of the total carbon in milk fatty acids. De novo fatty acid synthesis
creates a range of fatty acids with chain lengths of 4 to 16 carbons; mechanisms regulating chain
length termination are not clearly understood (McGuire and Bauman, 2002). Ruminant milk fat
is unique among mammals in that it contains a high proportion of short-chain fatty acids (4, 6, 8,
and 10 carbons). Milk fatty acids derived from circulating lipoprotein triglycerides and nonesterified
fatty acids (NEFA) include a portion of the 16 carbon fatty acids and all ≥ 18 carbons.
These circulating fatty acids originate from lipids absorbed from the digestive tract and from
mobilized body fat reserves. Dietary triglycerides are not soluble in water but are packaged in
lipoproteins within the blood. The specific lipoprotein that transports dietary triglycerides to the
mammary gland is very-low-density lipoproteins (VLDL). To obtain the fatty acids from the
VLDL, the enzyme lipoprotein lipase cleaves the triglyceride into glycerol and NEFA that then
are taken up by the mammary cell. NEFA liberated from body fat reserves are also taken up by
the mammary gland (Lock and Bauman, 2004).
Since milk fat is composed mostly of triglycerides, esterification of the fatty acids is also an
important feature of milk fat synthesis (McGuire and Bauman, 2002). Fatty acids from both
sources are esterified in the endoplasmic reticulum, where they are attached to the glycerol
molecule in an orderly and systematic fashion. There are three sites of attachment to the glycerol
molecule. Some fatty acids are positioned at random onto glycerol, while others occupy a
specific position. For example, lauric acid (C12) is randomly assigned, while butyric acid (C4) is
positioned primarily on the third carbon (sn-3) of the glycerol structure. Once the triglycerides
are formed, they coalesce into fat droplets, which move through the epithelial cell toward the
luminal side where they acquire the MFGM coat and pinched off into the lumen (Figure 2).
As mentioned previously, milk fat contains a multitude of fatty acids, with a large portion of
these produced as intermediates during lipid metabolism in the rumen (Lock and Bauman, 2004).
Saturated, monounsaturated, and polyunsaturated fatty acids are all present in ruminant milk fat.
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Non-nutritional Factors
genetics
stage of lactation
season
parity
ambient temperature