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Milk Composition & Synthesis
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Milk Synthesis
Milk Fat


Sources of Fatty Acids

Milk fat triglycerides are synthesized in the mammary epithelial cells. However, the fatty acids used to synthesize the milk triglycerides may arise from two sources:

  1. from breakdown of blood lipids
  2. from de novo synthesis within the mammary epithelial cells.

1. Blood Lipids -

From 40 to 60% of the fatty acids come from the blood. These are primarily derived from very low density lipoproteins (VLDL), which are synthesized in the intestine or liver. VLDL are composed of 90 to 95% lipid (55-60% triglyceride) on the inner core and 5 to 10% protein at the outer surface. Chylomicrons, containing ingested fatty acids from the intestine, also can act as a source of blood-derived fatty acids for the mammary gland.

Triglycerides in the VLDL are hydrolyzed in the mammary capillaries by an enzyme called lipoprotein lipase (LPL). The LPL can hydrolyze off one, two or all three of the fatty acids from the glycerol backbone, resulting in free fatty acids plus diacylglycerides, monoacylglycerides, or glycerol, respectively. The free fatty acids, monacylglycerides, diacylgycerides and glycerol can all be taken up by the mammary epithelial cell and reused for triglyceride synthesis.

The FAs contained in VLDL and cylomicrons are dependent upon dietary lipids and on mobilized fat from body adipose. In the nonruminant, the FA composition of the diet can directly affect the FA composition of the milk. Feeding supplemental dietary fat can increase milk fat yield and FA composition of the milk fat. However, in ruminants, diets are typically low in dietary lipid and that lipid is metabolized by the rumen. The result is that cow milk FA composition is not normally regulated much by diet. However, in cases of bypass fat in diets of ruminants the lipids pass directly to the intestine and become part of the FA profile of the VLDL and chlyomicrons. Cow milk FA composition can be altered by bypass dietary lipid.


2. De novo Fatty Acid Synthesis -

Synthesis of short and medium chain fatty acids in the mammary gland occurs by de novo synthesis (synthesis from the start, or synthesis of new molecules of fatty acids from precursors absorbed from the blood). De novo synthesis of fatty acids occurs in the cytoplasm of the mammary epithelial cell. In the ruminant, the carbon sources used for FA synthesis are acetate (the most important one) and b-hydroxybutyrate (BHBA). Glucose is a carbon source for FA synthesis in nonruminants. The reducing equivalents needed for FA synthesis come from NADPH2 (nicotinamide adenine dinucleotide phosphate, reduced form). The two key enzymes involved in fatty acid synthesis in the mammary gland are:

Acetyl-CoA Carboxylase, which is the rate limiting enzyme for the fatty acid synthesis pathway.
Fatty Acid Synthetase, which is a large complex of enzyme activities responsible for the chain elongation of the fatty acid chains.

The relative proportions of milk fatty acids derived from de novo synthesis or blood lipids is dependent on carbon chain length of the fatty acid. Typically, shorter chain fatty acids arise predominantly from de novo synthesis inthe epithelial cell, while longer chain fatty acids arise directly from blood lipids. An example is given in the table below for the cow.

Proportional contribution of sources of fatty acids in cow milk

Fatty Acid

% of FA from
De novo synthesis

% of FA from
VLDL Fatty Acid

C4 -C10

100

0

C12

80-90

10-20

C14

30-40

60-70

C16

20-30

70-80

C18

0

100


Milk Fat
Milk Composition & Synthesis
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