MILK COMPOSITION:
LIPIDS OR FATS
Leo Timms
Iowa State University
Why is fat important?
 Concentrated source of energy
2.25x as much E as glucose
 Provide insulation
Maintain body temperature
Protect vital organs
 Membrane synthesis
 Vitamin transport
MANY MORE!!!
MILK COMPOSITION
LIPIDS OR FATS
• Mostly triglycerides
• most variable component!!!
- across and within species!
• <1% - >50% fat: species differences
• variability in fatty acid profiles (handout)
• Fatty acid content is labile: diet changes
• major TASTE component in milk
LIPIDS OR FATS
.
Lipid
Triglyceride
Wt % of lipids
.
Cow Human
Rat
97-98
98.2
87.5
Diglyceride
.25-.48
Monoglyceride .02-.04
Free fatty acids .1-.4
Phospholipids .6 – 1
Cholesterol
.2 - .4
.7
T
.4
.25
.25
2.9
0.4
3.1
0.7
1.6
Triglyceride
Glycerol + 3 Fatty Acids
Triglyceride
Connected by an ester bond
Fatty acids can be the same or mixed
FATTY ACIDS
• Long chain: > C14 (14 carbons)
- diet
- adipose
• Medium chain: C10-C14 (10-14 carbons)
• Short chain: < C8 (< 8 carbons)
* made in the mammary gland
- glucose (NR) - acetate/butyrate (rum)
• Saturated: no double bonds
• Unsaturated: double bonds
LIPIDS OR FATS
LIPIDS OR FATS
Where do milkfat precursors come from?
1. Blood lipids: long chains
• diet and adipose tissue
• lipoproteins (liver)
• chylomicrons (gut)
• free fatty acids / ketones
2. Glucose: glycerol
fatty acids (non ruminants)
3. Acetate and butyrate (ruminants)
LIPIDS OR FATS
FATTY ACIDS
• Long chain: > C14 (14 carbons)
- diet
- adipose
• Medium chain: C10-C14 (10-14 carbons)
• Short chain: < C8 (< 8 carbons)
* made in the mammary gland
- glucose (NR) - acetate/butyrate (rum)
• Saturated: no double bonds
• Unsaturated: double bonds
FATTY ACIDS
• What do the fatty acids of triglycerides
look like that get to the mammary gland?
• long chain fatty acids (diet / adipose)
• diet = mammary gland (non-ruminants)
• diet is different than mammary gland in
ruminants
• Polyunsaturated fatty acids from feeds
are hydrogenated (saturated) in rumen
• Rumen by pass fats = mammary gland!
LIPIDS OR FATS
GETTING FATS INTO THE GLAND
• 35-50% of triglycerides (chylomicrons/
lipoproteins taken up by mammary gland)
• Triglycerides must be broken down to get
into mammary gland.
• Lipoprotein lipase: capillary walls
• 17 -45% of milk fat from acetate
• 8 - 22% of milk fat from butyrate
• Ruminants: 50% diet / 50% de novo syn.
• Acetate/ butyrate / ketones in milk (diag.)
Lipoprotein lipase (LPL)
• Anchored to capillary walls
• Hydrolyses triglycerides
– 1-3 times
• All cells have equal access to free FA’s
– Therefore must circulate in blood as
triglycerides
• Tissues with higher LPL activity can uptake
more fat from blood
Free fatty
acids
MILK FATTY ACIDS - COW
% from de novo % from VLDL
Fatty acid
synthesis
fatty acids
C4 - C10
100
0
C12
80 - 90
10 - 20
C14
30 - 40
60 - 70
C16
20 - 30
70 - 80
C18
0
100
DE NOVO FATTY ACID SYNTHESIS
IN THE MAMMARY GLAND
• Occurs in the cytoplasm!
• Need a few things:
* short carbon chains ( acetyl - CoA)
- acetate and butyrate ( ruminants)
- glucose (acetate) in non ruminants
* reducing equivalents ( NADPH2)
* proper enzymes:
Acetyl CoA carboxylase / fatty acid synthetase
DE NOVO FATTY ACID SYNTHESIS
IN THE MAMMARY GLAND
Acetyl CoA (2C) + HCO3 + ATP =
Malonyl CoA (3C) + ADP + P + H+
(acetyl CoA carboxylase - rate limiting)
Acetyl ACP (2 C) + Malonyl ACP (3C)
next 4 steps - fatty acid synthetase *
*Large single cpmplex of enzymes
DE NOVO FATTY ACID SYNTHESIS
IN THE MAMMARY GLAND
Acetyl ACP (2 C) + Malonyl ACP (3C)
condensation
ACP + CO
reduction
NADPH - NADP
dehydration
H2O
reduction
NADPH - NADP
4 carbon chain (Butyrate)
• Add 2 carbons at 1 time: C2 + C3 = C4 + CO2
• Lots of energy: 1 ATP and 2 NADPH / 2 carbons
•fatty acid chain attached to acyl carrier protein
MILK FATTY ACIDS
C4:0 Butyric acid C16:0 Palmitic acid
C6:0 Hexanoic
C18:0 Stearic
C8:0 Caprylic
C18:1 Oleic
C10:0 Caproic
C18:2 Lineoleic (ess.!*)
C12:0 Auric
C18:3 Linolenic (ess.)
C14:0 Myristic
• Most species: lots of palmitate / oleic
• More short chains in ruminants
• Conjugated linoleic acid (CLA)
• Other species diff: goats C8 + C10
MILK TRIGLYCERIDE
SYNTHESIS
• Occurs at cytoplasmic surface of
smooth endoplasmic reticulum
• Glycerol + fatty acids (esterified)
• Esterases (enzymes)
• Fatty acid specificity to C site
• Fatty acids coalesce to form droplets
• Grow and move to apical membrane
• Secreted with membrane (protein coating)
MILK COMPOSITION
Apical membrane
Basement
membrane