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Triacylglycerol (TAG) (triglyceride)
• Fat, primarily TAG, currently provides ~34% of total calories in average American diet
• TAG also serve as major energy reserve in human tissues
Energy Reserves of Humans After 12hr Energy Reserves of Humans After 12hr FastFast
Fuel reservesStored fuel Tissue (g) (kcal)
Glycogen Liver 70 280 Glycogen Muscle 120 480Glucose Body fluids 20 80Fat Adipose 15,000 135,00Protein* Muscle 6,000 24,000Data are for “normal” male subject weighing 70 kg. Carbohydrate, 4 kcal/g; Fat, 9 kcal/g; Protein, 4 kcal/g*There is no specific storage form of protein
DRI’s for FatDRI’s for Fat
• Except for infants, 2002 IOM-FNB recs set no DRI for total fat intake
• This is because humans can synthesize saturated and unsaturated fatty acids and cholesterol and therefore are not required in diet
• Adequate Intakes (AI) for infants 0 - 6 and 7 - 12 months are 30 and 31 g/day, respectively
• Will deal with AI for n-6 & n-3 later in this lecture
Fat Consumption in the United States1900 ~20% of calories1960’s ~40 - 42%mid 1980’s ~38%Current ~34%2002 recs 20 - 35%
• In 1900, butter and lard comprised ~75% of fat intake
• This has been replace to a great extent by vegetable oils, margarine, and shortening
• This has led to increase in proportion of unsaturated fatty acids (especially linoleic acid) to saturated fatty acids in American diets
• Consequence of increased use of vegetable
oils has been increased amount of trans fatty
acids in the diet to ~2.6% of total energy
intake
• Trans fatty acids produced by partial
hydrogenation of vegetable oils during
production of margarines and shortening
A doughnut contains ~6 g trans fatty acids and a large order of fast-food french fries contains ~7 g trans fatty acids.
Oreo® cookies contain 2 g trans fatty acid per serving of 3 cookies
From Lichtenstein, A. H. et al., New England Journal of Medicine 25 :
1933 (1999)
See p. 8 Lipids Handout
• By far major sources of trans fatty acids in
diet include anything prepared with partially
hydrogenated vegetable oil or vegetable
shortening (e.g. Crisco®)
• Note that current food labels do not list trans
fatty acids
• In November 1999 U. S. FDA proposed that
trans fats be added to food labels
• In July 2003 FDA issued final rules on trans
fat labeling, to take effect January 2006
• One type of food label
showing amount of trans
fat, effective January 2006
• Some manufacturers
starting to use this type of
label now, especially if no
trans fat in the product
Effect of Effect of TransTrans Fatty Acids Fatty Acids on Serum Cholesterol Levelson Serum Cholesterol Levels
• Convincing evidence that trans fatty acids
increase LDL:HDL ratios
• Indeed, they appear to increase the ratio
more than do saturated fatty acids
• They not only increase LDL, they also
markedly lower HDL
A Very Brief Review A Very Brief Review
ofof
Fat DigestionFat Digestion
and Absorptionand Absorption
Orlistat(Xenical™)
• Inhibits pancreatic lipase• Undigested TAG excreted in feces• (Reduces fat absorption by ~20 - 30%)• Requires prescription
• Another compound, not to be confused with Orlistat®, is Olestra®
• Olestra® is a non-digestible sucrose-fatty acid polyester that can replace fat in prepared foods
• Use has thus far been limited to use in snack foods (potato chips, etc.) by FDA
• Both Orlistat and Olestra have some untoward GI side effects
• Vitamin A supplementation is required when either is used
R
R R
R R
R R
R
C-(CH2)n-CH3 = R
O
Sucrose polyester, also called Olestra®
Fatty acids not removed by pancreatic lipase
Approved by FDA for use in snack foods
Serum Lipoprotein Classes
Chylomicrons
Very Low Density Lipoprotein VLDL
Intermediate Density Lipoprotein IDL
Low Density Lipoprotein LDL
High Density Lipoprotein HDL
Ketone Body MetabolismKetone Body Metabolism• Ketone bodies (acetone, acetoacetic acid,
-hydroxybutyrate) produced in liver during fast or reduced carbohydrate take
• Incomplete fatty acid oxidation (and TCA operation) leads to increased acetyl-CoA concentration
3 acetyl-CoA HMG-CoA ketone bodies
• The liver cannot use ketone bodies.
• They are exported to extrahepatic tissues, many of which can use them readily as fuel
Essential Fatty Acids Essential Fatty Acids (PUFA)(PUFA)
• Linoleic acid (18:29,12) (n-6; 6)
• -Linolenic acid (18:39,12,15) (n-3; 3)
• Humans can synthesize arachidonic acid(20:45,8,11,14)(n-6; 6) from linoleic acid
• AI for n-6 PUFA as linoleic acid for adults ages19-50 is 19g/day; AMDR is 5-10% of daily caloriefor ages 1-70
• AI for n-3 PUFA as -linolenic acid acid for adults ages 9-70 is 1.1g/day; AMDR is 0.6-1.2% of daily calories for ages 1-70
Essential Fatty Acid Deficiency
Clinical deficiency ofessential fatty acidsrare on most normaldiets.
This individual wason prolonged EFA-freeTotal parenteral nutrition
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