Sports Nutrition - HECO 336....
A Review of the Chemistry of
Dietary Lipids
Lipids are a
group of three chemical compounds including triacylglycerol (triglycerides),
phospholipids
and sterols. Triacylglycerols are composed of one molecule
of glycerol and three fatty acids. At rest, fatty acids provide at
least fifty percent of the energy for aerobic metabolism.
Fatty Acids (FA)
When
people talk about the fat in their diets they are actually referring
to the fatty acids they are consuming.
Fatty
acids are the building blocks for triacylglycerols. They are simply
a chain of carbon atoms connected to one another with hydrogen atoms taking
up the additional bonds. At one end of the fatty acid is a methyl
group (a carbon with three hydrogens attached [CH3]) while at the other
end is an acid group (carboxyl group [COOH)]
H H H H H H H
H H H H
| | | | | | | | |
| |
METHYL HC-C-C-C-C-C-C-C-C-C-C-C=O
GROUP |
| | | | | | | | | | \OH
ACID
GROUP
H H H H H H H H H
H H
Chemical Identification
To better describe the structure of
fatty acids a numeric classification is often given after the name of the
fat. The first number given is the length of the carbon chain, a
colon follows the number. The second number represents the the number
of double bonds found in the chain. Oleic acid is a fatty acid with
a 16 carbon chain and one double bond, its' numeric classification is 16:1.
Chain Length
the length of the carbon chain effects
the digestion, absorption and transport of fatty acids. Short chain
FA (SCFA) have a carbon chain length of up to four; medium chain FA (MCFA)
have a chain length of 6-12 and long chain FA (LCFA) have a chain length
greater than 12.
saturated
saturated
fatty acids have a carbon chain with no double (=) bonds.
H3C-C-C-C-C-C-C-C-C-C=O
\OH
Not all saturated fats are bad - new research finds that
stearic acid (18:0), a saturated fatty acid found in chocolate seems to
have no effect in raising serum cholesterol levels. For more information
see: www.chocolateinfo.com
However, guidelines
still recommend a diet should contain < 30% total kcalories as fat and
< 10 % as saturated fat.
monounsaturated
monounsaturated
fatty acids (MUFA)s have
one double bond in their
carbon chain.
Oleic
acid (16:1) is a common monounsaturated fatty acid found in
high
levels in
olive and canola oils.
* Health
Benefits of Olive Oil - Bertolli
Olive Oil
* Using
Canola Oil in Your Diet - Canadian
Canola Oil
* 60%
of the fat in Avocados is monounsaturated -
The California Avocado Commission
polyunsaturated
polyunsaturated
fatty acids have two or more double bonds in their chains.
H3C-C-C=C-C-C=C-C-C=C-C-C=O
\OH
Trans Fatty Acids
When
polyunsaturated fatty acids are partially hydrogenated one or more of the
double bonds not saturated alters its normal cis configuration to
a trans
configuration - rotating the position of the H atom to the
other side of the carbon. The body seems to recognize this new structure
as a saturated fat despite its double bond in the carbon chain.
H H
H
H3C-C-C=C-C-C-C-C-C-C=O
H3C-C-C=C-C-C-C-C-C-C=O
cis
\OH trans
H
\OH
Understanding trans fatty acids
Essential Fatty Acids
An essential fatty acid (EFA) is one which must be obtained in the diet.
Linoleic
Acid (18:2) is the essential fatty acid for humans. Individuals
on very low fat diets (<10%) may not receive enough linoleic acid.
Omega Classification
Phospholipids
Phospholipids
- are similar to triglycerides except the third fatty acid has choline
and and phosphate group. These differences make phospholipids emulsifiers
- able to mix fatty (nonpolar) substances with water soluble (polar) substances.
lecithin
- is a common phospholipid found in cell membranes and commercially used
as an emulsifier to disperse fat.
Sterols
Sterols are multi-ring structures
found in plant and animal products.
Cholesterol
is a major sterol important in human nutrition. Cholesterol is a waxy substance
produced by the liver and found in foods of animal origin.
It is essential for the production of vitamin D and gender hormones e.g..
testosterone and estrogen.
New research has shown that some plant
sterols may interfere with cholesterol absorption and thus reduce serum
cholesterol levels.
Information About Plant Sterols in Foods
Benecol Margarine
MORE INFORMATION ABOUT CHOLESTEROL
Check Out the American Egg
Board/
Food and Nutrition Information
Digestion and Absorption
Of all the energy components,
lipids take the longest to digest. Meals high in fat prior to training
or competition may cause intestinal discomfort.
Digestion
The
majority of lipid digestion occurs in the small intestine. The presence
of fat in the small intestine stimulates the release of the gastro- intestinal
hormone
cholecystokinin (CCK). Cholecystokinin stimulates
the gallbladder to release
bile into the small intestine.
The emulsification action of bile breaks up fats into small particles so
that pancreatic lipase can breakdown triacylglycerols into free
fatty acids (FAA), mono and diglycerides.
Absorption
Since
lipids are insoluble in the aqueous (water) environment of the body the
must be modified to be absorbed. Mixed micelles are created with
bile salts on the outside and monoacyglycerols and fatty acids on the inside
(the same idea as an M & M candy). These are transported to the
intestinal cell (enterocyte) wall and absorbed. Because short chain
fatty acids (SCFA) and glycerol are more water soluble than long
chain fatty acids (LCFA) the can diffuse directly into the enterocyte.
Lipoproteins
Lipids are repackaged in the enterocyte into compounds called lipoproteins
- a mixture of lipid components and protein. Phospholipids and protein
form the outside layer (more water soluble) and the LCFA and cholesterol
are on the inside. Chylomicrons are the major lipoprotein
made in the intestine to transport dietary triacylglycerols to the cells
for energy use. Chylomicrons are absorbed into the lymph system
for transport to the cells. Short and medium chain fatty acids and
glycerol are absorbed directly into the portal vein for transport.
Lipoprotein Metabolism
Lipoproteins are analyzed and categorized based on their density - the greater the fat content the lower the density. Lipoproteins include:
Chylomicrons are formed in the intestinal cell for
transport via the lymphatic system to the liver. TGís are removed
enroute and the TG reduced chylomicron becomes a chylomicron remnant
[chylo-R]. In the liver, chylomicrons and chylo -R are reformed into
VLDL,
Lipoprotein lipase [LPL] in capillary walls removes triacyglycerols from chylomicrons as they are needed; this primarily occurs as the chylomicrons pass through adipocytes and the liver, the remaining chylomicrons form chylomicron remnants. Chylomicrons and chylo-r in the liver are repackaged to form very low density lipoprotein (VLDL) and are then released.
Circulating VLDL is TG rich and a ready target for LPL which removes TG producing intermediate density lipoprotein (IDL), circulating IDL may also have TG removed generating low density lipoprotein (LDL), which has now become a cholesterol heavy substance. Low density lipoproteins are sometimes called "bad cholesterol". Freed TG enter adipocytes and muscle cells where they are stored for energy. Entrance into cells is facilitated by insulin. The consumption of carbohydrates in a meal or feeding raises insulin levels making making entrance into the cell possible.
High density lipoprotein (HDL) are primarily made in the liver and are protein heavy. They function as cholesterol scavengers and are also know as "good cholesterol".
High levels of dietary fat tend to
raise LDL levels while aerobic exercise tends to raise HDL levels.
American
Heart Association
Risk Factor Quiz and More
Cholesterol and Your Risk Assessment
Coronary Heart Disease And Your Family
National
Cholesterol Education Program:
including the step I and step II diets
Lipids in Energy Systems
Triacylglycerols represent the largest stored source of energy for athletes.
Training increases the body's ability to use fatty acids at higher levels
of VO2max and also increases the storage of fatty acids in muscle.
Lipolysis
- the breakdown of lipids
When needed,
epinephrine, norepinephrine, glucagon, adrenocorticotropic hormone [ACTH],
thyroid stimulating hormone, thyroid hormone, and growth hormone
stimulate cellular adenylate cyclase which promotes cAMP activity.
Cyclic AMP stimulates hormone sensitive lipase [HSL] in cells to
hydrolyze FA from TG so the FA may be oxidized in the cell or released
into the blood stream for use in other cells.
TCA
Cycle
Acetyl CoA is the
primary substrate of the TCA cycle. While carbohydrates and fatty acids
are the usual sources of acetyl CoA, amino acids may also contribute to
the supply when carbohydrate is near exhaustion.
MCT
Some advocate the use of MCT oil which
can be transported directly to the liver and does not require the use of
bile salts for absorption or incorporation into chylomicrons. But,
MCT consumption does not increase TG concentration in muscle fibers or
their release from adipose as FFA into the plasma. High fat diets
have been used increase serum FFA. However the results tend to reduce
glycogen levels, thus reduce total time to fatigue.
Fat Substitutes & Alternatives
AHA
Recommendations
Products
Butter Buds: A Fat Free Butter Alternative
I Can't Believe It's Not Butter
Lipids and Cancer
The
role of diet and exercise in cancer
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Cardiovascular |
Copyright © 2000 Department of Human Ecology [HECO] , The University
of Tennessee at Chattanooga. All rights reserved.
The University of Tennessee at Chattanooga is an equal employment opportunity/affirmative
action/Title VI/TitleIX/Section 504/ADA/ADEA institution.
Please send your questions, comments, and suggestions to: Holly-Dieken@utc.edu
Last Modified: Jul 18, 2000