| Perspectives in Nutrition, 5/e Gordon M. Wardlaw,
Ohio State University Margaret W. Kessel,
Ohio State University
Lipids
Chapter 6 Summary- Compared with carbohydrates and proteins, lipids are a group of relatively
oxygen-poor compounds that dissolve in organic solvents, such as chloroform,
benzene, and ether. Saturated fatty acids contain no carbon-carbon double bonds,
monounsaturated fatty acids contain one carbon-carbon double bond, and polyunsaturated
fatty acids contain two or more carbon-carbon double bonds in the carbon chain.
Triglycerides rich in long-chain saturated fatty acids tend to be solid at room
temperature, whereas those rich in polyunsaturated fatty acids are liquid at
room temperature.
- In omega-3 polyunsaturated fatty acids, the first of the carbon-carbon
double bonds is located three carbons from the methyl end of the carbon chain.
In omega-6 polyunsaturated fatty acids, the first carbon-carbon double bond
counting from the methyl end occurs at the sixth carbon. Both omega-3 and omega-6
fatty acids are essential fatty acids; these must be included in the diet to
maintain health.
- Body cells can synthesize hormone compounds called eicosanoids from
both omega-3 and omega-6 fatty acids. The eicosanoids produced from omega-3
fatty acids tend to reduce blood clotting, blood pressure, and inflammatory
responses in the body. Those produced from omega-6 fatty acids tend to increase
blood clotting.
- Triglycerides are formed from a glycerol backbone with three fatty
acids. Triglyceride is the major form of fat in both food and the body. It allows
for efficient energy storage, protects certain organs, transports fat-soluble
vitamins, and helps insulate the body. Phospholipids are derivatives of triglycerides.
Phospholipids are important parts of cell membranes, and some act as efficient
emulsifiers.
- Cholesterol forms vital biological compounds, such as hormones, components
of cell membranes, and bile acids. Cells in the body make cholesterol whether
we eat it or not. It is not a necessary part of an adult's diet.
- Fat digestion takes place primarily in the small intestine. Lipase
enzyme released from the pancreas digests the long-chain triglycerides into
smaller breakdown products--namely, monoglycerides (glycerol backbones with
single fatty acids attached) and fatty acids. The breakdown products are then
absorbed by the absorptive cells of the small intestine. These products are
mostly resynthesized into triglycerides and combined with cholesterol, protein,
and other substances to yield a chylomicron. Chylomicrons enter the lymphatic
system, in turn passing into the bloodstream.
- Lipids are carried in the bloodstream by various lipoproteins, which
are particles consisting of a central triglyceride core encased in a shell of
protein, cholesterol, and phospholipid. Chylomicrons are released from intestinal
cells and carry lipids arising from dietary intake. Very-low-density lipoprotein
(VLDL) and low-density lipoprotein (LDL) carry lipids synthesized in the liver.
High-density lipoprotein (HDL) picks up cholesterol from cells and acts in allowing
transport of it back to the liver.
- In the blood, elevated amounts of LDL and low amounts of HDL are
strong predictors of the risk for cardiovascular disease.
- Fat adds flavor and texture to foods and increases satiety after
meals. Hydrogenation is the process of converting carbon-carbon double bonds
into single bonds by adding hydrogen at the point of unsaturation. Hydrogenation
of fatty acids in vegetable oils changes the oils to solid fats and helps reduce
rancidity, which results from the breakdown of fatty acids. Hydrogenation also
increases the trans fatty acid content. High amounts of trans fatty acids in
the diet are discouraged, as these increase LDL and reduce HDL. When fatty acids
break down, food becomes rancid, emitting a foul odor and flavor. Some fats
are used in food as emulsifiers. These suspend fat in water.
- There is currently no RDA for fat. We need about 4% of total energy
intake from plant oils to obtain the needed essential fatty acids. Fish is a
good source of omega-3 fatty acids and should be consumed at least twice a week.
- The typical American diet contains about 33% of total energy as
fat. Many health agencies and scientific groups suggest reducing fat intake
to no more than 30% of energy intake. Some health experts advocate an even further
reduction to 20% of energy intake for some people, but such a diet requires
professional guidance. If fat intake exceeds 30% of total calories, the diet
should emphasize monounsaturated fat.
- Fat-reduced products aid in the goal of reducing fat intake, but
they still must be eaten in moderate amounts to maintain control of total energy
intake.
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