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Perspectives in Nutrition, 5/e
Gordon M. Wardlaw, Ohio State University
Margaret W. Kessel, Ohio State University
Lipids

Chapter 6 Summary

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. In the blood, elevated amounts of LDL and low amounts of HDL are strong predictors of the risk for cardiovascular disease.

  9. 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.

  10. 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.

  11. 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.

  12. 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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