Chapter
5 Lipids
What Are Lipids? • Hydrophobic/hydrop hilic • Lipophilic/lipophobic • Classes of lipids • Triglycerides • Phospholipids • Sterols © PhotoDisc
Fatty Acids Are Key Building Blocks • Chain Length • 4–24 carbons
Fatty Acids Are Key Building Blocks • Saturation • Saturated • All single bonds between carbons
• Monounsaturated • One carbon-carbon double bond
• Polyunsaturated • More than one carbon-carbon double bond
Fatty Acids Are Key Building Blocks • Geometric and Positional Isomers • • • •
Cis fatty acid Trans fatty acids Hydrogenation Conjugated linoleic acid
Fatty Acids Are Key Building Blocks • Types of fatty acids • Omega-3 • Omega-6 • Omega-9
Fatty Acids Are Key Building Blocks • Nonessential and Essential Fatty Acids • Elongation • Desaturation • Nonessential fatty acids • Essential fatty acids
Fatty Acids Are Key Building Blocks • Building Eicosanoids • Omega-6 fatty acids • Omega-3 fatty acids
Triglycerides • Structure • Glycerol + 3 fatty acids
• Functions • • • •
Energy source Energy reserve Insulation and protection Carrier of fat-soluble compounds • Sensory qualities in food
Triglycerides in Food • Sources of omega-3 fatty acids • Soybean, canola, walnut, flaxseed oils • Salmon, tuna, mackerel
• Sources of omega-6 fatty acids • Vegetable oils • Nuts and seeds © Photodisc
Triglycerides In Foods • Commercial Processing of Fats • Hydrolysis • Oxidation • Hydrogenation
Phospholipids • Structure • Glycerol + 2 fatty acids + phosphate group
• Functions • Component of cell membranes • Lipid transport as part of lipoproteins • Emulsifiers
• Food sources • Egg yolks, liver, soybeans, peanuts
Sterols: Cholesterol • Structure • Functions • Component of cell membranes • Precursor to other substances • Sterol hormones • Vitamin D • Bile acids
• Synthesis • Made in the liver
• Food sources • Found only in animal foods
Lipid Digestion and Absorption • Mouth and stomach • Minimal digestion of triglycerides
• Small intestine • Emulsified by phospholipids • Digested by pancreatic lipase • Absorbed into intestinal cells • Formed into chylomicrons and moved into lymphatic system
Lipid Digestion and Absorption • Digestibility • Steatorrhea
• Lipid Absorption • Lipoprotein • Chylomicron
• Digestion and Absorption of Sterols
Lipids in the Body • Lipoproteins carry lipids around the body • Chylomicrons • Delivers dietary lipids from intestines to cells and liver
Lipids in the Body • Very low density lipoproteins (VLDLs) • Deliver triglycerides to cells
• Intermediate-density lipoproteins • Remove some phospholipids
• Low-density lipoproteins (LDLs) • Deliver cholesterol to cells
• High-density lipoproteins (HDLs) • Pick up cholesterol for removal or recycling
Lipids in the Diet • Recommended intake • • • • •
Consume at least two weekly servings of oily fish Balance of calories from fat and carbohydrate Reduce sat and trans fat intake Total fat: 20–35 percent of calories Need ~2 percent of calories as essential fatty acids • Improve balance of omega-3: omega-6 fatty acids
Lipids in the Diet • • • •
Essential Fatty Acid Requirements Omega-6 and Omega-3 Balance Current Dietary Intakes Role of Fat Replacers
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Lipids in the Diet • Fat substitutes • The Olestra Controversy: Are Fat Replacers Safe? composition • Sucrose + fatty acids • Indigestible – provides zero kcals • Reduces absorption of fat-soluble vitamins
Lipids and Health • Obesity • High-fat diets promote weight gain
• Heart disease • High saturated and trans fat intake raises LDL cholesterol
• Cancer
AHA Position on Omega-3 Fatty Acids • The American Heart • Fatty fish such as Association recommends mackerel, lake trout, eating fish (particularly herring, sardines, fatty fish) at least two albacore tuna, and times a week. Fish is a salmon are high in two good source of protein kinds of omega-3 fatty and doesn’t have high acids: eicosapentaenoic saturated fat that fatty acid (EPA) and meat products do. docosahexaenoic acid (DHA).
Function of Triglycerides • Fat performs a number of essential functions in the body. • Fat is also a major energy source. When at rest, muscles prefer to use fat for fuel.
A Healthy Balance • An appropriate balance of calories from fat and carbohydrates can have multiple health benefits aside from controlling weight. • These include cholesterol levels, vitamin intake, etc.
Fat = Storage • Evolution has selected fat, rather than glycogen, as its primary energy storage medium. • A gram of fat stores more than six times as much energy as a gram of glycogen.
Micronutrient Carrier • Fat is a micronutrient carrier. • Fat holds more than just energy. • It also carries important nutrients, such as fat-soluble vitamins and carotenoids.
Recommended Fat Intake • The Dietary Guidelines for Americans recommends a fat intake of 20 to 35 percent of total calories. • Saturated fat should supply no more than 10 percent of our total calories, or about onethird of our fat calories.
Position on Fat Replacers • It is the position of the Academy of Nutrition and Dietetics that the majority of fat replacers, when used in moderation by adults, can be safe and useful adjuncts to lowering the fat content of foods and may play a role in decreasing total dietary energy and fat intake. • Moderate use of low-calorie, reduced-fat foods, combined with low total energy intake, could potentially promote dietary intake consistent with the objectives of Healthy People 2010 and the 2010 Dietary Guidelines for Americans.
The Women’s Health Initiative • This study sought to explore whether a dietary intervention that reduces intake of total fat and increases intakes of fruits, vegetables, and grains will reduce the risk of cardiovascular disease.
• The results found that the intervention had no effect on the risk of coronary heart disease, stroke, or overall cardiovascular disease. • Since this study was initiated, scientific thinking has evolved to focus on modifying intake of specific types of fat.
NCEP Tips for Healthful Eating Out • Choose restaurants that have low-fat, low-cholesterol menu items. • Don’t be afraid to ask for foods that follow your eating pattern. • Select poultry, fish, or meat that is broiled, grilled, baked, steamed, or poached rather than fried.
Diet and Cancer • The evidence linking dietary fat to cancer is inconclusive. The case looks strong when we compare cancer rates between countries: Overall cancer rates are generally higher in countries with high fat intake, and lower in countries where people eat less fat. But in population studies within those countries, the evidence linking fat to cancer is weaker.