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Transcript
Chapter 5 Lipids: Not Just Fat What Are Lipids? • Essential nutrients – Provide energy – Help transport fat-soluble nutrients throughout the body • Contribute greatly to the flavor and texture of food Lipids Include… • Triglycerides (most abundant lipids) – In body: stored in adipose tissue – In food: “fats and oils” • Phospholipids (~2% of dietary lipids) – Plant and animal origin – Body can make them – Soluble in fat and water • Sterols (very, very small % of lipids) – Most well known: Cholesterol Fatty Acids Are Key Building Blocks • Fatty acid – Determines whether a fat is solid or liquid at room temperature – Basic structure: (-COOH); (-CH3) Fatty Acids Are Key Building Blocks • Chain length – Fatty acids differ in chain length – Lengths vary from 4 to 24 carbons – Grouped as shortchain, mediumchain, and longchain Fatty Acids Are Key Building Blocks • Chain Length – Shorter fatty acids remain liquid at room temperature and even with refrigeration – Shorter fatty acids also are more water-soluble Fatty Acids Are Key Building Blocks • Saturation – Saturated fatty acids • All single bonds between carbons – Unsaturated fatty acids • One or more carbon bonds is a double bond 1. Monounsaturated fatty acid (MUFA) 2. Polyunsaturated fatty acid (PUFA) – Fats with more double bonds are generally more liquidy Fatty Acids Are Key Building Blocks • Cis vs. trans – Unsaturated fatty acids can vary in shape – Cis fatty acids • Chain is bent • Occur naturally – Trans fatty acids • Chain is straighter • Produced by hydrogenation Trans Fat • In health, trans fatty acids are known to raise LDL (“bad”) cholesterol, and lower HDL (“good cholesterol”), promote systemic inflammation, and increase triglycerides in your blood. • History and how it is made: – In Europe (1910): needed a cheaper butter substitute for soldiers – In US (1960s): People wanted to eat a “healthier and cheaper butter” food scientists decided to hydrogenate vegetable oil to solidify them Fatty Acids are Key Building Blocks • Nonessential and essential fatty acids 1. Nonessential fatty acids • Can be made in the body • Not “essential” to have in your diet Fatty Acids Are Key Building Blocks • Essential and Nonessential Fatty Acids – Essential fatty acids • Must come from food • Cannot be made by the body • Two families: –Omega-3 (alpha-linolenic acid) –Omega-6 (linoleic acid) • Pre-cursors to eicosanoids Triglycerides • Structure – Glycerol + 3 fatty acids – Most fatty acids exist as part of triglyceride molecules Triglycerides • Functions 1. Energy source: 9 kcal/g 2. Energy reserve: form of stored energy in adipose tissue 3. Insulation and protection: 1. Visceral fat 2. Subcutaneous fat 4. Carrier of fat-soluble nutrients 5. Sensory qualities (flavor, odor, texture) in food Triglycerides • Functions – Carrier of Fat-Soluble Compounds • Improves intestinal absorption and bioavailability during digestion – Sensory Qualities • Contributes to food’s flavor, odor, and texture Triglycerides in Food • Found in a variety of fats and oils – Classified by their most prevalent type Triglycerides in Food • Commercial processing of fats – Reduces waste, prevents spoilage, increases availability of calorie-rich oils – Removes damaging free fatty acids – Adds antioxidants to delay rancidity and extend shelf life Triglycerides in Food • Commercial processing of fats – Negatives • Removes potentially healthful phospholipids, plant sterols, and other phytochemicals • Further processing into solid fats increases the proportion of trans fatty acids Phospholipids • Structure – Glycerol + two fatty acids + phosphate group – Compatible with both fat and water Phospholipids • Functions – Ideal emulsifiers • Keep fat suspended in water • Keep oil and water mixed – Perfect structural element for cell membranes • Able to communicate with watery environments of blood and cell fluids • Allows other lipids to enter and exit cells Phospholipids • Cell membranes – Double layer of phospholipids – Selectively allow both fatty and water-soluble substances into the cell – Store fatty acids temporarily – Plays an important role in nerve cells Phospholipids • Lipid Transport – In the stomach • Break fats into tiny particles for digestion – In the intestine • Continue emulsifying fat – In the blood • Coat the surface of the lipoproteins that carry lipid particles to their destinations in the body Phospholipids • Emulsifiers (lecithin) – Lecithin used as an emulsifier to combine two ingredients that don’t ordinarily mix, such as oil and water – Allows ingredients in salad dressing to mix well and remain mixed, for example Phospholipids in Food • Occur naturally in plants and animals, but in much smaller amounts than triglycerides • Abundant in egg yolks, liver, soybeans, peanuts • Not a dietary essential Sterols • Have a multiple-ring structure • Best known example is cholesterol • Most contain no fatty acid Sterols • Cholesterol Functions – Major structural component of all cell membranes – Precursor of important substances, including • Vitamin D • Sterol hormones • Bile salts Sterols • Cholesterol Synthesis – Primarily made in the liver Sterols in Food • Found only in foods of animal origin • Typical American consumers between 250-700 mg of cholesterol and 250 milligrams of plant sterols per day Lipid Digestion and Absorption • Digestion of Triglycerides and Phospholipids – Mouth • Chewing and lingual lipase start digestion – Stomach • Breaks triglycerides down to diglycerides and free fatty acids – Small intestine • Bile and pancreatic lipase emulsify and break down the fats for absorption • Intestinal cells absorb glycerol and fatty acids into the bloodstream Lipid Digestion and Absorption • Lipid absorption – Micelles • Water-soluble globules with a fatty core • Carry monoglycerides and long-chain fatty acids to microvilli • Bile recycling pathway known as enterohepatic circulation Lipid Digestion and Absorption • Lipid absorption – Lipoprotein – Cylomicron • Deliver dietary lipids from intestines to cells and liver Lipid Digestion and Absorption • Digestion and absorption of sterols – Body absorbs about 50% of dietary cholesterol • Dietary fat increases absorption • Plant sterols and dietary fiber decrease absorption Transportation of Lipids in the Body • Lipids packaged into lipoprotein carriers in order to travel in the bloodstream • Lipoproteins differ by size, density, and the composition of their lipid cores Transportation of Lipids in the Body • Chylomicrons – Form in the intestinal tract – Enter the lymphatic system – About 90 percent fat – Liver uses remnants as raw material for very-lowdensity lipoproteins Transportation of Lipids in the Body • Very-Low-Density Lipoproteins (VLDLs) – Deliver triglycerides to cells Transportation of Lipids in the Body • Intermediate-Density Lipoproteins (IDLs) – Converted to lowdensity lipoproteins Transportation of Lipids in the Body • Low-Density Lipoproteins (LDLs) – Deliver cholesterol to cells Transportation of Lipids in the Body • High-Density Lipoproteins (HDL) – Pick up cholesterol for removal or recycling Recommendations for Fat Intake • Recommended intake – Reduce saturated fat, trans fat, and cholesterol – Total fat: 20–35% of calories – Less than 10% of calories from saturated fat – Less than 300 mg per day of cholesterol Recommendations for Fat Intake • Essential fatty acid requirements – Linoleic acid should provide about 2% of calories • Omega-6 and omega-3 balance – Ratio of 6:1 omega-6 to omega-3 fatty acids • Seafood (fatty fish), canola or soybean oil Recommendations for Fat Intake • Current dietary intakes – Americans eat ~33% of total calories from fat – Average calorie intake has increased = Americans consuming more total grams of fat – Saturated fat intake ~11% • Cheese, pizza, grain-based desserts, chicken, sausage, etc. Recommendations for Fat Intake • Role of fat replacers – Different types of composition – Olestra • Sucrose + fatty acids • Indigestible— provides no calories • Reduces absorption of fat-soluble vitamins Lipids and Health • Heart disease – Major risk factors • High blood cholesterol –High LDL and low HDL • Smoking • High blood pressure Lipids and Health • Reducing heart disease risk – AHA diet and lifestyle recommendations • Consume an overall healthy diet • Aim for a healthy body weight • Aim for a desirable lipid profile • Aim for normal blood pressure • Aim for normal blood glucose levels • Be physically active • Avoid use of and exposure to tobacco products Lipids and Health • Reducing heart disease risk – AHA recommendations • Balance calorie intake and physical activity to achieve or maintain a healthy body weight • Consume a diet rich in fruits and vegetables • Choose whole-grain, high-fiber foods • Consume fish, especially oily fish, at least twice a week • Limit your intake of saturated and trans fat and cholesterol Lipids and Health • Reducing heart disease risk – AHA recommendations (cont.) • Minimize your intake of beverages and foods with added sugars • Choose and prepare foods with little of no salt • If you consume alcohol, do so in moderation • Follow the AHA recommendations when eating outside of the home Lipids and Health • Obesity – High-fat diets promote weight gain – Significant within the U.S. population • 34.9% of American adults • 17% of American children and adolescents Lipids and Health • Metabolic syndrome – Affects ¼ of American adults – Cluster of at least three symptoms • Excess abdominal fat • High blood glucose • High serum triglycerides • Low HDL cholesterol • High blood pressure Lipids and Health • Cancer – Results from complex mix of lifestyle, hereditary, and environmental factors – Role of nutrition and diet complex • Evidence suggests 30-40% are due to poor food choices and physical inactivity • Some dietary factors act as promoters, while others serve a protective role Lipids and Health • Cancer – Dietary and lifestyle factors for reducing cancer risk • Maintain a healthful weight • Adopt a physically active lifestyle • Consume a healthy diet • Limit alcohol consumption Lipids and Health • Putting It All Together – Healthy People 2020 objectives target reducing deaths from heart disease and stroke and reducing the number of adults with high blood cholesterol levels