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The Lipids: Triglycerides, Phospholipids, and Sterols Chapter 5 Fig. 5-CO, p. 132 Introduction Poor health Too much fat Too little fat (unlikely in U.S.) Too much of some kinds of fat Family of lipids Triglycerides- fats and oils Phospholipids Sterols Overview of Fatty Acids and Triglycerides Energy provided per gram More carbons and hydrogens per oxygen Preview of lipids from diet Triglycerides: 1 glycerol plus 3 fatty acids Fatty acids have even number of carbons Fatty acids are saturated or unsaturated Omega-3 and omega-6 fatty acids Chemist’s View of Fatty Acids and Triglycerides Fatty acids Organic (carbon-based) acid Methyl group at one end; acid group at other end Usually even number of carbons, 4-24 18-carbon fatty acids abundant in food Saturations Saturated – full of hydrogens, no double bonds Unsaturated – missing hydrogens Acid end Methyl end Up to 22 more carbons p. 138 Saturated Fatty Acids NO double bonds Solid at 77 F Found in meat, dairy, tropical fat Max. allowed is 1/3 total daily fat Stearic Acid Fully saturated, no double bonds Stearic Acid Zero double bonds Polyunsaturated Fatty Acids (PUFA’s) Omega-3: 1st double bond is 3 carbons from methyl end; ex: linolenic acid • fatty fishes Omega-6: 1st double bond is 6 carbons from methyl end; ex: linoleic acid many vegetable oils Liquid at 77 F PUFA’s Linolenic acid, an omega-3 fatty acid Omega carbon at #3 Acid end Methyl end Linoleic acid, an omega-6 fatty acid Omega carbon at #6 Acid end Methyl end Monounsaturated Fatty Acids (MUFA’s) One double bond Most MUFA in diet are omega-9 Oleic acid most common MUFA Olive, safflower, canola oils Liquid at 77 F Impossible configuration One Double Bond Mono= one point of unsaturation Linoleic acid, an 18-carbon PUFA 2 double bonds Linoleic acid Double bond Saturated fatty acids tend to stack together. Consequently, saturated fats tend to be solid (or more firm) at room temperature. This mixture of saturated and unsaturated fatty acids does not stack neatly because unsaturated fatty acids bend at the double bond(s). Consequently, unsaturated fats tend to be liquid (or less firm) at room temperature. Fatty Acids Distinction by location of double bonds Omega number is 1st double bond nearest the methyl end of the carbon chain Linolenic acid 3 dbl bonds, minus 6 H+ Linoleic acid 2 dbl bonds, minus 4 H+ Monounsaturated fatty acids Omega-9 groups PUFA’s in Food Linolenic acid, an omega-3 fatty acid Omega carbon at #3 Acid end Methyl end Linoleic acid, an omega-6 fatty acid Omega carbon at #6 Acid end Methyl end Triglycerides (TG) Glycerol backbone Three fatty acids Formed via series of condensation reactions Usually contain mixture of fatty acids Glycerol How Triglycerides are Made Glycerol + three fatty acids An H atom from glycerol and an OH group from a fatty acid combine to create water, leaving the O on the glycerol and the C at the acid end of each fatty acid to form a bond. Saturation vs Unsaturation Firmness Poly and Monounsaturated fats Saturated fats Length of carbon chain- shorter Stability Oxidation and spoilage of fats Saturated fats are more stable Antioxidants BHA, BHT, Vitamin E softer Hydrogenation Adding H2 to PUFA’s to reduce double bonds, making them more saturated / solid and more resistant to oxidation which leads to rancidity. Hydrogenation produces trans fatty acids How to Make Trans from Poly with Hydrogen Gas Hydrogenation Advantages – Shelf life – Texture improvement Disadvantages Acts like saturated fat in the blood, only worse Cis- and Trans- cis-fatty acid trans-fatty acid A cis-fatty acid has its hydrogens on the same side of the double bond; cis molecules fold back into a U-like formation. Most naturally occuring unsaturated fatty acids in foods are cis. A trans-fatty acid has its hydrogens on the opposite sides of the double bond; trans molecules are more linear. The trans form typically occurs in partially hydrogenated foods when hydrogen atoms shift around some double bonds and change the configuration from cis to trans. Phospholipid (PL) • Compound similar to a triglycerides but has a phosphate group and choline in place of one of the fatty acids • Lecithin most common one Phospholipid From 2 fatty acids The plus charge on the N is balanced by a negative ion— usually chloride. From choline From glycerol From phosphate Phospholipids Phospholipids Solubility in fat and water Emulsifiers in food industry (lecithin) Food sources- egg yolk, liver, soy, peanuts Roles in the Body Part of cell membranes Emulsifiers Cell Membrane Outside cell (ECF) Glycerol heads Fatty acid tails Glycerol heads Inside cell (ICF) Sterols Cholesterol Food sources and production of 800-1500 mg/d by liver Plant sterols inhibit cholesterol absorption Body compounds made from cholesterol Bile acids Sex hormones Adrenal hormones Vitamin D Cholesterol Vitamin D3 Lipid Digestion Fats are hydrophobic Digestive enzymes are hydrophilic Goal of fat digestion Dismantle triglycerides into monoglycerides, fatty acids, and glycerol Lipid Digestion FAT Mouth and salivary glands Some hard fats begin to melt as they reach body temperature. The sublingual salivary gland in the base of the tongue secretes lingual lipase. Salivary glands Stomach The acid-stable lingual lipase initiates lipid digestion by hydrolyzing one bond of Gallbladder triglycerides to produce diglycerides and fatty acids. The degree of hydrolysis by lingual lipase is slight for most fats but may be appreciable for milk fats. The stomach’s churning action mixes fat with water and acid. A gastric lipase accesses and hydrolyzes (only a very small amount Common of) fat. Small intestine Bile flows in from the gallbladder (via the common bile duct): Bile Fat Emulsified fat Pancreatic lipase flows in from the pancreas (via the pancreatic duct): Pancreatic (and intestinal) Monoglycerides, Emulsified lipase fat glycerol, fatty (triglycerides) acids (absorbed) Large intestine Some fat and cholesterol, trapped in fiber, exit in feces. Mouth Tongue Sublingual salivary gland Stomach (Liver) Pancreatic duct Pancreas bile duct Small intestine Large intestine Fig. 5-12, p. 142 Lipid Digestion Mouth- minor Lingual lipase for dairy fat Stomach- minor Strong muscle contractions Gastric lipase hydrolyzes TG into diglycerides and fatty acids Fat Watery GI juices Enzymes In the stomach, the fat and watery GI juices tend to separate. The enzymes in the GI juices can’t get at the fat. Lipid Digestion Main Site Small intestine Cholecystokinin (CCK) signals gall bladder to release bile Bile acts as emulsifier Pancreatic lipases Hydrolysis Triglycerides and phospholipids Bile routes Blood cholesterol levels Bile as an Emulsifier Bile acid made from cholesterol (hydrophobic) Bound to an amino acid from protein (hydrophilic) Emulsification of Fat by Bile Fat Watery GI juices Enzymes In the stomach, the fat and watery GI juices tend to separate. The enzymes in the GI juices can’t get at the fat. Enzyme Fat Bile Emulsified fat When fat enters the small intestine, the gallbladder secretes bile. Bile has an affinity for both fat and water, so it can bring the fat into the water. Emulsified fat Bile’s emulsifying action converts large fat globules into small droplets that repel each other. Emulsified fat After emulsification, more fat is exposed to the enzymes, making fat digestion more efficient. Bile acting like Soap Fat Bile Emulsified fat Enterohepatic Circulation of Bile In the gallbladder, bile is stored. In the liver, bile is made from cholesterol. In the small intestine, bile emulsifies fats. In the colon, bile that has been trapped by soluble fibers is lost in feces. Enzyme Emulsified fat After emulsification, more fat is exposed to the pancreatic lipases, making fat digestion (hydrolysis) more efficient. Hydrolysis of a Triglyceride Bonds break Bonds break Triglyceride The triglyceride and two molecules of water are split. The H and OH from water complete the structures of two fatty acids and leave a monoglyceride. Monoglyceride + 2 fatty acids These products may pass into the intestinal cells, but sometimes the monoglyceride is split with another molecule of water to give a third fatty acid and glycerol. Fatty acids, monoglycerides, and glycerol are absorbed into intestinal cells. Lipid Absorption Directly into bloodstream Glycerol and short- & medium-chain fatty acids Micelles- fatty acids, monodiglycerides, bile, cholesterol diffuse into intestinal cells Reassembly of triglycerides from micelles Chylomicrons- protein vehicle picking up TG’s, cholesterol, phospholipids in S.I. Intestinal cells release chylomicrons into lymphatic system Absorption of Fat Small intestine Monoglyceride Stomach Short-chain fatty acids 1 Medium-chain fatty acids Glycerol Micelle 2 Protein Triglyceride Chylomicrons Chylomicron Long- Capillary network Blood vessels Lacteal (lymph) chain fatty acids 2 Large lipids such as monoglycerides and long-chain fatty acids combine with bile, forming micelles that are sufficiently water soluble to penetrate the watery solution that bathes the absorptive cells. There the lipid contents of the micelles diffuse into the cells. Via lymph to blood Via blood to liver 1 Glycerol and small lipids such as short- and medium-chain fatty acids can move directly into the bloodstream. Lipid Transport Four main types of lipoproteins Chylomicrons Largest and least dense (more fat, less prot.) Shrink as they transport diet-derived lipids Liver removes remnants from blood Very-low-density lipoproteins (VLDL) Made in the liver, 50% TG Cells take TG until VLDL becomes LDL Lipid Transport Four main types of lipoproteins Low-density lipoproteins (LDL) More cholesterol than TG Distribute Chol, TG and PL for cell needs Liver regulation High-density lipoproteins (HDL) Made by liver Removes cholesterol from cells Carry cholesterol to liver for recycling Anti-inflammatory properties Phospholipid Cholesterol Triglyceride 100 80 LDL VLDL Percent Chylomicron Protein A typical lipoprotein contains an interior of triglycerides and cholesterol surrounded by phospholipids. The phospholipids’ fatty acid “tails” point towards the interior, where the lipids are. Proteins near the outer ends of the phospholipids cover the structure. This arrangement of hydrophobic molecules on the inside and hydrophilic molecules on the outside allows lipids to travel through the watery fluids of the blood. Protein 60 40 Cholesterol 20 Phospholipid 0 Triglyceride Chylomicron VLDL LDL HDL Chylomicrons contain so little protein and so much HDL triglyceride that they are the lowest in density. Very-low-density lipoproteins (VLDL) are half This solar system of lipoproteins shows their triglycerides, accounting for their very low density. relative sizes. Notice how large the fat-filled Low-density lipoproteins (LDL) are half cholesterol, chylomicron is compared with the others and accounting for their implication in heart disease. how the others get progressively smaller as their proportion of fat declines and protein increases. High-density lipoproteins (HDL) are half protein, accounting for their high density. Lipid Transport Role of Triglycerides Provide the cells with energy 9 kcalories per gram Virtually unlimited ability to store fat energy in body Adipose tissue secrete adipokines regulate energy balance (leptin) Insulin resistance and inflammation (resistan) Skin insulation, shock absorption, cell membranes, and cell signaling pathways Essential Fatty Acids Linoleic acid – Omega-6 fatty acid Starter for arachidinic acid Sources- vegetable oils and meat Linolenic acid – Omega-3 fatty acid Sources- fish, flaxseed Starter for DHA , EPA Eicosanoids made from arachidinic and EPA, regulate blood pressure, clotting Fatty acid deficiencies Lipid Metabolism (Burning Fat) Adipose cells store fat after meals Lipoprotein lipase in adipose hydrolyzes triglycerides from lipoproteins passing by and releases them into adipose cells Triglycerides reassembled inside adipose cells for storage Fat supplies 60% of energy during rest 1 lb body fat = 3500 kcal Requires CHO to break down fat Health Effects of Lipids Heart disease Elevated blood cholesterol Saturated fat – increase LDL cholesterol, promote blood clotting Dietary choices Trans-fats – increase LDL cholesterol Dietary cholesterol Health Effects of Lipids Heart disease Monounsaturated fats Replace saturated and trans fats Reduces blood cholesterol Dietary sources Omega-3 fats Benefits Dietary sources Omega-6 to omega-3 ratio Health Effects of Lipids Cancer Promotion rather than initiation of cancer Dietary fat and cancer risk Differs for various types of cancer Obesity Cutting fat from diet reduces kcalories Dietary recommendations Recommended Intakes of Fat DRI and Dietary Guidelines Diet low in saturated and trans fat Diet low in cholesterol 20 to 35 percent of daily energy from fat AI set for linoleic and linolenic acids Daily Values (DV) on food labels Saturated fat and cholesterol Risk of insufficient fat intake From Guidelines to Groceries Fat-soluble vitamins A, D, E, and K Flavor, texture, and palatability Meats and meat alternatives Selections Milk and milk products Selections From Guidelines to Groceries Vegetables, fruits, and grains Lowers consumption of various fats in the diet Invisible fat Fried and baked goods Choose wisely Unprocessed foods From Guidelines to Groceries Fat replacers Types Risks Read food labels Total fat, saturated fat, trans fat, and cholesterol Compare products % Daily Value vs. % kcalories from fat Butter and Margarine Labels Compared Highlight 5 High-Fat Foods – Friend or Foe? Guidelines for Fat Intake Limit saturated fat and trans fat intake Moderate kcalories Enough fat for good health Not too much of the harmful fats DRI recommendations Compatible with low rates of disease High-Fat Foods and Heart Health Olive oil Benefits for heart health Replace saturated fats Nuts LDL cholesterol Fat composition Benefits for heart health Cautious advice for dietary inclusion High-Fat Foods and Heart Health Fish Omega-3 fatty acids Benefits for heart health Environmental contaminants Dietary recommendations High-Fat Foods and Heart Health High-Fat Foods and Heart Disease Saturated fat and LDL cholesterol Sources of saturated fat in the U.S. Meats Whole milk products Tropical oils Zero saturated fat is not possible Trans fat Limit hydrogenated foods High-Fat Foods and Heart Disease High-Fat Foods and Heart Disease High-Fat Foods and Heart Disease The Mediterranean Diet Traditionally Low in saturated fat Very low in trans fat Rich in unsaturated fat Rich in complex carbohydrate and fiber Rich in nutrients and phytochemicals Benefits for heart disease risk