The Lipids: Triglycerides, Phospholipids, and Sterols Chapter 5

Why do we need it?  Fat provides…  Ability to absorb fat-soluble vitamins A, D, E, and K  Flavor, texture, and palatability  Increased satiety  Essential nutrients: the essential fatty acids (EFA)

Fat and Health  Fat intake and health issues  Too much fat  Too little fat  Too much of some kinds of fat  Lipid family  Triglycerides  Phospholipids  Sterols

Chemist’s View of Fatty Acids and Triglycerides  Energy provided per gram: 9 kcals  More carbons and hydrogens  Lipid Structure  Triglycerides: Major lipid in foods  Glycerol and 3 fatty acids  Fatty acids: even number of carbons  Fatty acids: saturated or unsaturated

Structure of a Triglyceride Triglyceride

Triglyceride Condensation

Characteristics of Fatty Acids  Organic acid  Methyl group at one end (omega end)  Acid group at other end  Usually even number of carbons –Vary in length from a few to 20 or more carbons –Short-chain fatty acids: 4-7 carbons –18-carbon fatty acids abundant in food  Saturation  Saturated – full of hydrogens  Unsaturated – missing hydrogens

Saturated Fatty Acid Stearic acid

Monounsaturated Fatty Acid

Polyunsaturated Fatty Acid Docosahexaenoic Acid (DHA)

Double Bonds  Fatty acids  Location of double bonds  Numbered starting with bond nearest the methyl end of the carbon chain  Omega number  Linolenic acid: omega-3  Monounsaturated fatty acids  Omega-9 groups

Effects of Double Bonds  Degree of unsaturation  Firmness  Polyunsaturated fats  Saturated fats  Length of carbon chain  Fatty acid structure and stability  Oxidation and spoilage of fats  Influences  Antioxidants in oils

Structure and Physical Properties of Fatty Acids  Polyunsaturated fats vs. saturated fats  “Vegetable oils” are cheap for manufacturers, but vulnerable to oxidation (spoilage)  Solution: hydrogenation  Advantages  Disadvantages  Trans-fatty acids  Configurations – cis and trans  Trans: implicated in a growing number of disorders and illnesses - inflammation

Trans- fatty acid

Essential Fatty Acids  Linoleic acid – Omega-6 fatty acid  Sources  AA (arachidonic acid)  Linolenic acid – Omega-3 fatty acid  Sources  ALA  DHA  EPA  Cell membrane structure, eicosanoids

EFA: Metabolic Pathways

Omega-3 vs. Omega-6 Both essential fatty acids Americans tend to get lots of n-6, not so much n-3 Eicosanoids made from n-3: anti-inflammatory Eicosanoids made from n-6: pro-inflammatory

Eicosanoids from Omega-3 and Omega-6 Fatty Acids

Essential Fatty Acid Deficiency Frank deficiency is rare –Scaly, dry skin –Liver abnormalities –Poor wound healing –Impaired growth and neurological development in infants, young children –Impaired vision and hearing Suboptimal levels –Possible disorders affecting mood, cognition, and memory –Inflammatory conditions Skin Cardiovascular system Joints

Phospholipids and Sterols

Phospholipids  Phospholipids  Solubility in fat and water  Emulsifiers in food industry  Lecithin  Food sources  Roles  Part of cell membranes  Emulsifiers

Phospholipid Bilayer

Sterols  Sterols  Food sources  Cholesterol  Plant sterols  Roles of sterols  Body compounds made from cholesterol Cholesterol

Lipid Digestion  Fats are hydrophobic  Digestive enzymes are hydrophilic  Digestive juices are watery  Need bile to emulsify fats  Goal of fat digestion  Dismantle triglycerides  Monoglycerides, fatty acids, and glycerol

Hydrolysis of a Triglyceride

Lipid Digestion  Mouth  Lingual lipase  Stomach  Strong muscle contractions  Gastric lipase  Most of the action occurs in the small intestine

Lipid Digestion  Small intestine  Cholecystokinin (CCK) released  Stimulates gall bladder to release bile  Bile acts as emulsifier  Pancreatic lipase  Hydrolysis  Triglycerides and phospholipids  Bile routes  Blood cholesterol levels

Fat Watery GI juices Fat In the stomach, the fat and watery GI juices tend to separate. The enzymes in the GI juices can’t get at the 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. Bile’s emulsifying action converts large fat globules into small droplets that repel each other. After emulsification, more fat is exposed to the enzymes, making fat digestion more efficient. Enzyme Emulsified fat Bile Enzymes Stepped Art Emulsified fat Fig. 5-14, p. 143

In the gallbladder, bile is stored. In the liver, bile is made from cholesterol. In the colon, bile that has been trapped by soluble fibers is lost in feces. In the small intestine, bile emulsifies fats. Stepped Art Fig. 5-16, p. 144

Lipid Absorption  Directly into bloodstream  Glycerol and short- & medium-chain fatty acids  Lymphatic system  Micelles diffuse into intestinal cells  Reassembly of triglycerides  Packaged with proteins – chylomicrons  Bypass liver at first

Lipid Transport  Four main types of lipoproteins  Provide triglycerides, cholesterol, and phospholipids for cells  Build new membranes  Make hormones or other compounds  Storage

Lipoproteins  Chylomicrons  Largest and least dense  Transport diet-derived lipids (mostly TG)  Taken up by cells  Liver removes remnants from blood  Very-low-density lipoproteins (VLDL)  Made in the liver from chylomicron remnants  Released to rest of body  Cells take up TG, leaving cholesterol

Lipoproteins  Low-density lipoproteins (LDL)  Contain lots of cholesterol; also TG and phospholipids  Circulate cholesterol to the body’s cells  High-density lipoproteins (HDL)  Remove cholesterol from cells  Carry cholesterol to the liver for recycling  Anti-inflammatory properties  Health implications

Lipid Transport

Cardiovascular Disease Over 80 million Americans suffer from some form of cardiovascular disease Studies show that diet and lifestyle affect the risk of developing cardiovascular disease –Disagreement about the specifics! –Increasing evidence implicates diets high in refined CHO and low in fiber, antioxidants, and omega-3 fats Atherosclerosis is a disease in which lipids and fibrous materials are deposited in artery walls

Lipid Functions in the Body Most lipids in the body are triglycerides stored in adipose tissue, which helps to… –Define body shape –Provide stored energy –Insulate the body from temperature changes –Protect internal organs against physical shock Lipids are also important for lubricating body surfaces, such as the skin, hair, and the mucous membranes of the eyes

More Functions of Lipids Cholesterol is used to make several hormones, including estrogen and testosterone. Essential fatty acids are important for growth, skin integrity, fertility, the structure and function of cell membranes, and making eicosanoids Eicosanoids help regulate blood clotting, blood pressure and immune function. Fatty acids and glycerol can be used to produce energy in the form of ATP.

Fatty Acid Oxidation: Energy from fats

Energy from Fats