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Learning Outcomes List and describe the major types, functions, and structures of lipids. Differentiate among essential, conditionally essential, and nonessential fatty acids. Describe the differences among mono-, di-, and triglycerides. Discuss the functions of phospholipids and sterols. Explain the processes of triglyceride digestion, absorption, and circulation.

Learning Outcomes 6 List and describe the types and functions of the various lipoproteins. 7 Explain the impact of lipids on your health. 8 Understand and be able to implement dietary recommendations for lipids.

Organic macronutrients What Are Lipids? Organic macronutrients Insoluble in water Hydrophobic Roles of lipids Oil Liquid at room temperature Fat Solid at room temperature Major lipids

What Are Lipids? Fatty acids Most abundant type of lipid in body and diet Structure Carbon chain Alpha end – carboxylic acid group Omega end – methyl group Types Number of carbons Types and locations of chemical bonds

FIGURE 6.1 Fatty Acid Structure All fatty acids have three components: a methyl or ω end (–CH3), a fatty acid backbone made from carbon and hydrogen atoms and a carboxylic acid or α end (–COOH). 6

What Are Lipids? Fatty acids Chain length Most naturally-occurring fatty acids have even number Short-chain, medium-chain, and long-chain fatty acids Chemical properties Physiological functions Solubility in water

FIGURE 6.2 Fatty Acids Can Have Different Chain Lengths First image: Medium-chain fatty acid Second Image: Long-chain fatty acid 8

What Are Lipids? Fatty acids Chemical bonds Single bonds Double bonds Saturated fatty acids (SFAs) Double bonds Monounsaturated fatty acids (MUFAs) Polyunsaturated fatty acids (PUFAs) Chemical nature

FIGURE 6.3 Saturated and Unsaturated Fatty Acids First image: Saturated fatty acid - This double bond is part of the carboxylic acid group and does not make the fatty acid “unsaturated.” Second image: Monounsaturated fatty acid - The presence of a double bond bends the fatty acid backbone. There are two fewer hydrogens for each double bond. Third image: Polyunsaturated fatty acid - The presence of two double bonds causes two bends in the fatty acid backbone. The type of carbon–carbon bonds in a fatty acid determines whether it is saturated or unsaturated. 10

What Are Lipids? Fatty acids Cis double bonds Hydrogens on same side of double bond Trans double bond Hydrogens on opposite sides of double bond Trans fatty acids in food Naturally occurring Hydrogenation

FIGURE 6.4 Cis versus Trans Fatty Acids First image: A cis fatty acid - Hydrogens are on the same side of the fatty acid backbone. Second image: A trans fatty acid - Hydrogens are on opposite sides of the fatty acid backbone. Unsaturated fatty acids differ according to whether they have cis or trans carbon–carbon double bonds. cis bonds cause the fatty acid to bend, whereas trans bonds do not. 12

What Are Lipids? Naming fatty acids Characteristics Alpha naming system Relative to carboxylic acid end Omega naming system Double bond’s distance from the methyl end Omega-3 and omega-6 fatty acids Common names

FIGURE 6.5 The Alpha (α) Naming System The first double bond (which is cis) from the alpha end is on the ninth carbon atom. The second double bond (which is cis) from the alpha end is on the twelfth carbon atom. The alpha naming system for fatty acids requires that you count how far the double bonds are from the alpha (carboxylic acid) end of the carbon backbone. Using this system, this fatty acid’s name is cis9, cis12–18:2. 14

What Are Essential, Conditionally Essential, and Nonessential Fatty Acids? Linoleic acid 18 carbons; two double bonds; omega-6 fatty acid Arachidonic acid Linolenic acid 18 carbons; three double bonds; omega-3 fatty acid EPA DHA

What Are Essential, Conditionally Essential, and Nonessential Fatty Acids? Eicosanoids Roles Omega-6 eicosanoids Cause inflammation and constriction of blood vessels Omega-3 eicosanoids Reduce inflammation and stimulate dilation of blood vessels

FIGURE 6.6 Fatty Acid Metabolism and Eicosanoid Formation 1. The essential fatty acids can be elongated and desaturated to form new fatty acids such as arachidonic acid, EPA, and DHA. 2. Arachidonic acid and EPA can be further metabolized to 􀁚-6 and 􀁚-3 eicosanoids, respectively. Note that eicosanoids are not, themselves, fatty acids. Linoleic acid and linolenic acid can be converted, via elongation and desaturation, to longer-chain polyunsaturated fatty acids, some of which are used to make hormone-like eicosanoids. 17

What Are Essential, Conditionally Essential, and Nonessential Fatty Acids? Deficiency Primary Rare Secondary Diseases that disrupt lipid absorption Cystic fibrosis People who consume foods high in v-3 fatty acids (such as many cold-water fish) may have lower risk for conditions related to inflammation, such as heart disease.

Conditionally essential fatty acids What Are Essential, Conditionally Essential, and Nonessential Fatty Acids? Conditionally essential fatty acids Infancy Arachidonic acid DHA Dietary sources of fatty acids Linoleic acid Linolenic acid EPA and DHA

Nonessential fatty acids What Are Essential, Conditionally Essential, and Nonessential Fatty Acids? Nonessential fatty acids Dietary sources SFAs PUFAs MUFAs Roles

What Is the Difference between Mono-, Di-, and Triglycerides? Number of fatty acids in chemical structure Saturation of fatty acids Lipogenesis Fatty acids combine with glycerol

FIGURE 6.8 Triglyceride Structure These fatty acids can be saturated (SFA), monounsaturated (MUFA), polyunsaturated (PUFA), or a combination. A triglyceride consists of a glycerol molecule and three fatty acids. 22

What Is the Difference between Mono-, Di-, and Triglycerides? Richest source of energy 9 kcal per gram Lipolysis β-oxidation Ketones Ketogenesis

What Is the Difference between Mono-, Di-, and Triglycerides? Storage in adipose tissue Adipocytes Stored as triglycerides Advantages Location of adipose tissue Subcutaneous vs. visceral adipose tissue Role of insulin Insulation

What Are Phospholipids and Sterols? Roles Cell membranes Transport of lipids in bloodstream No dietary requirements for either of them

What Are Phospholipids and Sterols? Has two fatty acids Phosphate-containing hydrophilic head Amphipathic Roles Cell membranes Digestion, absorption, and transport of lipids Other functions

FIGURE 6.10 Cell Membrane Made from Phospholipid Bilayer Proteins are embedded in the cell membrane. Cell membranes are made of a bilayer of phospholipids with proteins and cholesterol dispersed. Hydrophobic fatty acids make up the interior portion of the cell membrane. Hydrophilic head groups point toward hydrophilic (watery) environments. The amphipathic nature of phospholipids allows cell membranes to carry out their functions. 27

What Are Phospholipids and Sterols? Multi-ring structure Cholesterol Synthesis of bile acid Component of cell membranes Reproductive hormones Other roles Sources Phytosterols

FIGURE 6.12 Cholesterol Content of Selected Foods Source: USDA National Nutrition Database for Standard Reference, Release 18, 2005. 29

How Are Triglycerides Digested, Absorbed, and Circulated? Basic goal of digestion Mouth Lingual lipase Stomach Gastrin Gastric lipase

How Are Triglycerides Digested, Absorbed, and Circulated? Small intestine Phase 1: Micelle formation Bile disperses large globules into smaller droplets Emulsification Gallbladder disease Removal of gallbladder

FIGURE 6. 14. Emulsification of Lipids to Form FIGURE 6.14 Emulsification of Lipids to Form Micelles in the Small Intestine The liver produces bile, which contains bile acids, cholesterol, and phospholipids. The gallbladder stores the bile and releases it into the small intestine. Emulsification occurs in the 1. Large lipid globules made of fatty acids, monoglycerides, diglycerides, triglycerides, and cholesterol enter the small intestine from the stomach. 2. The presence of lipids in the small intestine stimulates the release of cholecystokinin (CCK). CCK signals the gallbladder to release bile, which consists of bile acids, cholesterol, 3. Bile acids and phospholipids emulsify the fat globules by breaking them into smaller particles, called micelles, and stabilizing them in the small intestine. Intestinal emulsification of lipids requires bile acid- and phospholipid-containing bile, which is produced in the liver and stored in the gallbladder. 32

How Are Triglycerides Digested, Absorbed, and Circulated? Small intestine Phase 2: Pancreatic lipase Secretin Release of pancreatic lipase Final products of lipid digestion Fatty acids Glycerol Monoglycerides

How Are Triglycerides Digested, Absorbed, and Circulated? Absorption Two ways Unassisted transport into intestinal cells Short- and medium-chain fatty acids Repackaging into micelles in lumen Contents released into intestinal cell’s interior

How Are Triglycerides Digested, Absorbed, and Circulated? Circulation Depends on hydrophilic nature of lipid More hydrophilic Circulation in blood attached to albumin Circulates to liver Less hydrophilic Circulated in lymph Lipoproteins Liporotein lipase

FIGURE 6.15 Absorption and Circulation of Lipids 1. Short- and medium-chain fatty acids are absorbed easily and circulated, bound to albumin, in the blood to the liver. 2. Long-chain fatty acids, phospholipids, cholesterol, and monogylcerides are packaged into micelles and then taken up by the intestinal cell. 3. Triglycerides and phospholipids are reformed. 4. Triglycerides, cholesterol and phospholipids are then packaged into chylomicra and circulated in the lymph. The process by which a lipid is absorbed and circulated depends on how hydrophilic it is. In general, it is easier for the body to absorb and circulate hydrophilic lipids (like short-chain fatty acids) than hydrophobic ones (like long-chain fatty acids). 36

What Are the Types and Functions of Various Lipoproteins? Produced in the liver Transport lipids in the blood Complex globular structures Apoprotein Chylomicron Largest and least dense Produced in small intestine Transport dietary lipids exclusively

FIGURE 6.17 Origins and Major Functions of Lipoproteins Lipoproteins -- The liver makes very-low-density lipoproteins (VLDL). Major functions -- VLDLs transport endogenous and exogenous fatty acids from the liver to the body with the help of lipoprotein lipase. Lipoproteins -- Intermediate-density lipoproteins (IDL). Major functions -- IDLs continue to deliver dietary and other fatty acids to the body. Some are also taken up by the liver. Lipoproteins -- Low-density lipoproteins (LDL). Major functions -- LDLs deliver cholesterol to the body (especially liver, adipose, and muscle cells) by docking with LDL receptors located on the cell membranes of receptive cells. Lipoproteins -- The liver and small intestine make high-density lipoproteins (HDL). Major functions -- HDLs pick up (scavenge) excess cholesterol from nonhepatic (nonliver) cells and deliver it to the liver. This is called “reverse cholesterol transport.” Lipoproteins -- The small intestine makes chylomicra. Major functions -- Chylomicra deliver exogenous dietary fatty acids to the body with the help of llipoprotein lipase. Both the liver and small intestine make lipoproteins that circulate lipids in the body. 38

What Are the Types and Functions of Various Lipoproteins? Very low-density lipoprotein (VLDL) Lower lipid-to-protein ratio than chylomicra Primary function Low-density lipoproteins (LDL) LDL receptors “Bad” cholesterol Plaque Intermediate-density lipoprotein (IDL)

What Are the Types and Functions of Various Lipoproteins? High-density lipoproteins (HDL) Lowest lipid-to-protein ratio Collect excess cholesterol Transport it back to the liver “Good” cholesterol Different types Not equally effective in removing cholesterol

How Are Dietary Lipids Related to Health? High-fat foods and obesity Major public health concern Associated health risks Although consumption of a high-fat diet can increase risk for obesity, there are many other factors, such as genetics and inadequate exercise, that contribute to this health problem.

How Are Dietary Lipids Related to Health? Cardiovascular disease Common types Heart disease Stroke Atherosclerosis Blood clot Aneurysm Lipids of concern Genetics

FIGURE 6.18 Causes of Cardiovascular Disease Healthy blood vessel: Healthy arteries allow adequate blood flow. Atherosclerotic plaque, Blood clot, or Ruptured Aneurysm: Anything that reduces or blocks blood flow can cause cardiovascular disease. Atherosclerosis, blood clots, and aneurysms can all reduce or stop blood flow, leading to cardiovascular disease. 43

How Are Dietary Lipids Related to Health? Cardiovascular disease Nutritional guidelines Moderate overall energy intake Balance your macronutrients Whole grains Dietary fiber Cancer Recommendations

What Are Some Overall Dietary Recommendations for Lipids? Essential fatty acids Consume adequate amounts DRIs AIs Omega-3 fatty acid intake Limit cholesterol, SFAs, and trans fat Dietary guidelines Total lipid consumption AMDRs

FIGURE 6.19 Reading Nutrition Facts Panels You can tell how much total fat, saturated fat, and trans fat is in a food by checking its Nutrition Facts panel. 46