Chapter 5: The Lipids: Triglycerides, Phospholipids, and Sterols

The Lipids: Triglycerides, Phospholipids, and Sterols

Notes from NFS 141000 HUNTER COLLEGE, taught by CLARE PARME MILLER MS RD CDN.

The Lipid Family

• What are lipids?

  • A family of compounds primarily composed of Carbon (C), Hydrogen (H), and Oxygen (O).

  • They include triglycerides, phospholipids, and sterols.

Triglycerides and Fatty Acids

• Triglycerides:

  • The most abundant type of fat found in both food and the human body.

  • Composed of one glycerol molecule and three fatty acids, resembling an "E" shape.

• Fatty Acids:

  • Typically consist of a chain of 4 to 24 carbons, with 18 carbons being the most common length.

  • Always have an even number of carbon atoms.

  • Can be classified based on their saturation:

    • Saturated: No double bonds between carbon atoms.

    • Unsaturated: Contain one or more double bonds.

      • Monounsaturated: One double bond.

      • Polyunsaturated: Two or more double bonds.

  • Essential Fatty Acids:

    • The body cannot synthesize these and they must be obtained through the diet.

    • Omega-3 fatty acids (Linolenic acid): An 18-carbon polyunsaturated fatty acid.

    • Omega-6 fatty acids (Linoleic acid): An 18-carbon polyunsaturated fatty acid.

    • The body can use these essential fatty acids to create longer fatty acids necessary for regulating blood pressure and blood clotting.

18-Carbon Fatty Acids

Chemists use a shorthand notation where the first number indicates carbon atoms and the second indicates double bonds (e.g., 18:0).

• Stearic acid (18:0):

  • Number of Carbon Atoms: 18

  • Number of Double Bonds: 0

  • Saturation: Saturated

  • Common Food Sources: Most animal fats.

• Oleic acid (18:1):

  • Number of Carbon Atoms: 18

  • Number of Double Bonds: 1

  • Saturation: Monounsaturated

  • Common Food Sources: Olive and canola oils.

• Linoleic acid (18:2):

  • Number of Carbon Atoms: 18

  • Number of Double Bonds: 2

  • Saturation: Polyunsaturated

  • Common Food Sources: Sunflower, safflower, corn, and soybean oils.

• Linolenic acid (18:3):

  • Number of Carbon Atoms: 18

  • Number of Double Bonds: 3

  • Saturation: Polyunsaturated

  • Common Food Sources: Soybean and canola oils, flaxseed, walnuts.

Bonds of Fatty Acid Chains

• Double bonds: The location of a double bond is described relative to the methyl ( ext{CH}_3) end of the carbon chain.

  • Omega number: Represents the position of the closest double bond from the methyl end.

  • Omega-3 fatty acids: Have their closest double bond located three carbons away from the methyl end.

  • Monounsaturated fatty acids: Are typically part of the Omega-9 group, meaning their closest double bond is nine carbons away from the methyl end.

Condensation of Glycerol and Fatty Acids to Form a Triglyceride

• This is a chemical process where glycerol reacts with three fatty acids.

• An H atom from glycerol combines with an OH group from each fatty acid, forming a water molecule ( ext{H}_2 ext{O}).

• This results in the formation of a triglyceride and three molecules of water.

• The bond forms between the O on the glycerol and the C at the acid end of each fatty acid.

• E.g., a triglyceride can be formed from a saturated, a monounsaturated, and a polyunsaturated fatty acid.

Saturated and Unsaturated Fatty Acids Compared

• Saturated fatty acids:

  • Have no double bonds in their carbon chains.

  • Their linear structure allows them to stack neatly together.

  • Consequently, saturated fats tend to be solid (or more firm) at room temperature.

• Unsaturated fatty acids:

  • Contain one or more double bonds.

  • The presence of double bonds causes bends or kinks in the carbon chain, preventing them from stacking neatly.

  • Consequently, unsaturated fats tend to be liquid (or less firm) at room temperature.

Characteristics of Solid Fats and Oils

• Firmness:

  • Most polyunsaturated fats are liquid at room temperature.

  • Saturated animal fats are typically solid at room temperature.

  • The degree of saturation is the primary determinant of firmness.

  • Shorter carbon chains generally result in a softer fat at room temperature.

• Stability:

  • Fats can spoil through oxidation, leading to a rancid smell and taste.

  • The degree of saturation influences stability; unsaturated fats are more prone to oxidation due to their double bonds.

  • Antioxidants can be added to fats and oils to compete for oxygen, thereby protecting the oils from spoilage.

Fatty Acid Composition of Common Food Fats

• Saturated fatty acids rich sources: Animal fats (e.g., butter, beef tallow, lard, chicken fat) and tropical oils (coconut oil, palm oil).

• Monounsaturated fatty acids rich sources: Olive oil, canola oil, peanut oil.

• Omega-6 polyunsaturated fatty acids rich sources: Safflower oil (over 70% linoleic acid), sunflower oil, corn oil, soybean oil, walnut oil, cottonseed oil.

• Omega-3 polyunsaturated fatty acids rich sources: Flaxseed oil, fish oil (salmon), soybean and canola oils, flaxseed, walnuts.

Trans Fatty Acids

• Hydrogenation: A chemical process involving the addition of hydrogen atoms to unsaturated fatty acids.

  • This process removes some or all double bonds, making fats more solid at room temperature.

  • It also increases their resistance to oxidation, extending shelf life.

• Cis Fatty Acids:

  • Hydrogens around the double bond are on the same side of the carbon chain.

  • This configuration causes the molecule to bend into a U-like formation.

  • Most naturally occurring unsaturated fatty acids in foods are in the cis form.

• Trans Fatty Acids:

  • Hydrogens around the double bond are on opposite sides of the carbon chain.

  • This configuration results in a more linear molecule.

  • The trans form typically occurs during the partial hydrogenation of unsaturated fats, altering their configuration from cis to trans.

Phospholipids

• Properties: Possess unique solubility characteristics, being soluble in both fat and water due to their hydrophilic head and hydrophobic tails.

• Emulsifiers: Widely used as emulsifiers in the food industry to mix oil and water, as seen in products like mayonnaise and salad dressings.

• Lecithin: The best-known phospholipid.

• Structure: Consist of a glycerol backbone, two fatty acids, a choline group, and a phosphate group.

• Food Sources: Naturally found in eggs, liver, soybeans, wheat germ, and peanuts.

• Roles in the Body:

  • A critical component of all cell membranes, forming the lipid bilayer.

  • Act as emulsifiers in the body, helping to keep fats suspended in blood and other body fluids, facilitating their transport and digestion.

Sterols

• Food Sources:

  • Animal sources: Exclusively cholesterol (e.g., meat, poultry, shellfish, eggs, milk, and milk products).

  • Plant sources: Include plant sterols, which are structurally similar to cholesterol but can interfere with cholesterol absorption in the human gut, potentially offering health benefits.

• Roles of Sterols in the Body:

  • The liver is the primary site of cholesterol synthesis in the body (endogenous cholesterol).

  • Cholesterol serves as a precursor for vital compounds:

    • Bile acids, essential for fat digestion and absorption.

    • Sex hormones (e.g., testosterone, estrogen).

    • Adrenal hormones (e.g., cortisol).

    • Vitamin D.

  • Sterols are also important structural components of cell membranes, providing rigidity and stability.

Lipid Digestion

• Goal of Fat Digestion: To dismantle complex triglycerides into simpler absorbable units: monoglycerides (glycerol with one fatty acid attached), free fatty acids, and glycerol.

• Challenge: Fats are inherently hydrophobic (water-fearing), while digestive enzymes are hydrophilic (water-loving). This incompatibility requires special mechanisms for digestion.

Fat Digestion in the GI Tract

• Mouth and Salivary Glands:

  • Some hard fats begin to melt as they are warmed to body temperature.

  • The sublingual salivary gland secretes lingual lipase.

  • The degree of hydrolysis by lingual lipase is slight for most fats, but can be significant for milk fats, especially important for infant digestion.

• Stomach:

  • Strong muscle contractions churn the fat, mixing it with water and acid.

  • A gastric lipase is secreted, which contributes to a very small amount of fat hydrolysis.

  • Overall, little fat digestion occurs in the stomach.

• Small Intestine: The primary site of fat digestion.

  • Cholecystokinin (CCK): A hormone that signals the gallbladder to release bile via the common bile duct.

  • Bile: Acts as an emulsifier, breaking down large fat globules into smaller ones, thereby increasing the surface area for enzyme action.

  • Pancreatic lipase: Flows from the pancreas via the pancreatic duct.

  • Hydrolysis: Pancreatic lipase (and intestinal lipase) break down emulsified fats (triglycerides and phospholipids) into monoglycerides, glycerol, and fatty acids.

  • Sterols: Are absorbed in their original, intact form, without undergoing hydrolysis.

• Bile Routes and Blood Cholesterol:

  • Bile, after emulsifying fats, can be reabsorbed and recycled (enterohepatic circulation).

  • Alternatively, bile can be trapped by dietary fiber in the large intestine and excreted in feces. Increasing fiber intake can thus lower whole-body cholesterol levels by promoting greater bile excretion.

Lipid Absorption

• Micelles: In the small intestine, hydrolyzed fats (monoglycerides, fatty acids) combine with bile salts to form tiny spherical complexes called micelles.

  • Micelles are sufficiently water-soluble to penetrate the watery solution bathing the absorptive cells of the intestinal lining.

  • The lipid contents of the micelles then diffuse into these intestinal cells.

• Small Fats (Short- and Medium-Chain Fatty Acids) and Glycerol:

  • These smaller lipid molecules are water-soluble enough to move directly into the bloodstream and travel to the liver.

• Larger Lipids (Monoglycerides and Long-Chain Fatty Acids):

  • Once inside the intestinal cells, these are reassembled back into triglycerides.

  • Triglycerides, along with cholesterol and phospholipids, are then repackaged with proteins to form chylomicrons (a type of lipoprotein).

• Chylomicrons:

  • These large lipoproteins are too big to enter the capillaries directly.

  • They typically bypass the liver initially and travel through the lymphatic system.

  • Chylomicrons enter the bloodstream at the thoracic duct, which empties into the subclavian vein near the heart.

Lipoproteins

Lipoproteins are specialized structures designed to transport lipids through the watery environment of the blood.

• A typical lipoprotein has an interior containing hydrophobic triglycerides and cholesterol, surrounded by a shell of phospholipids and proteins.

• The fatty acid tails of the phospholipids point inwards towards the lipids, while the hydrophilic heads and proteins face outwards, allowing the structure to be stable in blood.

5.1 Chemistry of Fatty Acids and Triglycerides; Saturated and Unsaturated Fats

• What are lipids?- A family of compounds primarily composed of Carbon (C), Hydrogen (H), and Oxygen (O).

  • They include triglycerides, phospholipids, and sterols.

• Triglycerides:- The most abundant type of fat found in both food and the human body.

  • Composed of one glycerol molecule and three fatty acids, resembling an "E" shape.

• Fatty Acids:- Typically consist of a chain of 4 to 24 carbons, with 18 carbons being the most common length.

  • Always have an even number of carbon atoms.

  • Can be classified based on their saturation:

    • Saturated: No double bonds between carbon atoms.

    • Unsaturated: Contain one or more double bonds.

      • Monounsaturated: One double bond.

      • Polyunsaturated: Two or more double bonds.

18-Carbon Fatty Acids (Chemistry/Saturation Examples)

Chemists use a shorthand notation where the first number indicates carbon atoms and the second indicates double bonds (e.g., 18:0).

• Stearic acid (18:0):

  • Number of Carbon Atoms: 18

  • Number of Double Bonds: 0

  • Saturation: Saturated

  • Common Food Sources: Most animal fats.

• Oleic acid (18:1):

  • Number of Carbon Atoms: 18

  • Number of Double Bonds: 1

  • Saturation: Monounsaturated

  • Common Food Sources: Olive and canola oils.

• Linoleic acid (18:2):

  • Number of Carbon Atoms: 18

  • Number of Double Bonds: 2

  • Saturation: Polyunsaturated

  • Common Food Sources: Sunflower, safflower, corn, and soybean oils.

• Linolenic acid (18:3):

  • Number of Carbon Atoms: 18

  • Number of Double Bonds: 3

  • Saturation: Polyunsaturated

  • Common Food Sources: Soybean and canola oils, flaxseed, walnuts.

Bonds of Fatty Acid Chains (Chemistry)

• Double bonds: The location of a double bond is described relative to the methyl ( ext{CH}_3) end of the carbon chain.

  • Omega number: Represents the position of the closest double bond from the methyl end.

  • Omega-3 fatty acids: Have their closest double bond located three carbons away from the methyl end.

  • Monounsaturated fatty acids: Are typically part of the Omega-9 group, meaning their closest double bond is nine carbons away from the methyl end.

Condensation of Glycerol and Fatty Acids to Form a Triglyceride

• This is a chemical process where glycerol reacts with three fatty acids.

• An H atom from glycerol combines with an OH group from each fatty acid, forming a water molecule ( ext{H}_2 ext{O}).

• This results in the formation of a triglyceride and three molecules of water.

• The bond forms between the O on the glycerol and the C at the acid end of each fatty acid.

• E.g., a triglyceride can be formed from a saturated, a monounsaturated, and a polyunsaturated fatty acid.

Saturated and Unsaturated Fatty Acids Compared

• Saturated fatty acids:

  • Have no double bonds in their carbon chains.

  • Their linear structure allows them to stack neatly together.

  • Consequently, saturated fats tend to be solid (or more firm) at room temperature.

• Unsaturated fatty acids:

  • Contain one or more double bonds.

  • The presence of double bonds causes bends or kinks in the carbon chain, preventing them from stacking neatly.

  • Consequently, unsaturated fats tend to be liquid (or less firm) at room temperature.

5.2 Chemistry, Food Sources, and Roles of Phospholipids and Sterols

Phospholipids

• Properties: Possess unique solubility characteristics, being soluble in both fat and water (hydrophilic head and hydrophobic tails).

• Emulsifiers: Widely used as emulsifiers in the food industry (e.g., mayonnaise) and in the body.

• Lecithin: The best-known phospholipid.

• Structure: Consist of a glycerol backbone, two fatty acids, a choline group, and a phosphate group.

• Food Sources: Naturally found in eggs, liver, soybeans, wheat germ, and peanuts.

• Roles in the Body:

  • A critical component of all cell membranes, forming the lipid bilayer.

  • Act as emulsifiers in the body, helping to keep fats suspended in blood and other body fluids, facilitating their transport and digestion.

Sterols

• Food Sources:

  • Animal sources: Exclusively cholesterol (e.g., meat, poultry, shellfish, eggs, milk, and milk products).

  • Plant sources: Include plant sterols, which are structurally similar to cholesterol but can interfere with cholesterol absorption in the human gut.

• Roles of Sterols in the Body:

  • The liver is the primary site of cholesterol synthesis in the body (endogenous cholesterol).

  • Cholesterol serves as a precursor for vital compounds:

    • Bile acids, essential for fat digestion and absorption.

    • Sex hormones (e.g., testosterone, estrogen).

    • Adrenal hormones (e.g., cortisol).

    • Vitamin D.

  • Sterols are also important structural components of cell membranes, providing rigidity and stability.

5.3 Summarize Fat Digestion, Absorption, and Transport

Lipid Digestion

• Goal of Fat Digestion: To dismantle complex triglycerides into simpler absorbable units: monoglycerides (glycerol with one fatty acid attached), free fatty acids, and glycerol.

• Challenge: Fats are inherently hydrophobic (water-fearing), while digestive enzymes are hydrophilic (water-loving). This incompatibility requires special mechanisms for digestion.

Fat Digestion in the GI Tract

• Mouth and Salivary Glands:

  • Some hard fats begin to melt as they are warmed to body temperature.

  • The sublingual salivary gland secretes lingual lipase.

  • The degree of hydrolysis by lingual lipase is slight for most fats, but can be significant for milk fats, especially important for infant digestion.

• Stomach:

  • Strong muscle contractions churn the fat, mixing it with water and acid.

  • A gastric lipase is secreted, which contributes to a very small amount of fat hydrolysis.

  • Overall, little fat digestion occurs in the stomach.

• Small Intestine: The primary site of fat digestion.

  • Cholecystokinin (CCK): A hormone that signals the gallbladder to release bile via the common bile duct.

  • Bile: Acts as an emulsifier, breaking down large fat globules into smaller ones, thereby increasing the surface area for enzyme action.

  • Pancreatic lipase: Flows from the pancreas via the pancreatic duct.

  • Hydrolysis: Pancreatic lipase (and intestinal lipase) break down emulsified fats (triglycerides and phospholipids) into monoglycerides, glycerol, and fatty acids.

  • Sterols: Are absorbed in their original, intact form, without undergoing hydrolysis.

• Bile Routes and Blood Cholesterol:

  • Bile, after emulsifying fats, can be reabsorbed and recycled (enterohepatic circulation).

  • Alternatively, bile can be trapped by dietary fiber in the large intestine and excreted in feces. Increasing fiber intake can thus lower whole-body cholesterol levels by promoting greater bile excretion.

Lipid Absorption

• Micelles: In the small intestine, hydrolyzed fats (monoglycerides, fatty acids) combine with bile salts to form tiny spherical complexes called micelles.

  • Micelles are sufficiently water-soluble to penetrate the watery solution bathing the absorptive cells of the intestinal lining.

  • The lipid contents of the micelles then diffuse into these intestinal cells.

• Small Fats (Short- and Medium-Chain Fatty Acids) and Glycerol:

  • These smaller lipid molecules are water-soluble enough to move directly into the bloodstream and travel to the liver.

• Larger Lipids (Monoglycerides and Long-Chain Fatty Acids):

  • Once inside the intestinal cells, these are reassembled back into triglycerides.

  • Triglycerides, along with cholesterol and phospholipids, are then repackaged with proteins to form chylomicrons (a type of lipoprotein).

Lipoproteins (Lipid Transport)

Lipoproteins are specialized structures designed to transport lipids through the watery environment of the blood.

• A typical lipoprotein has an interior containing hydrophobic triglycerides and cholesterol, surrounded by a shell of phospholipids and proteins.

• The fatty acid tails of the phospholipids point inwards towards the lipids, while the hydrophilic heads and proteins face outwards, allowing the structure to be stable in blood.

5.4 Major Roles of Fats in the Body; Essential Fatty Acids and Omega Fatty Acids

Major Roles of Fats in the Body

• Energy Storage: Fats are the body's chief form of stored energy, providing 9 calories per gram, which is more than double that of carbohydrates or proteins.

• Insulation and Protection: Stored fat beneath the skin helps insulate the body from temperature extremes, and fatty pads surrounding vital organs protect them from shock.

• Absorption of Fat-Soluble Vitamins: Dietary fat is necessary for the absorption of fat-soluble vitamins (A, D, E, K).

• Structural Components: As seen with phospholipids and sterols, lipids are integral components of cell membranes.

• Cell Signaling: Lipids play a role in various cellular processes, including cell signaling and gene expression.

Essential Fatty Acids

• The body cannot synthesize these and they must be obtained through the diet.

• Omega-3 fatty acids (Linolenic acid): An 18-carbon polyunsaturated fatty acid.

• Omega-6 fatty acids (Linoleic acid): An 18-carbon polyunsaturated fatty acid.

• The body can use these essential fatty acids to create longer fatty acids necessary for regulating blood pressure, blood clotting, and inflammation.

Omega Fatty Acids

• Omega number: Represents the position of the closest double bond from the methyl ( ext{CH}_3) end of the carbon chain.

• Omega-3 fatty acids: Have their closest double bond located three carbons away from the methyl end. Important for heart health, brain function, and normal growth and development.

  • Food Sources: Soybean and canola oils, flaxseed, walnuts, fish oil (salmon).

• Omega-6 fatty acids: Have their closest double bond located six carbons away from the methyl end. Involved in inflammation and blood clotting.

  • Food Sources: Sunflower, safflower, corn, and soybean oils.

5.5 Relationships Among Saturated Fats, Trans Fat, and Cholesterol and Chronic Diseases; Recommendations

Saturated Fatty Acids

• Sources: Animal fats (e.g., butter, beef tallow, lard, chicken fat) and tropical oils (coconut oil, palm oil).

• Chronic Disease Relationship: High intake of saturated fats is strongly associated with an increased risk of cardiovascular diseases, primarily by raising LDL ("bad") cholesterol levels in the blood.

• Recommendations: Limit saturated fat intake to less than 10\% of total daily calories; some guidelines suggest as low as 7\%.

Trans Fatty Acids

• Formation: Occur during hydrogenation, a chemical process that adds hydrogen atoms to unsaturated fatty acids, making them more solid and increasing shelf life typically by altering the configuration from cis to trans.

• Chronic Disease Relationship: Trans fats significantly increase LDL cholesterol and decrease HDL ("good") cholesterol, leading to a higher risk of cardiovascular disease. They are considered one of the most harmful types of fat for heart health.

• Recommendations: Minimize or eliminate intake of trans fats. Food manufacturers are largely required to remove trans fats from products.

Dietary Cholesterol

• Sources: Exclusively from animal products (meat, poultry, shellfish, eggs, dairy).

• Chronic Disease Relationship: For most healthy people, dietary cholesterol has a lesser impact on blood cholesterol levels compared to saturated and trans fats because the body regulates its own cholesterol synthesis. However, individuals sensitive to dietary cholesterol or with specific genetic predispositions may see more significant increases in blood cholesterol.

• Recommendations: While previous guidelines recommended limiting dietary cholesterol to 300 \text{ mg/day}, current dietary guidelines primarily focus on limiting saturated and trans fat intake, as these have a greater impact on blood cholesterol for most individuals.

5.6 Relationships Between Monounsaturated and Polyunsaturated Fats and Health; Recommendations

Monounsaturated Fatty Acids (MUFA)

• Sources: Olive oil, canola oil, peanut oil, avocados, nuts.

• Health Relationship: Replacing saturated fats with MUFAs can help lower LDL cholesterol levels without lowering HDL cholesterol, thereby reducing the risk of heart disease.

• Recommendations: Include sources of MUFAs in the diet as alternatives to saturated fats.

Polyunsaturated Fatty Acids (PUFA)

• Sources:

  • Omega-6 PUFA: Safflower oil, sunflower oil, corn oil, soybean oil, walnut oil, cottonseed oil.

  • Omega-3 PUFA: Flaxseed oil, fish oil (salmon), soybean and canola oils, flaxseed, walnuts.

• Health Relationship: Both omega-3 and omega-6 fatty acids are essential. Replacing saturated and trans fats with PUFAs can significantly lower LDL cholesterol. Omega-3 fatty acids are particularly beneficial for heart health, reducing triglyceride levels, blood pressure, and inflammation, and supporting brain and eye health.

• Recommendations: Ensure adequate intake of both omega-3 and omega-6 fatty acids while maintaining a healthy ratio (often recommended to increase omega-3 intake). Replace solid fats with liquid vegetable oils rich in PUFAs.