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

Learning Objectives for Chapter 5: The Lipids

• 5.1 Chemistry: Identify the chemical structure and characteristics of fatty acids and triglycerides.
• 5.2 Phospholipids and Sterols: Describe the chemistry, primary food sources, and various roles of phospholipids and sterols within the body.
• 5.3 Digestion and Transport: Summarize the physiological processes involved in the digestion, absorption, and transport of lipids.
• 5.4 Body Roles and Essential Fatty Acids: Outline the major functions of lipids in the body, including a detailed discussion of essential fatty acids and omega fatty acids.
• 5.5 Saturated Fats and Cholesterol: Explain the biological relationships between saturated fats and cholesterol and the development of chronic diseases.
• 5.6 Unsaturated Fats and Health: Explain the relationships between monounsaturated and polyunsaturated fats and their impact on health.
• 5.7 Dietary Choices: Identify specific fats that support health versus those that impair it.

The Chemist’s View of Fatty Acids and Triglycerides

• Chemical Composition: Lipids are molecules composed of carbon ($C$), hydrogen ($H$), and oxygen ($O$).
• Energy Density: Lipids supply significantly more energy per gram than carbohydrates.
• Triglyceride Structure:
  • The most common type of lipid.
  • Structured like the letter "E".
  • Consists of one glycerol backbone with three fatty acids attached.
• Essential 18-Carbon Polyunsaturated Fatty Acids:
  • Omega-3 (linolenic acid): The body cannot manufacture this; it must be obtained from the diet.
  • Omega-6 (linoleic acid): Also essential and cannot be made by the body.

Detailed Classification and Structure of Fatty Acids

• Basic Chain Structure: Each fatty acid is a chain of carbon and hydrogen atoms.
  • One end features an acid group ($COOH$).
  • The other end features a methyl group ($CH_3$).
• Chain Length Categories:
  • Long-chain ($> 12$ carbons): Commonly found in meats, seafood, and vegetable oils.
  • Medium-chain ($8–12$ carbons): Found in small amounts in dairy products.
  • Short-chain ($\leq 6$ carbons): Primarily found in small amounts in dairy products.
• Degree of Saturation:
  • Saturated Fatty Acid: Contains only single bonds between carbon atoms; the chain is fully "loaded" with hydrogen atoms.
  • Unsaturated Fatty Acid: Contains at least one double bond where hydrogen atoms are missing.
  • Monounsaturated: Contains exactly one double bond.
  • Polyunsaturated: Contains two or more double bonds.
  • Point of Unsaturation: Defined as the location of the double bond.
• Comparison of 18-Carbon Fatty Acids:
  • Stearic acid: $18$ carbons, $0$ double bonds; Saturated; found in most animal fats.
  • Oleic acid: $18$ carbons, $1$ double bond; Monounsaturated; found in olive and canola oils.
  • Linoleic acid: $18$ carbons, $2$ double bonds; Polyunsaturated; found in sunflower, safflower, corn, and soybean oils.
  • Linolenic acid: $18$ carbons, $3$ double bonds; Polyunsaturated; found in soybean and canola oils, flaxseed, and walnuts.
• Double Bond Locations (Omega Numbers):
  • Omega-3 fatty acid: The first double bond is at the third carbon from the methyl end.
  • Omega-6 fatty acid: The first double bond is at the sixth carbon from the methyl end.
  • Omega-9 group: Characteristic of monounsaturated fatty acids.

Formation and Characteristics of Triglycerides

• Synthesis: Triglycerides are formed through a series of condensation reactions linking three fatty acids to a single glycerol molecule.
• Physical Firmness at Room Temperature:
  • Polyunsaturated fats: Generally liquid (e.g., most vegetable oils).
  • Saturated fats: Generally solid (e.g., most animal fats).
  • Exceptions: Saturated vegetable fats like coconut oil and palm oil are firmer than typical oils.
• Stability and Rancidity:
  • Oxidation of fats leads to rancidity.
  • Manufacturers often add antioxidants to compete for oxygen and protect the oil.
• Hydrogenation: The process of adding hydrogen to unsaturated fats to improve texture (e.g., making margarine spreadable, pie crusts flaky, and puddings creamy).
• Trans-Fatty Acids:
  • Configuration where hydrogens next to double bonds are on opposite sides of the carbon chain.
  • These fats behave biologically more like saturated fats.

The Chemist’s View of Phospholipids and Sterols

• Phospholipids:
  • Lecithin: The best-known example of a phospholipid.
  • Solubility: Phospholipids possess a hydrophobic (fat-soluble) end and a hydrophilic (water-soluble) end.
  • Food Sources: Found in eggs, liver, soybeans, wheat germ, and peanuts.
  • Roles: They act as emulsifiers in the body and facilitate the movement of fat-soluble substances in and out of cells.
• Sterols:
  • Definition: Compounds featuring a multiple-ring structure.
  • Cholesterol: The most famous sterol.
  • Food Sources: Found in animal-derived foods such as meats, eggs, seafood, poultry, and dairy products. Plant foods also contain sterols.
  • Types of Cholesterol:
    • Endogenous: Produced within the body.
    • Exogenous: Obtained from external food sources.
  • Roles: Used to synthesize bile acids, sex hormones (e.g., testosterone, estrogen), adrenal hormones (e.g., cortisol), Vitamin D, and structural cholesterol.
  • Health Hazards: Excess cholesterol contributes to arterial plaque formation, leading to atherosclerosis.

Digestion, Absorption, and Transport of Lipids

• Digestion Goal: To dismantle triglycerides into monoglycerides, fatty acids, and glycerol for absorption.
• Digestive Process by Organ:
  • Mouth: Some hard fats begin to melt as they reach body temperature.
  • Stomach: Churning actions mix fat with water and gastric acid.
  • Small Intestine: Cholecystokinin ($CCK$) signals the gallbladder to release bile. Pancreatic lipase is secreted from the pancreas to break down fats.
  • Large Intestine: Some fat and cholesterol become trapped in fiber and are excreted in the feces.
• The Role of Bile: Bile acts as an emulsifier to prepare fats for digestion by enzymes.
• Lipid Transport (Lipoproteins):
  • Chylomicrons: The largest and least dense lipoproteins; they transport diet-derived lipids from the small intestine to the rest of the body.
  • VLDL (Very-Low-Density Lipoproteins): Packaged in the liver with proteins; they transport lipids made in the liver and from chylomicron remnants to body tissues.
  • LDL (Low-Density Lipoproteins): Often called "bad" cholesterol; they circulate throughout the body, delivering contents to all tissues. High levels increase heart disease risk.
  • HDL (High-Density Lipoproteins): Known as "good" cholesterol; produced by the liver to remove cholesterol from cells and carry it back to the liver for recycling or disposal (reverse transport). High levels reduce heart disease risk.
• Factors Improving Lipid Profile:
  • Lowering $LDL$ or raising $HDL$ is supported by healthy body weight, substituting saturated fats with monounsaturated or polyunsaturated fats, consuming soluble dietary fibers and phytochemicals, moderate (if any) alcohol consumption, and regular physical activity.

Lipids in the Human Body

• Storage and Adipose Tissue: The body has a virtually unlimited capacity to store fat energy in adipose tissue.
• Adipokines: Adipose tissue secretes hormones called adipokines. In obesity, the levels of resistin increase, while adiponectin levels decrease.
• Protection: Fat pads serve as natural shock absorbers to cushion bones and vital organs.
• Essential Fatty Acid Families:
  • Omega-6 Family: Derived from linoleic acid.
  • Omega-3 Family: Derived from linolenic acid.
  • Eicosanoids: Hormone-like substances derived from these fatty acids.
  • Recommended Ratio: The suggested ratio of Omega-6 to Omega-3 is between $4:1$ and $10:1$.
  • Deficiency Symptoms: Include impaired growth, reproductive failure, skin lesions, kidney/liver disorders, and neurological or visual problems.
• Energy Metabolism:
  • Fat at Rest: Fat provides approximately $60\%$ of the body's energy needs during rest. This contribution may increase during prolonged light-to-moderate exercise but decreases during high-intensity exercise as carbohydrates become the primary fuel.
  • Lipoprotein Lipase ($LPL$): Hydrolyzes triglycerides to release fatty acids/monoglycerides into cells for energy.
  • Hormone-sensitive Lipases: Dismantle stored triglycerides to release glycerol and fatty acids directly into the bloodstream.

Health Effects and Recommended Intakes

• Chronic Disease Links:
  • Heart Disease: Elevated $LDL$ is a major risk factor for Cardiovascular Disease ($CVD$). Trans-fats are particularly harmful as they raise $LDL$ and lower $HDL$.
  • Cancer: The link between total fat and cancer is weak/inconclusive, though saturated fats may increase risk, while Omega-3 from fish may reduce risk due to anti-inflammatory effects.
  • Obesity: Fat provides more than twice the kcalories per gram compared to protein or carbohydrates ($9\,kcal/g$ vs $4\,kcal/g$).
• Recommended Dietary Intakes:
  • Total Fat: $20–35\%$ of daily energy. (On a $2,000\,kcal$ diet, this is $400–700\,kcal$ or $45–75\,g$ of fat).
  • Saturated Fat: Less than $10\%$ of daily energy.
  • Cholesterol: Less than $300\,mg$ per day.
  • Trans-fats: As little as possible.
  • Linoleic Acid (AI): $5–10\%$ of daily energy.
  • Linolenic Acid (AI): $0.6–1.2\%$ of daily energy.
• Omega-3 Supplementation: Routine supplementation of fish oil capsules is not generally recommended. Excessive intake can lead to lengthened bleeding time, impaired wound healing, raised $LDL$, and suppressed immune function.

Guidelines to Groceries

• Protein Foods:
  • Very Lean: Chicken breast (no skin), cod, flounder, trout, canned tuna (in water), egg whites, legumes.
  • Lean: Beef/pork "round" or "loin", chicken (drumsticks/legs, no skin), herring, salmon, canned tuna (in oil).
  • Medium-Fat: Ground beef, eggs, tofu.
  • High-Fat: Bacon, hot dogs, lunch meats, sausage, peanut butter, nuts.
• Dairy: Low-fat milk and fortified soy alternatives provide similar or higher nutrients than whole milk with significantly less saturated fat.
• Fruit, Veg, Grains: Most contain little to no fat.
• Fat Replacers:
  • Ingredients that provide the taste and texture of fat with fewer kcalories.
  • Artificial Fats: Provide sensory qualities with zero kcalories (e.g., Olestra).

Questions & Discussion

• What does the chemical structure of a triglyceride resemble, and what does it consist of?
  • It resembles the letter "E"; it consists of one glycerol molecule and three fatty acids.
• Describe the difference between cis- and trans-fatty acids.
  • These terms describe the configuration of hydrogens around the double bond. Trans-fatty acids have hydrogens on opposite sides. Cis-fatty acids have both hydrogens on the same side (Hint: "cis-" means "same").
• How much of the body’s ongoing energy need is supplied by fat during times of rest?
  • Approximately $60\%$.
• Sterols Reflect Activity (Summary):
  • Students evaluated different sterols such as cholesterol, plant-derived sterols, bile, cortisol, and Vitamin D to determine whether they are made by the body and how they affect health (e.g., raising or lowering disease risk).
• Lipoprotein Matching:
  • Chylomicrons: Contain the highest amount of triglycerides and are the least dense.
  • VLDL: Made in the liver and shrink as cells take up triglycerides.
  • LDL: Higher in cholesterol, lower in triglycerides; taken up by cells via receptors.
  • HDL: Carry cholesterol back to the liver for recycling or excretion.