Lipids: Triglycerides, Phospholipids, and Sterols - Lecture Notes

Recap and Introduction to Lipids

Recap from Last Week

  1. GI and Diabetes:

    • Using the Glycemic Index (GI) can help people with diabetes as it indicates the impact of food on blood glucose levels (BGL).

  2. Hormones in Glucose Homeostasis:

    • Insulin: Produced in the pancreas, it facilitates the uptake of glucose from the blood into cells, thus lowering BGL.

    • Glucagon: Also produced in the pancreas, it signals the release of glycogen when BGL is low, thus increasing BGL.

Learning Outcomes

  1. Understand the structure of fatty acids.

  2. Name and list food sources and roles of different fatty acids.

  3. Discuss the effects of dietary fat on health.

Lipids: Triglycerides, Phospholipids, and Sterols

What are Lipids?

Lipids include:

  • Triglycerides (fats and oils)

  • Phospholipids

  • Sterols

Role of Triglycerides

  1. Energy Storage: Primary function.

  2. Structural Components: Part of cell membranes.

  3. Hormone Secretion: Adipokines are secreted.

  4. Insulation: Provides thermal insulation.

  5. Shock Absorbers: Protects organs.

Basic Structure of a Triglyceride

A triglyceride (or triacylglycerol) consists of:

  • Glycerol + 3 fatty acids

    • Fatty acids vary in carbon chain lengths, degree of unsaturation, and number of double bonds.

    • Saturation affects the physical characteristics and storage properties of the fat.

C - FA1 C - FA2
C - FA_3

Types of Fatty Acids

Chain Length

  • Short: 1-5 carbons

  • Medium: 6-12 carbons

  • Long: 13-21 carbons

Type of Bonds

  • Saturated

  • Monounsaturated

  • Polyunsaturated (omega-6, omega-3)

Essential Fatty Acids (EFAs)

  • We cannot make these, so they must be supplied by the diet.

  • Polyunsaturated Fats:

    • Linoleic Acid (omega-6)

    • Alpha-linolenic Acid (omega-3)

Chemist’s View of Fatty Acids

  • Unsaturated Fatty Acids:

    • Lack hydrogen atoms and have at least one double bond.

    • The double bond is the point of unsaturation.

  • Monounsaturated Fatty Acids (MUFA):

    • Lack two hydrogen atoms and have one double bond.

  • Polyunsaturated Fatty Acids (PUFA):

    • Lack four or more hydrogen atoms and have at least two or more double bonds.

  • Omega Number:

    • Refers to the position of the first double bond, counting from the methyl end.

Fatty Acid Saturation

  • Saturated Fats:

    • Often found in animal-based food products.

    • Excess consumption can increase the risk of heart disease and weight gain.

  • Unsaturated Fats (Mono and Polyunsaturated):

    • Play an important role in a healthy diet and can help reduce the risk of heart disease.

Naming Fatty Acids

  • Named from the methyl end (omega system).

  • List the number of carbon atoms, then the number of double bonds, and the position of the double bond.

  • Example: 18:1 n-9 (oleic acid).

Examples of Fatty Acids

  • Stearic acid: Saturated fatty acid (C:18)

  • Linoleic acid: Polyunsaturated fatty acid (C:18:2 n-6)

Essential Fatty Acids - Omega-6 and Omega-3 Series

  • Linoleic Acid (Omega-6):

    • Can be supplied by vegetable oils and meats.

  • Linolenic Acid (Omega-3):

    • Must be supplied by food; important for the eyes, brain, and heart.

    • Found in oils, nuts, seeds, fish, and seafood.

  • n-6 Series:

    • Linoleic (LA) (18:2 n-6)

    • Gamma-Linolenic (GLA) (18:3 n-6)

    • Arachidonic (AA) (18:4 n-6)

  • n-3 Series:

    • Alpha-Linolenic (ALA) (18:2 n-3)

    • Eicosapentaenoic (EPA) (20:5 n-3)

    • Docosapentaenoic acid (DPA) (22:5 n-3)

    • Docosahexaenoic (DHA) (22:6 n-3)

  • Australian Diet Sources:

    • Vegetable oils & meats (Omega-6)

    • Oils, nuts, and seeds (ALA)

    • Fish and seafood (EPA and DHA)

Essential Fatty Acids - Eicosanoids and Deficiencies

  • Eicosanoids:

    • Made from arachidonic acid and EPA; hormone-like, including prostaglandins, thromboxanes, and leukotrienes.

    • Require adequate long-chain fatty acids for synthesis.

  • Fatty Acid Deficiencies:

    • Rare, but can occur in infants and children with fat-free or low-fat diets.

    • Symptoms include growth retardation, reproductive failure, skin lesions, kidney and liver disorders.

Physical Properties of Saturated Fats

  • Carry the maximum possible number of hydrogen atoms.

  • Solid at room temperature.

  • Resistant to oxidation.

Physical Properties of Unsaturated Fats

  • Polyunsaturated fats are liquid at room temperature.

  • Shorter fatty acid chains are softer at room temperature than longer chains.

  • MUFA are slightly less susceptible to spoilage; PUFA spoil most readily.

  • Hydrogenation: Protects against oxidation, prolonging shelf life and altering texture.

    • Creates Trans fats.

Trans Fatty Acids

  • Changed from cis to trans configuration.

Phospholipids

  • Food sources of lecithin include eggs, liver, soybeans, wheat germ, and peanuts.

  • Enable transport of lipids across cell membranes.

  • Structure generally consists of hydrophobic tails and a hydrophilic head.

  • Can be used as emulsifiers in the food industry (e.g., lecithin).

Sterols

  • Sterols have a multiple ring structure; cholesterol is a well-known sterol.

  • Found in plant and animal foods.

  • Cholesterol is found in animal foods only (meat, eggs, fish, poultry, and dairy products).

Roles of Sterols

  • Starting material for bile acids, sex hormones, adrenal hormones, and vitamin D.

  • Structural component of cell membranes.

  • Liver produces 800 to 1500 mg cholesterol per day.

  • Atherosclerosis occurs when cholesterol forms deposits in the artery wall, leading to heart attacks.

Digestion of Lipids

  • Mouth: Initial breakdown.

  • Stomach: Further breakdown.

  • Small Intestine: Main site of digestion and absorption.

  • Large Intestine: Minimal role.

Absorption of Lipids

  • Glycerol and short/medium-chain fatty acids diffuse into intestinal cells and then into the bloodstream.

  • Larger molecules merge with bile to create micelles, which are soluble in watery digestive fluid and transported into intestinal cells.

  • These are packed with protein to become chylomicrons, which enter the lymph and then the blood via the heart.

Transport of Lipids

  • Lipid transport is facilitated by lipoproteins.

  • Chylomicrons: Largest of the lipoproteins.

  • In the liver:

    • VLDL (Very-Low-Density Lipoproteins): Composed primarily of triglycerides; deliver triglycerides to cells.

    • LDL (Low-Density Lipoproteins): Composed primarily of cholesterol; deliver cholesterol to cells.

    • HDL (High-Density Lipoproteins): Composed primarily of protein; collect cholesterol from blood and bring it back to the liver.

Lipid Metabolism

  • Conversion of food fat to body fat.

  • Fat provides 60% of energy requirements for the body at rest.

  • 1 kg fat provides 30,000 kJ.

  • Require carbohydrate or protein for complete breakdown of fat.

  • If CHO is low, ketone bodies can be made from fats as an alternate fuel source, but they can accumulate in the blood and disrupt the body’s acid-base balance.

Short Chain Fatty Acids (SCFAs)

  • Fermentation of fiber produces:

    1. SCFAs

    2. Gases

    3. Energy

  • SCFAs stimulate colonic blood flow and fluid and electrolyte uptake, as well as the growth of bacteria in the colon.

    • Acetate: Taken up by the liver, may lower cholesterol.

    • Propionate: Enters circulation, metabolized by peripheral tissues, may regulate appetite.

    • Butyrate: Energy for cells lining the colon; may have anti-cancer/immune function.

Fats and Health

  • Getting enough fat in our diets is rarely an issue, but not all fats are created equal.

  • Certain fats can contribute to the risk of chronic diseases, while many fats are essential for good health.

  • Fats vary in their chemical shape and form, particularly the saturation of chemical bonds with hydrogen atoms.

  • Saturation affects the physical characteristics of fat and its storage properties.

  • Fats can be saturated, monounsaturated, and polyunsaturated.

  • Trans fats are a form of unsaturated fat that act like saturated fats; some occur naturally, while others are commercially created.

Blood Lipid Profiles

  • Blood lipid profiles reveal concentrations of lipids in the blood and should meet desirable levels:

    • Total cholesterol: < 4 mmol/L

    • LDL cholesterol: < 2.5 mmol/L

    • HDL cholesterol: > 1 mmol/L

    • Triglycerides: < 2.0 mmol/L

Cholesterol and Elevated LDL/VLDL

  • Elevated LDL/VLDL is a CVD risk factor as cholesterol accumulates in blood vessels and increases BP.

  • Cholesterol intake has much less effect on blood cholesterol than saturated fat and trans fat.

  • Genetics play a role.

  • People with high blood cholesterol may benefit from limiting cholesterol to <200 mg/day.

Trans Fats and Inflammation

  • Trans-fatty acids increase LDL cholesterol and decrease HDL cholesterol.

  • Increase inflammation and insulin resistance.

  • Food sources include industrial or ruminant (naturally occurring) sources, with possible differences in health effects.

    • Examples include deep-fried foods using vegetable shortening, cakes, biscuits, doughnuts, pastry, crackers, snack chips, margarine, meat, and dairy products.

Omega-3 and 6s

  • EPA/DHA eicosanoids help decrease blood pressure, clotting, and inflammation.

  • Some n6 promote inflammation, but others reduce it.

  • Potentially useful in arthritis, allergy, and nerve pain.

  • Focus on increasing n3s rather than aiming for an ideal ratio.

  • Omega-3 best sources:

    • EPA/DHA: Atlantic salmon, mackerel, sardines, tuna, human breast milk

    • ALA: Flaxseed, canola, walnut, wheat germ, soybean oil, nuts & seeds (almond, flaxseed, walnuts) and soybean kernel and soy beans.

Plant Sterols

  • Plant 'cholesterol' that can interfere with cholesterol absorption.

  • Examples include Logicol and Flora Proactiv.

  • Needs to be taken in sufficient quantities.

Fat Intake Recommendations from Australian Dietary Guidelines

  • Limit intake of saturated fats.

  • Saturated and trans fat together should be less than 8-10% of daily energy intake.

  • Moderate fat intake to a maximum of 35% of daily total energy intake.

  • On an average daily intake of 8000kJ, this equals 640-800kJ (17.5 to 21g) from saturated and trans fat combined.

  • Choose lean and very lean options of meat and meat alternatives.

  • Choose unsaturated fat instead of saturated fat (e.g., olive oil vs. butter).

  • Examples:

    • 10 g monounsaturated spread

    • 10 g polyunsaturated spread

    • 7 g mono/polyunsaturated oil

Sources of Long-Chain Omega-3 Fatty Acids

  • Order of highest to lowest source:

    1. 728 mg/100g

    2. 129 mg/100g

    3. 47 mg/100g

Review Questions

  1. The most usual form in which fats are transported in the blood is as lipoproteins.

  2. In which form are most dietary lipids found? Triglycerides.

  3. What percentage of stored body fat is in the form of triglycerides? 99%.

  4. How many double bonds are present in stearic acid? 0.

  5. What type of fatty acid is found in high amounts in olive oil? Monounsaturated.