LIPIDS

LIPIDS FUNDAMENTALS

Instructor: Melisa Medina-Rivera, Ph.D.

Course: NS1150 • Fall 2025

Learning Objectives

• Chemistry of Fatty Acids and Triglycerides
  • Recognize the chemistry involved in fatty acids (FA) and triglycerides.
  • Outline how structural differences impact the properties of lipids.
• Phospholipids and Sterols
  • Describe the chemistry, food sources, and biological roles of phospholipids and sterols.
• Fat Digestion and Absorption
  • Summarize the processes of fat digestion, absorption, and transport.
• Types of Lipoproteins
  • Differentiate between various types of lipoproteins: Chylomicrons, Very Low-Density Lipoprotein (VLDL), Low-Density Lipoprotein (LDL), and High-Density Lipoprotein (HDL).
• Roles of Fats in the Body
  • Outline the major roles of fats in human physiology.

Fatty Acids (FA)

• Overview
  • Fatty acids are the most abundant lipids in the human body and various food sources.
  • Comprised of:
  • Carbon atoms
  • Hydrogen atoms
  • Oxygen atoms
• Structure
  • Carbon Backbone:
  • Alpha (⍺) End: Contains a carboxylic group (–COOH)
  • Omega (ω) End: Contains a methyl group (–CH3)

Classification of Fatty Acids

By Chain Length

• Short-chain FA: Approximately 2-5 carbons, predominant in specific food sources.
• Medium-chain FA: Approximately 6-12 carbons.
• Long-chain FA: Greater than 12 carbons.
  • Impact of Chain Length:
  • Influences chemical properties and biological functions (e.g., solubility).
  • Affects digestion and absorption processes.

By Saturation

• Saturated FA
  • Solid at room temperature (RT).
  • Have the maximum number of hydrogen atoms bound to the carbon backbone.
• Unsaturated FA
  • Liquid at room temperature.
  • Classification includes:
  • Monounsaturated FA (MUFAs): One double bond.
  • Polyunsaturated FA (PUFAs): Multiple double bonds.

Unsaturated Fatty Acids Classifications

Hydrogen Atom Arrangement

• Cis FA
  • Hydrogen atoms are on the same side of the double bond.
  • Causes bending in the fatty acid chain; liquid at RT.
• Trans FA
  • Hydrogen atoms are on opposite sides of the double bond.
  • No bending; solid at RT.
  • Linked to increased risk of coronary cardiovascular disease (↑CVD).

Naming Unsaturated Fatty Acids

• Alpha (⍺) Naming System
  • Indicates the position of double bond relative to the α-carbon.
• Omega (ω) Naming System
  • Indicates the position of double bond relative to the ω-carbon.
  • Does not indicate if double bond is cis or trans.
• Examples
  • Linoleic acid:
  • α-nomenclature: cis9,cis12–18:2
  • ω-nomenclature: ω-6

Food Sources of Fatty Acids

• General Sources
  • Nuts, seeds, and oils.
  • Meats and animal-derived products such as butter, cheese, and lard.
• Fat Contents in Various Foods
  • Fats: 5g+/serving (e.g., oils, nuts, seeds, pastries).
  • Milk: 0-8g/serving.
  • Vegetables: ~0g/serving.
  • Animal-based foods: ~1-30g/serving (3 oz).
  • Fruits: Generally ~0g/serving (except for avocado).
  • Starches: Typically less than 1-2 g for unprocessed starches.

Types of Dietary Fats

Solid Fats vs. Oils

• Examples of Solid Fats and Oils:
  • Canola Oil: 64% unsaturated.
  • Avocado: 12% unsaturated.
  • Coconut Oil: Contains 91% saturated fats.
• Fatty Acid Composition:
  • Most fat-containing foods have a mixture of saturated and unsaturated fats.

Complex Lipid Forms

• Triglycerides
  • Also referred to as triacylglycerol (TAG).
  • Primary dietary lipid and significant energy source.
  • Composed of 1 glycerol and 3 fatty acids bound together via ester linkages.
• Phospholipids
  • Found naturally in most foods.
  • Comprise 1 glycerol, 2 fatty acids, and a phosphate-containing polar head group (e.g., choline, ethanolamine).
  • Amphipathic molecules (both hydrophobic and hydrophilic).
• Sterols and Sterol Esters
  • Multi-ring structured lipids.
  • Cholesteryl ester is the most abundant (1 cholesterol + 1 fatty acid bound via an ester linkage).
  • Can be synthesized in the body and are necessary for synthesizing bile acids and steroid hormones.

Fat Digestion and Absorption

• Processes Involved:
  • Begins in the mouth with minor chemical digestion (lingual lipase).
  • Gastric lipase in the stomach helps hydrolyze fat with churning action.
  • In the small intestine, CCK signals gallbladder to release bile acids for fat emulsification.
  • Pancreatic lipase plays a major role in digesting fats.
  • Some fat and cholesterol trapped in fiber exit in feces through the large intestine.
• Outcomes of Digestion:
  • Triglycerides are broken down into monoglycerides (MAGs) and free fatty acids (FFAs).
  • Forming micelles allows absorption into enterocytes via passive diffusion.
  • Inside the enterocyte, MAGs and FFAs re-esterify into TAGs and form chylomicrons (lipoproteins).

Lipid Transport

• Introduction to Lipoproteins:
  • Most lipids in circulation are transported using lipoproteins, which include:
  • Chylomicrons
  • Very Low-Density Lipoprotein (VLDL)
  • Intermediate-Density Lipoprotein (IDL)
  • Low-Density Lipoprotein (LDL)
  • High-Density Lipoprotein (HDL)
  • Apoproteins
  • Proteins associated with lipoproteins that play key roles in lipid transport.
• Chylomicrons:
  • Transport diet-derived lipids, primarily triglycerides from the small intestine through the lymph system, eventually draining into the subclavian vein.
  • Medium-chain fatty acids (MCFA) and short-chain fatty acids (SCFA) enter the bloodstream directly from enterocytes bound to albumin.

Circulation of Lipoproteins

• Role of Lipoprotein Lipase (LPL):
  • An enzyme located in capillary endothelium of adipose, liver, and muscle tissue that stimulates lipolysis.
  • Lipolysis breaks down triglycerides into fatty acids and glycerol, facilitating their entrance into cells.
  • Chylomicron remnants return to the liver for repacking with endogenously synthesized lipids.
• Nutrient Redistribution:
  • The liver redistributes lipids to other tissues via lipoproteins.

Lipoprotein Composition and Density

• Relative Composition by Type:
  • VLDL (Very Low-Density Lipoprotein):
  • High triglyceride content.
  • IDL (Intermediate-Density Lipoprotein):
  • Transitional lipoprotein.
  • LDL (Low-Density Lipoprotein):
  • Rich in cholesterol, sometimes referred to as the "bad" cholesterol due to its association with increased cardiovascular disease (CVD) risk.
  • HDL (High-Density Lipoprotein):
  • High protein content, often called "good" cholesterol as it helps remove cholesterol from circulation.
• Density Determination:
  • The lipid-to-protein ratio determines lipoprotein density and thus its classification.

Cholesterol and Health

• LDL (Low-Density Lipoprotein):
  • Transports cholesterol; excessive levels increase CVD risk by depositing cholesterol in blood vessel walls.
  • Often labeled as "bad" cholesterol.
• HDL (High-Density Lipoprotein):
  • Facilitates the removal of cholesterol and high levels reduce CVD risk.
  • Referred to as "good" cholesterol.

Functions of Lipids in the Body

• Energy Reservoirs
  • The body can store approximately 6 times more energy in fat compared to carbohydrates.
  • 1 gram of fatty acid = 9 kcal (over twice the energy yield of carbohydrates).
  • Fatty acids can be utilized as energy sources or combined with glycerol to form intramuscular triglycerides (IMTG) in muscle.
  • In adipocytes, fatty acids combine with glycerol to form triglycerides, which are the primary storage form of fat.
• Prevention of Protein Catabolism
  • During prolonged fasting, fatty acids can be converted into energy-yielding ketone bodies, preserving muscle mass.
• Insulation and Protection
  • Triglycerides (TAGs) protect internal organs from injury and help maintain body temperature.
• Structural Components of Cells
  • Phospholipids and cholesterol are vital for maintaining cellular membrane integrity.
• Signaling Molecules
  • Lipids serve as precursors for hormones and are essential for the absorption of fat-soluble vitamins.