Lipids: Structure, Function, Digestion, and Health Implications

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Types of Lipids

Overview of Lipids

Lipids are a structurally and functionally diverse group of substances that are insoluble in water but soluble in organic solvents. They are classified as fats (solid at room temperature) and oils (liquid at room temperature). The main types of lipids include triglycerides, phospholipids, and sterols.

Triglycerides: The most abundant lipid in the diet and body, composed of three fatty acids and one glycerol molecule.
Phospholipids: Essential components of cell membranes, composed of glycerol, two fatty acids, and a phosphate group.
Sterols: Lipids with a ring structure, including cholesterol and phytosterols.

Olive oil, olives, and bread as sources of dietary lipids

Triglycerides: Structure and Function

Triglycerides consist of three fatty acids attached to a glycerol backbone. Fatty acids are long chains of carbon atoms surrounded by hydrogen atoms.

Energy Provision: Triglycerides provide 9 kcal/gram and are the largest energy store in the body.
Structural Role: Building blocks for cell membranes.
Protection and Insulation: Protect internal organs and retain body heat.
Transport: Aid in the absorption of fat-soluble vitamins.
Satiety: Promote fullness and take longer to digest than carbohydrates or proteins.

Structure of triglyceride and fatty acid

Phospholipids: Structure and Function

Phospholipids are composed of a glycerol backbone, two fatty acids, and a phosphate group.

Cell Membrane: Structural components maintaining cell integrity.
Emulsification: Act as emulsifiers, keeping lipids soluble in water.
Amphiphilic Nature: Soluble in both water and fat.
Example: Lecithin, found in egg yolks, wheat germ, peanuts, soybeans, and organ meats.

Phospholipid structure and cell membrane Egg yolk as a source of lecithin

Sterols: Structure and Function

Sterols, such as cholesterol, have a multi-ring structure and are lipophilic.

Cholesterol: Found in animal products and synthesized by the liver and intestine.
Phytosterols: Found in plants and reduce cholesterol absorption.
Functions: Structural component of cell membranes, precursor for hormone synthesis (e.g., estrogen), bile acid synthesis, and vitamin D production. Cholesterol is not used for energy.

Sterol ring structure and cholesterol molecule

Classification of Fatty Acids

Chain Length

Fatty acids are classified by the length of their carbon chain:

Short chain: <6 carbons
Medium chain: 6–12 carbons
Long chain: >14 carbons

Chain length affects digestion, absorption, and transport.

Degree of Saturation

Fatty acids are also classified by their degree of saturation:

Saturated fatty acids (SFA): No double bonds; all carbons are saturated with hydrogen.
Monounsaturated fatty acids (MUFA): One double bond.
Polyunsaturated fatty acids (PUFA): Two or more double bonds.

Saturated and unsaturated fatty acid structures

Shape and Physical Properties

Saturated fatty acids: Straight chains, pack tightly, solid at room temperature.
Unsaturated fatty acids: Kinked/bent chains, do not stack well, liquid at room temperature.

Saturated fatty acid structure

Dietary Sources

Animal fats: High in SFAs (e.g., butter, cream, whole milk, beef, lard).
Plant fats: Usually lower in SFAs, except coconut and palm kernel oil.
MUFA and PUFA: Found in olive oil, canola oil, nuts, seeds, and fish; generally protective for health.

Bar graph of fatty acid composition in foods

Unsaturated Fatty Acids: Configuration

Cis and Trans Configurations

The hydrogen atoms at the unsaturated region can be arranged in different positions:

Cis: Hydrogens on the same side of the double bond; bent shape; most common in nature.
Trans: Hydrogens on opposite sides; straighter chain; acts like SFA; created by hydrogenation.

Cis and trans arrangement of polyunsaturated fatty acids

Essential Fatty Acids

Omega-6 and Omega-3 Fatty Acids

Essential fatty acids cannot be synthesized by the body and must be obtained from the diet.

Omega-6 (Linoleic acid): Found in vegetable and nut oils.
Omega-3 (Alpha-linolenic acid): Found in dark green leafy vegetables, flaxseeds, soybeans, walnuts, and canola oil. EPA and DHA are found in fish and fish oils.

Structure of linoleic acid and alpha-linolenic acid Salmon as a source of omega-3 fatty acids

Functions and Deficiency

Essential fatty acids are precursors to eicosanoids, which regulate blood clotting, blood pressure, inflammation, and immune function. Deficiency can result in growth failure, scaly skin, reproductive failure, and poor immunity.

Digestion and Absorption of Lipids

Overview of Lipid Digestion

Lipid digestion begins in the mouth and stomach but is primarily completed in the small intestine.

Mouth: Lingual lipase initiates minimal digestion.
Stomach: Gastric lipase and mixing break lipids into smaller droplets.
Liver: Produces bile, which emulsifies fats.
Pancreas: Secretes pancreatic lipases into the small intestine.

Digestive system overview for lipid digestion

Small Intestine: Bile and Lipase Action

Bile is secreted from the gallbladder into the small intestine, increasing the surface area for digestion. Pancreatic lipase breaks triglycerides into monoglycerides and fatty acids. Bile and lipase action in small intestine

Micelle Formation and Absorption

Micelles are water-soluble spheres of bile and phospholipids that trap digestion byproducts and transport them to enterocytes for absorption. Micelle formation and transport

Transport of Lipids

Lipoproteins

Lipoproteins are transport vehicles made of lipids and proteins, carrying triglycerides and cholesterol in the bloodstream. Types include:

Chylomicrons: Remove absorbed fats from the small intestine.
VLDL: Transport triglycerides from the liver to cells.
LDL: Deliver cholesterol to cells; high levels increase CVD risk.
HDL: Pick up cholesterol and bring it to the liver; high levels decrease CVD risk.

Lipoprotein structure and composition Pie charts of lipoprotein composition

Cardiovascular Disease and Lipids

Overview and Pathology

Cardiovascular disease (CVD) includes dysfunction of the heart or blood vessels, such as coronary heart disease and stroke. Atherosclerosis is the hardening of arteries due to plaque buildup, which can lead to angina, ischemia, and myocardial infarction. Heart with blocked artery Atherosclerosis and plaque formation

Risk Factors and Prevention

Nonmodifiable: Age, sex, heredity.
Modifiable: Diet, physical activity, tobacco use, sleep, weight, cholesterol, blood sugar, blood pressure.

Dietary Strategies for CVD Prevention

Reduce total fat and saturated fat intake.
Decrease trans fat and dietary cholesterol.
Increase omega-3 fatty acid and dietary fiber intake.
Consume vitamins B6, B12, and folate.
Maintain healthy blood glucose, avoid smoking, and stay active.

Lifestyle Strategies

Exercise most days of the week.
Maintain a healthful body weight.
Reduce caloric intake and increase physical activity.
Decrease stress and increase HDL levels.

Summary Table: Types of Lipids

Type	Structure	Function	Food Sources
Triglycerides	3 fatty acids + glycerol	Energy, insulation, protection	Animal fats, plant oils
Phospholipids	Glycerol + 2 fatty acids + phosphate	Cell membrane, emulsification	Egg yolk, soybeans
Sterols	Multi-ring structure	Cell membrane, hormone precursor	Animal products, plants

Key Equations

Energy from fat:
Fatty acid notation: (e.g., for oleic acid)