Metabolic Pathways for Lipids: Digestion, Absorption, Metabolism, and Cardiovascular Implications

Lipid Metabolic Pathways

Introduction

Lipids are essential macronutrients involved in energy storage, cellular structure, and signaling. Their metabolism encompasses digestion, absorption, catabolism, synthesis, and roles in health and disease, particularly cardiovascular health.

Fatty Acid Nomenclature

Systems of Naming Fatty Acids

• Delta (Δ) System: Denotes chain length and position of double bonds from the carboxyl end. Example: Linoleic acid is 18:2 Δ9,12.

• Omega (ω) System: Counts double bonds from the methyl end; double bonds are usually separated by three carbons. Example: Linoleic acid is 18:2 n-6.

Practice Examples

• 24:1 ω9: 1 double bond, Δ notation is 24:1Δ15

• 20:5 ω3: Double bonds at positions 5, 8, 11, 14, 17 from the methyl end; Δ notation is 20:5Δ5,8,11,14,17

• 18:3 ω3 (ALA): Δ notation is 18:3Δ9,12,15

Digestion and Absorption of Lipids

Overview of Lipid Digestion

Lipid digestion begins in the mouth and stomach but is most significant in the small intestine, where enzymes and bile facilitate breakdown and absorption.

Site	Main Enzyme	Substrate	Product
Mouth	Lingual lipase	Triglycerides	Diglycerides, free fatty acids
Stomach	Gastric lipase	Triglycerides	Diglycerides, free fatty acids
Small Intestine	Pancreatic lipase, bile	Triglycerides, phospholipids	Monoglycerides, free fatty acids, cholesterol

Short and medium-chain fatty acids are absorbed directly into the portal vein, while long-chain fatty acids are re-esterified and packaged into chylomicrons for lymphatic transport.

Triglyceride Metabolism: Sources of Circulating Lipids

Lipoprotein Lipase (LPL) and Hormone Sensitive Lipase (HSL)

• LPL: Uptake of fatty acids from bloodstream into tissues; regulated by insulin.

• HSL: Release of fatty acids from adipose tissue into bloodstream; inhibited by insulin, activated by glucagon.

Fatty Acid Catabolism: β-Oxidation

Pathway and Energy Yield

• Step 1: Activation of fatty acid by coenzyme A in the cytosol to form fatty acyl CoA. $ext{Fatty acid} + ext{CoA} + ext{ATP} ightarrow ext{Acyl CoA} + ext{AMP} + ext{PP}_i$ (Uses 2 ATP)

• Step 2: Transport of fatty acyl CoA into mitochondrial matrix via carnitine shuttle (required for long-chain FAs).

• Step 3: β-oxidation cycle: Each cycle removes 2 carbons as acetyl CoA, producing 1 FADH2 and 1 NADH. Example: Palmitic acid (16C) undergoes 7 cycles, yielding 8 acetyl CoA, 7 FADH2, and 7 NADH.

ATP Yield Calculation for Palmitate

• 7 FADH2 × 1.5 ATP = 10.5

• 7 NADH × 2.5 ATP = 17.5

• 8 Acetyl CoA × 10 ATP = 80

• Total: 108 ATP

• Net yield: 106 ATP (subtract 2 ATP for activation)

Anabolic Reaction: Fatty Acid Synthesis

Pathway

• Location: Cytosol (ER); major sites include liver, adipose tissue, lungs, brain, kidneys, lactating mammary glands.

• Step 1: Transport of acetyl-CoA from mitochondria to cytosol via citrate shuttle.

• Step 2: Acetyl-CoA carboxylase (ACC) converts acetyl-CoA to malonyl-CoA (rate-limiting step). $ext{Acetyl-CoA} + ext{CO}_2 + ext{ATP} ightarrow ext{Malonyl-CoA} + ext{ADP} + ext{P}_i$

• Step 3: Fatty acid synthase (FAS) complex elongates the chain to form palmitate.

Regulation

• Hormonal: Insulin stimulates, glucagon inhibits ACC and FAS.

• Allosteric: Cytosolic citrate stimulates, long-chain FA CoA inhibits.

• PUFAs: Decrease activity.

Lipid Metabolism in Adipose Cells and Liver

Adipose Tissue

• Postprandial state favors energy storage as triacylglycerol (TAG).

• TG synthesized from both dietary lipids and excess glucose.

Liver

• Endogenous lipids, glucose, fructose, and amino acids contribute to VLDL synthesis.

Eicosanoids

Definition and Function

• Eicosanoids: Signaling molecules derived from 20-carbon fatty acids (omega-3 and omega-6).

• Include prostaglandins, thromboxanes, leukotrienes, neuroprotectins, lipoxins, and resolvins.

• Regulate inflammatory responses and immunity.

Pathways

Omega-6 Pathway	Omega-3 Pathway
Linoleic acid (18:2 n-6) → Gamma-linolenic acid (18:3 n-6) → Dihomo-gamma-linolenic acid (20:3 n-6) → Arachidonic acid (20:4 n-6)	Alpha-linolenic acid (18:3 n-3) → Stearidonic acid (18:4 n-3) → Eicosapentaenoic acid (20:5 n-3) → Docosahexaenoic acid (22:6 n-3)

Enzymes

• COX (Cyclooxygenase): Produces prostaglandins and thromboxanes.

• LOX (Lipoxygenase): Produces leukotrienes.

Physiological Effects

Messenger Class	Arachidonic Acid (n-6)	EPA/DHA (n-3)
Prostaglandins	Pro-inflammatory	Anti-inflammatory
Thromboxanes	Promotes blood clotting, vasoconstriction	Suppresses blood clotting, vasodilation
Leukotrienes	Pro-inflammatory	Anti-inflammatory
Lipoxins, Resolvins, Neuroprotectins	Derived from DHA, anti-inflammatory, neuroprotective	Anti-inflammatory, neuroprotective

Eicosanoids Competition and Dietary Balance

• Essential fatty acids compete for desaturase enzymes; preference for most unsaturated (n-3).

• Excess n-6 decreases EPA; excess n-3 decreases ARA.

• Balance of n-6/n-3 is crucial for health.

Dietary Recommendations and DRIs for Fat

• AMDR: 20-35% of energy from fat.

• AI for Linoleic acid (n-6): Men 17g/d, Women 12g/d (ages 19-50).

• AI for α-Linolenic acid (n-3): Men 1.6g/d, Women 1.1g/d.

• Saturated, trans fats, and cholesterol: as low as feasible (cholesterol <300mg/d).

• Replace saturated fats with monounsaturated (MUFA) or polyunsaturated (PUFA) fats.

Serum Lipids and Cardiovascular Disease (CVD)

Learning Objectives

• Describe atherogenic potential of lipoprotein classes and apolipoproteins.

• Describe process of atherosclerosis.

• Explain relationship between fatty acids and serum cholesterol.

• Describe metabolic connections between lipids and carbohydrates.

Screening and Risk Factors

• Total Cholesterol: >5.2 mmol/L

• Serum Triglycerides: >1.7 mmol/L

Cholesterol Metabolism

• Synthesized predominantly in the liver.

• Key enzyme: HMG-CoA reductase (target of statin drugs).

• Not used for energy; eliminated by the liver.

Reducing Cholesterol

• Dietary changes (alter fats and sugars).

• Increased exercise (raises HDL, may lower LDL).

• Weight reduction if necessary.

• Drugs: phytosterols, statins, ezetimibe, bile reuptake inhibitors.

Lipoprotein Classes and Apolipoproteins

Composition and Transport

• Exogenous transporters: Chylomicrons (ApoA-1, ApoB-48)

• Endogenous transporters: VLDL, IDL, LDL (ApoB-100)

• HDL: Reverse cholesterol transport (ApoA-1, LCAT, ABCA1)

Key Apolipoproteins

• ApoB-100: On VLDL, IDL, LDL; marker for CVD risk.

• ApoA-1: On chylomicrons, HDL; involved in reverse cholesterol transport.

• LCAT: Esterifies cholesterol in HDL.

• ABCA1: Transports cholesterol and phospholipids to HDL.

Atherosclerosis: Pathogenesis

Development

• Initiated by injury to arterial wall, accumulation of LDL/cholesterol, formation of fatty streaks, plaque development, and eventual rupture leading to heart attack.

Risk Factors for Heart Disease

Category	Risk Factors
Blood Lipid Levels	High total/LDL cholesterol, low HDL, high triglycerides, high ApoB, low ApoA1
Other Factors	High blood pressure, diabetes, obesity, family history, smoking, unhealthy diet

Dietary Fat and Blood Lipids: Protection by n-3

• n-3 PUFAs exert anti-inflammatory effects, improve lipid profiles, and reduce CVD risk.

Key Takeaways from Clinical Study

• Lower n-6/n-3 ratio may improve blood lipid parameters in hyperlipidemic patients.

• Excessive reduction of n-6/n-3 ratio does not yield additional benefits.

References and Further Learning

• Videos on dietary cholesterol and atherosclerosis pathogenesis (see provided links).

Additional info: Expanded explanations, tables, and context were added to ensure completeness and academic quality.