4.2 Lipid Metabolism, Lipoproteins, and Cardiovascular Biomarkers

Lipids are water-insoluble, hydrophobic or amphipathic organic macromolecules soluble in non-polar organic solvents. Dietary lipids furnish approximately 9 kcal/g (37.7 kJ/g) of energy, provide structural building blocks for cell membranes, facilitate the absorption of fat-soluble vitamins, and supply precursors for bioactive lipid mediators (eicosanoids, docosanoids, and steroid hormones). In clinical practice and public health nutrition, balancing lipid subtypes is fundamental to preventing atherosclerotic cardiovascular disease (ASCVD), managing non-alcoholic fatty liver disease (NAFLD/MASLD), and treating acute pancreatitis secondary to severe hypertriglyceridemia.

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Lipid Chemistry and Fatty Acid Classification

Fatty acids consist of a terminal carboxyl group ($-COOH$, carbon 1) linked to an aliphatic hydrocarbon chain of variable length. Systematic nomenclature designates carbon chain length and the position of double bonds relative to the carboxyl carbon ($\Delta$ system) or terminal methyl carbon ($\omega$ or n- system).

1. Saturated Fatty Acids (SFAs)

SFAs contain no double bonds along the carbon chain, allowing tight intermolecular packing and high melting points (solid at ambient temperature):

• Lauric Acid ($C12:0$): Coconut oil, palm kernel oil.
• Myristic Acid ($C14:0$): Nutmeg butter, whole dairy fat, coconut oil.
• Palmitic Acid ($C16:0$): Palm oil, animal fats. The most abundant dietary SFA.
• Stearic Acid ($C18:0$): Cocoa butter, beef tallow.
• Atherogenic Potential: Lauric, myristic, and palmitic acids downregulate hepatic LDL receptor expression, decreasing LDL clearance and significantly raising plasma LDL-C. In contrast, stearic acid ($C18:0$) exerts a neutral effect on serum LDL-C because it is rapidly converted in hepatocytes into oleic acid ($C18:1, n-9$) by the microsomal enzyme stearoyl-CoA desaturase ($\Delta^9$-desaturase).

2. Monounsaturated Fatty Acids (MUFAs)

MUFAs contain a single double bond, almost universally in the cis geometric configuration in nature, introducing a 30-degree rigid bend into the acyl chain:

• Oleic Acid ($18:1, n-9$ or $18:1\Delta^9$): The predominant dietary MUFA, abundant in olive oil, canola oil, avocados, and nuts. When substituted for dietary SFAs, oleic acid lowers LDL-C while preserving anti-atherogenic HDL-C concentrations and improving insulin sensitivity.

3. Polyunsaturated Fatty Acids (PUFAs)

PUFAs contain two or more cis double bonds separated by a single methylene carbon ($-CH_2-$ bridge):

• Omega-3 ($n-3$) Fatty Acids:
  • $\alpha$-Linolenic Acid (ALA, $18:3, n-3$): Indispensable dietary fatty acid found in flaxseed, chia seeds, walnuts, and canola oil.
  • Eicosapentaenoic Acid (EPA, $20:5, n-3$) and Docosahexaenoic Acid (DHA, $22:6, n-3$): Long-chain marine omega-3 fatty acids synthesized poorly in humans from ALA ($< 5\%$ conversion efficiency for EPA, $< 0.5\%$ for DHA) due to low $\Delta^6$-desaturase activity. EPA and DHA incorporate into cell membranes, displacement of arachidonic acid, and serve as substrates for anti-inflammatory, anti-thrombotic series-3 prostaglandins ($PGE_3$), series-5 leukotrienes ($LTB_5$), and specialized pro-resolving mediators (SPMs: resolvins, protectins, maresins). High-dose EPA/DHA (2–4 g/day) reduces hepatic VLDL secretion and lowers plasma triglycerides by 20–30%.
• Omega-6 ($n-6$) Fatty Acids:
  • Linoleic Acid (LA, $18:2, n-6$): Indispensable dietary fatty acid found in soybean, corn, and sunflower oils.
  • Arachidonic Acid (AA, $20:4, n-6$): Abundant in meat, poultry, and egg yolks. Cleaved from membrane phospholipids by phospholipase A2 ($PLA_2$), AA enters the cyclooxygenase (COX-1/COX-2) and 5-lipoxygenase (5-LOX) pathways, producing pro-inflammatory, pro-thrombotic series-2 prostaglandins ($PGE_2$), thromboxane $A_2$ ($TXA_2$), and series-4 leukotrienes ($LTB_4$).

4. Trans Fatty Acids (TFAs)

Trans fatty acids contain double bonds in the trans geometric configuration (hydrogen atoms on opposite sides of the carbon-carbon double bond), causing the acyl chain to adopt a straight, rigid conformation resembling an SFA:

• Sources: Industrial partial hydrogenation of vegetable oils (historically used in shortening, margarines, and commercial baked goods) and ruminant biohydrogenation (vaccenic acid in dairy and beef fat).
• Metabolic Harm: Industrial TFAs markedly increase atherogenic LDL-C, decrease protective HDL-C, increase plasma triglycerides, elevate lipoprotein(a) [Lp(a)], and trigger systemic endothelial inflammation.
• Philippine Regulatory Framework: Following the WHO REPLACE package, the Department of Health issued Administrative Order No. 2021-0039 (June 18, 2021), the national policy on eliminating industrially produced trans fat. It prohibits partially hydrogenated oils (PHOs), oils and fats blended with PHOs, oils and fats with more than 2 g of trans fat per 100 g, and processed foods made with PHOs or high in trans fat, with the goal of keeping trans fat intake below 1% of energy.

5. Triglycerides, Phospholipids, and Sterols

• Triglycerides (Triacylglycerols, TAGs): Composed of a glycerol molecule esterified with three fatty acid chains. Represents 95% of dietary lipids and the primary dense metabolic energy reserve in adipose droplets.
• Phospholipids: Amphipathic lipids featuring a glycerol backbone esterified to two fatty acids at sn-1 and sn-2, and a polar phosphate headgroup at sn-3 linked to an amino alcohol (choline $\to$ phosphatidylcholine / lecithin; ethanolamine $\to$ phosphatidylethanolamine). Form the structural lipid bilayer of cellular membranes and the outer monolayer of plasma lipoproteins.
• Sterols: Characterized by a four-ring cyclopentanoperhydrophenanthrene cyclopentane core. Cholesterol is the principal animal sterol, serving as a structural membrane fluidity modulator and metabolic precursor for bile acids, steroid hormones, and 7-dehydrocholesterol. Plant sterols and stanols (phytosterols) possess modified side chains that competitively displace cholesterol from mixed micelles in the intestinal lumen, reducing dietary cholesterol absorption by 30–50% without being absorbed themselves.

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Lipid Digestion, Micelle Formation, and Absorption

Dietary Lipids (TAGs, Phospholipids, Cholesteryl Esters)
       │
       ▼ [Mouth: Lingual Lipase]
Short- and Medium-Chain Fatty Acid Cleavage initiated
       │
       ▼ [Stomach: Gastric Lipase + Mechanical Shearing]
10–30% of TAGs cleaved to 1,2-DAGs + FFAs (Emulsion Droplets)
       │
       ▼ [Duodenum: CCK stimulates Gallbladder & Pancreas]
Bile Salts Emulsify Lipids ──► Mixed Droplets
       │
       ▼ [Pancreatic Colipase anchors Pancreatic Lipase at droplet interface]
Pancreatic Lipase Cleaves sn-1 & sn-3 bonds
       │
       ▼
Products: 2-Monoacylglycerol (2-MAG) + 2 Free Fatty Acids
       │
       ▼ [Mixed Micelle Assembly (Bile Salts + 2-MAG + FFAs + Fat-Soluble Vitamins)]
Diffusion across Unstirred Water Layer to Enterocyte Brush Border
       │
       ├──────────────────────────────┬──────────────────────────────┐
       ▼                              ▼                              ▼
Long-Chain FFAs & 2-MAG        Short- & Medium-Chain FAs      Free Cholesterol
(Passive & CD36/FATP4)         (C6–C12: Direct Diffusion)     (NPC1L1 Transporter)
       │                              │                              │
       ▼ [ER Re-esterification]       ▼ [Enters Portal Blood]        ▼ [ACAT2 Re-esterification]
Resynthesized TAGs            Directly to Liver via Albumin   Cholesteryl Esters
       │                                                             │
       └──────────────────────────────┬──────────────────────────────┘
                                      │
                                      ▼ [Microsomal Triglyceride Transfer Protein (MTP)]
                              Nascent Chylomicrons (Apo B-48)
                                      │
                                      ▼ [Exocytosis into Lacteals]
                              Thoracic Duct ──► Systemic Blood

Luminal Enzymatic Hydrolysis

1. Lingual and Gastric Lipases: Secreted by Ebner's glands of the tongue and gastric chief cells. Acid-stable enzymes (optimum pH 4.0–5.5) that preferentially hydrolyze ester bonds at the sn-3 position of triglycerides containing short- and medium-chain fatty acids. Crucial in neonates, digesting up to 50% of dietary milk fat before pancreatic function matures.
2. Bile Salts and Duodenal Emulsification: Mechanical antral contractions pump coarse lipid droplets into the duodenum. Cholecystokinin (CCK) stimulates gallbladder contraction, releasing amphipathic bile salts (glycocholate, taurocholate). Bile salts reduce interfacial tension, dispersing large lipid droplets into fine microscopic emulsion particles.
3. Pancreatic Lipase and Colipase: Pancreatic lipase is normally inhibited and displaced from lipid droplets by bile salts. Pancreatic acinar cells co-secrete procolipase, which is cleaved by luminal trypsin into colipase. Colipase anchors pancreatic lipase to the oil-water interface, enabling it to cleave the sn-1 and sn-3 ester bonds of TAGs, yielding 2-monoacylglycerol (2-MAG) and two free fatty acids (FFAs).
4. Cholesterol Esterase and Phospholipase A2: Pancreatic cholesterol esterase hydrolyzes cholesteryl esters into free cholesterol and fatty acids. Phospholipase A2 cleaves the sn-2 fatty acid of lecithin, yielding lysophosphatidylcholine.

Mixed Micelles and Enterocyte Uptake

When bile salt concentrations exceed the critical micellar concentration (CMC) (typically 2–5 mM), bile salts aggregate with 2-MAG, FFAs, free cholesterol, and fat-soluble vitamins to form cylindrical mixed micelles (4–8 nm diameter). Mixed micelles diffuse across the acidic, unstirred water layer to the brush border:

• Monosaccharide/Lipid Absorption: Fatty acids and 2-MAG passively partition into the enterocyte apical membrane or are transported via fatty acid translocase (FAT / CD36) and fatty acid transport proteins (FATP4).
• Niemann-Pick C1-Like 1 (NPC1L1): A dedicated apical sterol transporter mediating the active uptake of free cholesterol into the enterocyte. Targeted clinically by the lipid-lowering drug ezetimibe, which selectively blocks NPC1L1 to reduce intestinal cholesterol absorption.

Medium-Chain Triglycerides (MCTs): Unique Clinical Transport

Medium-chain fatty acids ($C6:0$ to $C12:0$, found in MCT oil and coconut oil) bypass traditional lipid digestion:

• Hydrolyzed rapidly and completely by gastric and pancreatic lipases without requiring bile salt micellar incorporation.
• Absorbed directly into enterocytes and pass directly across the basolateral membrane into the portal venous blood, binding to circulating serum albumin and traveling straight to the liver.
• Do not require enterocyte chylomicron assembly, lymphatic lacteal drainage, or the carnitine shuttle for mitochondrial entry. Consequently, MCT oil is an indispensable medical nutrition therapy for patients with pancreatic exocrine insufficiency, biliary atresia, short bowel syndrome, chylothorax, and intestinal lymphangiectasia.

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Lipoprotein Metabolism and Apolipoprotein Functions

Lipoproteins are spherical macromolecular complexes composed of a hydrophobic core (triglycerides and cholesteryl esters) surrounded by an amphipathic monolayer of phospholipids, unesterified cholesterol, and specific surface proteins called apolipoproteins.

Apolipoprotein Functions

• Apolipoprotein B-48 (Apo B-48): Synthesized exclusively in enterocytes via post-transcriptional RNA editing of the Apo B mRNA (cytidine deaminase edits a CAA codon to a premature UAA stop codon). Apo B-48 lacks the LDL receptor-binding domain and serves as the obligatory structural scaffold for chylomicron assembly via microsomal triglyceride transfer protein (MTP).
• Apolipoprotein B-100 (Apo B-100): Synthesized in hepatocytes as the full-length protein. Serves as the structural protein of VLDL, IDL, and LDL. Contains the specific ligand domain recognized by the hepatic and peripheral LDL Receptor (LDLR).
• Apolipoprotein C-II (Apo C-II): Transferred from circulating HDL to nascent chylomicrons and VLDL. Functions as the obligatory allosteric cofactor and activator of Lipoprotein Lipase (LPL) anchored to capillary endothelial surfaces in skeletal muscle, myocardium, and adipose tissue. Congenital Apo C-II deficiency causes Familial Chylomicronemia Syndrome (Type I Hyperlipoproteinemia), characterized by massive hypertriglyceridemia ($> 1{,}000–2{,}000\text{ mg/dL}$), eruptive xanthomas, lipemia retinalis, and recurrent, life-threatening acute pancreatitis.
• Apolipoprotein E (Apo E): Serves as a high-affinity ligand mediating receptor-mediated endocytosis of chylomicron remnants and IDL by hepatic LDL receptors and LDL receptor-related protein 1 (LRP1). Polymorphisms in the APOE gene ($\epsilon2, \epsilon3, \epsilon4$) influence cardiovascular and neurological risk: $APOE\,\epsilon4$ significantly elevates the risk of late-onset Alzheimer's disease and premature atherosclerosis; $APOE\,\epsilon2/\epsilon2$ homozygosity causes Type III Hyperlipoproteinemia (Familial Dysbetalipoproteinemia).
• Apolipoprotein A-1 (Apo A-1): The major structural apolipoprotein of HDL. Activates Lecithin-Cholesterol Acyltransferase (LCAT), the plasma enzyme that esterifies free cholesterol with a fatty acid from phosphatidylcholine, trapping cholesteryl esters inside the HDL core and driving the maturation of nascent discoidal HDL into spherical mature $HDL_3$ and $HDL_2$.

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Central Pathways of Lipid Metabolism

1. Mitochondrial $\beta$-Oxidation and the Carnitine Shuttle

$\beta$-Oxidation is the cyclic catabolic pathway that cleaves two-carbon acetyl-CoA units sequentially from the carboxyl end of fatty acyl-CoA molecules within the mitochondrial matrix:

1. Activation: Cytosolic fatty acids are converted to fatty acyl-CoA by fatty acyl-CoA synthetase (thiokinase) at the outer mitochondrial membrane, consuming 2 ATP equivalents (ATP $\to$ AMP $+ PP_i$).
2. The Carnitine Shuttle: Inner mitochondrial membranes are impermeable to CoA. Carnitine Palmitoyltransferase-1 (CPT-1) on the outer membrane replaces CoA with carnitine, forming acylcarnitine. Carnitine-acylcarnitine translocase (CACT) shuttles acylcarnitine into the matrix in exchange for free carnitine. CPT-2 on the inner membrane regenerates fatty acyl-CoA and free carnitine.
   • Rate-Limiting Control: CPT-1 is the rate-limiting enzyme of $\beta$-oxidation and is allosterically inhibited by malonyl-CoA, the initial intermediate of fatty acid synthesis. This prevents futile simultaneous synthesis and degradation of fatty acids.
3. The Four-Step $\beta$-Oxidation Spiral:
   • Oxidation: Acyl-CoA dehydrogenase generates a trans-$\Delta^2$-enoyl-CoA and 1 $FADH_2$.
   • Hydration: Enoyl-CoA hydratase adds water, forming L-3-hydroxyacyl-CoA.
   • Oxidation: 3-Hydroxyacyl-CoA dehydrogenase forms 3-ketoacyl-CoA and 1 NADH.
   • Thiolysis: $\beta$-Ketothiolase cleaves the chain with CoASH, releasing 1 Acetyl-CoA and a fatty acyl-CoA shortened by two carbons.

2. Ketogenesis and Extrahepatic Utilization

Ketogenesis occurs exclusively within the mitochondrial matrix of hepatocytes when fatty acid $\beta$-oxidation generates acetyl-CoA at rates exceeding the capacity of the citric acid cycle:

• Triggers: Prolonged fasting, starvation, poorly managed type 1 diabetes mellitus, and very-low-carbohydrate ketogenic diets. Under these conditions, hepatic gluconeogenesis depletes mitochondrial oxaloacetate, causing acetyl-CoA to accumulate.
• Pathway: Two acetyl-CoA molecules condense to form acetoacetyl-CoA. Mitochondrial HMG-CoA Synthase (the rate-limiting enzyme of ketogenesis) condenses acetoacetyl-CoA with a third acetyl-CoA to form $\beta$-hydroxy-$\beta$-methylglutaryl-CoA (HMG-CoA). HMG-CoA lyase then cleaves HMG-CoA into acetyl-CoA and acetoacetate.
• Ketone Body Subtypes:
  • Acetoacetate: The primary metabolic ketone body.
  • $\beta$-Hydroxybutyrate: Formed by the reversible reduction of acetoacetate by mitochondrial $\beta$-hydroxybutyrate dehydrogenase utilizing NADH. It is the predominant ketone body circulating in blood.
  • Acetone: A volatile, non-metabolizable side product formed by spontaneous non-enzymatic decarboxylation of acetoacetate; excreted via expired air, imparting a characteristic fruity/sweet breath odor in diabetic ketoacidosis (DKA).
• Extrahepatic Ketone Oxidation: Acetoacetate and $\beta$-hydroxybutyrate are transported to extrahepatic tissues (skeletal muscle, myocardium, renal cortex, and brain during starvation). Acetoacetate is reactivated to acetoacetyl-CoA by the enzyme succinyl-CoA:3-ketoacid CoA transferase (thiophorase), transferring CoA from succinyl-CoA. The liver completely lacks thiophorase, ensuring that hepatocytes cannot utilize the ketone bodies they synthesize and must export them to peripheral tissues.

3. Fatty Acid Biosynthesis (De Novo Lipogenesis)

Occurs in the cytosol of hepatocytes, lactating mammary glands, and adipose tissue during energy surplus:

• Citrate Shuttle: Mitochondrial acetyl-CoA condenses with oxaloacetate to form citrate, which crosses into the cytosol via the tricarboxylate transporter. Cytosolic ATP-citrate lyase cleaves citrate back into acetyl-CoA and oxaloacetate.
• Rate-Limiting Step: Acetyl-CoA Carboxylase (ACC) carboxylates acetyl-CoA to form malonyl-CoA, consuming 1 ATP and requiring biotin ($B_7$). ACC is allosterically activated by citrate and dephosphorylated/activated by insulin; it is phosphorylated/inactivated by AMP-activated protein kinase (AMPK) and glucagon/epinephrine.
• Fatty Acid Synthase (FAS): A multifunctional enzyme complex with an acyl carrier protein (ACP) containing 4'-phosphopantetheine (Vitamin $B_5$). FAS sequentially condenses malonyl-CoA units, consuming NADPH (derived from the pentose phosphate pathway and malic enzyme), until releasing the 16-carbon saturated fatty acid palmitate (16:0).

4. Cholesterol Biosynthesis

Occurs in the cytosol and endoplasmic reticulum of all nucleated cells, especially hepatocytes:

• Three acetyl-CoA molecules form HMG-CoA via cytosolic HMG-CoA synthase.
• HMG-CoA Reductase: Catalyzes the rate-limiting conversion of HMG-CoA to mevalonate, consuming 2 molecules of NADPH. HMG-CoA reductase is the primary pharmacological target of statins (competitive HMG-CoA reductase inhibitors like atorvastatin, rosuvastatin).

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Cardiovascular Biomarkers and Clinical Lipid Profile Interpretation

Fasting Lipid Panel Reference Cutoffs

Standard lipid profiles assess Total Cholesterol (TC), High-Density Lipoprotein Cholesterol (HDL-C), and Triglycerides (TG), with LDL-C calculated or directly measured.

The Friedewald Formula and Clinical Limitations

In standard clinical laboratories, LDL-C is calculated rather than measured directly:

$\text{LDL-C (mg/dL)} = \text{Total Cholesterol} - \text{HDL-C} - \left( \frac{\text{Triglycerides}}{5} \right)$ $\text{LDL-C (mmol/L)} = \text{Total Cholesterol} - \text{HDL-C} - \left( \frac{\text{Triglycerides}}{2.2} \right)$

Where the term $\frac{\text{Triglycerides}}{5}$ estimates the cholesterol carried within VLDL particles (VLDL-C), assuming a constant $5:1$ ratio of triglyceride to cholesterol mass in VLDL.

Non-HDL Cholesterol and Atherogenic Dyslipidemia

• Non-HDL Cholesterol ($\text{TC} - \text{HDL-C}$): Quantifies the cholesterol content of all circulating atherogenic apolipoprotein B-containing lipoproteins (LDL, VLDL, IDL, and Lp(a)). It does not require a fasting sample and remains reliable even when triglycerides exceed $400\text{ mg/dL}$. In high-risk clinical guidelines, the non-HDL-C target is set at $\text{LDL-C goal} + 30\text{ mg/dL}$.
• The Atherogenic Dyslipidemia Triad: Highly prevalent in metabolic syndrome, type 2 diabetes mellitus, and visceral obesity. Characterized by:
  1. Elevated fasting triglycerides ($> 150\text{ mg/dL}$).
  2. Low HDL-C ($< 40\text{ mg/dL}$ in men, $< 50\text{ mg/dL}$ in women).
  3. Predominance of Small, Dense LDL particles (Pattern B). Small, dense LDL particles are prone to subendothelial arterial wall penetration, bind avidly to intimal proteoglycans, exhibit prolonged plasma half-life due to reduced LDL receptor affinity, and undergo rapid oxidation into cytotoxic oxidized LDL (oxLDL), initiating foam cell formation and atherosclerotic plaque progression.