3.1 Fatty Acids, Triglycerides, Cholesterol & Lipoproteins

3.1 Fatty Acids, Triglycerides, Cholesterol & Lipoproteins

[!NOTE] ASCP Exam Focus: Mastery of lipid metabolism requires thorough understanding of three interrelated concepts: (1) the chemical structures distinguishing free lipids from esterified storage forms, (2) the structural and functional composition of the five major lipoprotein classes (especially density, diameter, lipid payload, and apolipoproteins), and (3) analytical separation principles, notably agarose gel electrophoretic migration and the overnight standing plasma (refrigerator) test.

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Chemical Structure and Classification of Lipids

Lipids are a chemically heterogeneous group of organic compounds characterized by their insolubility in polar solvents such as water and high solubility in nonpolar organic solvents (e.g., ether, chloroform, and hexane). In clinical biochemistry, lipids are categorized into four primary classes: fatty acids, triglycerides (triacylglycerols), cholesterol (free and esterified), and phospholipids.

+---------------------------------------------------------------------------------------------------------+
|                                      Primary Clinical Lipid Classes                                     |
+---------------------------------------------------------------------------------------------------------+
| Class            | Basic Structure                             | Primary Clinical / Biological Role     |
+------------------+---------------------------------------------+----------------------------------------+
| Fatty Acids      | Aliphatic hydrocarbon chain with -COOH head | Metabolic fuel; membrane acyl groups   |
| Triglycerides    | 1 Glycerol + 3 Fatty acids (ester bonds)    | Primary cellular energy storage form   |
| Cholesterol      | Sterol nucleus (27 C) with C-3 -OH group    | Membrane fluidity; steroid/bile precursor|
| Cholesteryl Ester| Cholesterol + Fatty acid at C-3 position    | Nonpolar core storage in lipoproteins  |
| Phospholipids    | Diacylglycerol + Phosphate + Polar head     | Amphipathic membrane lipid bilayer     |
+---------------------------------------------------------------------------------------------------------+

1. Fatty Acids

Fatty acids consist of a straight aliphatic hydrocarbon chain terminating in a polar carboxylic acid group (-COOH). In human plasma, fatty acids predominantly exist with an even number of carbon atoms (ranging from 12 to 24 carbons):

• Saturated Fatty Acids (SFAs): Contain zero carbon-carbon double bonds (C=C). Examples include palmitic acid (16:0) and stearic acid (18:0). The absence of double bonds allows the hydrocarbon chains to adopt a straight, linear conformation that packs tightly, yielding high melting temperatures; they are solid at room temperature.
• Monounsaturated Fatty Acids (MUFAs): Contain exactly one double bond, typically in the cis geometric conformation, which introduces a 30-degree rigid kink into the acyl chain. The primary physiological example is oleic acid (18:1 cis-9). The cis bend disrupts tight crystalline packing, substantially lowering the melting point.
• Polyunsaturated Fatty Acids (PUFAs): Contain two or more double bonds separated by a methylene bridge (-CH2-). PUFAs are classified by the position of the first double bond counted from the terminal methyl (omega, $\omega$) carbon:
  • Omega-6 ($\omega$-6) Fatty Acids: Linoleic acid (18:2 cis-9,12) is an essential fatty acid because human hepatocytes lack $\Delta^{12}$ and $\Delta^{15}$ desaturase enzymes capable of inserting double bonds beyond carbon 9. Linoleic acid serves as the obligate dietary precursor for arachidonic acid (20:4 $\omega$-6), the primary substrate for eicosanoid synthesis (thromboxanes, leukotrienes, prostaglandins).
  • Omega-3 ($\omega$-3) Fatty Acids: Alpha-linolenic acid (18:3 cis-9,12,15) is also an essential fatty acid, converted through elongation and desaturation into eicosapentaenoic acid (EPA, 20:5 $\omega$-3) and docosahexaenoic acid (DHA, 22:6 $\omega$-3), which yield anti-inflammatory, anti-arrhythmic, and anti-thrombotic eicosanoids.
• Trans Fatty Acids: Formed artificially via partial industrial catalytic hydrogenation of vegetable oils. The trans double bond straightens the hydrocarbon backbone, causing trans fats to mimic saturated fats biophysically. Trans fatty acid consumption potently raises atherogenic Low-Density Lipoprotein Cholesterol (LDL-C) and lowers High-Density Lipoprotein Cholesterol (HDL-C).

2. Triglycerides (Triacylglycerols)

Triglycerides are composed of a single three-carbon glycerol backbone esterified to three fatty acids via covalent ester bonds at the sn-1, sn-2, and sn-3 positions. The fatty acid at sn-1 is typically saturated, sn-2 is usually unsaturated, and sn-3 can be either.

Triglycerides constitute over 95% of dietary lipid intake and represent the primary metabolic storage form of energy in human adipose tissue. Because triglycerides possess neither charged nor uncharged polar functional groups, they are completely hydrophobic and nonpolar ("neutral fats"). Consequently, they are stored in adipocyte droplets in an anhydrous state, delivering approximately 9 kcal/g of metabolic energy upon complete $\beta$-oxidation, compared to 4 kcal/g for hydrated glycogen.

3. Cholesterol and Cholesteryl Esters

Cholesterol is an alicyclic sterol composed of 27 carbon atoms arranged into a rigid four-ring cyclopentanoperhydrophenanthrene nucleus (rings A, B, C, and D), an eight-carbon branched hydrocarbon side chain attached at carbon-17 (C-17), two angular methyl groups at C-10 and C-13, a double bond between C-5 and C-6, and a single polar hydroxyl group (-OH) at carbon-3 (C-3).

                     [C-20 to C-27 Hydrocarbon Tail]
                                    |
                                 [Ring D] (5 carbons)
                               /        \
      [Ring A] ---- [Ring B] ------------- [Ring C]
         |             |
     (C-3 -OH)      (C=C at C5-C6)
   [Polar Head]   [Rigid Steroid Planar Ring System]

• Free (Unesterified) Cholesterol: Comprises approximately 30% of total circulating plasma cholesterol. The polar C-3 hydroxyl imparts amphipathic characteristics, allowing free cholesterol to orient itself on the outer monolayer of cell membranes and lipoproteins. It regulates membrane fluidity, organizes lipid rafts, and serves as the universal biochemical precursor for bile acids (cholic acid, chenodeoxycholic acid), steroid hormones (cortisol, aldosterone, testosterone, estradiol, progesterone), and vitamin D3 (7-dehydrocholesterol).
• Cholesteryl Esters: Comprise approximately 70% of total circulating plasma cholesterol. Formed when the polar C-3 hydroxyl group is esterified with a long-chain fatty acid (most commonly linoleic or oleic acid). This esterification completely eliminates the polar headgroup, converting cholesterol into a totally hydrophobic, nonpolar molecule that cannot reside in lipid bilayers. Instead, cholesteryl esters are sequestered exclusively within the interior core of circulating lipoproteins.

Enzymatic Esterification Mechanisms: LCAT vs. ACAT

Cholesterol esterification is regulated by two distinct enzymes operating in separate physiological compartments:

4. Phospholipids

Phospholipids are amphipathic molecules consisting of a glycerol backbone esterified to two hydrophobic fatty acid chains at C-1 and C-2, and a hydrophilic phosphoric acid ester at C-3 linked to a nitrogenous or polyol base:

• Glycerophospholipids: Major classes include phosphatidylcholine (lecithin), phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol. The polar headgroup faces the aqueous external environment, while the two fatty acid tails face inward, establishing the universal lipid bilayer of biological membranes and the outer shell of lipoproteins.
• Sphingomyelins: Contain a 18-carbon amino alcohol backbone (sphingosine) rather than glycerol. A fatty acid is linked via an amide bond to the amino nitrogen (forming a ceramide), while phosphorylcholine serves as the polar headgroup. Sphingomyelin is abundant in the myelin sheath of nerves and red blood cell membranes.
• Clinical Application (Amniotic Fluid L/S Ratio): The ratio of lecithin to sphingomyelin (L/S ratio) in amniotic fluid, measured via thin-layer chromatography (TLC), assesses fetal pulmonary surfactant maturity. Before 34 weeks of gestation, amniotic lecithin and sphingomyelin concentrations are roughly equal. Beyond 35 weeks, mature type II alveolar pneumocytes accelerate dipalmitoylphosphatidylcholine (lecithin) synthesis. An L/S ratio $\ge$ 2.0 indicates mature fetal lungs and minimal risk of Neonatal Respiratory Distress Syndrome (NRDS), whereas an L/S ratio < 1.5 indicates high risk of severe alveolar collapse.

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Lipoprotein Architecture and General Physiology

Because nonpolar lipids (triglycerides and cholesteryl esters) cannot circulate freely in aqueous blood, they are packaged into macromolecular pseudo-micellar complexes termed lipoproteins. Every lipoprotein particle shares a universal spherical architecture:

                           [ A P O L I P O P R O T E I N ]
                                     │
                       ┌─────────────┴─────────────┐
                   ▲   │  Unesterified Cholesterol │   ▲
                   │   │      & Phospholipids      │   │   Hydrophilic Surface Monolayer
                   │   └───────────────────────────┘   │   (Thickness ~2.0 nm)
         Aqueous   │                                   │   Direct contact with plasma
         Plasma ───┼───────────────────────────────────┼───
                   │   ┌───────────────────────────┐   │
                   │   │     Triglycerides &       │   │   Hydrophobic Core
                   ▼   │    Cholesteryl Esters     │   ▼   (Completely nonpolar)
                       └───────────────────────────┘

Functions of Apolipoproteins

The protein components of lipoproteins—termed apolipoproteins (or apoproteins)—are amphipathic $\alpha$-helical proteins embedded in the outer monolayer. They fulfill three essential functions:

1. Structural Scaffolding: Solubilize hydrophobic lipids in plasma and maintain the physical geometry of the particle (e.g., Apo B-48, Apo B-100, Apo A-I).
2. Enzyme Activation and Inhibition: Serve as obligate cofactors or inhibitors for key metabolic enzymes in lipid processing:
   • Apo C-II: Activates Lipoprotein Lipase (LPL) on capillary endothelial surfaces.
   • Apo C-III: Inhibits LPL, slowing triglyceride clearance.
   • Apo A-I: Activates Lecithin-Cholesterol Acyltransferase (LCAT).
3. Receptor Ligands: Bind with high affinity to cell-surface endocytic receptors to govern tissue uptake and clearance:
   • Apo B-100: Primary ligand for the LDL Receptor (LDLR) on hepatocytes and peripheral cells.
   • Apo E: High-affinity ligand for the LDL Receptor (LDLR) and hepatic LDL Receptor-Related Protein (LRP) for remnant clearance.

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The Five Major Lipoprotein Classes

Lipoproteins are classified into five major operational classes based on their hydrated density determined by preparative ultracentrifugation. Density is inversely proportional to particle diameter and lipid content: larger particles contain more neutral lipids (lower density), whereas smaller particles possess a higher protein-to-lipid ratio (higher density).

+-----------------------------------------------------------------------------------------------------------------------------+
|                                 Comprehensive Characteristics of the Five Lipoprotein Classes                               |
+-----------------------------------------------------------------------------------------------------------------------------+
| Lipoprotein Class | Density (g/mL) | Diameter   | % Protein | % Triglyceride | % Cholesterol | Key Apolipoproteins          |
+-------------------+----------------+------------+-----------+----------------+---------------+------------------------------+
| Chylomicrons      | < 0.950        | 80-1000 nm | 1-2%      | 85-90% (exog)  | 3-7%          | Apo B-48, C-II, C-III, E, A-I|
| VLDL              | 0.950-1.006    | 30-80 nm   | 8-10%     | 50-65% (endog) | 15-20%        | Apo B-100, C-II, C-III, E    |
| IDL               | 1.006-1.019    | 25-35 nm   | 15-20%    | 25-30%         | 35-40%        | Apo B-100, Apo E             |
| LDL               | 1.019-1.063    | 18-28 nm   | 20-25%    | 5-8%           | ~50% (esters) | Apo B-100 (sole apoprotein)  |
| HDL               | 1.063-1.210    | 5-12 nm    | 45-55%    | 3-5%           | 15-20%        | Apo A-I, Apo A-II, Apo C, E  |
+-----------------------------------------------------------------------------------------------------------------------------+

1. Chylomicrons

• Origin & Assembly: Synthesized exclusively by the mucosal epithelial cells of the small intestine (enterocytes) following dietary fat absorption.
• Structural Apolipoprotein: Contains Apolipoprotein B-48 (Apo B-48). In humans, Apo B-48 is produced from the same pre-mRNA as hepatic Apo B-100 through an intestinal RNA-editing enzyme (APOBEC-1) that deaminates cytidine to uridine at codon 2153, converting a glutamine codon (CAA) into an in-frame stop codon (UAA). Apo B-48 represents exactly the amino-terminal 48% of the full-length Apo B-100 protein and lacks the carboxyl-terminal LDL receptor-binding domain.
• Metabolic Pathway: Nascent chylomicrons are secreted into intestinal lacteals, enter the systemic bloodstream via the thoracic duct, and acquire Apo C-II and Apo E from circulating HDL. In capillary beds of adipose tissue and cardiac/skeletal muscle, Apo C-II activates endothelial Lipoprotein Lipase (LPL). LPL hydrolyzes core triglycerides into free fatty acids (taken up for energy production or re-esterification into fat) and glycerol (transported to the liver).
• Remnant Clearance: As core triglycerides are depleted (>90% loss), the particle collapses into a Chylomicron Remnant, shedding Apo C back to HDL while retaining Apo B-48 and Apo E. Hepatic parenchymal cells rapidly clear remnants from circulation via Apo E binding to the LDL-Receptor-Related Protein (LRP) and hepatic LDLR. Under normal conditions, chylomicrons are cleared completely within 10 to 12 hours after a meal; their presence in a fasting serum specimen is abnormal.

2. Very Low-Density Lipoproteins (VLDL)

• Origin & Assembly: Synthesized and secreted by hepatocytes to package and export endogenous triglycerides synthesized via de novo hepatic lipogenesis or assembled from circulating free fatty acids.
• Apolipoproteins: Contains one single molecule of full-length Apolipoprotein B-100 (Apo B-100) as its structural backbone, alongside Apo C-I, Apo C-II, Apo C-III, and Apo E.
• Metabolic Cascade: Like chylomicrons, VLDL interacts with LPL on vascular endothelial surfaces in muscle and adipose tissue. Hydrolysis of core triglycerides progressively converts VLDL into Intermediate-Density Lipoproteins (IDL). During this conversion, surface phospholipids, Apo C-II, and Apo C-III are transferred to HDL.

3. Intermediate-Density Lipoproteins (IDL / VLDL Remnants)

• Characteristics: Transient, highly atherogenic particles formed during the catabolism of VLDL. IDL particles contain roughly equimolar quantities of triglycerides and cholesterol (~30% each) and retain Apo B-100 and Apo E.
• Metabolic Fate: Approximately 50% of circulating IDL particles are cleared directly by hepatocytes via Apo E binding to the LDL receptor. The remaining 50% undergo further lipolysis on hepatic sinusoidal endothelial surfaces by Hepatic Lipase (HL), which hydrolyzes remaining triglycerides and phospholipids, sheds Apo E to HDL, and converts the particle into LDL.

4. Low-Density Lipoproteins (LDL)

• Physiological Role: The terminal catabolic product of VLDL and IDL, LDL represents the primary circulating carrier of cholesterol in human plasma (~50% cholesterol by dry weight, predominantly cholesteryl esters). Its sole structural protein is a single molecule of Apo B-100.
• Receptor-Mediated Endocytosis: Approximately 70% to 80% of circulating LDL is cleared by the liver, with the remainder taken up by peripheral extrahepatic tissues (adrenals, gonads, vascular smooth muscle) through the cell-surface LDL Receptor (LDLR, Apo B/E receptor):
  1. Apo B-100 binds with high affinity to LDLR localized in specialized clathrin-coated pits.
  2. The LDL-receptor complex is internalized into an endosome via clathrin-dependent endocytosis.
  3. Endosomal ATP-driven proton pumps lower the luminal pH to <5.5, causing a conformational change that dissociates LDL from its receptor.
  4. The LDLR recycles back to the cell surface (a single receptor recycles ~100 times before degradation).
  5. The endosome fuses with a lysosome, where acid lipases hydrolyze cholesteryl esters into free cholesterol, and proteases degrade Apo B-100 into free amino acids.

          Extracellular Space                 Clathrin-Coated Pit       Endosome (pH < 5.5)
     [LDL Particle (Apo B-100)] ───► Binds to [ LDLR ] ───► Internalization ───► Dissociation
                                                                                   │
     ┌─────────────────────────────────────────────────────────────────────────────┘
     │
     ├───► [ LDLR Recycled ] ───► Returns to Cell Membrane (~100 cycles)
     │
     └───► [ Lysosome Fusion ] ───► Lysosomal Acid Lipase: Cholesteryl Esters ──► Free Cholesterol
                                   Proteases: Apo B-100 ──► Free Amino Acids
                                                                │
     ┌──────────────────────────────────────────────────────────┘
     ▼
  [ Cytosolic Free Cholesterol Pool ]
     ├── (1) Inhibits HMG-CoA Reductase (suppresses de novo synthesis)
     ├── (2) Down-regulates LDLR transcription (prevents cholesterol overload)
     └── (3) Activates ACAT (converts excess cholesterol to cholesteryl esters for storage)

• Intracellular Regulatory Feedback: The surge in intracellular free cholesterol coordinates three homeostatic responses: (1) allosteric inhibition and accelerated degradation of HMG-CoA Reductase (the rate-limiting enzyme of endogenous cholesterol synthesis); (2) down-regulation of LDLR gene transcription via the SREBP-2 pathway (protecting the cell against toxic cholesterol overload); and (3) allosteric activation of ACAT, promoting storage of excess cholesterol as nonpolar lipid droplets.
• Small, Dense LDL (sdLDL, Phenotype B): When hypertriglyceridemia is present, CETP enriches LDL with triglycerides in exchange for cholesteryl esters. Hepatic lipase subsequently hydrolyzes these core triglycerides, collapsing the particle into small, dense LDL (<25.5 nm diameter). Compared to large, buoyant LDL (Phenotype A), sdLDL particles exhibit lower binding affinity for LDLR, prolonged circulation half-life, increased penetration into the arterial subendothelial space, avid binding to intimal proteoglycans, and marked susceptibility to oxidation (oxLDL) by reactive oxygen species (ROS). Oxidized LDL binds to macrophage Scavenger Receptors (SR-A, CD36), which lack feedback down-regulation. Uncontrolled ingestion converts macrophages into lipid-laden foam cells, initiating atherosclerosis.

5. High-Density Lipoproteins (HDL)

• Origin & Architecture: Synthesized and secreted by both the liver and small intestine as nascent, discoidal, lipid-poor particles composed primarily of Apolipoprotein A-I (Apo A-I) and phospholipids ("pre-$\beta$-HDL"). HDL is the densest lipoprotein (1.063–1.210 g/mL) due to its high protein content (~50%).
• Reverse Cholesterol Transport (RCT): The primary cardioprotective function of HDL, facilitating the clearance of excess cholesterol from peripheral tissues and macrophages and transporting it to the liver for excretion:
  1. Efflux: Lipid-poor Apo A-I interacts with the cell-surface transporter ATP-Binding Cassette Transporter A1 (ABCA1) on peripheral macrophages, stimulating the active efflux of cellular free cholesterol and phospholipids to the nascent HDL particle.
  2. Esterification: The acquired free cholesterol is esterified by LCAT (which is activated by Apo A-I). The resulting hydrophobic cholesteryl esters migrate from the surface into the core, transforming the flat disk into spherical, mature HDL3, and subsequently into larger, less dense HDL2 via additional cholesterol uptake via ABCG1.
  3. Hepatic Delivery (Direct Pathway): HDL2 binds selectively to Scavenger Receptor Class B Type 1 (SR-B1) on the sinusoidal membrane of hepatocytes. SR-B1 selectively extracts core cholesteryl esters into the hepatocyte without internalizing or degrading the HDL particle, allowing the lipid-depleted HDL to return to circulation.
  4. Hepatic Delivery (Indirect Pathway via CETP): Cholesteryl Ester Transfer Protein (CETP) facilitates an equimolar neutral lipid exchange, transferring cholesteryl esters from HDL2 to Apo B-containing lipoproteins (VLDL and LDL) in exchange for triglycerides. The liver subsequently clears these Apo B particles via the standard LDLR pathway. In the liver, cholesterol is converted into primary bile acids or secreted directly into bile for fecal elimination.

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Electrophoretic Mobility of Lipoproteins

Lipoproteins are separated clinically by agarose gel electrophoresis (or cellulose acetate) in an alkaline barbital buffer at pH 8.6. Under these conditions, the apolipoproteins confer a net negative charge to each particle, driving migration from the cathode (negative terminal, application origin) toward the anode (positive terminal). Migration speed depends on net surface charge density and particle size.

Cathode (-) [Origin]                                                        Anode (+)
        │                                                                       │
        ▼                                                                       ▼
    ┌────────┐              ┌────────┐              ┌────────┐              ┌────────┐
    │ Origin │              │  Beta  │              │Pre-beta│              │ Alpha  │
    └────────┘              └────────┘              └────────┘              └────────┘
   Chylomicrons                LDL                    VLDL                     HDL
  (Stays at origin)      (Beta-globulins)      (Alpha-2 globulins)     (Alpha-1 globulins)
   Size: 80-1000 nm        Size: 18-28 nm         Size: 30-80 nm          Size: 5-12 nm
   Net Charge: ~0         Apo B-100 (mod -)       Apo C/E (high -)        Apo A-I (highest -)

Migration Sequence (Cathode to Anode)

1. Origin (Application Point): Chylomicrons remain stationed at the origin. Because of their massive physical diameter (80–1000 nm), chylomicrons cannot penetrate the agarose gel pore matrix. Furthermore, their low protein content (1–2%) confers virtually no net surface charge.
2. Beta ($\beta$) Zone: LDL migrates with the $\beta$-globulin fraction. Although LDL is smaller than VLDL, its single Apo B-100 molecule confers a relatively modest net negative charge density.
3. Pre-beta (Pre-$\beta$) Zone: VLDL migrates with the $\alpha_2$-globulin fraction. Crucial ASCP concept: Even though VLDL particles (30–80 nm) are physically larger than LDL particles (18–28 nm), VLDL migrates faster (further toward the anode) than LDL because of a significantly higher negative surface charge density contributed by Apo C and sialic-acid-rich surface apolipoproteins.
4. Alpha ($\alpha$) Zone: HDL migrates fastest and furthest toward the anode, aligning with the $\alpha_1$-globulin fraction. HDL exhibits the highest negative charge-to-mass ratio due to its dense concentration of Apo A-I (~50% protein) and its small particle size (5–12 nm).
5. Pathological Broad-Beta Band: In Familial Dysbetalipoproteinemia (Fredrickson Type III), accumulation of intermediate-density lipoproteins and $\beta$-VLDL creates a continuous, confluent band spanning across the $\beta$ and pre-$\beta$ regions ("broad-$\beta$" pattern).

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The Standing Plasma Test (Refrigerator Test)

The standing plasma test is an overnight visual inspection of serum or EDTA plasma. The specimen is placed in a clear glass tube and incubated undisturbed at 4°C for 16 to 24 hours.

Physical Principles

• Chylomicrons have a hydrated density of <0.95 g/mL, which is significantly less than the density of plasma water (~1.006 g/mL). In the absence of centrifugal force, chylomicrons float to the surface, coalescing into a buoyant, opaque, curd-like creamy layer.
• VLDL has a density between 0.95 and 1.006 g/mL. Because its density is close to plasma water and its diameter (30–80 nm) scatters visible light, VLDL remains evenly suspended in the aqueous matrix, producing diffuse opalescent or milky turbidity throughout the tube.
• LDL particles (18–28 nm) are too small to scatter visible light; thus, isolated LDL elevation leaves plasma completely clear, though it may impart a deep golden-orange hue due to lipophilic carotenoids.

+-------------------------------------------------------------------------------------------------------------+
|                             Standing Plasma Test (Overnight at 4°C) Interpretation                           |
+-------------------------------------------------------------------------------------------------------------+
| Visual Tube Appearance                  | Lipoprotein Elevated      | Fredrickson Type | Primary Lipid Elevation    |
+-----------------------------------------+---------------------------+------------------+----------------------------+
| Creamy floating top layer;              | Chylomicrons              | Type I           | Triglycerides (extreme,    |
| Clear infranatant                       |                           | (or non-fasting) | >1000-2000 mg/dL)          |
+-----------------------------------------+---------------------------+------------------+----------------------------+
| Completely clear throughout             | LDL                       | Type IIa         | Total Cholesterol          |
| (Often distinct orange/yellow tint)     |                           |                  | (Triglycerides normal)     |
+-----------------------------------------+---------------------------+------------------+----------------------------+
| Clear to slightly hazy;                 | LDL + VLDL                | Type IIb         | Total Cholesterol and      |
| No floating surface layer               |                           |                  | Triglycerides              |
+-----------------------------------------+---------------------------+------------------+----------------------------+
| Turbid / hazy throughout;               | IDL / Beta-VLDL           | Type III         | Cholesterol and            |
| Occasionally faint surface veil         |                           |                  | Triglycerides (~1:1 ratio) |
+-----------------------------------------+---------------------------+------------------+----------------------------+
| Uniformly turbid / milky throughout;    | VLDL                      | Type IV          | Triglycerides              |
| NO floating cream layer                 |                           |                  | (typically 200-800 mg/dL)  |
+-----------------------------------------+---------------------------+------------------+----------------------------+
| Creamy floating top layer;              | Chylomicrons + VLDL       | Type V           | Triglycerides (extreme)    |
| Turbid / milky infranatant              |                           |                  | and Total Cholesterol      |
+-----------------------------------------+---------------------------+------------------+----------------------------+