Lipid Metabolism and Dyslipidemias

Lipoprotein Structure and Transport

• Lipoproteins consist of hydrophobic lipids (cholesterol esters, triglycerides) surrounded by hydrophilic phospholipids and apolipoprotein shells
• Apolipoprotein B-100 (ApoB-100) is the structural protein for VLDL, IDL, and LDL; each particle contains exactly one ApoB-100 molecule, making ApoB a marker of atherogenic particle number
• Apolipoprotein A-I (ApoA-I) is the major protein of HDL and mediates reverse cholesterol transport
• Apolipoprotein E (ApoE) mediates remnant uptake and has three isoforms (E2, E3, E4) with distinct metabolic properties; E4 is associated with higher LDL-C and Alzheimer's disease risk

LDL Metabolism and Atherogenesis

• Hepatic 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase catalyzes the rate-limiting step of cholesterol synthesis from acetyl-CoA
• Dietary cholesterol suppresses HMG-CoA reductase activity and reduces LDL receptor (LDLR) expression via sterol regulatory element-binding proteins (SREBPs)
• VLDL particles are secreted by the liver and converted to IDL through lipoprotein lipase (LPL) action in capillary beds; IDL is further catabolized to LDL
• LDL particles accumulate in the bloodstream when LDLR expression is inadequate or LDL production is excessive
• Oxidized LDL (oxLDL) is taken up by scavenger receptors on macrophages (not regulated by cholesterol content), leading to foam cell formation and atherosclerotic plaque development
• Each LDL-C reduction of 1 mmol/L (39 mg/dL) reduces major vascular events by approximately 22%

Triglyceride and VLDL Metabolism

• Triglycerides are synthesized in the liver from glycerol-3-phosphate and fatty acids via the glycerol-3-phosphate pathway
• Apolipoprotein C-II (ApoC-II) is a cofactor for lipoprotein lipase, which hydrolyzes triglycerides within VLDL and chylomicrons
• Elevated triglycerides reflect increased hepatic VLDL production (from excess carbohydrates, alcohol, or free fatty acids) or decreased clearance (due to reduced LPL activity)
• Remnant lipoproteins (VLDL remnants and chylomicron remnants) are direct atherogenic particles that deposit cholesterol in arterial walls

HDL Metabolism and Reverse Cholesterol Transport

• Apolipoprotein A-I is synthesized by the liver and intestines; nascent discoidal HDL particles acquire cholesterol from peripheral tissues and other lipoproteins
• Lecithin-cholesterol acyltransferase (LCAT) esterifies cholesterol on HDL surface, allowing additional cholesterol uptake
• Cholesteryl ester transfer protein (CETP) transfers cholesterol esters from HDL to ApoB-containing lipoproteins and triglycerides to HDL
• HDL returns cholesterol to the liver for excretion via scavenger receptor B1 (SR-B1) and LDLR-related proteins
• Low HDL-C (<40 mg/dL in men, <50 mg/dL in women) is an independent risk factor for ASCVD; HDL increases by only 2-3 mg/dL per 1% weight loss

Lipoprotein(a) and Additional Atherogenic Particles

• Lipoprotein(a) [Lp(a)] consists of an LDL-like particle with apolipoprotein(a) disulfide-bonded to ApoB-100; levels are primarily genetically determined and largely unresponsive to lifestyle modification
• Lp(a) promotes atherosclerosis through oxidative stress, inflammation, and impaired fibrinolysis (due to structural homology to plasminogen)
• Elevated small dense LDL particles are more atherogenic than larger, buoyant LDL particles due to increased arterial infiltration and resistance to LDLR-mediated clearance

Primary (Genetic) Dyslipidemias

• Familial Hypercholesterolemia (FH): Autosomal dominant mutations in LDLR (~85% of cases), ApoB-100 (~8% of cases), or PCSK9 (~7% of cases) genes; heterozygous FH affects 1 in 250-500 individuals, while homozygous FH affects 1 in 160,000-1,000,000 individuals; causes severe LDL-C elevation (heterozygous: 350-550 mg/dL; homozygous: >600 mg/dL) and premature ASCVD
• Familial Defective Apolipoprotein B-100: Mutations in ApoB-100 reduce LDLR binding, causing modest LDL-C elevation (200-350 mg/dL) with ASCVD risk intermediate between general population and FH
• Autosomal Recessive Hypercholesterolemia (ARH): Rare mutations in LDLRAP1 (LDL receptor adapter protein); presents similarly to homozygous FH with severe elevation in LDL-C and xanthomas
• Familial Hypertriglyceridemia: Autosomal dominant disorder affecting hepatic VLDL secretion; causes triglyceride elevation (200-1000 mg/dL) without markedly elevated LDL-C; increases pancreatitis and ASCVD risk
• Familial Combined Hyperlipidemia (FCH): Complex genetic disorder with variable phenotypes (elevated LDL-C, triglycerides, or both); affects 1 in 100-200 individuals; associated with insulin resistance and increased ASCVD risk
• Familial HDL Deficiency: Rare mutations in ApoA-I, LCAT, or ABCA1 genes; causes severe HDL-C reduction with variable ASCVD risk depending on genetic cause
• Type III Hyperlipoproteinemia (Dysbetalipoproteinemia): Results from ApoE2/E2 genotype with additional environmental triggers (obesity, diabetes, hypothyroidism, nephrotic syndrome); causes simultaneous elevation of cholesterol and triglycerides with eruptive xanthomas and palmar crease xanthomas

Secondary Dyslipidemias (Metabolic/Systemic Causes)

• Hypothyroidism: Decreases LDLR expression and LDL catabolism, raising LDL-C by 20-40 mg/dL; TSH >10 mIU/L warrants lipid reassessment after thyroid hormone replacement
• Nephrotic Syndrome: Increases hepatic lipoprotein synthesis due to albumin loss and oncotic pressure changes; LDL-C may rise 200-600 mg/dL with lipid wasting in urine
• Chronic Kidney Disease: Decreases LPL activity, impairs remnant clearance, and increases VLDL production; triglycerides often markedly elevated (200-1000+ mg/dL)
• Diabetes Mellitus: Hyperglycemia and insulin resistance decrease LPL activity and increase hepatic VLDL production; triglycerides elevated while LDL-C may be paradoxically normal or low (atherogenic dyslipidemia pattern)
• Obesity: Increases free fatty acid flux to liver, promoting VLDL production and hepatic steatosis; weight gain increases triglycerides and decreases HDL-C
• Alcoholism: Increases hepatic triglyceride synthesis and VLDL secretion; heavy alcohol consumption can raise triglycerides to 300-1000+ mg/dL
• Cholestasis: Impairs bile acid excretion and increases plasma lipoprotein X (Lp-X), a cholestasis-specific abnormal lipoprotein; markedly elevated cholesterol (500-1000+ mg/dL) with preserved HDL-C
• Medication-Induced: Thiazide diuretics and beta-blockers increase LDL-C and triglycerides; oral estrogens increase triglycerides (especially in those with baseline hypertriglyceridemia); antiretroviral agents cause mixed dyslipidemia; corticosteroids increase LDL-C and triglycerides
• Liver Disease: Cirrhosis reduces cholesterol synthesis and LDLR expression; total cholesterol often paradoxically low despite impaired lipid metabolism
• Systemic Illness: Acute myocardial infarction, sepsis, and major trauma acutely decrease LDL-C and HDL-C for 4-12 weeks due to systemic inflammation

Lifestyle and Dietary Risk Factors

• Saturated Fat Intake: Each 1% increase in calories from saturated fat raises LDL-C by ~2 mg/dL; trans fats are even more atherogenic
• Refined Carbohydrates and Simple Sugars: Increase hepatic VLDL production and triglycerides while decreasing HDL-C
• Sedentary Lifestyle: Lack of physical activity decreases LPL activity, lowers HDL-C, and increases triglycerides and body weight
• Smoking: Lowers HDL-C by 10-15% and increases LDL oxidation and foam cell formation
• Excessive Alcohol: Increases triglycerides through increased hepatic VLDL synthesis

Asymptomatic Presentations

• Most dyslipidemias detected on routine lipid panel obtained during primary prevention screening or as part of cardiovascular risk assessment
• Absence of symptoms does not indicate absence of atherosclerotic disease; silent myocardial ischemia and stroke are common presentations

Symptomatic Presentations and Physical Exam Findings

• Tendon Xanthomas: Firm nodular thickening of tendons (especially Achilles tendon and extensor tendons of hands) due to lipid deposition; highly specific for familial hypercholesterolemia; often palpable as a firm nodule 1-2 cm proximal to the Achilles insertion; may be accompanied by mild tenderness
• Corneal Arcus: Grayish-white ring at the corneal periphery representing lipid infiltration; appears before age 50 in patients with markedly elevated cholesterol; nonpathologic arcus senilis occurs in older individuals
• Xanthelasmas: Yellow plaques on the eyelids medial to the inner canthus; less specific than tendon xanthomas but seen with significant cholesterol elevation (LDL-C >300 mg/dL); may be the first clue to FH
• Eruptive Xanthomas: Yellow papules with erythematous halos appearing on the extensor surfaces of elbows, knees, and buttocks; pathognomonic for severe hypertriglyceridemia (typically >1000 mg/dL); can appear acutely during periods of poor glycemic control or alcohol excess
• Palmar Crease Xanthomas: Striped yellowish discoloration of palmar creases; highly specific for Type III hyperlipoproteinemia (dysbetalipoproteinemia)
• Tuberose Xanthomas: Nodular xanthomas on elbows, knees, and Achilles tendons appearing as clusters; also specific for Type III hyperlipoproteinemia and very severe hypercholesterolemia
• Lipemia Retinalis: White or cream-colored appearance of retinal vessels due to circulating triglyceride-rich particles; seen with triglycerides >1000-1500 mg/dL; reversible with triglyceride reduction

Acute Presentations Related to Severe Dyslipidemias

• Acute Coronary Syndrome: Myocardial infarction (STEMI or NSTEMI) may be the first manifestation of long-standing dyslipidemias; high-risk patients present with chest pain, dyspnea, or diaphoresis
• Acute Pancreatitis: Triggered by severe hypertriglyceridemia (typically >1000 mg/dL, occasionally as low as 500 mg/dL); presents with acute epigastric pain, nausea, vomiting, and elevated pancreatic enzymes; life-threatening complication requiring aggressive triglyceride lowering
• Stroke and Transient Ischemic Attack: Elevated LDL-C promotes carotid atherosclerosis and plaque rupture; patients present with focal neurologic deficits or transient neurologic symptoms

Lipid Panel Components and Interpretation

• Total Cholesterol (TC): Sum of LDL-C, HDL-C, and triglycerides/5; desirable <200 mg/dL; levels 200-239 mg/dL considered borderline high; ≥240 mg/dL indicates high cholesterol; however, TC is a crude measure and LDL-C is superior for ASCVD risk assessment
• Low-Density Lipoprotein Cholesterol (LDL-C): The primary target of lipid-lowering therapy; calculated using Friedewald equation [LDL-C = TC - HDL-C - (TG/5)] when triglycerides <400 mg/dL; direct measurement preferred when triglycerides >400 mg/dL; optimal <100 mg/dL, <70 mg/dL in ASCVD patients, <55 mg/dL in very high-risk patients; each 39 mg/dL reduction decreases major vascular events by ~22%
• High-Density Lipoprotein Cholesterol (HDL-C): "Protective" cholesterol; desirable ≥40 mg/dL in men and ≥50 mg/dL in women; each 1 mg/dL increase in HDL-C reduces ASCVD risk by ~2%; low HDL-C is independent risk factor for ASCVD
• Triglycerides: Fasting measurement standard; desirable <150 mg/dL; 150-199 mg/dL borderline high; 200-499 mg/dL high; ≥500 mg/dL very high with markedly increased pancreatitis risk; non-fasting triglyceride measurements increasingly used as they better reflect post-prandial lipemia and may better predict ASCVD risk
• Non-High-Density Lipoprotein Cholesterol (non-HDL-C): Calculated as TC - HDL-C; includes all atherogenic particles (LDL, VLDL, remnants, Lp(a)); goals are 30 mg/dL lower than LDL-C goals; especially useful in patients with elevated

Immediate stabilization (triglyceride-induced pancreatitis)

• Supportive care first: NPO, aggressive isotonic IV fluids, and analgesia; removing the substrate (oral fat) plus treating the precipitant (alcohol, uncontrolled diabetes) allows lipoprotein lipase to clear chylomicrons.
• Insulin infusion (with dextrose to prevent hypoglycemia) activates LPL and lowers triglycerides rapidly in the hyperglycemic patient; plasmapheresis/apheresis is reserved for refractory or critically ill cases. Long-term prevention uses a fibrate (fenofibrate) plus fat restriction and alcohol abstinence.

First-line ASCVD risk reduction — 2018 AHA/ACC Multisociety Cholesterol Guideline

• Lifestyle therapy in everyone: saturated/trans fat restriction, weight loss, aerobic exercise, tobacco cessation; treat secondary causes (hypothyroidism, nephrotic syndrome, poorly controlled diabetes) before labeling a lipid disorder primary.
• HMG-CoA reductase inhibitors (statins) are first-line for all four benefit groups. Blocking the rate-limiting step of hepatic cholesterol synthesis upregulates LDL receptors and increases LDL clearance.
• Clinical ASCVD → high-intensity statin (atorvastatin 40–80 mg or rosuvastatin 20–40 mg) targeting ≥50% LDL-C reduction.
• LDL-C ≥190 mg/dL (e.g., familial hypercholesterolemia) → high-intensity statin regardless of calculated risk.
• Diabetes, age 40–75 → at least moderate-intensity statin, consistent with the ADA Standards of Care.
• Primary prevention, age 40–75, LDL-C 70–189 → Pooled Cohort Equations, risk-enhancer review, and a clinician–patient discussion; coronary artery calcium scoring may reclassify intermediate-risk patients.

Escalation

• Ezetimibe (NPC1L1 inhibitor blocking intestinal sterol absorption) is added first in very high-risk ASCVD not at threshold on maximally tolerated statin.
• PCSK9 monoclonal antibodies (evolocumab, alirocumab) prevent LDL-receptor degradation; inclisiran (siRNA) and bempedoic acid (ATP-citrate lyase inhibitor) are alternatives.
• Icosapent ethyl is added for persistent hypertriglyceridemia in statin-treated high-risk patients.

Definitive/contraindicated

• Homozygous FH: LDL apheresis, lomitapide, evinacumab.
• Statins are avoided in pregnancy and lactation; bile acid sequestrants (which raise triglycerides) are the traditional option. Avoid gemfibrozil with a statin (impaired statin glucuronidation → myopathy).

Complications of the disease

• Atherosclerotic cardiovascular disease: retained ApoB particles are oxidized, taken up by macrophage scavenger receptors, and form foam cells; signaled by angina, acute coronary syndrome, ischemic stroke, or claudication with diminished pulses. Acute coronary syndrome and stroke are emergencies.
• Acute pancreatitis from severe hypertriglyceridemia: chylomicron hydrolysis liberates free fatty acids that are directly toxic to acinar cells and cause capillary sludging; signaled by epigastric pain radiating to the back with lipemic (milky) serum and eruptive xanthomas. Emergency. Note that serum amylase may be spuriously normal, and pseudohyponatremia is a classic accompanying lab artifact.
• Aortic valve disease: lipid deposition and calcific remodeling, strongly linked to elevated Lp(a); homozygous FH may produce supravalvular aortic stenosis and childhood myocardial infarction.
• Hepatic steatosis and metabolic dysfunction from excess free fatty acid flux; signaled by transaminase elevation with a hyperechoic liver.

Complications of therapy

• Statin-associated muscle symptoms: the most common adverse effect; symmetric proximal myalgia with normal or mildly elevated CK. Risk rises with CYP3A4 inhibitors (macrolides, azoles, protease inhibitors), fibrates, and hypothyroidism.
• Rhabdomyolysis: myoglobin-induced tubular injury; signaled by severe weakness, tea-colored urine, markedly elevated CK, and urine dipstick positive for blood with no red cells. Emergency — stop the statin and give IV fluids.
• Statin hepatotoxicity and new-onset diabetes: transaminase elevation is usually mild and transient; the small dysglycemia signal does not outweigh ASCVD benefit per the 2018 AHA/ACC Multisociety guideline.
• Fibrates: myopathy (worse with gemfibrozil), cholesterol gallstones from increased biliary cholesterol saturation, and a reversible creatinine rise.
• Niacin: prostaglandin-mediated flushing (blunted by aspirin), hyperglycemia, hyperuricemia/gout, hepatotoxicity.
• Bile acid sequestrants: constipation, triglyceride elevation, and malabsorption of fat-soluble vitamins and co-administered drugs.
• PCSK9 inhibitors: injection-site reactions.

• Tendon xanthoma (Achilles, extensor tendons of the hand) is the near-specific physical finding of familial hypercholesterolemia; combined with LDL-C ≥190 mg/dL and a family history of premature coronary disease, the single best next step is a high-intensity statin plus cascade screening of first-degree relatives — not repeat lifestyle counseling alone.
• Palmar crease xanthomas (xanthoma striatum palmare) with roughly equal elevation of cholesterol and triglycerides = type III dysbetalipoproteinemia, the ApoE2/E2 genotype unmasked by diabetes, obesity, or hypothyroidism. This is the single most reliably tested lipid–genotype association.
• Eruptive xanthomas plus lipemia retinalis plus milky serum = triglycerides in the thousands. The exam wants you to fear pancreatitis, not myocardial infarction. Lipoprotein lipase or ApoC-II deficiency (type I) presents in childhood with recurrent pancreatitis and, characteristically, no premature atherosclerosis.
• Friedewald fails when triglycerides exceed 400 mg/dL — order a direct LDL-C or use non-HDL-C. Non-HDL-C captures every ApoB-containing particle and is the better target in hypertriglyceridemia and diabetes.
• Before labeling a dyslipidemia primary, check TSH (and consider urine protein and A1c). Treating hypothyroidism can normalize LDL-C and also removes a major statin-myopathy risk factor.
• Common distractor — raising HDL is not a treatment target. Niacin and CETP inhibitors raise HDL-C without reliable outcome benefit; the guideline-endorsed lever is lowering ApoB/LDL-C. Similarly, do not order routine surveillance LFTs or CK in an asymptomatic statin user — check CK only if muscle symptoms occur.
• Statins are stopped in pregnancy; a woman of childbearing age started on a statin needs contraception counseling. Bile acid sequestrants are the classic pregnancy-compatible choice.
• Recheck a lipid panel 4–12 weeks after starting or changing therapy to confirm adherence and percent LDL-C reduction — this is the 2018 AHA/ACC Multisociety guideline's monitoring answer.