Lipid Metabolism, Cholesterol & Lipoproteins LM

Dietary triacylglycerol is hydrophobic and must be emulsified and enzymatically digested before efficient absorption. Bile salts increase the effective interface between lipid and aqueous digestive contents, while pancreatic lipase-related activity releases absorbable products. Mixed micelles deliver lipid products to the intestinal surface.

Within enterocytes, absorbed long-chain fatty acids are re-esterified and packaged with other lipids and apoproteins into chylomicrons. Chylomicrons enter lymph and then blood. Lipoprotein lipase at capillary surfaces releases fatty acids from circulating triacylglycerol for uptake by muscle and adipose tissue.

Adipose tissue re-esterifies fatty acids for storage in the fed state. During fasting, hormone-sensitive lipolytic regulation releases fatty acids and glycerol. Albumin carries non-esterified fatty acids in plasma.

Long-chain fatty acyl-CoA cannot freely cross the inner mitochondrial membrane. The carnitine shuttle transfers the acyl group through coordinated carnitine acyltransferase and translocase steps. This transport is regulated so that synthesis and oxidation of fatty acids do not run maximally at the same time.

Inside mitochondria, beta-oxidation repeatedly performs oxidation, hydration, a second oxidation and thiolytic cleavage. Each cycle shortens the fatty acyl chain by two carbons and produces acetyl-CoA, NADH and FADH2. The reduced cofactors support oxidative phosphorylation, while acetyl-CoA enters TCA metabolism or, in liver under appropriate conditions, ketogenesis.

Odd-chain and unsaturated fatty acids require additional biochemical handling, illustrating that the basic spiral is a framework rather than the entire story.

When hepatic fatty-acid oxidation generates acetyl-CoA faster than it can be handled through ordinary TCA flux—especially during prolonged fasting or severe insulin deficiency—liver mitochondria convert acetyl units into ketone bodies. Acetoacetate and beta-hydroxybutyrate can be exported and oxidized by extrahepatic tissues.

The liver produces ketones but does not use them as a major fuel because it lacks the necessary transferase step for ketone utilization. During prolonged fasting, increased brain ketone use reduces but does not abolish glucose requirement.

Physiological ketosis and pathological ketoacidosis differ in degree and regulatory context. In diabetic ketoacidosis, severe insulin deficiency with counter-regulatory drive promotes lipolysis and ketogenesis to a level that contributes to high-anion-gap metabolic acidosis and major fluid/electrolyte disturbance.

Fatty-acid synthesis occurs mainly in the cytosolic compartment when energy and carbon are abundant. Mitochondrial acetyl-CoA carbon is exported indirectly through citrate. Acetyl-CoA carboxylase produces malonyl-CoA, the committed regulatory step, and fatty-acid synthase uses repeated condensation and reduction reactions to build the chain.

NADPH supplies reducing power. Malonyl-CoA also helps coordinate metabolism by inhibiting mitochondrial entry of long-chain fatty acids through CPT-I, reducing simultaneous synthesis and oxidation.

Triacylglycerol synthesis esterifies fatty acids to a glycerol backbone. Liver can export triacylglycerol in VLDL, whereas adipose tissue specializes in storage. Insulin favours the fed-state conditions that support these anabolic processes.

Cholesterol is required for cell membranes and serves as precursor for steroid hormones, bile acids and vitamin D-related metabolism. It is synthesized from acetyl-CoA through a multistep pathway whose major regulated step involves HMG-CoA reductase.

Cellular cholesterol homeostasis integrates synthesis, uptake through lipoprotein receptors, esterification/storage and export. When cellular cholesterol is adequate, regulatory mechanisms can reduce synthesis and receptor-mediated acquisition.

This regulation explains why cholesterol metabolism cannot be understood from dietary intake alone. Endogenous synthesis, receptor function, lipoprotein trafficking and clearance all influence circulating concentrations.

Chylomicrons transport exogenous dietary triacylglycerol from intestine. After lipoprotein-lipase action removes much of their triacylglycerol, remnants are taken up by liver. VLDL is secreted by liver to transport endogenous triacylglycerol and is progressively remodeled toward IDL and LDL-like particles.

LDL carries substantial cholesterol and delivers it to cells through receptor-mediated uptake. Excess atherogenic particle burden can promote cholesterol deposition in arterial walls. HDL participates in exchange and reverse-transport-related processes, but its biology is more complex than simply labelling it 'good cholesterol.'

Apolipoproteins serve structural, receptor-binding and enzyme-regulatory roles. Lipoprotein disorders can therefore arise from altered production, apoproteins, receptors, enzymes or clearance.

Lipids provide concentrated energy, membrane components and precursors for signalling molecules and steroid hormones. Because triacylglycerol is hydrophobic, it can be stored with little associated water. During fasting or increased energy demand, adipose lipolysis releases fatty acids that can be transported to tissues for oxidation.

Fatty acids enter mitochondria for beta-oxidation through regulated transport steps, especially for long-chain fatty acids. Repeated beta-oxidation cycles produce acetyl-CoA, NADH and FADH2. In liver during prolonged fasting or marked insulin deficiency, abundant acetyl-CoA can support ketone-body production; extrahepatic tissues can use ketone bodies as fuel under appropriate conditions.

Fatty-acid synthesis occurs in a different cellular and metabolic context from beta-oxidation and uses reducing power, particularly NADPH. Cholesterol is required for membranes and is a precursor for steroid hormones and bile acids, but its synthesis and transport must be regulated.

Because lipids are poorly soluble in plasma, lipoproteins transport triacylglycerol and cholesterol. Chylomicrons primarily carry dietary lipid from the intestine; VLDL carries endogenous triacylglycerol from liver; LDL is a major cholesterol-delivery particle; HDL participates in complex cholesterol transport and exchange processes.

Mechanism map: stored triacylglycerol → fatty-acid release → transport → mitochondrial entry → beta-oxidation → acetyl-CoA → TCA/ketogenesis depending on hepatic and hormonal state. For lipoproteins, identify origin, principal cargo, tissue interaction and remnant fate.

Clinical integration includes dyslipidaemia, atherosclerotic risk, fatty liver, ketosis and inherited defects of lipid transport or oxidation.

Common confusion: dietary fat, body-fat storage, plasma lipoprotein cholesterol and cellular cholesterol are connected but not interchangeable concepts. Ketone bodies are an adaptive fuel in fasting but can accumulate pathologically when production greatly exceeds utilization.