Lipid Metabolism Notes (Exogenous & Endogenous Pathways)

Lipid Metabolism: Exogenous and Endogenous Pathways

  • Overview

    • Lipids serve roles in energy metabolism and structural/functional biology. The largest lipid reservoir in mammals is triacylglycerols (TAG).

    • Typical body fat content ranges from 5%to25%5\% to 25\% of body weight, with about 90%90\% of this fat stored as TAG in adipose tissue.

    • TAGs come from two primary sources:

    • Diet: digestion, absorption, and transport to adipose tissue.

    • Mobilization of fat stored in adipocytes (adipose lipolysis).

  • Exogenous pathway (dietary fats)

    • Input: Dietary fat and cholesterol enter via the intestine and reach circulation as chylomicrons.

    • Emulsification and digestion facilitation

    • Bile salts are essential for normal digestion/absorption of lipids. They are salts of bile acids (e.g., cholic and chenodeoxycholic acid) synthesized from cholesterol in hepatocytes.

    • Bile acids have 24 carbon atoms with 2–3 hydroxyl groups; the carboxyl group is ionized at pH ~7.0.

    • Primary bile acids conjugate via amide bonds to glycine or taurine to form glycocholic or taurocholic acid, which are secreted into bile.

    • Detergent action and micelle formation

    • A bile salt molecule has a hydrophobic surface and a hydrophilic surface, enabling it to emulsify TAGs at the oil–water interface to form micelles (~1 μm in diameter).

    • Micelles increase the surface area and allow pancreatic enzymes to access lipids and aid absorption.

    • Digestion by pancreatic lipase

    • Pancreatic lipase hydrolyzes TAG at the 1 and 3 positions to yield 2-monoacylglycerol (2-MAG) and long-chain fatty acids (FAs).

    • Mixed micelles and absorption

    • Digestion products (2-MAG, long-chain FAs, lysophosphoglycerols) form mixed micelles with bile salts, enabling transport through the aqueous lumen to the enterocyte surface.

    • The micelle then dissociates at the brush border, delivering high local concentrations of digestion products for absorption.

    • Clinical note on bile salts

    • Low luminal bile salts lead to diminished fat absorption and steatorrhea.

    • Causes include biliary obstruction (blocked bile duct) or liver disease reducing bile production.

    • Fat malabsorption can also reduce absorption of fat-soluble vitamins A, D, E, and K; these may require injectable supplementation.

    • Absorption and chylomicron formation

    • Inside absorptive cells, fatty acids are re-esterified to TAGs.

    • TAGs, phospholipids, cholesterol, and apolipoprotein assemble into spherical chylomicrons (>75 nm75\,\text{nm} diameter).

    • Chylomicrons contain roughly: 85% TAG, 8% phospholipid, 2% cholesterol, 3% cholesterol ester, 2% protein85\% \text{ TAG},\ 8\% \text{ phospholipid},\ 2\% \text{ cholesterol},\ 3\% \text{ cholesterol ester},\ 2\% \text{ protein}.

    • They are released into the bloodstream and are transported to peripheral tissues and liver.

    • Lipolysis and fate of chylomicrons

    • At capillary beds (e.g., in muscle and adipose), lipoprotein lipase (LPL) hydrolyzes TAG in chylomicrons to glycerol and free fatty acids.

    • Glycerol and fatty acids released can be used for energy (β-oxidation) or re-esterified to TAG for storage in adipose tissue.

    • Transport overview

    • Chylomicrons primarily carry dietary TAG. They deliver lipids to peripheral tissues and to the liver via remnants that are cleared from the circulation.

    • Remnant chylomicrons and cholesterol can return to the liver (and portions may be used for glucose synthesis).

  • Endogenous pathway (lipid transport from liver)

    • Liver biosynthesis

    • The liver synthesizes fats and cholesterol (de novo biosynthesis) that are packaged into very-low-density lipoproteins (VLDL).

    • VLDL and peripheral delivery

    • VLDL transports triglycerides from the liver to peripheral tissues.

    • Lipoprotein lipase in capillaries hydrolyzes VLDL-TAG to glycerol and free fatty acids, which are taken up by tissues and either oxidized for energy or re-esterified in adipose tissue.

    • HDL, LDL receptor, and cholesterol trafficking

    • HDL participates in cholesterol transport (e.g., reverse cholesterol transport), with LCAT contributing to the maturation of HDL by converting free cholesterol to cholesterol esters.

    • LDL receptor mediates uptake of cholesterol from circulating lipoproteins into tissues.

    • Apolipoprotein B-100 (Apo B-100) is a key structural protein of VLDL/LDL particles.

    • Interconnections with exogenous pathway

    • Remnants of chylomicrons and cholesterol return to the liver for processing, storage, or conversion to glucose via metabolic pathways.

    • In peripheral tissues, cholesterol can be delivered for steroid hormone synthesis in endocrine glands.

    • Peripherally, fatty acids released from triglycerides enter cells and are transported in the blood bound to albumin, and are oxidized via β-oxidation or re-esterified for storage.

  • Chylomicrons: Synthesis and transport (summary)

    • Formed in intestinal absorptive cells with dietary lipid content

    • Diameter: > 75 nm75\,\text{nm}; rich in TAG (~85%85\%)

    • Transport via lymphatic system, then bloodstream to tissues and liver

    • Lipolysis by LPL releases glycerol and fatty acids for energy or storage

  • Adipose tissue and mobilization of stored fats

    • Hormonal control of fat mobilization from adipocytes

    • Postprandial state (high glucose, high insulin): glycolysis, glycogenesis, and fatty acid synthesis are stimulated; fatty acid oxidation (β-oxidation) and gluconeogenesis are inhibited. Lipolysis is reduced, so fat mobilization is limited.

    • Stress/starvation/diabetes/trauma (low glucose, low insulin; high glucagon and adrenaline): lipolysis is stimulated, providing fatty acids as an energy source; gluconeogenesis and fatty acid oxidation are promoted, while glycolysis and glycogenesis are inhibited.

    • Mechanism of mobilisation

    • The cyclic AMP (cAMP) cascade mediates adipose lipolysis (Figure 4 reference in the source): increased cAMP activates hormone-sensitive lipase, promoting TAG breakdown to glycerol and fatty acids.

    • Practical implications

    • Fat mobilization supplies energy during fasting or stress when glucose is scarce.

  • Key terms and components (glossary-style)

    • TAG: Triacylglycerol, main dietary fat; stored in adipose tissue; hydrolyzed by LPL to release glycerol and FFAs.

    • Bile salts: Emulsify fats; form mixed micelles aiding digestion and absorption; include conjugated bile acids such as glycocholic and taurocholic acids.

    • Micelles: Mixed bile salt micelles (~1 μm) transport digestion products to enterocytes.

    • Chylomicrons: Intestinal lipoproteins carrying dietary TAG; cleared by liver and adipose tissue after delivering lipids.

    • VLDL: Liver-derived lipoprotein carrying endogenous TAG to tissues.

    • HDL/LDL: Lipoproteins involved in cholesterol transport and receptor-mediated uptake; LCAT and ApoB-100 are key proteins in these pathways.

    • LPL: Lipoprotein lipase; hydrolyzes TAG in chylomicrons and VLDL in capillaries.

    • LCAT: Lecithin–cholesterol acyltransferase; esters cholesterol on HDL, promoting reverse cholesterol transport.

    • Albumin: Carrier protein for fatty acids in plasma.

  • Connections to physiology and real-world relevance

    • Dietary fat digestion and absorption are essential for energy and fat-soluble vitamin uptake; malabsorption has clinical consequences (steatorrhea, vitamin A, D, E, K deficiencies).

    • Hormonal regulation of fat metabolism links with energy balance, feeding, stress responses, and metabolic diseases such as diabetes and obesity.

    • The liver coordinates endogenous and exogenous lipid pools via VLDL, chylomicron remnants, HDL, and LDL pathways, affecting systemic lipid homeostasis.

  • Formulas and numerical references (LaTeX)

    • Dietary fat intake range: 60−100 g/day60-100\ \text{g/day}

    • Body lipid content range: 5%−25%5\%-25\% of body weight

    • Proportion of dietary fat as TAG: >90\%

    • Chylomicron composition (approximate): TAG=85%, phospholipid=8%, cholesterol=2%, cholesterol ester=3%, protein=2%\text{TAG}=85\%,\ \text{phospholipid}=8\%,\ \text{cholesterol}=2\%,\ \text{cholesterol ester}=3\%,\ \text{protein}=2\%

    • Chylomicron diameter: >75\ \text{nm}

    • Mixed micelle diameter: ≈1 μm\approx 1\ \mu\text{m}

    • Bile acid structure: C24 skeleton with 2−3 OH groups\text{C}_{24}\text{ skeleton with }2-3\ \text{OH groups}; conjugation forms glycocholic\text{glycocholic} or taurocholic\text{taurocholic} acids

    • Emulsification mechanism: micelles enable pancreatic lipase access to TAGs, yielding 2-monoacylglycerol and fatty acids

    • Transport and processing: glycerol and free fatty acids after lipolysis can be oxidized via β-oxidation or re-esterified for storage in adipose tissue

  • Summary takeaways

    • Lipid digestion relies on emulsification (bile salts), enzymatic hydrolysis (pancreatic lipase), and micellar transport to enterocytes.

    • Absorbed lipids are reassembled into TAGs and packaged into chylomicrons for lymphatic and bloodstream transport to tissues and liver.

    • The liver produces VLDL for endogenous lipid transport; lipolysis by LPL in capillaries delivers fatty acids to tissues; HDL participates in cholesterol transport and homeostasis via LCAT and receptor pathways.

    • Hormonal status governs fat mobilization from adipose tissue, balancing energy needs with glucose availability, and shaping overall metabolism.