What are the components of bile?

Components of Bile

Bile is primarily composed of bile salts, phospholipids (mainly lecithin), cholesterol, proteins, and bilirubin, with bile salts being the predominant component ranging from 3-20 mM concentration depending on fed or fasted state. 1

Primary Components of Bile

Bile Salts/Acids

• Bile salts are the end products of cholesterol metabolism and are the main functional components of bile 2
• Concentration ranges from 3 mM (fasted state) to 20 mM (fed state) in the small intestine 1
• Common bile salts include:
  • Sodium glycodeoxycholate
  • Sodium deoxycholate
  • Taurocholate
  • Taurodeoxycholate
  • Cholate
  • Glycocholate 1
• Bile acids are synthesized in the liver from cholesterol and conjugated with glycine or taurine before secretion 2
• Conjugation makes bile acids impermeable to cell membranes and allows high concentrations to persist in bile 3

Phospholipids

• Phospholipids constitute a significant portion of bile, with phosphatidylcholine (lecithin) being the predominant type (>96%) 4
• Phospholipids can provide a protective effect when combined with bile salts 1
• The presence of lecithin can reduce the cytotoxicity of bile salts like sodium deoxycholate 1

Cholesterol

• Unesterified cholesterol is a major component of bile 4
• Hypersecretion of hepatic cholesterol into bile is the primary pathophysiological defect in cholesterol gallstone disease 4
• The molar concentrations of cholesterol, bile salts, and lecithin determine the solubility of cholesterol in bile 2

Bilirubin

• Bile contains bilirubin conjugates, also known as lipopigments 4
• Bilirubin has been shown to inhibit calcium carbonate precipitation in gallbladder bile 5
• It can form complexes with albumin and interfere with calcium carbonate precipitation 5

Proteins

• Bile contains various proteins, with over 222 unique proteins identified in human gallbladder bile from a cholesterol stone patient 6
• Albumin is one of the proteins present in bile that can interact with bilirubin 5

Physiological Functions of Bile Components

• Bile salts are essential for:

  • Inducing biliary lipid secretion 3
  • Solubilizing cholesterol in bile, promoting its elimination 3
  • Solubilizing dietary lipids in the small intestine, promoting their absorption 3
  • Absorption of lipids and lipophilic components 1

• Phospholipids contribute to:

  • Forming mixed micelles with bile salts and cholesterol 4
  • Protecting against the cytotoxic effects of bile salts 1

Clinical Significance of Bile Components

• Bile salt toxicity is well documented, with different bile salts showing varying levels of cytotoxicity 1
• Disruptions in bile formation and flow can lead to cholestasis, jaundice, pruritus, and liver damage 7
• Genetic defects in bile transporters can lead to various cholestatic disorders, including Progressive Familial Intrahepatic Cholestasis (PFIC) 7
• Ursodeoxycholic acid (UDCA) is commonly used to treat cholestatic conditions by changing bile composition from hydrophobic to more hydrophilic 7
• Bile acid sequestrants like cholestyramine can be used to treat bile acid reflux by forming an insoluble complex with bile acids in the intestine 8

Bile Transport Mechanisms

• Bile acids undergo enterohepatic circulation, being absorbed from the small intestine and recycled multiple times before excretion 2
• The ileal bile acid transporter (IBAT) is responsible for bile acid reabsorption in the terminal ileum 1
• Disruption of the enterohepatic circulation can be therapeutic in certain conditions, such as using IBAT inhibitors (maralixibat, odevixibat) to treat cholestatic pruritus 1