Bile Salts and Blood Cholesterol: The Core Connection

1. Bile Acids Are Made From Cholesterol

• Classical (neutral) pathway — initiated by cholesterol 7α-hydroxylase (CYP7A1), the rate-limiting enzyme; favours cholic acid (CA)
• Alternative (acidic) pathway — initiated by sterol 27-hydroxylase (CYP27A1); favours chenodeoxycholic acid (CDCA)

"Total body cholesterol metabolism is managed predominantly by the liver; the two principal pathways by which cholesterol is eliminated are cholesterol excretion and bile acid synthesis from cholesterol." — Yamada's Textbook of Gastroenterology

2. The Hepatocyte Is the Central Hub

3. The Enterohepatic Circulation: A Feedback Loop That Regulates Blood Cholesterol

• >95% of bile salts are reabsorbed in the terminal ileum via the apical sodium-dependent bile acid transporter (ASBT) and returned to the liver via portal blood.
• The liver extracts them efficiently → newly synthesised bile acids contribute <5% to biliary secretion under normal conditions.
• When the enterohepatic circulation is interrupted (e.g., by disease, ileal resection, or bile acid–binding resins like cholestyramine), fecal bile acid loss rises. This forces the liver to upregulate CYP7A1 and consume more cholesterol to replenish the pool → blood LDL cholesterol falls.

4. How This is Exploited Therapeutically

1. Upregulating CYP7A1 → more cholesterol converted to bile acids
2. Upregulating LDL receptors (LDLR) → more LDL cleared from blood

5. Feedback Regulation of Bile Acid Synthesis (FXR–FGF19 Axis)

• Bile acids activate the nuclear receptor FXR (farnesoid X receptor) in enterocytes
• FXR stimulates release of fibroblast growth factor 19 (FGF19)
• FGF19 signals the liver to inhibit CYP7A1 → less bile acid synthesis from cholesterol

"Bile acids in the intestine stimulate the release of fibroblast growth factor 19 (FGF19), which suppresses the hepatic synthesis of bile acids from cholesterol by inhibiting the rate-limiting enzyme cytochrome P450 7A1." — Harrison's Principles of Internal Medicine 22E

6. Biliary Cholesterol Secretion and Gallstones

• When bile salt secretion is low or biliary cholesterol is high, cholesterol supersaturates and crystallises → cholesterol gallstones
• The relationship between bile salt secretion rate and cholesterol secretion is curvilinear: at low bile salt secretion rates, more cholesterol is secreted per bile salt molecule than at higher rates.

Summary Table

Names of Bile Salts

Primary Bile Salts

Conjugated Bile Salts

Structure of glycocholic acid and taurochenodeoxycholic acid — examples of conjugated bile salts — Lippincott's Illustrated Reviews: Biochemistry

Secondary Bile Salts

• Glycodeoxycholic acid, Taurodeoxycholic acid
• Glycolithocholic acid, Taurolithocholic acid

Note: Lithocholic acid is only poorly absorbed from the colon.

Tertiary Bile Salts

Ursodeoxycholic acid (UDCA) is used therapeutically to dissolve cholesterol gallstones and treat primary biliary cholangitis.

Summary Tree

Cholesterol
│
├── Cholic acid (primary)
│   ├── Glycocholic acid  ┐ conjugated
│   ├── Taurocholic acid  ┘
│   └── → Deoxycholic acid (secondary, by gut bacteria)
│       ├── Glycodeoxycholic acid  ┐ conjugated
│       └── Taurodeoxycholic acid  ┘
│
└── Chenodeoxycholic acid (primary)
    ├── Glycochenodeoxycholic acid  ┐ conjugated
    ├── Taurochenodeoxycholic acid  ┘
    └── → Lithocholic acid (secondary, by gut bacteria)
        └── → Ursodeoxycholic acid (tertiary / UDCA)

What Insoluble Fibre Does in the Body

What It Is

1. Accelerates Gut Transit (Laxative Effect)

• Insoluble fibre causes rapid gastric emptying and decreased intestinal transit time.
• Coarser insoluble fibre mechanically stimulates the colonic mucosa, triggering mucus secretion and increasing stool bulk, making defecation easier.
• This "softens" stool and reduces pressure on the colonic wall.

"Some larger coarse fiber has a mechanical stimulating effect on the colonic mucosa, triggering mucus secretion, resulting in increased stool bulk." — Yamada's Textbook of Gastroenterology

Caution: Small-particle insoluble fibre does NOT have this irritating/stimulating effect and may actually cause a constipating effect by adding dry stool mass.

2. Reduces Risk of Diverticular Disease

3. Reduces Risk of Colorectal Cancer

4. Cardiovascular Protection (Reduced CHD/CVD Risk)

"Soluble and insoluble fibers both appear effective in lowering the risk [of CHD and CVD]." — Yamada's Textbook of Gastroenterology

5. Reduced Risk of Type 2 Diabetes

6. Partial Fermentation → Short-Chain Fatty Acids (Minor Role)

How Insoluble vs. Soluble Fibre Compare

Soluble vs. Insoluble Fibre: Which Do You Need More?

Official Guidelines: No Ratio Specified

"The 2015 Dietary Guideline Advisory Committee issued guidelines for fiber ingestion — anywhere from 22 to 34 g/day in adults, depending on age and sex. No distinction was made between soluble and insoluble fibers." — Basic Medical Biochemistry: A Clinical Approach 6e

What Each Type Does Better

What the Evidence Shows

• Soluble fibre has more directly proven metabolic benefits — lowering cholesterol, improving blood sugar, and reducing CVD risk through measurable mechanisms (gel formation, bile acid trapping).
• Insoluble fibre is equally protective for colorectal cancer, CVD, and type 2 diabetes through mechanisms that are less well understood but confirmed in large population studies.
• Both types together are more beneficial than either alone. Mixed fibre (e.g., psyllium, which is uniquely soluble yet resistant to fermentation) is often recommended clinically.

"The combination of soluble and insoluble fiber has been proposed to be as effective as soluble fiber alone and perhaps slightly better tolerated." — Sleisenger and Fordtran's Gastrointestinal and Liver Disease

Practical Answer

• ~75% of fibre intake in a balanced diet is insoluble (from grains and vegetables)
• ~25% is soluble (from fruits, oats, legumes)