Key Facts

The GI tract processes approximately 9L of fluid daily (2L ingested + 7L secretions); only ~100mL is excreted in faeces. Each region has specialised functions in digestion and absorption.

Epidemiology

GI diseases are extremely common in UK clinical practice. Coeliac disease affects approximately 1% of the population (many undiagnosed). IBS affects 10-20% of the population. Peptic ulcer disease affects ~5-10% lifetime risk.

Aetiology

GI function depends on:

• Motility: Coordinated smooth muscle contraction (enteric nervous system — "second brain")
• Secretion: Acid, enzymes, bile, mucus, bicarbonate
• Digestion: Mechanical and chemical breakdown of macronutrients
• Absorption: Specific transport mechanisms for nutrients, electrolytes, water
• Neuroendocrine regulation: Vagus nerve, hormones (gastrin, CCK, secretin, GIP, motilin)

Pathophysiology

Gastric acid secretion has three phases:

1. Cephalic phase (30%): Vagal stimulation — sight, smell, taste of food
2. Gastric phase (60%): Gastric distension, peptides → gastrin release → acid secretion
3. Intestinal phase (10%): Initially stimulatory, then inhibitory (secretin, GIP)

Defence mechanisms against acid: Mucus-bicarbonate barrier, prostaglandins (PGE2 promotes mucus/HCO3⁻ secretion, mucosal blood flow), epithelial cell turnover (3-5 days)

Hepatic function: Metabolism (drugs, bilirubin, ammonia), synthesis (albumin, clotting factors, bile), storage (glycogen, vitamins, iron), detoxification

Malabsorption Syndromes

• Diarrhoea, steatorrhoea, weight loss, nutritional deficiencies
• Iron deficiency (duodenal disease — coeliac)
• B12 deficiency (ileal disease — Crohn's, pernicious anaemia)
• Fat-soluble vitamin deficiency (A, D, E, K) — pancreatic insufficiency, biliary obstruction

Dyspepsia and Acid-Related Disease

• Epigastric pain, heartburn, early satiety, nausea
• Duodenal ulcer pain: Relieved by eating, worse at night
• Gastric ulcer pain: Worse with eating

Hepatobiliary Disease

• Jaundice (bilirubin >35 µmol/L), pruritus, pale stools, dark urine (obstructive)
• Coagulopathy (reduced synthesis of factors II, VII, IX, X)
• Ascites, encephalopathy (decompensated liver disease)

Red Flags

• Dysphagia — urgent 2-week-wait OGD referral
• Unexplained weight loss with GI symptoms — malignancy screen
• Haematemesis or melaena — upper GI bleeding
• Iron deficiency anaemia in men or post-menopausal women — investigate GI tract

Bedside

• Stool analysis: MC&S, faecal calprotectin (>100 µg/g suggests IBD), faecal elastase (<200 = pancreatic insufficiency)
• Urea breath test: H. pylori (stop PPI 2 weeks before)

Bloods

• FBC: Microcytic (iron), macrocytic (B12/folate) anaemia
• Iron studies, B12, folate: Malabsorption screen
• LFTs: Bilirubin, ALT, ALP, GGT, albumin
• tTG-IgA + total IgA: Coeliac disease screening
• Clotting: PT prolonged in liver disease/vitamin K deficiency

Imaging

• OGD: Upper GI pathology, duodenal biopsy (coeliac)
• Colonoscopy: Lower GI pathology, IBD assessment
• USS abdomen: Gallstones, liver assessment
• MRCP: Biliary tree, pancreatic duct assessment

Special Tests

• SeHCAT scan: Bile salt malabsorption
• Hydrogen breath test: SIBO, lactose intolerance
• Capsule endoscopy: Small bowel pathology

Non-pharmacological

• Gluten-free diet: Coeliac disease — lifelong, with dietitian support
• Low FODMAP diet: IBS symptom management
• Pancreatic enzyme replacement: Creon 25,000-50,000 units with meals for exocrine pancreatic insufficiency

Pharmacological

• PPI: Omeprazole 20mg OD for acid-related disease (NICE NG12)
• H. pylori triple therapy: PPI + amoxicillin 1g BD + clarithromycin 500mg BD for 7 days
• Loperamide: 2-4mg PRN for symptomatic diarrhoea (max 16mg/day)
• Bile acid sequestrants: Colestyramine 4g OD-QDS for bile salt malabsorption
• B12 replacement: Hydroxocobalamin 1mg IM on alternate days × 2 weeks, then every 3 months
• Iron replacement: Ferrous sulphate 200mg BD-TDS (65mg elemental iron per tablet)

Referral Criteria

• Red flag symptoms — urgent 2-week-wait referral
• Positive coeliac serology — gastroenterology for biopsy
• Suspected IBD (raised calprotectin) — gastroenterology
• Deranged LFTs >3 months — hepatology

• Coeliac disease: Excellent prognosis with strict GFD; small increased risk of enteropathy-associated T-cell lymphoma (~6-9× relative risk)
• Peptic ulcer disease: >95% cure rate with H. pylori eradication and PPI
• Chronic pancreatitis: 10-year survival ~70%; pancreatic cancer risk approximately 4%
• Pernicious anaemia: Lifelong B12 replacement required; ~5% risk of gastric carcinoma
• Untreated coeliac disease associated with osteoporosis, infertility, and neurological complications

GI Hormones

Site-Specific Absorption