10.2 Gastrointestinal Diseases

What are the main functions of the small intestine?

Nutrient absorption, water/electrolyte reabsorption (~8 L/day), infection barrier

What factors are required for normal intestinal absorption?

• Intact mucosa with large surface area

• Proper digestion

• Normal GI transit time

What provides the large absorptive surface area?

Folds, villi, microvilli

What is malabsorption?

Impaired digestion and absorption due to GI, pancreatic, or biliary dysfunction

Name diseases associated with malabsorption

• Disaccharidase deficiency

• Coeliac disease

• Inflammatory bowel disease

Where are disaccharidases located?

Brush border of the small intestine

Examples of disaccharidase deficiencies

Lactase, sucrase-isomaltase deficiencies

Symptoms of disaccharidase deficiency

Diarrhoea, bloating, flatulence, abdominal pain

Why does diarrhoea occur in disaccharidase deficiency?

Osmotic effect of unabsorbed sugars

Why is gas produced in disaccharidase deficiency?

Bacterial fermentation → H₂ production

What does the lactose tolerance test detect?

Lactase deficiency

What does faecal elastase test assess?

Pancreatic function

What does the Schilling test assess?

Vitamin B12 absorption

What is the purpose of intestinal permeability tests?

Assess mucosal integrity

What is the D-xylose test used for?

Assess proximal small intestine absorption

Why is D-xylose useful?

Does not require digestion

What does decreased D-xylose excretion indicate?

Malabsorption (e.g. coeliac disease)

What is steatorrhoea?

Excess fat in faeces

Causes of steatorrhoea

• Biliary (lack of bile)

• Pancreatic (lack of enzymes)

• Intestinal (malabsorption)

What is coeliac disease?

Autoimmune response to gluten

Gold standard test for coeliac disease?

Intestinal biospy

Most sensitive serological test for coeliac disease?

Anti-tissue transglutaminase antibodies

What do disaccharidases do?

Break down disaccharides into monosaccharides

Lactose is broken down into what?

Glucose + galactose

Sucrose is broken down into what?

Glucose + fructose

Maltose is broken down into what?

Two glucose molecules

How is disaccharidase deficiency diagnosed?

Breath hydrogen test, biopsy, dietary exclusion

Principle of the breath hydrogen test?

Measures H₂ from bacterial fermentation of unabsorbed sugars

Why does hydrogen appear in breath?

Produced in colon → absorbed → exhaled

What indicates lactose intolerance in BHT?

Increased hydrogen within 30–60 minutes

What happens in lactose-tolerant individuals?

No rise in hydrogen

What improves specificity of BHT?

Measuring hydrogen + methane

What is IBD?

Chronic inflammation of GI tract

Types of IBD

Ulcerative colitis and Crohn’s disease

Symptoms of IBD

Diarrhoea, bleeding, abdominal pain, anaemia

How is IBD diagnosed?

Colonoscopy

What causes cystic fibrosis?

Defective chloride transport

Effect of CF on secretions?

Thick, viscous secretions

What GI issue occurs in CF?

Pancreatic insufficiency

Diagnostic test for CF?

Sweat chloride test

Screening methods for colorectal cancer

FOBT, colonoscopy, sigmoidoscopy

Tumour marker for colorectal cancer?

CEA (carcinoembryonic antigen)

What is colonoscopy used for?

Visualise colon, biopsy, remove polyps

What is flexible sigmoidoscopy?

Examination of lower colon only

What is FOBT used for?

Detect hidden blood in stool

Who should get FOBT?

• Iron deficiency anaemia patients

• Colorectal cancer screening

How does guaiac FOBT work?

Detects peroxidase activity of haem

Limitation of guaiac FOBT?

Affected by diet (false positives)

What is required before guaiac FOBT?

Dietary restrictions

What do immunochemical FOBTs detect?

Human haemoglobin specifically

Advantage of immunochemical FOBT?

No dietary interference, higher specificity