Describe the factors regulating gastric emptying.

Definition

Normal Time

• Order — carbohydrate > protein > fat
• Emptying is exponential for liquids and linear for solids

Factors Promoting Emptying

• Gastric distension — via local myenteric and vagovagal reflexes
• Gastrin — increases antral pump activity
• Vagal stimulation
• Liquid, isotonic, non-fatty meals

Factors Delaying Emptying

Chyme enters duodenum → Duodenal receptors sense fat, acid, hypertonicity, protein → Enterogastric reflex (vagal and enteric) + enterogastrones → CCK, secretin, GIP released → Pyloric sphincter contracts; antral pump inhibited → emptying slows

• Fat is the most powerful inhibitor — acting through CCK
• This feedback ensures the duodenum is never overloaded
• Also delayed by pain, fear, sadness, and by anticholinergics and opioids

Measurement

• Radionuclide scintigraphy — the gold standard; a technetium-labelled meal is followed by gamma camera
• Barium meal, ultrasound, and the 13C breath test are alternatives

Applied Aspects

• Delayed emptying (gastroparesis) — commonly diabetic autonomic neuropathy; also post-vagotomy, opioids, hypothyroidism. Treated with prokinetics (metoclopramide, erythromycin)
• Pyloric stenosis — in infants, projectile non-bilious vomiting with a palpable olive-shaped mass and hypochloraemic hypokalaemic alkalosis
• Dumping syndrome — after gastric surgery, rapid emptying of hypertonic chyme → fluid shifts into the gut → early symptoms (faintness, palpitations) and late reactive hypoglycaemia
• Rate of emptying determines the rate of drug absorption — hence many drugs are given on an empty stomach