Gastric Emptying Physiology
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Direct answer
The stomach is a blender and a metering pump: liquids leave first (500 mL of water about half-empties in 20-30 minutes), while solids are retained and retropelled against the antrum until triturated to particles under 1-2 mm, then released at a near-constant caloric rate of roughly 2-3 kcal per minute regardless of the meal's composition. The duodenum applies powerful brakes — the enterogastric reflexes and hormones that slow emptying when chyme is too fatty, too acidic or too osmolar: CCK (fat and protein), GIP, secretin and GLP-1 all inhibit antral motility and tighten the pylorus, while acid and hyperosmolarity trigger duodenal osmoreceptor and pH feedback. During fasting, the migrating motor complex sweeps the stomach every 90-120 minutes under motilin's command — the housekeeper wave that erythromycin mimics as a prokinetic. Gastroparesis, typically diabetic, is confirmed by scintigraphy showing more than 10 per cent of a solid meal retained at 4 hours.
What you must remember
- Order of exit: liquids before solids, carbohydrate-rich before protein-rich before fat-rich — fat is the strongest single brake; a fatty meal can slow emptying for hours.
- Caloric metering: the duodenum receives calories at about 2-3 kcal per minute; doubling a meal's calories doubles emptying time rather than speed — the "duodenal brake" principle that exams quote directly.
- Proximal-distal division: fundus and body provide receptive relaxation and tonic accommodation (vagovagal, mediated by nitric oxide and VIP) — the reservoir; antrum and pylorus grind and meter — the pump; vagotomy historically caused dumping because accommodation was lost.
- Hormonal brakes: CCK (fat-stimulated, also relaxes the fundus and contracts the pylorus), secretin (acid), GIP and GLP-1 (glucose and fat) — all inhibit emptying; only motilin accelerates, driving phase III of the migrating motor complex.
- Migrating motor complex: every 90-120 minutes in fasting, phase III housekeeping waves sweep undigested residue; motilin peaks with it; erythromycin (motilin agonist) and domperidone are the prokinetics of gastroparesis.
- Pyloric sieve: particles over about 1-2 mm are rejected back for more antral grinding — the reason tablets designed to stay intact can linger, and why some drugs empty only with the next MMC.
- Gastroparesis numbers: diabetics dominate (autonomic neuropathy); scintigraphic criterion is greater than 10 per cent gastric retention of a standard solid meal at 4 hours; symptoms — postprandial fullness, nausea, early satiety, erratic glycaemia.
A worked case in glycaemic chaos
A young type 1 diabetic reports unpredictable glucose swings — high after dinner, crashing at midnight. Scintigraphy shows 35 per cent retention at 4 hours: diabetic gastroparesis from vagal autonomic neuropathy. Food arrives in the small intestine in unpredictable surges, mismatching her pre-meal insulin timing; the fix is physiological — prokinetics before meals (metoclopramide, domperidone or erythromycin as motilin agonist), smaller low-fat low-fibre meals (fat being the strongest brake), and insulin adjusted for delayed absorption.
Contrast the post-gastrectomy patient who sweats and faints 20 minutes after eating: the pylorus is gone, so a hyperosmolar load dumps straight into the jejunum, pulling water into the lumen and releasing vasoactive peptides — early dumping. Two hours later, rapidly absorbed glucose triggers exaggerated insulin release and reactive hypoglycaemia — late dumping. Same surgery, two physiologies, two timings.
Where students slip
Students say the stomach "pushes" chyme out; mostly the duodenum permits it — the rate-limiting step is duodenal feedback, and the classic proof is that instilling fat or acid directly into the duodenum slows emptying of a meal still in the stomach. The second slip is forgetting vagal participation in accommodation: truncal vagotomy, an old Indian surgery-exam staple, produced rapid emptying and dumping — which is why it was always paired with a drainage procedure. Third, the 4-hour scintigraphy criterion is the number to quote for gastroparesis; quoting only "delayed emptying" without the 10 per cent at 4 hours reads as a memorised phrase without the payload.
Frequently asked questions
Why do liquids empty faster than solids from the stomach?
Liquids flow down a pressure gradient from the accommodated fundus with no trituration required, whereas solids are retained until ground by antrial retropulsion to particles below 1-2 mm that the pyloric sieve can pass.
What is the enterogastric reflex?
Duodenal receptors for fat, acid, osmolarity and distension slow gastric emptying through vagovagal reflexes and hormones — CCK, secretin, GIP and GLP-1 — matching chyme delivery to digestive and absorptive capacity.
Which hormone accelerates gastric emptying, and when?
Motilin, released cyclically during fasting, drives phase III of the migrating motor complex every 90-120 minutes — the housekeeping wave that erythromycin mimics as a prokinetic.
How is gastroparesis diagnosed?
By gastric emptying scintigraphy showing more than 10 per cent retention of a radiolabelled solid meal at 4 hours, in a patient with postprandial fullness, nausea and early satiety — most commonly from diabetic autonomic neuropathy.
What distinguishes early from late dumping syndrome?
Early dumping occurs within 30 minutes from osmotic fluid shifts and vasoactive release after rapid gastric emptying; late dumping occurs 1-3 hours after meals from reactive hypoglycaemia driven by exaggerated insulin secretion.