Gastric Emptying

Gastric emptying is the rate at which the stomach delivers its contents into the duodenum, and it is regulated rather than mechanical. Liquids leave roughly exponentially; solids show a lag phase while the antrum grinds particles below a few millimetres, then a near-linear phase. The distal gut applies the brake: nutrients reaching the ileum trigger GLP-1 and peptide YY release, which slows antral contraction and raises pyloric tone. Scintigraphy is the reference measurement, with carbon-13 breath tests and paracetamol absorption used as practical substitutes.

GLP-1 receptor agonists slow emptying markedly, but not uniformly across the class. Short-acting agents such as exenatide twice daily and lixisenatide sustain the delay with each dose, whereas long-acting agents show tachyphylaxis of the gastric effect over a few weeks while the appetite effect persists. The clinical consequences have been formalised: the FDA added ileus to labelling for semaglutide in 2023, and anaesthesia bodies issued guidance the same year on residual gastric contents before elective procedures.

The rate change explains most of the postprandial glucose benefit of the short-acting agents and much of the early nausea across the class. It also sets a boundary on mechanistic claims: because the gastric effect wanes on a long-acting agonist while weight continues to fall, sustained weight loss on a weekly drug cannot be attributed to delayed emptying.

The error to watch is reasoning backwards from symptoms. Nausea is taken as proof of gastric delay, though it is also generated directly at the area postrema, and an absence of nausea is taken as proof of normal emptying before sedation, which the guidance exists precisely to contradict. Spec sheets quoting a single emptying half-time also hide that the measurement varies severalfold with meal composition.