3The physiology of gastric emptying
Section snippets
Physiology of gastric emptying
The main function of the stomach is to act as a reservoir of ingested food and to perform mechanical and chemical breakdown of the contents to a fluid chyme that is delivered to the duodenum at a controlled rate. In the small bowel the nutrient chyme is further degraded by digestive enzymes to molecular components that can be absorbed through the gut wall, a process facilitated by the fed motility pattern.
The stomach can be divided into two different functional regions. After ingestion of a
Test meal
The composition of the test meal can include both a liquid and a solid component. If only one component is radiolabelled, the solid phase is considered to be more sensitive and thus preferable. The caloric load should exceed 300 kcal.*6, 7
Data acquisition and corrections
Gastric emptying is affected by body posture. It is significantly slower in the supine position compared with sitting or standing, which is why the latter are preferable.8 Infrequent data sampling can affect the estimation of the initial lag phase.9 Utilizing
Fed motility
After food intake, the gastric fundus and upper part of the corpus function as a reservoir for the stomach contents. The adaptive relaxatory reflex, described by Cannon and Lieb in 191119, successively activates the proximal stomach to accommodate an increasing volume of contents with little change in luminal pressure. This is followed by a tonic contraction propelling gastric liquids and redistribution of solids to the distal stomach. From their origin in the mid-corpus, motor waves travel
Motilin
The gut peptide motilin has a cyclic plasma level pattern which is in synchrony with the MMC. The plasma motilin peak precedes the MMC phase III in the stomach, suggesting that motilin is involved in the formation of the MMC activity front45, 46, but the mechanism of cyclic motilin release is still unclear, even though it has been suggested that bile is the releaser of motilin42 which then stimulates gastric emptying.
Later research has shown that a novel gastrointestinal peptide hormone, ghrelin
Conclusion
The metabolic impact of gastric emptying is obvious as all metabolic processes in the body are in need of substrate delivery from the gastrointestinal tract. The stomach is the primary reservoir where foodstuffs can be stored for prolonged periods of time in order to serve the continuous metabolic needs for energy substrates. Gastric emptying is a slow process that takes place at a very finely tuned rate guided by blood sugar levels as well as enteric nervous signalling and gastrointestinal
Acknowledgements
This work was supported by the Swedish Research Council, NovoNordisk, the Nanna Svartz fund and funds of the Karolinska Institutet.
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