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In biology class, we learned this familiar story: Food enters the stomach, the stomach churns it with acid and enzymes, and then, somehow, the stomach senses when the stew is ready and releases it into the small intestine.

But this simple narrative implicitly assumes that all the food of a meal comes into the stomach at about the same time, and only then does the digestion process begin. This is often not the reality. What happens when we eat continuously over many hours, such as at a Super Bowl party, or an all-day summer barbecue, or a long afternoon at an all-you-can-eat crab shack?

Does every new bite cause the stomach to reset its process to zero? "Uh oh, detecting fresh, undigested food... better start over..." But if the stomach resets to zero with every new bite, wouldn't it eventually grow overfull?

Or does the stomach eventually get fed up with all the new introductions and just start releasing whatever it has into the small intestine, ready or not? In that case, when and how did it decide to proceed with incomplete processing?

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First of all, the stomach releases the digested food continuously. Simply put, during digestion the pylorus regularly opens a bit so that small food particles (< 1-2mm) are able to leave the stomach – it's not a batch process (see also this question).

However, because humans tend to ingest a lot of indigestible materials (e.g. bones), there is a need to pass the larger indigestible particles – and this process only happens after all digestible food has left the stomach and thus is "reset" every time you ingest new digestible food.

So if you would continuously ingest food that contains indigestible parts (e.g. mice or other small mammals), the larger bones of those mice would accumulate until you take a longer break and the stomach could complete it's digestion – but for normal food, this does not matter since it will be broken down enough to leave your stomach continuously.

Source: According to Estimation of gastric residence time of the Heidelberg capsule in humans: Effect of varying food composition, 1985 (PDF), the interdigestive migrating myoelectric complex (IMMC) is responsible for the emptying of of large undigested particles:

Recently, Hinder and Kelly (1), using radiolabeled techniques, demonstrated differences in the gastric emptying patterns of a liquid, a digestible solid, and an indigestible solid in dogs. Based on such experiments, it has been suggested that the emptying of large (>1 mm) indigestible objects from the stomach is dependent on the interdigestive migrating myoelectric complex (IMMC)

Furthermore the study shows that the IMMC gets triggered only after all digestible matter has leaved the stomach:

The longer GRT of the Heidelberg capsule compared with the t1/2 of the 99mTC-DTPA is consistent with the finding that large nondigestible solids are emptied by the IMMC once all of the digestible materials have passed through the pylorus into the duodenum

and

the interdigestive migrating myoelectric complex can be markedly delayed by frequent feedings with solids, and the interdigestive migrating myoelectric complex is delayed by both liquid and solid meals

and

Feeding has been shown to interrupt the IMMC; resumption of myoelectric activity is necessary for the passage of large nondigestible particles such as the Heidelberg capsule

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Does the stomach need to start over after every bite? No. When the stomach digests food and some new food comes in, it just continues to digest the old and new food. The stomach releases small particles first and continues to crush large particles.

In the stomach and small intestine, there are mechanoreceptors and receptors that detect gastric acid, amino acids, etc., which stimulate the release of gastrointestinal hormones and the vagus nerve, which all together act as brakes or accelerators of stomach emptying (Nutrients, 2016).

In both continuous and "few meals a day" pattern of eating, the stomach emptying can be greatly affected by various food factors:

  • Consistency. Liquid foods empty faster than solid foods.
  • Particle size. Small food particles empty faster than the bigger ones.
  • Volume. Greater volume of food will stimulate gastric emptying sooner than a small volume.
  • Energy value: 300 Cal meal will empty faster than 600 Cal meal; the stomach tends to release a certain amount of calories per time
  • Water and drinks up to ~7% of sugar empty faster than those with >8% sugar (regular fruit juices and sodas)
  • Macronutrients:
  • Fat and soluble fiber (such as glucan from barley or pectin from apples) can slow gastric emptying.

Human factors that can slow down gastric emptying:

  • Psychological factors, such as anxiety
  • Physical factors, such as exercise
  • Diseases, such as diabetic neuropathy
  • Medications, such as antacids and opioids

Other sources:

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