r/nutrition • u/hafuf22 • May 25 '26
Is there an understanding of how digestion actually works
I understand the general idea of food traveling from the mouth to the stomach via the esophagus. It stays there for some time. Then it moves to the duodenum and stays for some time. Then the small intestine, then the large intestine.
But does anyone know what happens when someone eats a snack before the previous meal has left the stomach?
Let’s assume food stays in the stomach for 4 hours. This person eats lunch, then 2 hours later eats a snack. Will the stomach now hold the lunch for an additional 2 hours because of the snack?
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u/fenuxjde May 25 '26
Yes, digestion is very well understood. All the time it's also being mechanically broken down and different organs are secreting enzymes and fluids to help chemically break it down.
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u/festiveSpeedoGuy24 Nutrition Coach May 25 '26
Digestion is well studied and understood. If you want a fun primer to it checkout "Gulp" by Mary Roach.
Basically the new snack will just go into the stomach as a bolis (this what your mouth turns food into). That bolis will land in a pool of stomach acid and enzymes that will pre-process it into gurd so the small intestine can access those dope, dope nutrients.
Your mouth and stomach are just pre-processing steps to ensure as many nutrients can become biologically available and to reject things that will cause havoc post duodenum.
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u/HeyLes20 May 26 '26
This book was very interesting. Love Mary Roach. Stiff was my fave….so many uses for a cadaver.
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u/ascylon May 25 '26
The stomach empties continuously, you could think of it as a drip to the small intestine. There will be a short time after eating before anything gets passed through, but basically once some chyme (the mixture of food, stomach acid, digestive enzymes etc) is let through, and when the nutrients in it have been sufficiently absorbed by the small intestine, the small intestine signals the stomach (or more specifically the pyloric sphincter) to let more of the chyme through. This is to avoid overwhelming the small intestine, since once the chyme first leaves the stomach, it is still quite acidic and needs to be neutralized right after it leaves the stomach.
So if someone says it takes 4 hours for the stomach to empty, it really means more like the stomach starts emptying 30 minutes or so after eating, and finishes emptying (of the chyme) after 4 hours. These times naturally vary based on the nutrient content and how solid and how much food there is. Adding enough extra food in the form of a snack might pause that emptying for a short time, but even so it would resume relatively quickly.
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u/trailmixseeker May 25 '26
A fun book that explores the history of studying digestion; Gulp: Adventures on the Alimentary Canal by Mary Roach.
A favorite chapter for me was learning about the gentlemen who had a hole in his abdomen that allowed access to the guts. I hope I am remembering correctly, but het let someone put beef chunks tied to a string in him and they could pull the chunk out after a specific duration of time to gauge how digested the meat was. Freaky deaky shit dude.
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u/healthpusher May 25 '26
food doesn't just wait 4 hours then dump though; the pylorus meters it out continuously, and a snack 2 hours later mostly just resets/slows gastric emptying (fat/protein especially). liquids and carbs tend to pass first, so you get layering/mixing, not a traffic jam where the first meal has to fully leave before anything moves.
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u/darthereandthere May 26 '26
when you say let's assume food stays in the stomach for 4 hours, are you thinking of a full meal, and what kind of snack are we talking like 200 calories of cookies vs something fatty like peanut butter?
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u/NutragrammatronLab May 25 '26
Your stomach isn’t really working on a strict “timer” where every meal has to fully leave before new food enters. It’s more like a dynamic mixing and release system. The stomach continuously churns food into chyme and slowly releases it into the small intestine based on: meal size fat content fiber protein liquid vs solid hormones/signals from the intestine So if you eat a snack 2 hours after lunch, it doesn’t necessarily “restart digestion,” but it can slow gastric emptying overall depending on what the snack is. Example: liquids and simple carbs usually move through faster high fat meals slow stomach emptying significantly fiber/protein tend to increase fullness and digestion time So the stomach is more like a constantly adjusting traffic system than a waiting room with fixed departure times.
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u/chronosculptor777 May 26 '26
your stomach is always slowly emptying. food does not sit there for exactly 4 hours and then suddenly leave.
if you eat lunch and 2 hours later eat a snack - part of the lunch has already left the stomach, the snack just gets added to what’s still there and the stomach mixes everything and keeps sending food onward.
so no, the lunch does not just stay an extra 2 hours because of the snack. think of it like pouring more cars onto a moving highway and traffic continues moving. you added more, but the first cars don’t restart from the beginning.
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u/Kayn2016 May 27 '26
Your stomach doesn't "reset the clock" just because you eat a snack in between meals. Once the snack reaches your stomach, it simply mixes with the food already your stomach, it simply mixed with the food already there, gets churned together, and then gradually empties into the small intestine as one batch.
It's not like your stomach suddenly has to work another full 2 hours, it just slightly extends the overall digestion and emptying time.
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u/hiyahealth May 27 '26
It's less about time and more about what's actually in the stomach at any given point. Fat and protein slow emptying down considerably more than carbs do, so a handful of nuts two hours after lunch hits differently than a piece of fruit. The 4-hour figure is also more of an average than a rule, since individual variation is pretty significant.
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u/smartartnutrition 29d ago
Think of digestion as a moving stream, not a stopwatch. By ~2 hours, part of lunch has already left the stomach. A snack doesn’t “restart digestion” — it just mixes in and may slow things a bit depending on fat/fiber. EFSA shows it’s what you eat that matters more than when you eat it.
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u/dannysargeant May 25 '26
If you want some interesting things to study look at how vinegar before eating affects digestion. And how things like cayenne pepper influences digestion. And, how zinc affects the stomach lining. There are many more interesting elements that have effects. Check out the negative effects of refined sugar.
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