How does the stomach digest protein?
The stomach digests protein with a very strong acid, about pH 1.7 when empty (Dressman, 1990), which unfolds proteins and activates pepsin, an enzyme that cuts them into pieces. Single amino acids are only released further on, in the intestine. Compare a normal meal with a meal plus an acid reducer.
Relative levels, simplified diagram for a healthy person, for a large meal with meat. Each curve has its own scale. Acidity goes up when the pH goes down. Dotted curves are estimates.
How does the stomach digest protein?
The stomach makes a very strong acid. The acid unfolds proteins and unlocks pepsin, an enzyme that cuts them into pieces. Single amino acids are only released further along, in the intestine. Start the meal, then compare with an acid-reducing medicine.
pH on an empty stomach and after a meal: Dressman et al. 1990. Pepsin is mostly active between pH 1.5 and 3, almost inactive above 5 (review on pepsin, 2020). With a PPI, pH above 4 for 10 to 17 h out of 24 (Miner et al. 2003). The emptying speed and the number of cuts are our own estimates.
Frequently asked questions about the stomach and protein
Why doesn't stomach acid damage the stomach wall?
Because a layer of mucus covers the wall and traps bicarbonate, which neutralizes the acid. In the middle of the stomach, the pH is about 2; against the wall, under the mucus, it stays around 5 (Quigley and Turnberg, 1987). Surface cells are also replaced every few days. An ulcer forms when this defense weakens, most often because of the bacterium Helicobacter pylori or anti-inflammatory drugs such as ibuprofen or aspirin.
Why doesn't pepsin digest the stomach itself?
It leaves the wall in a locked form, pepsinogen, like closed scissors. Acid, inside the stomach cavity, unlocks it. Against the wall, the mucus keeps the pH too high for it to work well: it is mostly active between pH 1.5 and 3.
Does an acid reducer stop you from digesting protein?
Hardly. The stomach does only about 10 to 20% of the work; the rest happens in the intestine, done by pancreatic enzymes that don't need acid. On the other hand, with a PPI-type acid reducer taken for years, vitamin B12 levels can drop (Lam et al., 2013). Don't stop a treatment without talking to your doctor.
Why does a meat-heavy meal stay in the stomach so long?
Protein and fat slow down emptying. The stomach exit, the pylorus, only lets through pieces of 1 to 2 mm, so the stomach churns the meat with acid until it turns into a paste. A large meal can stay 3 to 4 h, while a glass of water passes in a few tens of minutes.
What causes heartburn?
Most often, acid rising into the esophagus, whose wall doesn't have the same protection as the stomach's. A muscle at the bottom of the esophagus normally closes after food goes through; if it relaxes too often (large meals, lying down right after eating), acid comes back up.
True or false?
The stomach digests most of the protein you eat.
False. Stomach pepsin breaks down only 10 to 20%; the pancreas and the intestine do the rest (Guyton and Hall).
A meal makes your stomach more acidic right away.
False. At first it is the opposite: food neutralizes the acid and stomach pH rises toward 6 to 7, before slowly coming back down (Dressman, 1990).
Taking antacids for years can lower vitamin B12.
True. Acid-blocking drugs taken for 2 years or more are linked to more vitamin B12 deficiency, because acid is needed to free it from food (Lam, 2013).
This page as text
The same information as the charts, to read without animation: for each chart, the values at the main moments, then the notes with their level of evidence.
Acid and pepsin during digestion
On an empty stomach, the stomach is already very acidic. The meal first makes the pH go up, because food neutralizes the acid. The stomach then makes more acid, and pepsin gets to work as the pH comes back down.
| Minutes since the meal | Stomach acidity | Gastrin (the hormone that controls acid) (dotted) | Pepsin at work (dotted) | Food in the stomach (dotted) | What happens |
|---|---|---|---|---|---|
| −30 min | 88 | 15 | 8 | 0 | 30 min before the meal |
| Meal | 88 | 15 | 8 | 0 | The meal |
| +1 h | 67 | 72 | 70 | 95 | 1 h 00 min after the meal |
| +2 h | 85 | 38 | 83 | 75 | 2 h 00 min after the meal |
| +3 h | 88 | 22 | 59 | 50 | 3 h 00 min after the meal |
| +4 h | 88 | 18 | 35 | 25 | 4 h 00 min after the meal |
Key points
- The stomach waits, already acidic (Before and during the meal, Shown in humans)
On an empty stomach, the pH is about 1.7: that's very acidic. The lower the pH, the more acidic. Source: Dressman et al., « Upper gastrointestinal (GI) pH in young, healthy men and women », Pharm Res, 1990 - The meal lowers acidity (Up to 30 min after, Shown in humans)
Food neutralizes the acid: right after a large meal, the stomach pH can rise to about 6 to 7. Source: Dressman et al., « Upper gastrointestinal (GI) pH in young, healthy men and women », Pharm Res, 1990 - Gastrin boosts acid (From 5 min to 1 h 20 min after, Shown in humans)
Protein and a filling stomach release gastrin, a hormone that pushes the wall to make more acid. Source: Guyton and Hall, « Textbook of Medical Physiology », chapter on protein digestion; see also StatPearls, « Physiology, Pepsin » - Pepsin cuts (From 40 min to 2 h 30 min after, Shown in humans)
Below about pH 5, pepsin unlocks; it works best around pH 2. In less than 2 h, the stomach is back to its starting acidity. Source: « Pepsin properties, structure, and its accurate measurement: a narrative review », Annals of Esophagus, 2020 - The stomach empties little by little (From 2 h after, Shown in humans)
The stomach exit, the pylorus, only lets through pieces of 1 to 2 mm: the stomach churns the meat until it becomes a paste. A large meal can stay 3 to 4 h. Source: « In silico study of the dynamics of solid food particles in the stomach during gastric digestion », 2025
From protein to amino acids
The stomach only starts the job: it cuts proteins into pieces. Enzymes from the pancreas and the intestine release the amino acids, which then pass into the blood.
| Minutes since the meal | Whole proteins (dotted) | Protein pieces (peptides) (dotted) | Amino acids in the blood (dotted) | What happens |
|---|---|---|---|---|
| −30 min | 0 | 0 | 20 | 30 min before the meal |
| Meal | 0 | 2 | 20 | The meal |
| +1 h | 70 | 30 | 44 | 1 h 00 min after the meal |
| +2 h | 40 | 55 | 76 | 2 h 00 min after the meal |
| +3 h | 20 | 50 | 76 | 3 h 00 min after the meal |
| +4 h | 10 | 35 | 55 | 4 h 00 min after the meal |
Key points
- Acid unfolds proteins (Up to 1 h after, Shown in humans)
A protein is a long chain folded on itself. Acid unrolls it, which lets pepsin reach the inside of the chain. - A small share in the stomach (From 40 min to 2 h after, Shown in humans)
Pepsin does only about 10 to 20% of protein digestion. It cuts proteins into shorter pieces, peptides. Source: Guyton and Hall, « Textbook of Medical Physiology », chapter on protein digestion; see also StatPearls, « Physiology, Pepsin » - Most of the work in the intestine (From 1 h to 3 h 30 min after, Shown in humans)
In the intestine, pancreatic enzymes such as trypsin cut the peptides into small bits. The intestinal wall finishes the job, then the amino acids pass into the blood. Source: Guyton and Hall, « Textbook of Medical Physiology », chapter on protein digestion; see also StatPearls, « Physiology, Pepsin » - Amino acids in the blood (From 0.5 h after, Shown in humans)
They rise in the first hour and stay high for several hours. The speed depends on the protein: in older men, ground beef reaches the blood faster than steak. Source: Pennings et al., « Minced beef is more rapidly digested and absorbed than beef steak », Am J Clin Nutr, 2013
With a PPI-type acid reducer
A PPI (omeprazole, esomeprazole...) blocks the pump that makes acid. The pH stays higher, and pepsin works less. Proteins are still digested, by the intestine.
| Minutes since the meal | Stomach acidity (dotted) | Pepsin at work (dotted) | Amino acids in the blood (dotted) | What happens |
|---|---|---|---|---|
| −30 min | 42 | 2 | 20 | 30 min before the meal |
| Meal | 42 | 2 | 20 | The meal |
| +1 h | 25 | 10 | 32 | 1 h 00 min after the meal |
| +2 h | 35 | 18 | 62 | 2 h 00 min after the meal |
| +3 h | 39 | 16 | 74 | 3 h 00 min after the meal |
| +4 h | 42 | 12 | 62 | 4 h 00 min after the meal |
Key points
- Less acid (Throughout, Shown in humans)
With a PPI, the stomach pH stays above 4 about half the day: 10 to 17 h out of 24 depending on the drug. Source: Miner et al., « Gastric acid control with esomeprazole, lansoprazole, omeprazole, pantoprazole, and rabeprazole », Am J Gastroenterol, 2003 - Pepsin slows down (From 30 min to 2 h 50 min after, Shown in humans)
Above pH 5, pepsin's scissors stay almost closed. It cuts much less. Source: « Pepsin properties, structure, and its accurate measurement: a narrative review », Annals of Esophagus, 2020 - The intestine takes over (From 1 h after, Shown in humans)
Pancreatic enzymes don't need acid: amino acids still reach the blood. Source: Guyton and Hall, « Textbook of Medical Physiology », chapter on protein digestion; see also StatPearls, « Physiology, Pepsin » - Over the years, vitamin B12 (Throughout, Shown in humans)
Acid helps release vitamin B12 from food. With a PPI taken for years, B12 levels can drop. Don't stop a treatment without talking to your doctor. Source: Lam et al., « Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency », JAMA, 2013
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