A cat's stomach holds a fluid at pH 1 to 2, which means 10 to 100 millimoles of hydrogen ions per litre — between 100,000 and 1,000,000 times the concentration in blood at pH 7.4. That acidity is produced in 3 phases, each with its own trigger and its own share of the total, and it is controlled by 2 loops that a feeding plan can work with or against. The sections below give the phase timing, the pH range with the millimole arithmetic behind it, and the defences that keep 0.2 to 0.5 mm of mucus between the acid and the cells making it.
Three phases, one pump
All of the acid comes from a single enzyme, H+/K+-ATPase, in the membrane of the parietal cell. The pump exchanges 1 hydrogen ion outward for 1 potassium ion inward at the cost of 1 ATP, and it works against a gradient of 6 pH units: cytosol at pH 7.3, secretory canaliculus at pH 0.8, or 150 mmol/L H+. A parietal cell can lift gastric pH from 6 to 1 within 5 to 10 minutes of stimulation, which is why meal timing matters more than any single ingredient.
The 3 phases and their share of the acid
| Phase | Timing | Share (%) | Main mediator | Luminal pH |
|---|---|---|---|---|
| Cephalic | −15 to 0 min | 20–30 | Vagal acetylcholine, GRP | 3–5 |
| Gastric, early | 0–60 min | 35–45 | Vagal reflex, distension | 2–3 |
| Gastric, late | 60–180 min | 25–35 | Gastrin, histamine | 1–2 |
| Intestinal | 180–300 min | 5–10 | Enteric gastrin, amino acids | 1.5–3 |
| Interdigestive | 300+ min | 5–15 per day | Vagal tone, ghrelin | 1–3 |
Phase by phase
The cephalic phase starts before food arrives: the sight, smell, and sound of a can opening, then chewing and swallowing, send vagal efferents to the stomach. It contributes 20 to 30 percent of the meal's acid, and it starts the moment a cat hears the fridge. The gastric phase is the main event at 60 to 70 percent: distension of the body and antrum, plus peptides and amino acids in the lumen, drive a vagal-vagal reflex, the enteric plexus, and gastrin release from G cells in the antrum. It lasts 1 to 3 hours. The first hour belongs to the vagal reflex and distension and reads pH 2 to 3; the second and third hours belong to gastrin and histamine and read pH 1 to 2. The intestinal phase adds 5 to 10 percent as chyme reaches the duodenum, and it is quickly braked by secretin, cholecystokinin, and gastric inhibitory peptide once pH in the duodenum falls below 4.5. Between meals, basal secretion supplies 5 to 15 percent of the daily total at pH 1 to 3.
Two control loops
Loop one is neural. Vagal efferents release acetylcholine onto M3 receptors of parietal cells and GRP onto G cells; vagotomy drops acid output by 50 to 70 percent. Loop two is hormonal and paracrine, and it runs on a triad: gastrin from G cells binds CCK2 receptors and raises intracellular calcium; histamine from enterochromaffin-like cells binds H2 receptors and raises cyclic AMP; somatostatin from D cells shuts both down when luminal pH falls below 2. Both loops converge on the same pump, and histamine is the amplifier: gastrin alone produces a fraction of the acid that gastrin plus histamine produces.
What the mucosa does to survive pH 1
Between the lumen and the epithelium sits a mucus-bicarbonate layer 0.2 to 0.5 mm thick. Its luminal face reads pH 1 to 2; its epithelial face stays near pH 7, so the parietal cell lives in the same 0.5 mm as the acid it makes. Surface epithelial cells renew every 3 to 5 days, and mucosal blood flow carries away the bicarbonate that neutralises what diffuses back. Pepsinogen, secreted by chief cells, becomes active pepsin below pH 4 and is denatured above pH 4.5 to 5, so the enzyme and the acid share the same operating window.
Observable scale: strips, meters, and 1 drop
pH is a logarithmic scale, and the step size of the tool matters. A 0.5-unit test strip cannot tell pH 1.5 from 2.0, which is a 3.2-fold difference in acid. A glass electrode reads to 0.01 units, and a 0.3-unit move is a doubling of hydrogen ion concentration. Sample size matters as much: 1 drop of gastric fluid is 0.05 mL, and at pH 1.5 that drop holds 1.6 micromoles of acid — small enough that a trace of bile moves the reading by 0.5 units.
Worked conversion: pH to millimoles to milligrams
Hydrogen ion concentration is 10 to the minus pH, in moles per litre. pH 1 is 0.1 mol/L, or 100 mmol/L; pH 2 is 10 mmol/L; pH 3 is 1 mmol/L; pH 4 is 0.1 mmol/L. Take 40 mL of gastric content at pH 1.5: the concentration is 10⁻¹·⁵ = 0.0316 mol/L, or 31.6 mmol/L, and 0.04 L × 31.6 = 1.26 mmol of acid. To neutralise it you need 1.26 mmol of bicarbonate; sodium bicarbonate at 84 mg/mmol means about 106 mg, roughly a quarter of a 500 mg antacid tablet. Now run the same fluid at pH 4: 0.1 mmol/L × 0.04 L = 0.004 mmol, and 3 mg of bicarbonate would do — the same 40 mL, 315 times less acid.
Reference values, and how much they move
Different individuals and laboratories differ slightly: fasting gastric pH in adult cats is usually quoted as 1 to 2, but values up to 3 appear in cats that nibble all day, because each small meal buffers the lumen for 30 to 60 minutes. After a meal, pH rises to 4 to 6 within 15 to 30 minutes and returns to 2 by 2 to 4 hours. A cat fed 4 small meals instead of 1 large one spends more of the day above pH 3, which is about meal pattern, not disease. Measurement also depends on where the sample is taken: fundic juice is 0.5 to 1.0 pH units more acid than fluid sitting in the antrum next to partly digested food.
When the number stops explaining the cat
Vomiting that brings up food 8 to 12 hours after eating, a pH that stays above 4 while the cat is fasting, or weight loss of more than 5 percent in a month are reasons to look beyond acid — hairballs, foreign body, chronic enteropathy, or lymphoplasmacytic gastritis look like this. Blood in vomit, vomiting more than 3 times in 24 hours, or a cat that will not drink for 12 hours needs a veterinarian the same day. Acid suppression is a tool, not a diagnosis: omeprazole, dosed once daily by the veterinarian, raises gastric pH above 4 within 2 to 3 days, and stopping it abruptly can rebound acid output for 1 to 2 weeks.
Four steps that follow from the numbers
- Split the daily ration into 3 to 4 meals so the lumen spends less time below pH 2 — the buffered window after each meal is 30 to 60 minutes.
- Do not read a fasting pH taken 2 hours after the last meal as a baseline; wait 8 to 12 hours for a true fasting sample.
- Convert every pH you record into millimoles (10 to the minus pH × 1000) before comparing 2 cats, because 0.3 of a unit is a 2-fold difference.
- If acid suppression is prescribed, plan the taper over 1 to 2 weeks and read gastric pH in practice alongside the bile acid cycle; the breed quiz covers the body shapes behind reflux risk.
Terms mentioned in this article
Frequently asked
Why does a cat's stomach sit at pH 1 to 2 instead of something milder?
The H+/K+-ATPase of the parietal cell pumps hydrogen ions against a 6-unit gradient, from cytosol at pH 7.3 into a canaliculus at pH 0.8, so the lumen settles at pH 1 to 2 or 10 to 100 mmol per litre. That acidity activates pepsinogen below pH 4, kills much of what arrives on raw food, and costs the stomach a 0.2 to 0.5 mm mucus layer plus epithelial renewal every 3 to 5 days.
How much of the acid comes from each of the 3 phases?
The cephalic phase, triggered by sight, smell, and chewing before food arrives, supplies 20 to 30 percent. The gastric phase, driven by distension plus peptides and amino acids acting through vagal reflexes, the enteric plexus, and gastrin, supplies 60 to 70 percent over 1 to 3 hours. The intestinal phase adds 5 to 10 percent and is quickly braked once duodenal pH falls below 4.5.
What does a 0.3 change in measured pH actually mean?
pH is logarithmic, so 0.3 of a unit is a doubling or halving of hydrogen ion concentration, and a full unit is 10-fold. Convert before comparing: pH 1.5 is 31.6 mmol per litre, pH 2 is 10, pH 4 is 0.1. That is why a 40 mL sample at pH 1.5 needs about 106 mg of sodium bicarbonate to neutralise while the same volume at pH 4 needs about 3 mg.