What is the normal pH of stomach acid?

Normal Gastric Acid pH

The normal pH of stomach acid typically ranges from 1.5 to 3.5, with a mean value of approximately 1.7 in the gastric fundus. 1

Gastric pH Measurement and Normal Values

Gastric acid pH varies by location within the stomach and physiological state:

• Gastric fundus: The fundus is the optimal location to evaluate gastric acid, with highly reproducible pH values 1

  • Normal values (mean ± 95th percentile) for percentage time gastric pH < 4 in the fundus:
    • Total: 95.6 ± 1.5%
    • Upright: 94.8 ± 1.8%
    • Supine: 96.5 ± 2.3%

• Regional differences:

  • Fundus and body: Similar pH profiles with mean percentage total time pH < 4 at 92.2% and 90.1%, respectively 1
  • Antrum: Less acidic with pH < 4 only 54.6% of the time 1

Factors Affecting Gastric pH

Meal Effects

• Postprandial changes: After meals, gastric pH temporarily rises due to food buffering effects 2

  • High-fat meals have the strongest buffering effect (pH rising to 4.9 in proximal stomach)
  • Spicy meals have moderate buffering effect (pH rising to 4.2)
  • Bland meals have minimal buffering effect (pH rising to 3.0)
  • Buffering effects are significantly higher in the proximal than in the mid/distal stomach

• Acid pockets: Despite meal buffering, focal areas of high acidity (pH 1.5 ± 0.9) can be observed in the cardia-gastroesophageal junction during the postprandial period 2

Clinical Relevance

• GERD assessment: In gastroesophageal reflux disease (GERD) evaluation, a fall below pH 4 in esophageal pH is taken to indicate acid reflux 3

• Acid exposure measurement: Esophageal acid exposure time (AET), defined as the percent time spent at pH of 4.0 or less, is a key marker for phenotyping patients with GERD 3

  • Normal acid exposure: AET < 4.0%
  • Borderline: AET 4.0-6.0%
  • Abnormal: AET > 6.0%

• Hypochlorhydria: Defined as resting gastric pH > 2.9, found in approximately 11% of patients undergoing pH testing 4

  • There is an inverse relationship between gastric pH and esophageal acid exposure

Clinical Implications

• pH monitoring: When performing esophageal pH monitoring, the probe should be placed 5 cm above the upper border of the lower esophageal sphincter (LOS) 3

• Diagnostic significance: Understanding normal gastric pH is essential for:

  • Evaluating efficacy of acid-suppressing medications
  • Interpreting ambulatory pH monitoring results
  • Assessing potential for medication absorption (many drugs require acidic environment)

• Pitfalls to avoid:

  • Failure to consider regional differences in gastric pH when interpreting results
  • Not accounting for postprandial buffering effects when evaluating gastric acidity
  • Overlooking that most acid and non-acid reflux episodes occur at acid gastric pH, as measured 10 cm below the LOS 5

Understanding normal gastric pH values provides a foundation for diagnosing and managing acid-related disorders and properly interpreting pH monitoring studies.