In the past, the pH chosen by most formularies to represent the conditions in the stomach is 1.0 (100 mM HCl). However, there is some evidence that the basal gastric pH can be surprisingly high. It has been reported that 35% of humans have a resting gastric pH of 6 or above103. Less than 2% of the human subjects had a resting pH below 1.5. Basal gastric pH in normal healthy students is around 1.8, but occasional cases of achlorhydria are seen. Meals markedly alter the pH, which can increase to 3-5 after eating, particularly if the meal contains large amounts of easily digested protein.
These variations in pH will be especially important when developing products designed to be gastro-resistant, e.g. enteric coatings for acid-labile or potentially irritant drugs. In these cases gamma scintigraphy may be combined with in vivo pH measurement to investigate the efficiency and operation of the enteric coating. In a study which administered both radiolabelled pH radiotelemetry capsules and radiolabelled enteric coated naproxen tablets to fed subjects72, it was found that the pH remained below 2 within the stomach, except for a transient rise after food. Five tablets disintegrated in the small intestine approximately 1.2 h after gastric emptying, 1 disintegrated in the stomach at pH 1.1 and 1 tablet remained intact in the stomach for 9 h.
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