In vitro tests overestimate in vivo neutralizing capacity of antacids in presence of food.

The neutralizing capacity of two antacids (Alucol = A, Syntrogel = S), differing both in their composition and theoretical neutralizing capacity, was evaluated in vitro and in vivo. In vitro at pH 3.5, 1 ml of A or S neutralizes 3.9 and 1.6 meq of acid, respectively, in an aqueous solution. When tested in vivo in the absence of food during near maximal acid secretion, induced by impromidine, 60 ml of either A or S reduced the 4-hr mean H+ activity by 83% and 65%, respectively. In contrast, the reduction of the 12-hr H+ activity observed after repeated administration of 30-60 ml of A or S at the end of the postprandial hour failed to reach significance with both preparations. This suggests that interaction with food produces a considerable loss of in vivo antacid neutralizing capacity, not quantitatively predictable from in vitro tests.