The enzymes which aid in the digestion of proteins in the stomach is called pepsin. Pepsin needs an optimum pH of about 2 to be effective and that is why the hydrochloric acid is usually secreted in the stomach.
In the situation where an antacid is used and the acidity of the stomach is neutralized, then, the pepsin enzyme will not be able to perform its function because of altered pH and the protein content of the eaten food will not be digested leading to indigestion in the affected person.
