Lowering stomach acid would in theory lower bioavailability of vitamin C - since acid is required to promote the quickest route to the blood stream - through the stomach lining (See the Hickey/Roberts book THE RIDICULOUS DIETARY ALLOWANCE.)
I have read Cheraskin books describing methods of measuring vitamin C in the urine, from an old memory, silver nitrate comes to mind, but I would have to review the book. In the meantime a gross measure are the vitamin C strips sold by Wholesale Nutrition.
Owen R. Fonorow, Orthomolecular Naturopath
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