Not only is the calcium carbonate in Tums reducing the absorption of Calcium because you need acid in order to absorb Calcuim but the lack of acid also affects your absorption of Vitamin B12 and Zinc.
http://ezinearticles.com/?Getting-Enough-Calcium-Into-Your-Bones?-Ten-Facts-You-Need-to-Know!&id=851711
And low gastric acid is thought to predispose your stomach H. Pylori and H. pylori colonization increses gastric ph ( from acidic to non-acidic or basic)
And the proton pump inhibtor and H2 blockers that they give for GERD cause the same effect
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Read the Article at HuffingtonPost
