The New Complete Book of Food

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Food/Drug Interactions

False-positive test for occult blood in the stool. The active ingredient in the guaiac slide test for
hidden blood in feces is alphaguaiaconic acid, a chemical that turns blue in the presence of
blood. Alphaguaiaconic acid also turns blue in the presence of peroxidase, a chemical that
occurs naturally in turnips. Eating turnips in the 72 hours before taking the guaiac test may
produce a false-positive result in people who do not actually have any blood in their stool.

Turnips