Probiotics—live microorganisms that are consumed as supplements or food for purported health benefits—are a popular OTC remedy for various gastrointestinal (GI) ailments and other conditions, but the evidence supporting their use is mixed. Probiotics interact with the normal flora of the human body. They are believed to act by multiple mechanisms to deliver beneficial effects, including providing a protective barrier, altering intestinal pH to favor the growth of nonpathogenic bacteria, enhancing the host’s immunologic response, producing antimicrobial substances, and directly competing with pathogenic bacteria for receptors in the GI tract1 (see “The normal human intestinal flora,”).
In the United States, Lactobacillus and Bifidobacterium are the probiotic genera that are most commonly used. (For a list of the specific probiotic species found in five popular products, see Table 1.2-6) The review that follows examines the evidence for using probiotics for select GI ailments, including several types of diarrheal illness, inflammatory bowel disease (Crohn disease and ulcerative colitis), and irritable bowel syndrome (IBS). These findings are summarized in Table 2.1,7-21
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