Chey, who worked on the study (and who has worked for p&g as a consultant), says the impetus for the research came from patients asking about probiotics for various gastrointestinal disorders. Chey regularly recommends probiotics to patients with certain chronic gastrointestinal problems, and he says there's good evidence to support the use of Align for irritable bowel syndrome, for example, and some evidence for the use of the yogurt Activia for irregularity. Activia's maker, Dannon, spikes the product with Bifidobacterium animalis lactis (trademarked as Bifidus regularis in the United States). One of several studies that have tested it found that women who consumed three servings a day of Bifidus regularis-containing milk moved food through their colons about 18 percent faster.
Probiotics are potential immunity boosters, too. The dairy drink DanActive, another Dannon product, contains Lactobacillus casei immunitas. A trial published last summer in the British Medical Journal showed that seniors who drink DanActive during and after antibiotic treatment are less likely to develop diarrheal illnesses such as Clostridium difficile infections, which are hazards of antibiotic use. Of patients who drank DanActive, 12 percent developed diarrhea, and none developed C. difficile; 34 percent of a control group got diarrhea, and 17 percent fell ill with C. difficile.
Other food companies have jumped on the probiotics bandwagon, adding bacteria to new kinds of products. Naked's probiotic juice contains a strain of Bifidobacterium lactis that several studies have found to protect the elderly against gastrointestinal infections. Last year, Kashi released a probiotic-enriched cereal, but the strain it contains has been subjected to few peer-reviewed published studies. (The company asserts that the bugs reach the intestine alive and promote an increase of friendly microbes.) Even baby foods have followed the trend. An infant formula from Nestlé is one of several such probiotic products to hit shelves recently. It contains bacteria that the company says are similar to those found in the digestive systems of breast-fed babies. Nestlé-sponsored studies suggest the formula boosts levels of key antibodies that help the immune system develop. Some experts are optimistic that probiotics may help formula-fed babies gain some of the immunity-building benefits of breastfeeding.
Prebiotics too. As probiotics have grown popular, prebiotics have emerged as another category of so-called functional food. Recent studies support the idea that certain compounds—the nondigestible carbohydrates inulin (also known as oligofructose), galactooligosaccharide, and lactulose—can stimulate the growth of friendly bugs in the gut. Prebiotics might have health benefits similar to probiotics, and they're easier to incorporate into one's diet: Consumers can get prebiotics without even buying special products, Huffnagle says, because many fruits, vegetables, whole grains, and other high-fiber foods naturally contain the stuff.
When choosing a probiotic product, experts advise, look for specifics, including the bacterial genus, species, and strain as well as the number of bacteria, usually measured in millions or billions of colony-forming units or CFUs. Vague statements like "proprietary formula" should be a red flag, they say. Since many probiotic products don't list the exact strain they contain—leaving consumers with no way of knowing whether the product is really effective—the best way to find an effective product is in some cases to experiment with several, Huffnagle says. Such trial and error, he says, "is tedious, but it's the only way to do it right now."
That's partly because the Food and Drug Administration hasn't established a definition for probiotic—so the presence of that word on a label has little practical meaning. Groups such as the International Probiotics Association, an organization of manufacturers, healthcare professionals, and scientists, are working to develop an independent certification system for probiotic products. Such a system would help ensure safety and quality, says Chey. "It's a little bit like the Wild West right now."