It also appears that a veggie-laden diet can encourage beneficial bacteria naturally.
Despite these uncertainties, probiotics are big business, with 2012 sales up 24.5 percent to $947 million in the United States, according to Nutrition Business Journal. A 2012 survey of more than 10,000 supplement users by the independent testing service ConsumerLab.com found that 37.4 percent of women and 30.5 percent of men used probiotics. ConsumerLab.com also noted that probiotics were "one of the most expensive dietary supplements, with a daily dose often costing more than one dollar."
Two recent meta-analyses, examining previous research found positive results from to pairing probiotics with antibiotics. The Cochrane Collaboration, an independent research group, evaluated probiotics for treating Clostridium difficile-associated diarrhea (CDAD), which can develop when taking antibiotics. "C. diff." can cause chronic diarrhea, abdominal pain, and intestinal inflammation, all of which can be fatal. The analysis found that probiotics given with antibiotics, cut the risk of developing CDAD by 64 percent.
A second meta-analysis from the nonprofit Rand Corp. found a "clear beneficial effect of probiotics in preventing or treating antibiotic associated diarrhea," says study coauthor Sydne J. Newberry, a nutritional researcher at Rand. Close to a third of patients who take antibiotics suffer from diarrhea that prevents them from following through with a full course of treatment. Interest in whether probiotics might help or prevent this type of diarrhea has grown in recent years, resulting in many studies.
"The evidence suggests that there is little risk for people without serious health issues," Newberry says, "and there is at least the potential for a beneficial effect."
Yet she also notes it can be hard for consumers to figure out which strain of probiotics can be most effective in treating a specific condition. Researchers often use their own blends of probiotic strains that may not be available commercially. Because of this, the Rand data suggest that "a product that contains multiple types of organisms has a better chance of being effective than a product with just one type of organism," she said.
Although probiotics can be beneficial, they are not drugs and are not regulated by the FDA as drugs. It falls to doctors and consumers to figure out what type will be most effective for what medical issues.
Faten N. Aberra, a gastroenterologist at the University of Pennsylvania, recommends specific brands based on experience: Align probiotic supplement for irritable bowel, Florastor for C. diff. infections.
"I don't tell them to take just any probiotic," she says. "There is an amazing variability in benefits among different brands."
In a recent report for subscribers, ConsumerLab.com named products that live up to their labels. The bottles listed anywhere from one to more than 30 strains of Lactobacillus, Bifidobacterium, Streptococcus, or Saccharomyces (a yeast). Tests also found that, "out of 19 probiotics, five contained only 16 percent to 56 percent of the listed amounts of organisms."
"Supplements may or may not have the counts that are advertised, because there isn't any oversight of supplements," said Marion Nestle, professor of nutrition at New York University. "Regulation would involve setting standards for the type of bacteria and their levels. I'm not sure we have the science yet on what it takes for probiotic bacteria to have an effect."
Since probiotics are live cultures, shipping or handling at the warehouse or on store shelves can create problems. And then there is the question of whether cultures remain live as they travel to the stomach.
"Most studies of yogurt show that the bacterial counts are very high as advertised," Nestle says, "but drop precipitously when the yogurt is frozen."
Consumers should carefully read the strain of the probiotic in the bottle and check the amount of probiotic cells in a bottle or container, and pay close attention to the CFUs (colony-forming units) to see how many live microorganisms are in each dose. They should also consider refrigerating the bottle once they get home, and be aware that some companies stamp bottles with a notice that the lists of organisms are live "at the time of manufacture," which is no guarantee they will be live when sold.
For people who take probiotic supplements to promote health, Aberra suggests getting probiotics from food, rather than "flooding your GI tract with supplements." She recommends Greek yogurts and certain vegetables that produce "pre-biotics" that breed probiotics, such as leeks, garlic, and onions.
"If people want to maintain good health, focus on diet," she says. "A good diet will change the flora of the GI tract to be less inflammatory, creating good antibodies that will protect you from pathogens. Good nutrition will lead to good bacteria."