The problem comes when it collides with a food industry itching to exploit the multibillion-dollar market for functional foods. Many of the health promises that companies make about their products sound awfully vague, in part because of Food and Drug Administration restrictions on medical claims about food. Dannon, for instance, says that Activia "helps regulate your digestive system." But regulate it how? It turns out that two company-sponsored studies show that Activia may help to relieve "slow transit linked to occasional irregularity." (A layperson might call that mild constipation.)The Japanese company Yakult also says that its fermented dairy drink helps " balance your digestive system." But that mainly seems to mean it helps prevent diarrhea. The point is that "balancing digestion" or promoting GI health can mean different, even opposite, things. Consumers with a particular problem wouldn't have any idea which sort of digestive balance they might be in for. Of course, people may also take probiotics based on the hazy notion that they're good for you, like organic vegetables or raw honey. But are they really better at regulating your GI tract than the unsexy alternatives, like prune juice or high-fiber cereal?
Other claims, meanwhile, are simply bloated, especially when it comes to the immune system. Dannon is not outrageous for suggesting that its DanActive drink has an effect on that system: Some research does suggest that the relevant strain can give particular immune cells a boost. But that doesn't automatically mean it will keep you healthier. Company researchers in Europe have tried to get at that possibility—for instance, by giving a probiotic drink to elderly people and looking at their rates of common infectious diseases like colds, flus, and stomach viruses. (The strain they used, called Lactobacillus casei DN-114001, is the same one found in DanActive.) They found that each episode of sickness was shorter, on average, in people taking the drink: about six and a half days instead of eight days for those in the control group. So the probiotic did seem to spare them about a day-and-a-half of illness. Still, it didn't change the number of times they got sick or the severity of their illness. All of which might prompt consumers to give a bit of a shrug. (And some extra skepticism is always in order when so many studies in a field are company-funded.)
Questions of dosage also loom large, when companies try to blend the categories of food and medicine. When Dannon found that Activia improved occasional irregularity and slow transit through the gut, its research subjects ate the product three times a day. That's a lot of yogurt.
The more worrisome question, though, is what happens if a random mix of probiotics starts showing up all over our diets, making it hard to calibrate how much we're getting, and of what. Enthusiasts argue that common probiotic strains have excellent safety records. "Dealing with any living organism is not without risk," says Gregor Reid of the University of Western Ontario. "Considering the hundreds of millions of people who take them every day, we'd certainly know already if they caused any major problems." Still, amazingly few studies have been designed to assess safety, as a new report by the RAND corporation points out. According to its authors, no clinical trial of probiotics has yet to report an infection resulted from the treatment. However, studies seldom checked for the types of infections that have been identified in isolated case reports. In fact, they concluded that most of the major trials "did not state what adverse events were monitored and did not systematically address the safety of probiotic products." People with weakened immune systems or severe illness may also experience problems that others don't. In a notorious trial of patients with severe pancreatic disease, researchers found that those who received probiotics through a feeding tube were more likely to die than those who didn't. In a hospital context, as well, the probiotic yeast Saccharomyces boulardii often seems to remain on doctors' or nurses' hands after washing and can cause fungal blood infections, especially in patients who are very sick.
The point is that probiotics may have potent effects on the body. Like medication. We ought to give them their due—and not assume they'll only do what we want them to simply because they're natural, or served up sweet.