Is Yogurt Good for You?
The pros and cons of probiotics.
Good bacteria are having their moment—in yogurt, pizza, ice cream, even mattresses and aftershave. Dannon says that its probiotic yogurt Activia " helps regulate your digestive system," while its yogurt drink DanActive helps " strengthen your body's defenses." Jala asserts that its bacterially dosed ice cream fudge bars have "all the benefits of probiotics," again citing healthy digestion and an immune boost. Over at Naked Pizza, which boasts baked-in, heat-resistant bacteria, one of its co-founders blogs about how humans "came barefoot, naked and covered in bugs," even conjuring a woman squatting in childbirth until her newborn lands in some all-natural maternal feces. Then there are probiotic products purporting to fight bed bugs or alleviate the symptoms of autism.
No wonder the backlash is roiling. The Federal Trade Commission has gone after Dannon for overplaying the benefits of its probiotics. (The company reached a settlement though it did not concede any wrongdoing.) An army of lawyers has also attacked General Mills for advertising that its Yoplus-brand yogurt provides digestive health benefits. They argue that these claims lack scientific support and are " reasonably likely to mislead the public." The New York Times recently pilloried " foods with benefits," including probiotics, as well, questioning many of the rosy health claims made on their behalf. Even Stephen Colbert quipped last week about yogurt "supposedly full of good bacteria," saying, " Don't trust 'em. I always throw in a spoonful of Purell first."
But we shouldn't be too quick to throw the good bacteria out with the bad. Probiotics are, by definition, "live microorganisms which when administered in adequate amounts confer a health benefit on the host." But the diversity of these organisms—and the scope of their possible benefits—is much more extensive and complex than a quick stroll through the grocery store might suggest. Even different strains of a well-known genus like Lactobacillus can have different effects on the body, which makes careful research—and precise communication—critically important. Academic work suggests, for instance, that certain strains of probiotics may help to address a range of gastrointestinal ills, especially diarrhea.Alluring, if nascent, work also looks at different bugs' effects on the immune system, including their potential to fend off common infections or prevent allergic conditions in babies.Some papers speak to preventing sickness in healthy people. Others focus on treating disease. The problem isn't with any of this budding science. It's with marketing claims that exaggerate or are too vague to offer real guidance. (What does it even mean to "balance" digestion?) Beyond the fuzzy ad-speak of functional foods, it's time to pin down the real potential of these critters, attending to questions of safety and dosage and taking them seriously as medicine.
The literature on probiotics holds particular promise when it comes to the gut. Strong evidence suggests that several strains can help to treat or prevent diarrhea associated with viral infection or antibiotics. Less established, though fascinating, is the possibility that some may help relieve irritable bowel syndrome or treat disease linked to the superbug Clostridium difficile. The hope is that when our gut's ecosystem is thrown out of whack, specific probiotics might help rejigger the balance, allowing us to tend our own buggy gardens.
What about the immune system? Good bacteria may tweak the balance of immune cells or cause more cells to become activated, at least temporarily. In theory, this might help to fend off disease.Of course, "most people aren't as interested in, for example, how activated their macrophages might be as they are in keeping from getting sick," as Mary Ellen Sanders, a probiotics consultant who runs the company Dairy and Food Culture Technologies, puts it. The few studies that look at whether probiotics can help prevent common illness tend to find very modest benefits: A randomized trial of Finnish toddlers, for instance, suggested that those drinking a specific probiotic milk three times a day, five days a week, had about one sick day fewer over the course of seven months. It remains to be seen whether different strains (or combinations) might pack a bigger punch. At the same time, researchers are asking whether various bugs might help to prevent allergy if given early enough to breast-feeding mothers and babies, or whether they might reduce inflammation. None of this work is definitive, but it is intriguing early science.
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.
Amanda Schaffer is a science and medical columnist for Slate.
Photograph of woman eating yogurt by Stockbyte/Thinkstock.