If you think that your digestive system—and its regularity or lack thereof—have lately been the focus of an awful lot of TV commercials, you're right. Probiotics, live microbes that may bring health benefits, are clearly a hot topic in nutrition (U.S. News has written about them here and here). Predictable food industry marketing hype aside (the New York Times had a great take on this last year), the bulk of the evidence does suggest that for some conditions, especially digestive ailments, they can be helpful when taken in the proper amounts.
But two recent studies illustrate the point that very few things—OK, except maybe rainbows and puppies—are absolutely good for absolutely everyone. A study out of the Netherlands, published online by the Lancet last week, set out to find whether probiotics might prevent infections in a very sick group of people: those with acute pancreatitis. The hypothesis was that the probiotics, delivered via a feeding tube, might fend off some of the infectious complications that can make acute pancreatitis deadly. The result was surprising: The group of patients who took probiotics had more infections than those who received a placebo. And 16 percent of the patients in the probiotics group died, compared with 6 percent of the placebo group.
Despite the scary headlines that announced the news, there are a few important things to note. First, it's not at all clear what actually killed those patients—the probiotics aren't necessarily the culprit. Second, researchers have known for a while that introducing bacteria into very sick people can have bad results. So while the authors are correct to say that administering probiotics to people at risk of severe pancreatitis isn't a good idea and shouldn't be considered as "harmless" additions to liquid nutrition via a feeding tube, especially for the critically ill, it's important to remember that this study's recommendations pertain to a small group of people.
"These were not just regular people walking to the grocery store," says Gary Huffnagle, a probiotics researcher (and physician) at the University of Michigan who coauthored The Probiotics Revolution. "When people are this sick, your body is broken down so badly that you can die of infections from bread yeast." The severely ill or immunocompromised shouldn't be using probiotics without a doctor's supervision, he says.
In contrast to that study was another published last week, this one in the British Journal of Sports Medicine, which found that giving the probiotic Lactobacillus fermentum to elite male distance runners cut down on the number of days they experienced respiratory tract infection symptoms. I've written before about the impact of heavy endurance training on your body, including the immune system; according to an expert I spoke with for that story, immunity goes up when you work out moderately for about an hour, but immune function decreases when you work out for more than about 90 minutes. These young men qualified for being at risk; they trained more than 62 miles a week. Taking the supplements (in capsule form) didn't change their running performance, but it did cut down on their symptoms and illness severity compared with the placebo treatment.
Study author David Pyne, of the department of physiology at the Australian Institute of Sport, says his group is conducting follow-up studies in athletes to figure out what dose will bring the best effects and whether there's any performance benefit to be gained, as well as exactly how the probiotics work to boost immunity.
Most of us, of course, are neither critically ill nor elite runners—and so you should check out the broader stories I linked to up top if you're wondering whether probiotics might help you.