Probiotics are all the rage—the so-called "good bacteria" teeming naturally in foods such as yogurt and sauerkraut or worked into drinks and pills comprise a growing industry that Fortune reports posted more than $1 billion in annual sales in the US last year. It's expected to grow a whopping 15% this year alone. But the science of studying how billions of good and bad bacteria alike interact in our bodies is obviously complicated and slow, and critics argue that there isn't sufficient evidence to market probiotics as having any positive health benefits—some even suggest that these bacteria are killed by stomach acids before reaching the intestine. Now a meta study out of Denmark published in the journal Genome Medicine reflects this skepticism.
"There is little, if any, evidence of an effect of probiotic treatment in circumstances where the microbiota is unperturbed," researchers write. There are, however, some caveats. First, all studies reviewed were extremely small, with sample sizes ranging from 21 to 81, which calls into question any results, reports the Guardian. And while there doesn't appear to be any change in the fecal matter of healthy adults using probiotics, some effect was observed in people with "perturbed" microbiota, including "alleviation of gastrointestinal symptoms." People on antibiotics, for instance, may reap some benefits from eating yogurt. But until better studies are done, the researchers conclude that if you're healthy, you shouldn't waste your money. (Researchers have found a way to help restore some of the good bacteria that C-section babies are missing by not navigating the birth canal.)