These products take in about $1.3 billion annually, but some researchers say they don't do anything.

Probiotics are hot. People are spending a lot of money on supplements and foods like yogurt and kefir—about $1.3 billion annually—in hopes of improving their microbial profiles. Unfortunately, a group of researchers thinks these products might be "a waste of money."

Long held to be digestive aids, probiotics are live bacteria and yeasts thought to maintain or improve the balance of organisms—by displacing the "bad bacteria"—in your gut. However, researchers at the University of Copenhagen are now suggesting that these stomach helpers aren't quite as effective as previously thought, particularly in people who don't have bacteria-related illnesses.

This conclusion was based on a review a variety of small studies (in groups ranging from 21-81 participants), in which subjects were fed different strains of bacteria from varying foods and monitored for the resulting effects on their bodies.

The scientists in Copenhagen noted that among participants suffering from gut-bacteria-related illnesses, some found relief in the probiotics. However, for people with healthy, functioning digestive systems, the probiotics didn't tend to have any particular health benefits—meaning that healthy people shelling out in the yogurt aisle might be better off putting their money elsewhere.

While the scientists themselves noted that a study with a larger test group is needed to back up their research on probiotics, their most recent conclusion was that "no convincing evidence exists for consistent effects of examined probiotics... in healthy adults, despite probiotic products being consumed to a large extent by the general population."

As Fortune points out, some research has shown that diets high in fruits and vegetables and low in meats and carbohydrates have the potential to improve the state of your microbiome (the civilization of bacteria living in your intestines). Other work suggests that leafy greens, coffee and even beer have benefits. But there's no evidence to say that any of these foods are any more or less beneficial to your gut than products explicitly marketed for their intestinal flora-altering abilities.