Whether you're taking them to recover from a round of antibiotics, or simply to boost overall gut health, they may not have the intended effects.
Credit: Nungning20

Over the last few years, researchers have learned more and more about just how important our microbiome—the collection of bacteria that live both on and inside us—has to do with our overall health. Studies have found they affect everything from our cholesterol and how we store fat, to our chances of contracting Parkinson's disease, and more.

What research hasn't found, however, is whether eating foods that are high in specific microbes—foods that are often marketed as probiotic—help your own microbiome in any way. Despite the lack of research, a giant industry has sprung up, pitching everything from classic fermented foods like sauerkraut and yogurt, to microbe-infused foods like tea, trail mix, chips, and more.

Now a couple of studies out yesterday, from the journal Cell, are suggesting that people be a little cautious before embracing this profusion of probiotic-supplemented packaged food.

One study, by researchers at the University of Israel, looked at how the microbiomes of both humans and mice fared on probiotic foods after a round of antibiotics. It's known that antibiotics, while necessary to treat certain illnesses, can wreak havoc on a person's gut microbiome, so the researchers tested how well probiotic foods helped to reconstitute the personalized collection of bacteria.

What they found was that regular use of probiotics actually delayed a full reconstitution of the microbiome—so that it took longer for the gut to return to normal.

At first blush, this may seem counterintuitive—how would adding microbes delay things? But on closer inspection, it makes sense. After all, the microbes you're ingesting when you eat probiotic foods aren't your microbes. They're not necessarily the ones that your own gut wants or needs in order to rebuild itself. So by taking probiotics, there's a good chance that you're simply adding microbes that your gut system has to then fight off at the same time that it's attempting to reconstitute itself.  

And, in fact, the study also found that a round of antibiotics significantly helps bacteria from probiotics colonize a human gut—but that a gut colonized by probiotic bacteria was not the same as one that had fully recovered.

The study found that there was one surefire way to quickly reconstitute a microbiome after a round of antibiotics, however: An aFMT, or "autologous fecal microbiome transplantation." This is also known as a "fecal transplant," and is exactly what you are now imagining it is: Taking bacteria from a healthy stool sample, and introducing it directly into the colon.

Another study by some of the same researchers found that when people who simply take probiotics as a way to stay healthy, the microbe strains often have little to no effect on a person's microbiome—and when they do have an effect, it is often transient, meaning that without continued additions of probiotics, the new bacteria disappear and are replaced by the original bacteria, so they have a very limited impact.

This also makes a certain amount of sense, because not all bacteria are the same. In fact, the microbiotic makeup of your gut is highly personalized—as much so as a fingerprint—and contains a wide variety of bacteria. In contrast, the microbes in your average cup of yogurt, are pretty commonly one of only a few varieties: Lactobacillus delbrueckii, Streptococcus thermophilus, and maybe one or two others. While these strains make delicious fermented milk, and are certainly not dangerous, it's never been obvious that they can make up for the 500-1,000 different species of bacteria that live in the average healthy person's gut.
All of which is to say that, yes, yogurt is nutritious and certainly delicious. But whether you're recovering from a round of antibiotics and want to rebound as quickly as possible, or are just trying to do a solid for the little guys that keep things running smoothly inside you, it may make more sense to focus on prebiotics (the foods that your microbes actually digest, like fiber) instead.