Two new studies, published Thursday in the journal Cell, call into question the universal effectiveness of taking probiotic foods or supplements to boost digestive health.
The research also suggests that consuming probiotics after antibiotic use (to restore the gut’s ecosystem) may not be completely harmless, as was previously thought.
This research isn’t the first to show that probiotics — foods or supplements that contain live microorganisms to supposedly enhance the “good” bacteria in the gastrointestinal (GI) tract — don’t live up to the health hype that’s surrounded them. The two studies are apparently the first, however, to test the supplements’ effectiveness by examining microbe activity within the GI tract itself rather than in stool samples.
“People have thrown a lot of support to probiotics, even though the literature underlying our understanding of them is very controversial,” said Eran Elinav, the study’s senior author and an immunologist at the Weizmann Institute of Science in Tel Aviv, in a press release. “We wanted to determine whether probiotics such as the ones you buy in the supermarket do colonize the gastrointestinal tract like they’re supposed to, and then whether these probiotics are having any impact on the human host.”
Probiotic supplements are very popular with American consumers. According to background information in the study, almost 4 million adults take either probiotics or prebiotics (foods that are said to feed healthy bacteria in the gut), and about 60 percent of U.S. health care providers prescribe probiotics to their patients.
The food and supplement industries have taken note. Probiotic supplements are now sold not only in pill form and in “traditional” probiotic foods, such as yogurt, but also in such products as “sodas, chews, cubes, liquids, gummies, fizzy powders, and prefilled straws.”
Americans spent $1.4 billion on probiotic supplements in 2014.
Persisters vs. resisters
In the first study, Elinav and his colleagues had 15 volunteers undergo upper endoscopies and colonoscopies to obtain baseline samples of their microbiome, the mix of bacteria, viruses, fungi and other microbes that live in the digestive tract. They then randomly divided the volunteers into two groups. One group received a daily supplement containing 11 of the most popular strains of probiotics. The other received a placebo.
After three weeks, the volunteers underwent another round of endoscopies and colonoscopies. The researchers found that in some of the people — dubbed “persisters” — the probiotics took hold and successfully colonized their GI tracts. But in others — “resisters” — the probiotics were expelled without altering their GI tracts in any meaningful way.
Elinav and his colleagues were also able to predict from a person’s baseline microbiome and their gut gene expression profile which group they would fall into. They also found that the resisters seemed to have a strong immune response to the probiotics.
The researchers then analyzed the volunteers’ stool samples. They found that those samples only partially correlated with what was going on inside the GI tract.
“Although all of our probiotic-consuming volunteers showed probiotics in their stool, only some of them showed them in their gut, which is where they need to be,” said Eran Segal, one of the study’s authors and a computational biologist at the Weizmann Institute, in the press release. “If some people resist and only some people permit them, the benefits of the standard probiotics we all take can’t be as universal as we once thought.”
In the second study, the researchers looked at whether probiotics help repopulate the gut microbiome after it is altered by antibiotics. It’s been believed (but not proven) that this use of probiotics helps reduce the risk of diarrhea, which is a common side effect of antibiotics.
Twenty-one volunteers were given a course of broad-spectrum antibiotics for a week and then randomly divided into three groups. One group (the “control”) was left to let their microbiome recover on its own, without any kind of intervention. The second group was given the same 21 strains of probiotics used in the first study. The third group was given a fecal transplant containing their own gut bacteria, which was collected before they took the antibiotics.
The volunteers underwent endoscopies and colonoscopies. Those examinations showed that the probiotics easily colonized the guts of the people in the second group. But, much to the surprise of the researchers, a worrying development occurred. The probiotics significantly delayed — by months — the gut microbiome from returning to normal. The probiotics also appeared to alter the expression of the genes in the GI tract — changes that, if they persisted, could put people at greater risk of allergies and inflammation, according to the researchers.
By contrast, the microbiomes of the people in the other two groups returned to normal much more quickly. In fact, the microbiomes of those who received transplants of their baseline gut bacteria recovered within days.
“Contrary to the current dogma that probiotics are harmless and benefit everyone, these results reveal a new potential adverse side effect of probiotic use with antibiotics that might even bring long-term consequences,” said Elinav. “In contrast, replenishing the gut with one’s own microbes is a personalized mother-nature-designed treatment that led to a full reversal of the antibiotics’ effects.”
Of course, these studies have a major limitation: They involved a relatively small group of participants. Still, they are the only ones to look at the effect of probiotics on what is going on in the gut itself. And they support other research that has also questioned the widespread belief that probiotics benefit everybody.
The consumption of probiotics is “useless in many cases,” stressed Segal. For such products to be effective, he added, they will most likely need to be personalized for each individual.
FMI: You can read both studies on Cell’s website.