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Probiotics might not be doing what people think or hope they are doing to “correct” the balance of gut bacteria, Israeli researchers said Thursday.
In at least some people, the products pass straight through without leaving behind any “good” bacteria, and in others, they might actually disrupt the normal balance, the team reports in the journal Cell. This includes when probiotics are used to replace bacteria killed off by taking antibiotics, the team reported.
"Our results suggest that probiotics should not be universally given to the public as a 'one size fits all' supplement," immunologist Eran Elinav of the at the Weizmann Institute of Science said in a statement.
Millions of people globally take probiotics in the form of pills or drinks, both for general good health and after taking antibiotics that can indiscriminately kill both bad and beneficial bacteria.
The idea is to replace a faulty microbiome balance with a supposedly better one.
But no studies have shown which bacteria are truly beneficial, which are harmful, and whether adding certain microbes to the diet makes a difference. All that has been shown for certain is that transplants of fecal matter from healthy people can help patients with severe Clostridium difficile infections, by replacing the bad C. difficile germs with a different balance of bacteria.
A study out earlier this year suggested probiotic supplements could even be dangerous to some people with suppressed immune systems.
The Obama administration launched a national microbiome project in 2016, saying we need comprehensive understanding of what lives in our bodies, what the organisms do, and whether manipulating them makes any difference to health.
It’s hard to know what probiotics do and don’t do because what’s going on takes place entirely inside the digestive tract. So the Israeli team recruited about 40 volunteers for two intense and intimate experiments.
The first 25 had colonoscopies and stomach endoscopies using instruments to sample the bacteria in their upper and lower digestive systems. Then 15 of them got either a month’s worth of placebo or probiotic pills containing a typical mix of bacteria found in such products, including B. bifidum, L. rhamnosus, L. lactis, L. casei, and others.
Three weeks later, the researchers went back in to see what had changed.
In some people, nothing had changed at all. The supplements had just passed through their bodies, leaving little or no trace of the bacteria they are advertised as supplying.
In others, the bacteria did take up residence, they found, and in some cases changed the makeup of the population of microbes in the gut.
There’s not a lot of research showing that probiotics have a real benefit, even after people take antibiotics, so the researchers did a second experiment.
They gave antibiotics to 21 volunteers and again examined their stomachs and colons using endoscopes.
Then, seven of the volunteers did nothing, to see if their microbiomes recovered on their own. A second group got the probiotic cocktail, while the last seven got what’s called an autologous fecal microbiome transplant using bacteria taken from their own poop before they got the antibiotics.
As expected, the antibiotics killed off many bacteria in the gastrointestinal systems of the volunteers.
The probiotics took hold strongly in the 14 people who got them, showing that the supplements do indeed replace bacteria after someone takes antibiotics.
But the researchers were surprised to see that this may not necessarily be good. The balance changed in the volunteers who got probiotics, and their original balance of gut germs was not restored for at least five months, the team reported.
Those volunteers who got nothing eventually got their normal balance back, and the natural balance returned very quickly in those who got transplants of their own gut bacteria.
"These results reveal a new and potentially alarming adverse side effect of probiotic use with antibiotics that might even bring long-term consequences,” Elinav said.
“In contrast, personalized treatment — replenishing the gut with one's own microbes — was associated with a full reversal of the drugs' effects."
The researchers said probiotics may not necessarily be harmless, although they’ll need to study more people over time to see what health effects do come after taking probiotics.