Yoghurt drinks and probiotic supplements are bought by millions to top up the “good” bacteria in their gut.

A host of health benefits have been ascribed to having a well cultured “microbiome”, including tackling heart disease to improving mental health.

But Israeli researchers have conducted one of the first studies to actually sample the bacterial mix that resides in the lining of the stomach and intestines instead of what is excreted.

They found that most people given probiotics in the trial these bacteria were only fleetingly resident in the gastrointestinal tract and were mostly unable to establish a new population amongst all the other bacterial species.

“Surprisingly, we saw that many healthy volunteers were actually resistant in that the probiotics couldn’t colonise their gastrointestinal tracts,” said Professor Eran Elinav, an immunologist at the Weizmann Institute of Science in Israel.

“This suggests that probiotics should not be universally given as a ‘one-size-fits-all’ supplement,” he added.

The team also found that taking probiotics after receiving antibiotics could disrupt this internal environment and potentially cause problems that can add to metabolic disorders like diabetes and obesity.

Because antibiotics wipe out the natural mix of bacteria in the gut, the probiotic supplement microbes rapidly take over, and take months to return to a healthy balance.

“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,” Professor Elinav said.

The study, published in the journal Cell, says that 3.9 million people in the US alone regularly take probiotic supplements and 60 per cent of health providers prescribe them to patients.

But despite expert support the scientific literature has very conflicting findings about their benefits and that was particularly true about the supermarket-bought brands.

To investigate the effects the authors recruited 25 volunteers to undergo deep endoscopy (a tube from the mouth into the stomach) and colonoscopy (along the large intestine) to build a picture of the bacteria at different stages of the tract.

From this group 15 volunteers were assigned to receive either a placebo or a probiotic pill containing common supermarket strains like lactobacillus and bifidobacterium, twice a day for a month.

From stool samples and further endoscopies they showed that some of the 11 bacterial strains in the probiotic established themselves, but many of the participants were entirely “resistant” and had no benefit.

“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,” says Professor Eran Segal, another author of the study. “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.”

The team found they were able to predict from genetic markers and samples who was likely to be resistant and this raises the prospect of tailoring probiotic drinks to people to help tackle metabolic disorders like obesity and diabetes.

The Independent

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