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No clear evidence probiotics can help with human anxiety, study finds

Lactobacillus bacteria observed in a scanning electron microscope
Lactobacillus bacteria observed in a scanning electron microscope. Photograph: AP

There is no clear sign that taking probiotics can help dampen feelings of anxiety in humans, according to new research, despite evidence that it works for rodents.

A wide range of conditions, from obesity to asthma, have been linked to the microbes living in our guts, with a number of studies suggesting a link to mood and behaviour. As a result, there is a burgeoning interest in psychobiotics: using beneficial bacteria known as probiotics to tinker with the gut’s microbes to affect brain health.

But researchers who have examined evidence from previous studies say that while probiotics appear to reduce anxiety in rodents with various problems, there is little to show that they offer similar benefits to humans, whether healthy or not.

“If people are suffering from anxiety ... probiotics should not be the solution they look for. They should definitely seek professional treatment,” said Daniel Reis, first author of the research from the University of Kansas, noting that both therapy and medication are available.

Reis and colleagues looked at 22 studies involving a total of 743 rats and mice, and 14 involving a total of 1,527 humans, and analysed the data to see if, overall, probiotics reduce anxiety.

The results, published in the journal Plos One, show that while such a link was seen among “diseased” rodents – where researchers had exposed the animals to early life stress, infection or other induced conditions – it was not seen overall in healthy animals. Beneficial effects in animals were consistently linked to one type of bacteria – Lactobacillus rhamnosus – although individual studies suggested a number of other species and strains might have an anxiety-reducing effect.

By contrast, when the studies were taken together, no beneficial effect was seen for humans, whether healthy or with conditions such as cancer, irritable bowel syndrome or mood disturbance.

That said, the team note that none of the studies involved individuals diagnosed with an anxiety disorder, adding that it might be that probiotics only help once certain levels of anxiety have been reached. They also note that for humans, anxiety was based on self-report – which can be unreliable – and that follow-up might not have lasted long enough for effects to be seen.

“Before we make any firm conclusions, we really do need to see these probiotics being tested in people who had clinically significant anxiety,” said Reis.

The team add that the doses of probiotics given to rodents were up to 100 times larger than those given to humans, once body weight was taken into account, suggesting that researchers should explore whether the lack of effect seen in humans might at least in part be down to probiotics not being given in high enough doses.

What’s more, one of the few human studies that did show some evidence of an effect involved Lactobacillus rhamnosus – a finding the team says should be further explored.

Prof John Cryan, a neuropharmacologist and microbiome expert from University College Cork who was not involved in the research, disagreed with the study’s conclusion, saying there was some evidence that probiotics might help tackle anxiety in humans.

“What [this study] highlights is the importance of bacterial strain selection in mediating such effects. Researchers have long known that only specific strains will have beneficial effects and that most don’t,” he said, adding that clinical trials are needed to explore whether particular microbes offer a health benefit and can hence be termed “probiotics”.

Cryan noted that while his team have previously found that one type of Bifidobacteria longum reduces anxiety in humans and animals, other types of Bifidobacteria do nothing. “By ‘lumping’ all strains together in the one analysis, any potential effect is completely masked,” he said.