Probiotics and Mental Health: A Review of Human Studies

As regular readers of this blog know, one of my particular areas of interest is the relationship of the gut biome to mental health, including memory and cognition. Very early on, if you remember, I told you the story of losing my grandmother to dementia, which was truly hell on earth. So today, another interesting review study for you, just published.   Researchers at the University of Reading, in the UK, combed through the medical literature to determine what we actually know to date about the relationship of probiotics to cognitive functioning.[i]  They looked at studies done on humans that involved at least one probiotic strain and that measured at least one cognitive outcome in memory, attention or executive functioning.  With these parameters, they were able to isolate 30 appropriate studies and using the data provided from them, grouped participants into 3 age categories:  infants and children; young and middle aged adults; older adults.

Their results were interesting.  Firstly, they followed the health of premature infants who received probiotics in the neo-natal intensive care units until they were discharged from the hospital for 18 months up to 5 years, and found that probiotics had no significant impact on their development. Two studies on full term babies also found the same:  probiotics seemed to have no significant effects.  They conclude though that these findings are not as significant as they might have been because no information was available in the data sets on whether or not the babies were breast fed.  Breast milk contains prebiotics which help good bacteria establish in the gut, so knowing what the infants were fed is pretty crucial in knowing how probiotics might affect infants and children in the future.  Also, we already know that giving antibiotics early in life often has last repercussions, but this doesn’t seem to have been taken into account when analyzing this data:  that is, for infants and young children given antibiotics, do probiotics make a difference?  Remember:  the probiotics were only given while in the neo-natal unit – not after (as far as these researchers know).  So to sum up:  no real conclusions could be drawn.

The results in the studies performed on young through middle aged adults were more significant.  Four studies were done on people with cirrhosis of the liver, which can lead to brain inflammation that manifests as impaired memory and intellectual functioning.  3 of the 4 studies failed to show, after 8-12 weeks of supplementation, that probiotics were in any way helpful.  One though found that in those with this kind of encephalopathy that probiotic supplementation caused significant improvement in cognitive scores after only 30 days.  In those with HIV, 6 months of probiotics led to significant improvements in standardized tests of memory, verbal fluency and working memory.  Similar results were found in those with fibromyalgia and chronic fatigue syndrome and/or major depression disorder.  Interestingly, in 1 study on those with chronic fatigue, four weeks of a probiotic along with an antibiotic (erythromycin) demonstrated improved attention, processing speed, memory, verbal fluency and cognitive flexibility.

In healthy adults though, results were mixed.  That is, some studies found that probiotics did absolutely nothing for cognitive tasks while others have shown some improvements in different measurements including, interestingly, stress-related processing.

In older adults, with mild cognitive impairment,  one study showed that 24 weeks of probiotics led to improvements in standardized tests and another showed significant improvements in 3 tasks related to memory and attention in those with mild cognitive issues after only 12 weeks of probiotic supplementation.  Three studies done on those with confirmed Alzheimer’s Disease showed mixed results.  Two found improvements in test scores after 12 weeks of probiotics, but another did not.   Results have also been mixed on studies on healthy aging adults:  one found significant improvement in tasks involving executive functioning but a second found no differences between test subjects and controls.

So, how to sum up the results?  Overall, there is definitely enough evidence at this point to state that there is indeed a relationship between probiotic use and improved cognitive functioning in adults:  “The evidence suggests there may be potential for probiotics to enhance cognitive function or attenuate cognitive decline, particularly in clinically relevant adult populations for whom cognitive dysfunction may be present.”  Of course, some of the results also show though that this may be more important in those with inflammatory-type illnesses than those who are healthy.  On the other hand, because these studies didn’t examine lifestyles and other co-founding factors (after all, studies can only test so much!), the results need to be understood with their limitations.  For example, in the studies done on healthy adults:  were the studies long enough?  What probiotics were people taking and how much?  What did their diets look like?  Did they smoke?  Were they overweight?  Did they exercise?  The list of things that can affect the biome is endless, really.

One doctor, who was not involved in the research, is quote as saying, “This seems to be a safe approach in middle-aged and older adults. […] I don’t think anyone is saying that probiotics will cure cognitive dysfunction in and of themselves, but they may provide a significant piece of the puzzle and may be significant, in terms of their contribution to improving these kinds of symptoms.”[ii]  He goes on to state, “This gives us something that we can do to stem the rising tide of […] dementia. Hopefully, [it] can be a key target of something that we can affect, either through lifestyle changes — what we eat and what we don’t eat — or through supplementation with different prebiotics, probiotics, and synbiotics.”

And that is, I think, is the take-away point for today!

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[i] Eastwood, J, Walton, G, Van Hemert, S, Williams, C, Lamport, D. The effect of probiotics on cognitive function across the human lifespan: A systematic review, Neuroscience & Biobehavioral Reviews, Volume 128, 2021,

Pages 311-327, https://doi.org/10.1016/j.neubiorev.2021.06.032.

[ii] https://www.medicalnewstoday.com/articles/could-probiotic-supplementation-improve-cognitive-function