- A recent review suggests that probiotics may improve cognitive function or lessen cognitive decline in adults but not children.
- The current studies available for analysis had limitations affecting data interpretation and conclusions.
- Robust evidence from large, well-designed randomized-controlled trials is needed to determine the effects and real-world applications conclusively.
The human microbiome is the community of bacteria, fungi, and viruses that inhabit our bodies, directly affecting our health.
Amazingly, scientists estimate that there could be similar numbers of microbes in the body as there are human cells.
The microbes that live on or in our bodies help protect us from organisms that cause disease. Some also promote immune system development and aid in digestion. Scientists believe that a two-way relationship between the brain and the gut exists, though they do not entirely understand it yet.
The brain-gut connection
When scientists modify the gut microbiome in mice experiments, it induces behavioral and cognitive changes. These changes also alter levels of substances essential to learning, reasoning, and memory in the mice.
Other studies suggest that changes in the gut microbiome make-up may contribute to the development and progression of cognitive impairment seen in Parkinson’s disease, schizophrenia, major depressive disorder (MDD), and Alzheimer’s disease.
Dr. Scott Kaiser, director of geriatric cognitive health for the Pacific Neuroscience Institute at Providence Saint John’s Health Center, who was not involved in the paper, spoke with Medical News Today:
“There’s a mounting body of evidence showing clear connections between our gut health and our brain health, and specifically within gut health […] the constellation of different types and proportions of bacteria.”
The new systematic review came from researchers at the University of Reading, in the United Kingdom. Their aim was to determine whether supplementation with probiotics — dietary supplements containing live microbes — benefit cognitive function.
They analyzed human studies with at least one live probiotic strain and at least one cognitive outcome performance measure: memory, attention, or executive function.
Their findings appear in the journal Neuroscience & Biobehavioral Reviews. The work was partly funded by Winclove Probiotics.
After applying the criteria, the researchers identified 30 studies that met their eligibility requirements. They grouped the results by age groups: infants and children, young and middle-aged adults, and aging adults.
Effects in infants
Three studies examined the effect of probiotic supplementation in infants born prematurely with very low birth weight. They received the probiotics from when they were first able to feed until they were discharged from the hospital.
The scientists followed these infants for periods ranging from 18 months to 5 years and found that probiotics had no significant impact on cognitive development.
Two studies conducted in full-term infants also reported similar results — probiotics had no significant effect on cognitive outcomes.
However, the review found that the studies in infants were limited due to a lack of data about whether the babies received breast milk. This is important because breast milk contains natural prebiotics, while formula does not.
Younger and middle-aged adults
Four studies assessed whether probiotic supplementation in people with cirrhosis — permanent liver scarring — affects the severity or development of hepatic encephalopathy.
Hepatic encephalopathy is a complication of cirrhosis that may cause impaired memory and intellectual functioning.
In one study, participants with hepatic encephalopathy who received probiotics for 60 days demonstrated a significant improvement after 30 days on all three cognitive tasks measured.
However, three studies found that in people with cirrhosis but no hepatic encephalopathy, probiotic supplementation failed to improve cognitive assessment scores after 8–12 weeks.
In a pilot study and a larger placebo-controlled trial that was not randomized, adult participants with HIV received probiotics for 6 months. Both studies found a significant improvement in standardized tests of immediate and delayed memory, verbal fluency, and visuospatial working memory.
In three studies that included people with fibromyalgia, chronic fatigue syndrome, or MDD, probiotic interventions were linked with improved cognitive outcomes. In the fibromyalgia study, participants took an 8-week probiotic treatment and demonstrated a reduction in impulsive choices.
Participants with chronic fatigue syndrome who received a 4-week course of probiotics plus an antibiotic, erythromycin, showed improved attention, processing speed, story memory, cognitive flexibility, and verbal fluency.
An 8-week study demonstrated improvements in short-term memory and visual search in participants with MDD who received probiotics with a serotonin reuptake inhibitor, a medication used for depression, compared with those who only received the serotonin reuptake inhibitor.
However, several studies testing for cognitive improvements in healthy adults had mixed results. Five showed inconsistent improvements in different cognitive measures, including stress-related processing, while two studies demonstrated no effect of probiotic supplementation.
Three studies investigated the effects of probiotic supplementation in adults with mild cognitive impairment. A pilot study found that 24 weeks of probiotic treatment improved mini-mental state exam (MMSE) scores. This is a screening test for cognitive impairment.
However, a larger follow-up placebo-controlled study showed significant improvement in MMSE scores in both the placebo and probiotic groups before treatment at 12 weeks.
A third study demonstrated significant improvements in three tasks related to memory and attention in participants with mild cognitive impairment receiving a probiotic supplement after 12 weeks.
Three studies in participants with Alzheimer’s disease who received probiotics for 12 weeks showed various effects on cognition. Two studies showed an improvement in MMSE scores, but one study, which used another test to measure cognition, did not find a significant effect.
Two additional 12-week studies measured the cognitive effects of probiotic supplementation in healthy aging adults.
The first study demonstrated a significant improvement in information processing and tasks involving executive functioning, compared with a placebo. The second found no difference in cognitive functioning between the group that received probiotics and the control group.
Dr. Vernon Williams, director of the Center for Sports Neurology and Pain Medicine at the Cedars-Sinai Kerlan-Jobe Institute, in Los Angeles, spoke with MNT about the review’s findings. He was not involved in the investigation.
As Dr. Williams explained, the review demonstrates that “There is evidence, particularly in adults, of a relationship between gut health and the use of probiotics and cognitive functions.”
“I’m happy to see that there’s more and more objective evidence,” he continued, “and I’m hopeful that this kind of information becomes more widespread and used by practitioners who are treating individuals across the lifespan.”
Although he expressed excitement about the results, Dr. Kaiser noted the difficulties of drawing conclusions from a systematic review. In particular, he was concerned that participants had illnesses with different degrees of severity. “So you’re kind of comparing apples and oranges — particularly when you include people with [mild cognitive impairment] and AD.”
He added, “It’s very difficult to rule out confounding factors — whether it was the gut microbiome or the probiotics, or some other dietary or lifestyle factor.”
Dr. Williams commented on the need for additional research, particularly larger controlled trials of specific probiotics with a wider range of cognitive assessments. “There’s still a lot to be learned, in terms of the mechanisms of action,” he observed.
“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.”
– Dr. Vernon Williams
Dr. Kaiser added: “Right now, we have nearly 6 million people in the [United States] with Alzheimer’s disease, and it’s predicted that within the coming decades, globally, there will be over 130 million people with dementia. This type of work that looks at potential modifiable risk factors and potential targets for prevention and treatment is critical.”
“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,” he concluded.
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