Dementia: Can probiotics improve cognitive function?

cross section illustration of a human head with pills on top.
A review asks whether probiotics might improve cognitive function for people with dementia or mild cognitive impairment. Nikola Nastasic/Getty Images
  • Alzheimer’s disease is the most common type of dementia in the United States, affecting more than 6 million people.
  • The results of a meta-analysis suggest that probiotic supplementation may improve cognitive function in people with mild cognitive impairment.
  • The review adds extra information about the correlations between gut health and brain health.

Some studies hint that probiotics and prebiotics might slow the progression of some neurodegenerative diseases.

However, there are limited data on their effects in humans with Alzheimer’s disease or mild cognitive impairment (MCI).

Scientists from Jiangnan University in China collaborated to review the evidence to date. Their systematic review and meta-analysis appears in the journal Foods.

Overall, the authors report: “Consistently, our findings suggest that probiotic intervention at early stages of Alzheimer’s disease, such as MCI, could improve cognitive function and delay disease progression.”

Probiotics, prebiotics, and synbiotics

The International Scientific Association for Probiotics and Prebiotics (ISAPP) is an international nonprofit established in 2002. It seeks to advance scientific research regarding probiotics, prebiotics, and synbiotics.

The ISAPP defines probiotics as living bacteria “which when administered in adequate amounts confer a health benefit on the host.”

Prebiotics are a group of nutrients, or substrates, that encourage the growth of gut microbiota. Synbiotics are a combination of probiotics and prebiotics.

Identifying relevant studies

The team explored several databases for studies published between 1984 and early 2021 involving subjects with Alzheimer’s or MCI.

Zhu and colleagues sifted through 294 studies, ultimately including just eight articles for the systematic review and seven for the meta-analysis.

Cognitive function was the primary outcome of all the studies, measured by Mini-Mental State Examination (MMSE) scoring. Secondary outcomes included nutritional status, inflammation biomarkers, metabolic profiles, and oxidative stress.

Benefits revealed

The review’s authors found that probiotic supplements improved cognitive function in participants with MCI. However, the results were less impressive for participants with Alzheimer’s disease. Overall, the authors conclude:

“Compared with placebo or control interventions, probiotic supplementation considerably improved cognitive function in the participants with MCI, but it only caused a modest cognitive improvement in those with Alzheimer’s disease.”

The number of probiotic strains administered, their dosage, and the length of treatment played a role in the extent of cognitive improvement.

Probiotic supplementation also altered the structure and composition of fecal microbiota in people with Alzheimer’s.

The co-authors cite studies that show a reduced fecal microbial diversity in Alzheimer’s. However, only one study in the review showed that probiotic supplementation altered the structure and composition of fecal microbiota in people with Alzheimer’s.

Significant addition to research

Dr. David A. Merrill, Ph.D., is an adult and geriatric psychiatrist and director of the Pacific Neuroscience Institute’s Pacific Brain Health Center at Providence Saint John’s Health Center in Santa Monica, CA. He was not involved with this review.

In an interview with Micro B Life, Dr. Merrill explained:

“This meta-analysis — a study of studies — provides an important update about recent evidence showing that gut health may help brain function in older adults, especially those with mild cognitive impairment […] This is an important update on a topic which shows that our gut health impacts not only our systemic health but our emotional and cognitive well-being, as well.”

Dr. Merrill was intrigued by the review’s finding that prebiotics may enhance the production of brain-derived neurotrophic factor (BDNF):

“We know that BDNF levels decline in the brain of Alzheimer’s patients, and this loss of BDNF is thought to play a role in the increased death of neurons in the brain. So, perhaps indirectly, prebiotics may support the circulating levels of BDNF in the body, leading to greater cell survival in the brain.”

The study’s limitations

Zhu and fellow researchers listed several limitations to their meta-analysis. Firstly, some of the trials used small sample sizes.

Also, the chosen studies did not consider how other dietary supplements and lifestyles may have impacted the gut microbiota and metabolic profiles.

The studies included in the analysis also used different rating scales for cognitive impairment to measure cognitive function. Only four of the included studies referenced the commonly used NINCDS-ADRDA criteria for dementia.

Four of the studies failed to report any adverse effects of probiotic supplementation. While probiotics have been generally safe, older people are more susceptible to serious gastrointestinal, skin, and systemic side effects.

Zhu and the team also expressed concern that experimental design flaws and commercial funding may have skewed some results.

Guidance for future studies

The review’s authors hope that future research will discuss the adverse effects of probiotic supplements. They also want to see clinical trials that group and evaluate people with Alzheimer’s disease and MCI according to disease severity.

Further, Dr. Merrill stated: “Knowing the most effective dose of specific probiotic strains for different disease conditions like memory loss remains a challenge.”

The psychiatrist cautions that this meta-analysis does not prove that probiotics will help everyone with MCI. However, he said:

“If you’ve had a health stress to your gut like having surgery or needing to take antibiotics, one may consider taking probiotics for a time to increase the diversity of the good bacteria in the gut and return the gut to its healthiest possible state.”

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