By using systematic review method, scientists revealed that higher amount of two types of gut microbiota and a diverse microbiota composition link to better cognitive ability among cognitively healthy seniors. These findings suggest a future possibility of using gut microbiota as a biological marker for detecting Alzheimer’s before cognitive symptoms occur.  

Microbiota is consist of microorganisms, such as bacteria, fungi, and viruses. The ones that live in our gastrointestinal system is called gut microbiota. It generally contains six types, including Actinobacteria, Bacteroidetes, Firmicutes, Fusobacteria, Proteobacteria, and Verrucomicrobia, among which Bacteroidetes and Firmicutes are the main types. 

There have been extensive studies on gut microbiota and cognitive ability among the elderly with impaired cognitive ability. However, data on how gut microbiota affects cognitive ability in healthy older adults are lacking currently.

To fill this gap, a research team from Poland performed a systematic review. Researchers use systematic review method to review all available literature published within a certain time period to answer a specific research question. Evidence generated with this method are the strongest, compared with other types of study in the scientific evidence hierarchy system, such as randomised controlled trial. 

With the predefined criteria, from the initial over 1,000 papers identified, they eventually confirmed six studies that met all the criteria after four rounds of screening. A total of 1,835 individuals, with an average age ranging from 63 to 74, were from those studies. 

The researchers reviewed the study participants’ microbiota data analysed based on their stool samples. The team also reviewed two methods, neuropsycological test and electroencephalography (EEG), used in the six studies for evaluating individuals’ cognitive ability. Specifically, neuropsycological test is for measuring a person’s executive function, memory, information processing speed, and language abilities. EEG was for detecting unusual changes of electrical activities in the brain or brain waves. 

Among the six types of gut microbiota, Verrucomicrobia and Actinobacteria showed consistent patterns in their links to cognitive ability changes. A higher number of Actinobacteria associated with improved short-term memory of spatial information and memorization speed. Higher amount of Verrucomicrobia linked to better verbal memory, visual scanning, short-term memory, and cognitive flexibility.  

The older adults with diverse gut microbiota had better attention function, meaning better ability to allocate their mental resources and prioritize information for the brain to process. The participants also showed better ability to focus on one activity for a long period of time, also known as sustained attention. 

This study was the first to explore the associations between gut microbiota composition and cognitive ability among older adults with normal cognitive ability. While, the six studies reviewed here are all cross-sectional type, which means that researchers analysed the data from a population at a single time point. And systematic review method does not involve statistical analysis. Future research will require longitudinal studies that have a long follow-up period among the participants, and meta-analysis that combines and analyses published evidence with statistical methods. 

This study was published in Nutrients. Image credit: Canva

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