Gut-Brain Connection May Slow Cognitive Decline in Older Adults

The human body is sometimes described as having ‘two’ brains: the large one inside the skull and a smaller counterpart in the gut.

Known as the gut-brain connection, the link between them has increasingly been viewed as a potential way to slow the cognitive decline that commonly accompanies ageing.

Gut-brain connection and cognitive decline

A new review bringing together 15 human studies published from 2012 to 2025 presents some of the most robust trials yet available on this topic. Although the combined evidence remains preliminary, it indicates that restoring balance to the gut microbiome could help prevent or lessen cognitive decline in older people.

The clinical-trial review included 4,275 participants aged over 45 from Europe, Asia, North America and the Middle East. All had dementia, cognitive impairment, or conditions associated with a heightened risk of cognitive decline.

Some participants received treatments intended to alter gut microbes. These ranged from indirect dietary approaches, including the Mediterranean diet, ketogenic diet and omega-3 supplements, to more direct methods such as probiotics, prebiotics and faecal microbiota transplants.

Control groups were given a placebo, usual care, or different nutritional interventions.

Overall, people receiving an intervention designed to modify the gut had more diverse gut microbes. They also experienced larger gains in memory, executive function and overall cognition. This was especially apparent among people with early or mild cognitive impairment, while benefits were limited in advanced Alzheimer's disease.

"The reviewed interventions appear to mediate cognitive benefits by modulating the gut microbiota and its metabolic outputs," the review authors conclude. The team was led by researchers in Italy and Spain.

While encouraging, the findings require longer-term randomised controlled trials to establish more confidently how each intervention works and why.

Gut microbiome interventions, including FMT

Faecal matter transplants are leading-edge procedures that remain largely experimental, yet they produced some of the most notable results in the review.

In one study included in the review, five patients with Alzheimer's received a single transplant. Subsequent stool samples from these patients showed greater gut-microbe richness, and all five participants improved on two cognitive assessments covering memory, attention, language and problem-solving.

"Compared with dietary or probiotic interventions, FMT appears to induce more rapid and pronounced microbial shifts, though long-term stability and safety remain uncertain," explain the review authors.

Dietary adjustments and nutritional supplements carry fewer risks by comparison. Although they can take longer to have an effect, they may nevertheless be beneficial.

For example, some recent twin studies have reported that particular plant-fibre prebiotics, which encourage ‘good bacteria’ in the gut, can improve brain function in older adults.

Other research indicates that probiotics, which introduce ‘good bacteria’ directly into the gut, may assist with mood disorders or stress, though high-quality clinical research is still scarce.

Several small randomised trials in the current review also backed the view that probiotic and other ‘synbiotic’ interventions "improve executive function, memory, and verbal fluency, alongside increased microbial diversity and altered neurotransmitter pathways."

In addition, older adults following a Mediterranean diet containing olive oil or mixed nuts achieved significantly higher cognition scores than low-fat control groups.

How the gut microbiome may affect the brain

The review did not investigate why some interventions improved the gut microbiome and brain function more effectively than others, but its authors set out several possible explanations.

Certain substances made by gut microbes, including short-chain fatty acids, are thought to potentially offer anti-inflammatory and neuroprotective effects.

Increasing beneficial gut bacteria could also help repair ‘leakiness’ in the intestinal barrier. Without this protection, microbes can escape from the gut and trigger inflammation.

Alternatively, gut microbes may influence the immune system or sleep, both of which are closely linked with dementia.

Research remains limited, but it is becoming apparent that unseen rhythms in the gut can influence the brain, just as rhythms in the brain can affect the gut. Some scientists now contend that this two-way communication is powerful enough to count as a separate sense: the sixth in our species’ repertoire.

A fuller understanding of this hidden sense could create opportunities to treat not only dementia, but also a plethora of other health conditions.

The study was published in Nutrition Research.