Do intestinal bacteria play a role?

Dementia is a common neurocognitive disease, but scientists are still learning what factors may predispose to – or protect against – its development. Recent research indicates that the gut microbiota can play a complex role. We spoke to two experts in neurocognitive and neurological disorders to learn more.

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What do we know about the role of the gut in dementia? Image credit: Lisa Schaetzle/Getty Images.

Dementia is an umbrella term for a range of progressive diseases that affect the brain. The most common types of dementia are Alzheimer’s disease, vascular dementia, mixed dementia, and dementia with Lewy bodies.

These conditions have similar characteristics, including reduced ability to think, remember, and make decisions and potential problems with communication and visual perception.

Biologically, dementia results from the damage or loss of nerve cells called neurons in the brain and their connections. When damage prevents neurons from sending and receiving messages effectively, it affects how the body functions. The symptoms experienced by each person vary depending on the type of dementia and the damaged neurons.

Infectious bacteria, viruses or fungi can damage neurons by activating inflammatory cells in the brain, called microglia.

The gut is home to a large number of microorganisms, including bacteria and archaea, which are collectively referred to as the gut microbiota or microbiome.

The four main groups of bacteria in the human gut are Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria.

However, the types and numbers of each group are not constant; instead the microbiota constantly changing. Various factors, such as diet, medication and disease, can affect its composition.

The human microbiota plays an important role in controlling normal body functions, including resistance to infection, improvement of metabolism, inflammation, and prevention against autoimmune diseases. Indeed, it has become clear that the microbiota affects gut and central nervous system (CNS) processes.

The communication network between the gut and the brain is called the gut-brain axis (GBA). The GBA is responsible for linking the emotional and cognitive centers of the brain to digestive functions. Recent studies have shown that these interactions can change depending on the composition of the gut microbiota.

Medical News Today spoke to Dr. Verna R. Porter, neurologist and director of dementia programs at Providence Saint John’s Health Center in Santa Monica, California, on the impact of the gut-brian axis on dementia.

Dr. Porter said gut microbes help break down nutrients from our food, explaining that “the digestive process of these microbes results in many different by-products, some of which eventually enter the brain.”

Specifically, she highlighted the neurotransmitter serotonin, which affects mood, cognition, learning and memory, noting that “90% of serotonin [..] is a metabolic by-product of the gut microbiota. [A]Appropriate levels of serotonin in the brain influence brain health attributes, such as happiness, mood regulations (eg, anxiety levels), and even sleep. [A] a healthy gut microbiome may have important implications for optimal brain health and well-being.

The interaction of intestinal microorganisms with body processes can be mutually beneficial. The immune system evolved to protect the body from resident microorganisms which, in turn, help process the food we eat.

Intestinal bacteria produce short chain fatty acids, which break down indigestible fiber from food. Compounds or metabolites produced by the process can reduce inflammation and strengthen the gut barrier. Specifically, butyrate molecules can strengthen the blood-brain barrier, which neurodegeneration can disturb.

If the microbiota loses its balance – a state called dysbiosis — it can promote systemic inflammation. Dysbiosis has connections cardiovascular disease, autism, anxiety and depression, dementia and gastrointestinal disorders, such as irritable bowel syndrome.

In addition to dysbiosis, metabolites generated by the microbiome can interact with normal body processes. Scientists have also identified these “bioactive” molecules as disease risk factors.

Indeed, some types of heart failure have connections to the swelling of the intestinal wall and the movement of bacteria into the systemic circulation, which, in turn, increases inflammation, contributing to the worsening of heart failure and atherosclerosis.

Obesity and poor diet also have links to cognitive decline and dementia. The mechanisms responsible remain largely unknown, but research has targeted changes in GBA and inflammation.

There are links between diets high in fat and sugar and changes in the composition of the gut microbiota, which can trigger inflammation.

Experiences in mice have shown that the increase Bilophila wadsworthia bacteria in the gut worsen cognitive impairment by directly affecting the hippocampus, the region of the brain responsible for critical thinking and learning.

Scientists have shown that mice fed a high-fat, low-carb diet made 30% more errors navigating a maze than mice fed a standard diet.

Diets high in sugar, high in fat, and low in fiber reduce the number of fiber-digesting bacteria in favor of less beneficial bacteria. The production of essential metabolites, like butyrate, also drops.

A recent study by Dr Naoki Saji, Posted in Scientific reports, supports the concept that changes in the microbiota affect dementia risk. The study of microbiome metabolites found a different gut microbiota in people with dementia than in those without, specifically linking a higher concentration of fecal lactic acid to a 60% lower risk of dementia.

Talk to DTM, Dr. Saji explained:

“In the past, we never [thought] of the association between the gut and dementia. However, a new technology concerning the analysis of the microbiome revealed it. This new risk factor other than amyloid beta, a well-known factor, could play an important role in the next decade.

He went on to note that the current study “shows [..] data” to support the idea that people with dementia have a different gut microbiota than those without.

According to Dr Porter:[T]The gut microbiomes of patients with dementia have an overrepresentation of pro-inflammatory bacterial strains that may contribute to increased inflammation in the brain. It is thought that this pro-inflammatory milieu may, in turn, promote the development of amyloid plaques in the brain – a pathophysiological “signature” associated with Alzheimer’s disease. »

Alzheimer’s disease is the most common form of dementia, biologically characterized by bundles of amyloid and TAU proteins impeding message transmission from the brain.

Studies in rodents have shown how changes in the animal’s intestinal microbiome promote the formation of amyloid bundles, a risk factor for Alzheimer’s disease.

Analysis of the gut microbiota of people with Alzheimer’s disease revealed reduced microbial diversity. Specifically, researchers showed lower levels of Firmicutes and Bifidobacterium and a higher level of Bacteroidetes.

Research has also shown that the gut microbiomes of people with dementia contain more pro-inflammatory bacteria and less anti-inflammatory bacteria.

In his interview with DTM, Dr. Porter discussed a “recent pivot to studywhere scientists compared data from people with Alzheimer’s disease – with and without amyloid – to a control group without Alzheimer’s disease.

The researchers found that the “high presence of pro-inflammatory bacterial strains [and] decreased presence of anti-inflammatory strains in gut microbiomes had a positive correlation with increased inflammatory status. [They also found] higher levels of cognitive impairment and a greater concentration of amyloid deposits in the brain”.

Inflammation also has associations with microbiome metabolites in the circulation system. To research suggests that the increase in fatty acids produced by gut bacteria can disrupt the cells that line the gut.

This can lead to systemic inflammation, which is associated with increased amyloid protein in the brain.

There’s no cure for dementia, but there’s growing evidence that certain foods and nutrients can support cognitive health.

the Alzheimer Society recommends “a diet rich in fruits, vegetables and grains, and low in red meat and sugar may help reduce the risk of dementia”.

They write that “[t]he best way to reduce your risk of dementia is to adjust various aspects of your lifestyle, including eating certain foods, exercising regularly, not smoking, and maintaining normal blood pressure and cholesterol levels.

“Research suggests that certain lifestyle choices […] may help reduce the risk of dementia. Good gut balance is achieved by eating a diet high in fibrous, green and leafy foods, getting enough sleep and exercising regularly,” Dr. Porter added.

Adherence to healthy eating habits such as the Dietary Approach to Arrest Hypertension (DASH) and the Mediterranean Diet-DASH Intervention for Neurodegenerative Delay (MIND) has associations with reduced cognitive decline and risk of Alzheimer’s disease.

In addition, eating monounsaturated and polyunsaturated fatty acids and fish has connections reduced cognitive decline.

Dr. Saji echoed these findings, commenting that “[s]diet pattern[s] such as DASH, MIND and a Japanese-style diet could influence” the microbiome.

He also claimed that a “Japanese-style diet has [a] causal relationship [with] risk of dementia.

The exact mechanism of the impact of the microbiome and its metabolites on cognitive function is unclear. Research has shown conflicting results. What is certain is that people with dementia have different proportions of microorganisms in their microbiota.

Dr. Porter believes that “[u]Understanding how a gut-mediated inflammatory response combined with aging and poor diet may contribute to the pathogenesis of Alzheimer’s disease could provide a unique therapeutic target in our fight against this disease.

Extending lifespan is a public health issue, diet being a factor likely to prevent age-related diseases and preserve good health during ageing.

Future treatments for cognitive decline and dementia may include altering the gut microbiota. There is growing evidence that dietary factors may delay dementia. Although the underlying mechanismsare not yet fully understood, small changes in diet can lead to a healthier future.

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