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More and more studies are highlighting the associations between what is known as the gut-brain axis and there are also more relationships on the influence of the microbiome on different systems of the body.
The neurological field is one of those that have been incorporated into the influence of bacteria in pathologies, especially neurodegenerative ones. In many cases, scientists look for interrelationships that may offer potential causes of the pathological process, as well as preventive or therapeutic approaches.
One of the latest contributions, published inScience Translational Medicine, shows that people who are in the earliest stage of Alzheimer's disease – a period in which brain changes have begun but cognitive symptoms have not yet manifested – harbor a variety of bacteria in their different gut microbiome. to those of people who do not have this neurodegenerative disease, according to a study by researchers at the School of Science of the University of Washington, United States.
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Gautam Dantas, professor of Genomic Medicine at the aforementioned university and co-author of the work, believes that these observations "open the possibility of analyzing the intestinal bacterial community to identify people with an increased risk of developing dementia. This would also allow the design of potential preventive treatments: microbiome modifiers capable of preventing cognitive decline."
What are microbiome modifiers?
Dantas also points out that it is still unknown which system pivots the influence. "We don't know if the gut is influencing the brain or if the brain is influencing the gut. In any case, the knowledge of this association is very valuable."
The researcher suggests that it could be that changes in the gut microbiome are just a reading of pathological variations in the brain. Another alternative is that the gut microbiome is contributing to Alzheimer's disease, in which case "altering or modifying the gut microbiome with probiotics or fecal transplants could help change the course of neurodegeneration."
The idea to study the connection between the gut microbiome and Alzheimer's came at a youth soccer game, where Dantas and neurologists from the same university, Beau M. Ance and Daniel J. Brennan, chatted while their children played.
The scientific community knows that the gut microbiomes of people with symptomatic Alzheimer's differ from the microbiomes of healthy people of the same age. But, the University of Washington professors noted, "no one had yet examined the gut microbiomes of people in the presymptomatic critical phase."
According to Ances, co-author of the work, "by the time people have cognitive symptoms there are already significant changes that are often irreversible. But if it were possible to make an early diagnosis, that would be the optimal time to intervene therapeutically effectively."
During the early stage of Alzheimer's disease, which can last two decades or more, affected individuals accumulate clusters of amyloid beta and tau proteins in their brains, but show no signs of neurodegeneration or cognitive decline.
Dantas, Ances and Aura L. Ferreiro, first author of the study, and a member of the University Laboratory, evaluated participants who volunteered for studies at the Charles F. and Joanne Knight Alzheimer's Disease Research Center at the University of Washington. All participants were cognitively normal. As part of this study, participants provided stool, blood, and cerebrospinal fluid samples; as well as daily monitoring of food consumed. Imaging tests such as PET and MRI brain scans were also performed.
To distinguish participants who were already in the early stage of Alzheimer's disease from those who were healthy, the researchers looked for signs of amyloid beta and tau accumulation through brain scans and cerebrospinal fluid. Of the 164 participants, about one-third (49 people) had signs of early Alzheimer's.
The various analyses revealed that healthy people, versus those with preclinical Alzheimer's disease, have markedly different gut bacteria, referring to the species of bacteria present and the biological processes in which those bacteria are involved, despite eating basically the same diet.
"These differences correlated with amyloid and tau levels, which increase before cognitive symptoms appear, but did not correlate with neurodegeneration, which becomes apparent when cognitive abilities begin to decline. These differences could potentially be used to detect early Alzheimer's," Ferreiro said. "The good thing about using the gut microbiome as a screening tool is its simplicity and ease," Ances said.
An altered microbiota, cause or result of Alzheimer's?
The same researchers have launched a five-year follow-up study designed to determine whether differences in the gut microbiome are a cause or a result of the brain changes seen in early Alzheimer's disease.
According to Dantas, if there is a causal link, it is most likely inflammatory. "Bacteria are these amazing chemical factories and some of their metabolites affect inflammation in the gut or even get into the bloodstream, where they can influence the immune system of the whole organism."
These statements currently sit in the realm of speculation, but "if it turns out that there is a causal link, we might start thinking about whether promoting the 'good' bacteria or eliminating the 'bad' bacteria would slow or even stop the development of symptomatic symptoms of Alzheimer's."
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