Related Research Review - July 2024
Making Sense of Research
About once a month, the i-Forget team reviews scientific research that relates to our study.
Research shows that the gut microbiome is a family affair
Numerous studies have shown significant differences in gut microbial composition and diversity among individuals. The diversity in gut microbiomes is much greater compared to genomic variation. While humans share about 99.9% similarity in their host genome, the composition and abundance of microbial species within their microbiome can vary significantly from person to person. Estimates suggest that two randomly selected people will share only 20% of intestinal bacterial types on average.
We acquire the microorganism population in our gut at birth. Our mother's intestinal bacteria seeds what will become our own microbiome as we pass through the birth canal. Similarities between a mother's bacteria and those of her young children can be as high as 50 percent, even later in life.
What causes an individual's microbiome to change? Research reveals that factors such as diet and lifestyle can modify the composition of the human microbiome. New bacteria can also come from other individuals.
Do people who live in the same household share the same bacteria? Surprisingly, yes. For example, the longer we live with someone, the more it affects our oral (mouth) microbiome. As many as one-third of oral bacterial strains become common between cohabiting individuals. A similar "drift" toward similarity occurs in the intestinal microbiome, with the degree of similarity increasing with the time of cohabitation. Cohabitation is a stronger determinant of bacterial strain similarity than age or genetics. Choosing whom you live with may have deep implications.
How this research relates to the
i-Forget study
The i-Forget pilot study will compare intestinal microbiomes between people in the same household who have different genetic predispositions to the development of Alzheimer's disease. By reducing random variation as much as possible, differences between spouses may yield clues to the influence of such genes on the microbiome.
References:
Valles- Collomer and other co-authors: Nature volume 614, pages 125–135 (2023)
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Causation vs. correlation and what we know about the microbiome’s role in Alzheimer’s disease
It is easy to think that things occurring simultaneously or before a disease's onset may cause it, but that is not always true. For example, it is well-accepted that cigarette smoking during the years before the onset of lung cancer caused the disease. However, not every exposure to an environmental factor preceding the onset of a disease is causative. For instance, a 1981 publication in the prestigious New England Journal of Medicine identified a strong association between coffee drinking and pancreatic cancer. The authors imagined a causal link, but many subsequent studies failed to uphold this conclusion. There might be some other reason those who go on to develop pancreatic cancer also drink coffee.
The role of the intestinal microbiota in Alzheimer’s disease could be similar to the role of coffee in pancreatic cancer. There are studies claiming that a particular type of intestinal bacteria is more often seen in patients with Alzheimer’s disease—suggesting that this could be causative. Unfortunately, these types of small studies, of which there are many, don't agree on many aspects related to the role intestinal bacteria might play. It is also possible that people with Alzheimer’s have reasons for gut microbe differences that have nothing to do with the disease. For instance, cognitively impaired people may eat differently and exercise less—two things known to affect the microbiome.
The highlighted research paper we list our references below adds a valuable new perspective. The authors reasoned that if the intestinal microbiome caused cognitive impairment in people with Alzheimer’s disease, then exposing cognitively normal experimental animals to the microbiome of affected humans should affect their mental capacities. After fecal transplants from Alzheimer's patients into rats, researchers found impairments in memory functions and mood. Notably, the severity of a given rat's impairment matched the donor patient's clinical cognitive test scores.
How this research relates to the
i-Forget study
The i-Forget study will attempt to find intestinal microbiota signatures associated with future risk of cognitive decline. While the study is essentially looking for correlations, it will also be a portal to further research. We hope that the i-Forget findings could lead to early intervention and new hope for people with a genetic predisposition to Alzheimer’s disease.
References:
Stefanie Grabrucker and other authors: Brain, Volume 146, pages 4916-4934 (2023)
July 2024