The human microbiome is mostly developed by the age of three, at which point it pretty closely resembles the microbiome of a full adult. This time and the transition into old age mark the biggest shifts in human microbial composition.
Interestingly these are also the two points at which the human immune system is at its weakest. This is due to the large role the human microbiome plays in the development and upkeep of the immune system. In fact as more and more is discovered about the microbiome, it is uncovered that more and more human functions are connected to it.
Fairly recent technology has allowed researchers to discover that of the DNA present in our body, the human kind barely accounts for 43% of it. The other 57% is the DNA of the bacteria and other microbes living in our bodies. A fact which has shone a light on the potential of the microbiome as a measurement of age and overall health.
The changes that occur in the microbiome as someone ages are still far from fully understood but scientists have been able to determine certain patterns and connect these microbial shifts with common old age ailments. All of which we will discuss in the following sections:
People of old age have been found to have a different microbial profile compared to adults in good health. These differences generally present as lower amount of microbial diversity in older adults, putting their microbiome in a state known as dysbiosis. These changes may be due to several factors, including changes in lifestyle, hospitalization, addition of medications, infections, less mobility, etc.
As people age, some microbes tend to become more abundant while other populations decrease. For example Eubacterium hallii has been found to increase, as do certain opportunistic bacteria like C. difficile, while bacteroides, bifidobacteria and lactobacilli decrease.
Note that these patterns may vary according to geographical region.
Further research has been done by Alex Zhavoronkov and colleagues at InSilico Medicine. The team created a computer program that was able to accurately predict the age of a participant within 4 years, based on their examination of the gut bacteria of 1165 participants. What this means is that like human genes, the human microbiome is a very good indicator of biological age.
This technology could be used to assess someone's overall health and also to compare healthy people with those who are ill to see if there are microbial deviations. And although the differences in the microbiomes of older individuals are clear to researchers, it is still not yet clear whether the changes play a role in ageing or are a side effect of ageing.
There are a number of illnesses and conditions that have long been associated with old age that are now also being linked to dysbiosis in the microbiome. Again raising the question of whether these microbial changes are the cause or effect of an ageing body. Some common conditions that have already been related to the microbiome include Parkinson’s and Alzheimer’s.
A study from Tufts University done on a group of older adults showed differences in bacterial profiles in those demonstrating high physical function versus those with lower physical function. The study examined 18 older adults deemed to have “favorable body composition” and 11 older adults whose bodies were lesser functioning.
They found higher levels of bacteria such as Prevotellaceae, Barnesiella and Prevotella (all considered good bacteria) in the higher physical functioning participants. Further experimentation had them inoculating germ free mice with fecal samples from the older adults. Mice given the samples from the higher functioning older adults saw an increase in grip strength.
A study out of the Wisconsin Alzheimer’s Disease Research Center studied the microbiome differences between those with Alzheimer’s and their controlled counterparts. The study found that on the whole those with AD had a reduced level of bacterial richness in their guts. Specifically they had decreased amounts of Firmicutes and an increased level of Bacteroidetes. Although the study was focused on those with Alzheimer’s dementia patients, there is reason to believe that similar causation is at the core of other dementias.
Research has uncovered that those with Parkinson’s have a unique microbiome composition. And other studies have revealed that Parkinson’s prevalence is 28% higher in those with IBD. There are a few proposals as to how the gut is affecting the brain in the way it does. It is possible Parkinson’s is triggered through the vagus nerve which connects the GI tract to the central nervous system. The inflamed gut may also be triggering elevated levels of alpha-synuclein, which has been associated with Parkinson’s.
Overall, the research into ageing microbiomes suggests that dysbiosis of the microbiome is a common factor of older age, one that may be influencing the progression of age related conditions and diseases. It also suggests that there is potential for exploitation of the microbiome in order to better the ageing process.
FMT’s which are a popular area of study in many of the conditions mentioned above, may also be considered for those who see deteriorating microbial conditions as they age.
May 18, 2023
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