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The microorganisms that dwell in our guts are known collectively as the microbiome, and they are coming under increasing scientific scrutiny as a target for promoting health and longevity. Research suggests that the composition of the microbiome changes with age, and that these changes may promote age-related diseases. Gut bacteria play a role in digestion, help fend off infection, bolster the immune system and may even affect cognitive function.
With increasing scientific interest comes increasing media reporting, but there is some misinformation out there. In fact, even scientists are not immune to the illusory truth effect, and common claims often get repeated in scientific papers, even when they aren’t entirely correct. Fortunately, this paper is here to set us all right with a summary of microbiome myths and misconceptions. We here at Gowing Life were certainly surprised by a few of these.
The microbiome and its interaction with human health is often described as a new frontier, but this is far from the case – concepts like the gut-brain axis (the intercommunication between the gut and the central nervous system) have been around for centuries. We’ve also known about the health impacts of gut bacteria products (like short-chain fatty acids) for over 40 years.
While the field might not be new, the microbiome has become much easier to study recently thanks to high-throughput DNA sequencing. These techniques allow researchers to read the DNA sequences from entire populations of gut bacteria simultaneously. This lets us study the gut microbiome and its role in human health in ways we previously couldn’t.
This is perhaps the most commonly repeated microbiome myth – it’s just impressive enough to be worth quoting while remaining quite believable. Unfortunately, it doesn’t seem to be true – detailed analyses suggest that the true ratio is closer to 1:1. The 10:1 figure seems to originate from a ‘back of the envelope’ calculation made in the 1970s that turned out to be wildly off.
In general, the size of the microbiome tends to be overstated. For example, it is often claimed that there are 10^12 bacterial cells per gram of human faeces, whereas actual measurements are usually 10 to 100 times lower. Another common claim in the literature is that the gut microbiome weighs 1-2kg, but the origin of this claim is uncertain, and the true figure is unlikely to surpass 500g.
This one is more a matter of poor wording on the part of some authors. While some bacterial species do get transferred from mother to child during birth, this represents a very small proportion of the child’s fully developed microbiome. The diversity of the gut microbiome is mostly shaped by environmental factors during the first few years of life, and every adult will end up with a unique microbiome – even identical twins.
Perhaps a question of wording once again, but the description of bacteria as good for health or bad for health is perhaps an oversimplification. While it is true that some bacteria produce metabolites that are beneficial or harmful to health, their impact is context-dependent. We all have so-called ‘bad’ bacteria in our guts, but so long as they stay there and do not crowd out other bacteria, they are completely harmless. Likewise, ‘good bacteria’ may be capable of crowding out other ‘good’ bacteria and thereby having a negative health impact.
What really matters for health is the overall balance of the gut microbiome and how it interacts with the host organism and their diet. In other words, what constitutes a healthy gut microbiome is going to be different for everyone, and will remain an active area of research.
The bacteria contained within probiotic supplements are not native to the human gut, but are similar species derived from fermented foods. This means they do not actually survive within the human gut for very long. Bacteria already present in the gut are better adapted to survive there and will soon outcompete the probiotics. So, why do probiotics not contain human-native bacteria? The often cited reason is that human gut bacteria cannot be cultured in the lab, but we might just not be trying hard enough. Many human-native species have been successfully cultured, and while it is currently very expensive, this is at least partly due to lack of previous effort. There does not seem to be anything inherently ‘unculturable’ about human gut bacteria.
These five myths aren’t the only ones listed in the paper, merely those we think the average person is most likely to have heard of. Check out the full paper in Nature Microbiology.
Title image by CDC, Upslash
Human microbiome myths and misconceptions https://doi.org/10.1038/s41564-023-01426-7