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Dysbiosis just means the gut changed
Unknown good and bad
Scientists don't yet know which gut species are positive or negative. In a 10-week sucralose study that drew a lot of press, the species that changed most compared with the control was Blautia coccoides, which rose three- to fourfold.
Blautia and sucralose
That species is associated with lower adiposity and better insulin sensitivity, and people who are obese, including children, tend to have less of it. By that evidence, sucralose could be argued to improve the gut microbiome. No one is making that claim, since a healthy microbiome is still poorly defined.
The narrow takeaway
The study's main takeaway is narrower: some non-nutritive sweeteners are not metabolically inert. Whether those effects are good, bad, or neutral has yet to be fully worked out. Companies that analyze stool samples face the same limit, since a healthy microbiome is hard to define even though an unhealthy one may look like one with little diversity.
Why is a healthy microbiome so hard to define?
The Human Microbiome Project, a National Institutes of Health program that published its main results in 2012, sampled hundreds of healthy adults. It found that the mix of bacterial species differed widely from person to person. What stayed similar across people was the set of metabolic functions the microbes performed. Researchers therefore tend to describe health by function rather than by a list of species.
How do researchers read the species in a stool sample?
Most surveys sequence a gene called 16S ribosomal RNA, which all bacteria carry. Parts of it are the same across bacteria and parts differ between species. The differences work like a barcode, and the counts show how much of each group is present. Carl Woese's work in the 1970s on this gene established it as the standard way to classify bacteria.
Why does low diversity get linked with disease?
Gut diversity has been found to be lower in people with obesity, inflammatory bowel disease, and recurrent Clostridioides difficile infection. In a 2013 study in Nature, Emmanuelle Le Chatelier and colleagues in the MetaHIT project found that people with low gene richness in the gut had more body fat and inflammation. The studies were observational.
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