Two Diets, One 10-Week Window
In 2021, researchers published a trial in the journal Cell that put 36 healthy adults on one of two diets for 10 weeks: a high-fibre diet or a high-fermented-food diet.
The design let the researchers compare two very different ways of eating side by side, rather than testing fibre against a no-change control alone.
Both groups were tracked with blood and stool samples across the 10 weeks, which let the team follow changes in immune markers and in the gut microbes themselves.
What Fell in the Fermented-Food Group
The trial found that in the fermented-food group, 19 separate measured inflammatory proteins fell over the 10 weeks, including interleukin-6.
Interleukin-6 is a signalling protein the immune system uses to coordinate inflammation, and lower circulating levels were associated with the fermented-food diet in this trial.
This drop across 19 proteins at once is a notably broad shift for a 10-week dietary change, though the study does not show why any single protein moved.
Why the Fibre Group Looked Unchanged, At First
In the fibre group, the trial found that overall microbiome diversity did not change across the 10 weeks.
That result could easily read as a null finding, but the researchers also measured what the existing microbes were doing, not just which species were present.
They found that microbes in the fibre group increased their carbohydrate-processing enzymes, even though the roster of species stayed largely the same.
Function Before Composition
The Cell 2021 authors interpreted this pattern as fibre changing what the microbiome does before it changes what the microbiome is.
In practice, that means the same bacterial species were producing more of the enzymes needed to break down carbohydrate, a functional shift happening ahead of any shift in which species were present.
This distinction matters because most public discussion of gut microbes focuses on diversity or species counts, which this trial found to be the slower-moving measure over 10 weeks.
What This Trial Does Not Show
The sample size was 36 adults, which is small for drawing firm conclusions about how fibre or fermented foods work across different ages, health conditions or longer time periods.
The trial was association, not causation: lower inflammatory protein levels were associated with the fermented-food diet, but the study cannot say fibre or fermented foods caused every downstream effect in a person's body.
Ten weeks is also a short window against a lifetime of eating habits, and the 2021 paper does not report what happens if either diet stops or continues for years.
None of this tells a reader what to eat for any particular condition, but it does offer a concrete, checkable fact: 19 measured proteins changed in one direction in one small trial, and that is worth knowing on its own terms.