Fiber is often talked about as though it has a predictable effect on the gut. Eat more of it, feed certain bacteria and the microbiome changes.
A new study suggests it may not be that simple.
In laboratory experiments published in Nature Microbiology, researchers found that the effects of plant-based carbohydrates depended partly on which bacteria were already present. The findings may help explain why people can respond differently to the same foods.
The study does not show that certain fibers are better for certain people, and it does not provide a way to personalize a diet based on a microbiome test. Much of the research was done in controlled bacterial communities rather than in people.
Researchers built a community of 21 human gut bacteria and exposed it to 94 dietary components, including many complex carbohydrates.
One bacterium, Segatella copri, often found in people eating more plant-rich diets, grew more under many of those conditions. But its response changed depending on which other bacteria were present.
That was the key finding.
The researchers found that a plant carbohydrate called arabinan helped S. copri thrive in a larger bacterial community, but not when it was competing with a smaller group of bacteria on its own. Adding E. coli shifted the balance back in its favor.
In other words, the carbohydrate itself was only part of the story. The surrounding microbial community mattered too.
“We wanted to understand which factors actually influence the competition between these bacteria in the gut,” said Dr. Caroline Tawk, one of the study’s first authors.
The team saw similar patterns with other plant-based carbohydrates. They also compared their laboratory findings with microbiome data from healthy adults and found that S. copri and the bacterial groups that appeared to support it were more common in non-industrialized populations.
That does not prove the same interactions were happening inside those people, but it suggests the laboratory findings may reflect real-world patterns.
“Our results show that the effect of a dietary component should not be considered in isolation from the existing bacterial community,” said study leader Till Strowig. “We need to clarify in further studies whether this knowledge can be used for personalized dietary or microbiome approaches.”
The study adds to growing evidence that there may be no single microbiome response to fiber.
Two people can eat the same food but start with very different gut bacteria. Those differences may influence which microbes grow, which decline and how the overall community responds.
For now, the findings do not change general advice to eat fiber-rich foods or suggest that people need microbiome testing before choosing them.
Instead, they offer a better explanation for why diet and the gut microbiome can be so personal: what we eat matters, but so may the microbial community already there.
The research was supported by the German Research Foundation, the RESIST Cluster of Excellence and the Amplifying Funds in Infection Biology program.