In a clinical trial, 36 healthy adults were randomly assigned to a 10-week diet that included either fermented or high-fiber foods. The two diets resulted in different effects on the gut microbiome and the immune system, according to the study published in the scientific journal Cell.
The researchers analyzed blood and stool samples collected during a three-week pre-trial period, the 10 weeks of the diet, and a four-week period after the diet when the participants were free to eat as they chose.
Fermented food diet results
According to researchers, eating fermented foods such as yogurt, kefir, cottage cheese, kimchi, other types of fermented vegetables, and kombucha led to an increase in overall microbial diversity, with stronger effects from larger servings.
While authors of this particular study did not state why a diverse microbiome is important for human health, past research suggests that a lack of gut microbiome diversity or an unbalanced microbiome has been associated with the occurrence of certain digestive and immunological diseases in some individuals.
"This is a stunning finding," said one of the study's authors Justin Sonnenburg, PhD, an associate professor of microbiology and immunology. "It provides one of the first examples of how a simple change in diet can reproducibly remodel the microbiota across a cohort of healthy adults."
Researchers found that four types of immune cells showed less activation in the group of consumers who ate a fermented foods-rich diet for 10 weeks. In addition, the levels of 19 inflammatory proteins measured in blood samples also decreased among the fermented foods cohort.
"Microbiota-targeted diets can change immune status, providing a promising avenue for decreasing inflammation in healthy adults," said study author Christopher Gardner, PhD, the Rehnborg Farquhar Professor and director of nutrition studies at the Stanford Prevention Research Center.
"This finding was consistent across all participants in the study who were assigned to the higher fermented food group."
Fiber-rich diet results
For the study participants that followed a fiber-rich diet comprising legumes, seeds, whole grains, nuts, vegetables, and fruits, the results and impact to gut microbiome diversity were less pronounced, noted researchers.
"We expected high fiber to have a more universally beneficial effect and increase microbiota diversity," said study author Erica Sonnenburg, PhD, a senior research scientist in basic life sciences, microbiology and immunology.
"The data suggest that increased fiber intake alone over a short time period is insufficient to increase microbiota diversity."
Results also showed that greater fiber intake led to more carbohydrates in stool samples pointing to incomplete fiber degradation by gut microbes.
"It is possible that a longer intervention would have allowed for the microbiota to adequately adapt to the increase in fiber consumption," added Erica Sonnenburg.
"Alternatively, the deliberate introduction of fiber-consuming microbes may be required to increase the microbiota's capacity to break down the carbohydrates."
Researchers said they plan to do further research in mice to explore the molecular mechanisms which alter the microbiome and reduce inflammatory proteins.
They also plan to investigate whether high-fiber and fermented foods synergize to influence the microbiome and immune system of humans. Another goal is to examine whether the consumption of fermented food decreases inflammation or improves other health markers in patients with immunological and metabolic diseases, as well as pregnant women and older individuals.
"There are many more ways to target the microbiome with food and supplements, and we hope to continue to investigate how different diets, probiotics and prebiotics impact the microbiome and health in different groups," said Justin Sonnenburg.