Higher levels of gut microbiome diversity are generally thought to be a good thing. Studies have linked it to lower rates of obesity, Type 2 diabetes, metabolic disease and other ills. People who live in industrialized nations tend to have less microbial diversity in their guts than those living in more traditional, nonindustrialized societies. Some scientists speculate that modern lifestyle factors like diets high in processed foods, chronic stress and physical inactivity may suppress the growth of potentially beneficial gut microbes. Others argue that the correlation between diverse microbiomes and good health is overblown, and that the low levels of microbiome diversity typically seen in people living in developed nations may be suitably adapted to a modern world.
One subject on which there is usually little disagreement among nutrition experts is the benefits of a high-fiber diet. In large studies, people who consume more fruits, vegetables, nuts and other fiber-rich foods tend to have lower rates of mortality and less chronic disease. Fiber is considered good for gut health: Microbes in the gut feed on fiber and use it to produce beneficial byproducts like short-chain fatty acids, which can reduce inflammation. Some studies also suggest that eating a lot of fiber promotes a diverse microbiome.
The Stanford researchers expected that consuming a high-fiber diet would have a big impact on the makeup of the microbiome. Instead, the high-fiber group tended to show few changes in their microbial diversity. But when the scientists looked closer, they discovered something striking. People who started out with higher levels of microbial diversity had reductions in inflammation on the high-fiber diet, while those who had the least microbial diversity had slight increases in inflammation when they ate more fiber.
The researchers said they suspect that the people with low microbiome diversity may have lacked the right microbes to digest all the fiber they consumed. One finding that supports this: The high-fiber group had unexpectedly large amounts of carbohydrates in their stool that had not been degraded by their gut microbes. One possibility is that their guts needed more time to adapt to the high-fiber diet. But ultimately this finding could explain why some people experience bloating and other uncomfortable gastrointestinal issues when they eat a lot of fiber, said Christopher Gardner, another author of the study.
“Maybe the challenges that some people have with fiber is that their microbiomes aren’t prepared for it,” said Dr. Gardner, the director of nutrition studies at the Stanford Prevention Research Center.
One question that the researchers hope to answer in the future is what would happen if people simultaneously ate more fermented foods as well as more fiber. Would that increase the variety of microbes in their guts and improve their ability to digest more fiber? Would the two have a synergistic effect on inflammation?
Suzanne Devkota, the director of Microbiome Research at Cedars-Sinai Medical Center in Los Angeles, who was not involved in the new study, said it has long been assumed that eating fermented foods had health benefits but that the new research provides some of the first “hard evidence” that it can influence the gut and inflammation. “We were always a little reluctant to make comments about fermented foods being beneficial, particularly from an inflammatory standpoint, because there was really no data behind that,” she said.