Fiber-Rich Foods and Fermented Foods Are Good for Gut Health

In a new study published in the journal Cell, researchers looked at how high-fiber foods and fermented foods influence the human microbiome and the immune system in healthy adults.

The microbiome is defined as the sum of the microbes and their genomic elements in a particular environment. In this case, researchers looked specifically at the human gut and the microbes that live there.

“The gastrointestinal (GI) tract contains trillions of microorganisms that make up the gut microbiota,” says EA Stewart, MBA, RD, gut health and autoimmune specializing dietitian at Spicy RD Nutrition in San Diego. “Some of these microorganisms are bad, but many are good.”

Stewart explains that most immune cells are in the GI tract as well, so having healthy gut microbiota fights off harmful bacteria and stops the immune system from turning against us.

Researchers are interested in exploring the connection between diet, the microbiome and immunity, to see if certain foods can help or hinder our health.

Recent studies show that diet is an important factor in microbiota composition and function. The hope is that understanding how foods affect the microbiome can one day help create personalized and precise nutrition plans based on how certain microbes affect health conditions.

In this particular study, the researchers wanted to specifically see the impact of high-fiber diets and fermented foods on the microbiome. They were also curious whether diets that target the gut microbiome can reduce inflammation.

“Chronic inflammation in the body can significantly increase disease risk as the body’s inflammatory response over time can damage healthy cells, tissues, and organs,” explains dietitian Erin Palinski-Wade, RD, CDCES, author of “Belly Fat Diet for Dummies.”

How Was the Study Conducted?

This was a 17-week diet intervention study with a randomized prospective design. Researchers monitored the microbiome and immune status in healthy adults based on two diets:

1. High-fiber diet: 18 people
2. High-fermented food diet: 18 people

People following the high-fiber diet increased fiber consumption from an average of 21.5 grams (g) per day to 45.1 g per day. Examples of fiber-rich foods they ate include fruits, vegetables, legumes, grains, nuts and seeds.

People in the high-fermented food group boosted fermented foods from 0.4 servings per day to 6.3 servings per day. Examples of fermented foods include yogurt, kefir, fermented cottage cheese, fermented vegetables, vegetable brine drinks and kombucha.

Stool samples were assessed for microbiota composition, function, and metabolic output. Blood samples were used to generate a systems-level view of the immune system.

What Did the Study Find?

Overall, the researchers found that “high-fiber and high-fermented-food consumption influence the microbiome and human biology in distinct ways.”

Notably, the high fiber diet did not increase microbiota diversity, but that may have been due to the short duration of the study. There were some indications that microbiome remodeling was occurring during the study, and may have been more pronounced if the study was longer.

But the high fiber diet did increase microbiome function, microbial proteins and the density of microbes within the microbiota—all beneficial effects. The researchers said that fiber may fuel the growth of bacteria that help with fiber degradation by increasing enzymes.

The fermented food diet did increase microbiota diversity, and also reduced markers of inflammation.

All About Fiber

Fiber is the non-digestible part of foods. Palinski-Wade says that one of the best ways to eat more fiber is to focus on whole foods. She suggests adding fruits and vegetables, whole grains (such as rolled oats) and plant-based proteins into the diet, such as beans and soy.

Stewart lists some high fiber superstars including avocado, raspberries, acorn squash, collard greens, navy beans, split peas, chia seeds, flax seeds, bulgur, and teff. 

“When increasing fiber intake, I recommend doing it gradually over time to prevent any GI discomfort,” says Palinski-Wade. “Strive to increase your fiber intake by 3–5 grams every few days. In addition, be sure to increase your fluid intake as you increase fiber.”

Eat Fermented Food

Fermented foods have undergone a process that allows for microbial growth, says Palinski-Wade. She explains that microbes in fermented foods are considered the ‘good bacteria’ that can improve gut health.

“Since it is estimated that about 80% of our immune system comes from the gut, feeding beneficial bacteria to the gut may improve the function of the immune system and reduce inflammation, and possibly reduce disease risk,” explains Palinski-Wade.

Studies have linked the consumption of fermented foods with decreased risk of developing type 2 diabetes and cardiovascular disease. 

“Probiotic-rich foods to enjoy regularly include what I call the three K’s—kefir, kimchi, and kombucha,” says Stewart. “Other nourishing fermented foods to try are sauerkraut, miso, tempeh, and yogurt.”

Stewart adds that you should choose kimchi and sauerkraut from the grocer’s refrigerated section, as shelf-stable products do not contain live organisms.