Activating good bacteria and increasing the diversity of intestinal bacteria (the abundance of types of microorganisms in the intestine) are important in order to improve the intestinal environment. It is also essential to balance the responses of immune system in the intestines to protect ourselves from viruses and pathogenic bacteria. Recently, worldwide attention has been paid to the diversity of intestinal bacteria and intestinal immunity. In order to improve these conditions with dietary approaches, many people consider that taking dietary fiber and fermented foods are helpful to achieve so. 

Dietary fiber is a nutrient source for the good gut bacteria which will be beneficial for improving the condition of the intestines. Fermented foods contain a variety of nutrients that are produced by the microorganisms fermenting the original ingredients and have beneficial effects on our health. In July 2021, the Stanford School of Medicine in the United States published the results of a clinical trial with 36 healthy adults about whether dietary fiber or fermented foods had a beneficial effect on the intestinal environment. A 10-week dietary study found that a diet containing fermented foods increased the diversity of gut microbiota and reduced the excess inflammatory immune responses which may cause harmful effects to the body. 

Eating yogurt, kefir, fermented cheese, kombucha and fermented vegetables such as kimchi and pickles, can increase the overall microbial diversity in the intestine. We can obtain more effects when we consume them in large volume. Furthermore, in the group of high fermented food diet, the activation of 4 types of immune cells was suppressed. The concentration of 19 inflammatory proteins in the blood was also reduced. 

In fact, stronger immunity is not always better. A reaction that attacks foreign substances is necessary for the body in order to prevent from infections. On the other hand, an excessive immune response can hurt our own body or even react to harmless pollen and house dust which are not supposed to trigger such a reaction. If said reactions are mild, we can notice it as hay fever or allergies. Severe reactions present themselves with symptoms such as rheumatoid arthritis, type 2 diabetes, and inflammatory bowel disease. 

When the symptoms of COVID-19 infection become more severe, cytokine storm occurs. Cytokine storms are also called out-of-control immune systems. Experts are saying that suppressing the exaggerated immune response can reduce the risk of aggravation. The fact that fermented foods suppressed the activation of immune cells, as suggested by the results of this study, is an important finding that leads to the suppression of an exaggerated immune response.

Researches have shown that dietary fiber is also a component that increases the diversity of intestinal bacteria and regulates immunity. In this study, 19 inflammatory proteins were reduced in the group of high fermented food diet, but none of the inflammatory proteins were reduced in the group that ingested a fiber-rich diet containing a lot of legumes, seeds, whole grains, nuts, vegetables, and fruits. In addition, the diversity of intestinal bacteria remained stable without change. This shows that increasing the intake of fiber in the short term of 10 weeks was not enough to increase microbial diversity. 

There are several possible reasons why dietary fiber did not work in the short term. The first reason is that even if there is a change in the balance of intestinal bacteria during a short period of time, the state previous to this change prevails. Therefore, the change was insufficient when the dietary fiber intake period was short. Another reason is that the gut microbiota of people living in developed countries is depleted of beneficial microorganisms that can break down dietary fiber. In other words, it is conceivable that bacteria that use dietary fiber for fermentation activity have been decreased in their intestine. Fermented foods contain useful ingredients that are produced from decomposed dietary fiber, so it can work more directly to improve the intestinal environment. 

From these data, we can reaffirm the importance of continuous consumption of fermented foods. In addition, it is advisable to ingest dietary fiber together with useful bacteria (probiotics) that utilize it for fermentation activity. Our intestinal environment maintains its good health when we ingest various foods, lactic acid bacteria and bifidobacteria, and they work synergistically, instead of just ingesting a single component.