A large body of evidence has shown that the diet shapes the gut microbiome, which can affect the immune system and overall health. And more evidence continues to come out on a near-daily basis. Low microbiome diversity is known to have been linked to obesity and diabetes. A diet rich in fermented foods improves the diversity of gut microbes and reduces molecular signs of inflammation, according to researchers from the Stanford School of Medicine. In one clinical study, 36 healthy adults were randomly assigned to a 10-week diet that included fermented or high-fiber foods. The two diets had different effects on the gut microbiome and the immune system. Eating foods such as koumis, fermented cottage cheese, kimchi and other fermented vegetables, drinks such as yogurt, kefir and kombucha tea, led to an increase in overall microbial diversity with stronger effects from larger portions. This is a surprising finding, according to Dr. Justin Sonnenburg, PhD, associate professor of microbiology and immunology, as it provides one of the first examples of how a simple change in diet can reproducibly reshape the microbiota in a cohort of healthy adults.
Researchers focused on fiber and fermented foods due to previous reports of their potential health benefits. While high-fiber diets have been associated with lower death rates, consuming fermented foods can help maintain weight and can reduce the risk of diabetes, cancer, and cardiovascular disease. The researchers analyzed blood and stool samples collected during a three-week period before the test, the 10-week period of the diet, and a four-week period after the diet when the participants ate as they wanted. The findings paint a blurry picture of diet’s influence on gut microbes and immune status. On the one hand, those who increased the consumption of fermented foods showed similar effects on microbiome diversity and inflammatory markers, in line with previous research showing that short-term changes in diet can rapidly alter the gut microbiome. On the other hand, the limited change in the microbiome within the high-fiber group matches the researchers’ previous reports of general resilience of the human microbiome over short periods of time.
Furthermore, four types of immune cells showed less activation in the fermented food group. Levels of 19 inflammatory proteins measured in blood samples also decreased. One such protein, interleukin-6, has been linked to conditions such as rheumatoid arthritis, type 2 diabetes, and chronic stress. In contrast, none of these 19 inflammatory proteins decreased in participants assigned a high-fiber diet rich in legumes, seeds, whole grains, nuts, vegetables, and fruits. On average, the diversity of their gut microbes has also remained stable, which is odd since there is also scientific evidence that the high fiber content has a more universally beneficial effect of diversifying the microbiota. The data suggests that an increase in fiber intake alone over a short period of time is not enough to increase microbiota diversity. The results also showed that higher fiber intake led to more carbohydrates in stool samples, indicating incomplete degradation of fiber by gut microbes. These findings are consistent with other research suggesting that the microbiome of people living in the industrialized world is depleted of fiber-degrading microbes.
The research team thinks it is possible that a longer intervention would have allowed the microbiota to adapt adequately to the increase in fiber consumption. Alternatively, the deliberate introduction of fiber-consuming microbes may be required to increase the microbiota’s ability to break down carbohydrates. In addition to exploring these possibilities, the researchers plan to conduct studies in mice to study the molecular mechanisms by which diets alter the microbiome and reduce inflammatory proteins. They also aim to test whether high-fiber and fermented foods come together to affect the microbiome and immune system of humans. Another focus is to examine whether consuming fermented foods reduces inflammation, or improves other health markers, in patients with immunological and metabolic diseases, pregnant women, and the elderly. The principle could also be applied to autoimmune diseases. The information that an imbalance of the microbiota in predisposed individuals can contribute to the onset of autoimmunity is not new. A new study from the University of Iowa shows that a diet rich in isoflavones, phytoestrogens that resemble estrogen and mostly represented in soy, protects against multiple sclerosis-like symptoms in a mouse model of the disease.
Importantly, the isoflavone diet was protective only when the mice had gut microbes capable of breaking down isoflavones. Interestingly, previous human studies have shown that multiple sclerosis patients lack these bacteria compared to individuals without MS, but conclusive evidence was lacking. Isoflavones are found in soy, peanuts, chickpeas and other legumes. The study also found that mice fed the isoflavone diet have a microbiome similar to the microbiome found in healthy people and includes bacteria that can metabolize isoflavones. Conversely, an isoflavone-free diet promotes a microbiome in mice that is similar to that seen in MS patients and lacks beneficial bacteria capable of metabolizing isoflavones. In the current study, the team found that bacteria missing in MS patients are able to suppress inflammation in an experimental model. The team compared the effects of an isoflavone-based diet and an isoflavone-free diet on disease in the mouse model of MS, and found that the isoflavone-based diet led to disease protection.
However, when the team put the mice on the isoflavone-based diet but removed the isoflavone-metabolizing gut bacteria, the isoflavone-based diet was no longer able to protect against MS-like symptoms. When the bacteria were reintroduced, the protective effect of the isoflavone diet returned. Therefore, it is reasonable to think that for some autoimmunities such as Crohn’s disease, rheumatoid arthritis, and in this context multiple sclerosis, the type of diet can contribute to both prevention and an attempt to repress the disease, indirectly modulating the immune system through a intervention on the intestinal bacterial flora. The importance of the intestinal microbiota in maintaining general health is a topic often addressed in this site, both by topic of interest and by direct requests from readers. For further information, it is possible to search by “keyword” in the archive of this website.
- Edited by By Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
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