Gut microbes get most of their nutrients from your diet and help you digest a lot of the food you eat. It probably comes as no surprise that diet has a huge impact on the composition of gut microbiome and, consequently, on your immune system. Diet influences many aspects of the microbiome-immune system communication, including, for example, the permeability of the intestinal barrier and the types of microbes targeted by IgA.

Modern diets, particularly those of the Western world, are characterized by an excessive intake of highly palatable energy-dense foods, including high levels of processed and refined grains, manufactured and artificial saturated fats, simple sugars and refined salt, but low amounts of plant-derived fibers. Sadly, this is the dietary pattern that is being increasingly linked to immune dysfunctions that are directly associated with the microbiome. For example, high dietary intake of refined salt or of long-chain fatty acids may stimulate the harmful actions of certain immune cells in the gut, which, in turn, may increase the risk of autoimmune reactions.

On the other hand, short-chain fatty acids (SCFAs), which are produced from insoluble dietary fibers by certain microbial species, promote the activity of regulatory T cells (Treg). SCFA, like acetate and butyrate, are made when gut microbes ferment insoluble fiber and carbohydrates (e.g., resistant starches, oat bran, pectins, fructooligosaccharides). Treg cells prevent inflammatory reactions against harmless intestinal microbes by suppressing the abnormal activation of other immune cells. Treg cells are thus particularly important in maintaining immune tolerance to the gut microbiome, dietary components, and self-antigens.

SCFAs also support the integrity of the epithelial barrier, increase the production of IgA, and promote gut homeostasis by increasing intestinal secretions and decreasing inflammatory responses. SCFAs can reach other organs, such as the brain, for example, where they also decrease neuroinflammatory responses that are known to underlie many neurodegenerative diseases. SCFAs are one of the reasons why fiber-rich diets support our immune system. Eating a diet rich in plant foods that can be converted into SCFA helps keep the brain and immune system healthy.