**6. The effect of dietary fiber on the immune system of the gut**

Carbohydrate polymers naturally occur in edible plants and are used up as vegetables, fruits, seeds, cereals, and tubers. Dietary fibers travel from the small to the large intestine, where they perform a physiological role. Fibers consist of two types soluble and insoluble. Soluble fibers undergo total fermentation in the colon whereas insoluble fiber undergoes fermentation to some extent. Dietary fiber consists of a range of organic polymers, each of which contains various monomers coupled by different glycosidic linkages, resulting in a complex and heterogeneous

structure. Many methods of classifying dietary fiber, such as solubility, viscosity, and fermentability, have been formulated to aid in the correlation of physicochemical features of dietary fiber with their physiological roles. Although particular nutrients are known to play a role in the immune system's development and function, little is known about the impression of dietary fibers on immunological function. Dietary fiber is essential for good health. Higher dietary fiber consumption is linked to a lower risk of disease and mortality, according to several meta-analyses. Dietary fiber consumption is linked to a higher risk of Western diseases with immune system abnormalities, implying that dietary fiber is vital for immunological homeostasis. The preservation of the gut immune barrier is one direction through which fibers may protect against disease development4 . The innate immune system that includes physical barriers such as the skin and mucous membranes, cell-mediated barriers such as phagocytic cells, inflammatory cells, dendritic cells, and natural killer cells, and soluble mediators such as cytokines, complement, and acute-phase proteins, delivering immunity to invading organisms without the need for prior exposure to these antigens. During the 4–5 days it takes lymphocytes to become activated, this arm of the immune system supplies the early steps of host defense that safeguard the organism. Macrophages and their precursor monocytes, as well as polymorphonuclear leukocytes (neutrophils), frame the innate immune system's core cellular component.

The human body's janitor is the gastrointestinal immunological barrier. It is made up of a mucus layer and an epithelial cell layer that keeps luminal molecules out of the immune-cell-filled lamina propria beneath. Improving intestinal barrier function by increasing dietary fiber intake could thus be a useful therapy for preventing or delaying Western immune-related illnesses. Moreover, Dietary fibers may interact directly with immunological barrier cells in the small intestine before being destroyed by microbial enzymes in the colon. The small intestine has a thin and loose mucus layer that boosts nutrient absorption while also allowing food compounds like fibers to interact directly with intestinal epithelium and immune cells. The ramifications of these interactions are that the mucus layer is strengthened, epithelial cell barrier function is improved, and intestinal immune responses are modulated as a result of these direct interactions with intestinal immune barrier cells. Dietary fibers' direct contact with the gut immune system could be one of the processes by which they improve health and prevent disease. Dietary fiber can also have an indirect positive effect on the gastrointestinal immunological barrier by stimulating the proliferation and metabolic activities of gut microbiota communities.

The large intestine holds the place of most populated in terms of microbiota and immune cells. As a result of this research, it is becoming increasingly clear that intestinal metabolism has a significant impact on human physiology. Together with a mucus layer, the vast intestinal layer of specialized epithelial cells joined by tight junction proteins serves as a barrier that separates the host's mucosa from the luminal environment. Enterocytes, which are in control of nutritional absorption, and goblet cells, which create, store, and exude mucin glycoproteins, are two key cells of the intestinal epithelium. The maximum density of goblet cells can be found here, which in turn leads to a wide range of microbiota and their subsequent conversions into products, and hence, these products lead to a large no. of consequences. SCFAs (short-chain fatty acids) are made mostly by the fermentation of non-digested carbohydrates. This fermentation produces not only the primary SCFAs of acetate, propionate, and butyrate but also lactate, a crucial intermediary in the synthesis of SCFA.
