As stated in a recent book, "a relationship exists between nutrition, gut flora, immunology, and health."63 Food components that directly modulate the intestinal mucosal functions (including immunity) or act indirectly by modifying the composition or activity of the intestinal microflora (known to modulate some of the major gastrointestinal defense mechanisms) help by increasing activities that suppress pathogens, inactivate toxic chemicals, or down-regulate the response that will reduce the risk of allergic and inflammatory reactions. These food components belong to the functional foods (see Chapter 1) and their effects are in enhancing functions (i.e., the defense functions) and in reducing the risk of diseases like infectious diseases, inflammation, and inflammatory diseases but also neoplasia and cancer.
The relationship between nutrition and defense mechanisms was first observed in malnutrition (starvation, but also protein deficiency and specific deficiencies of micronutrients especially iron, zinc, copper, selenium, and B vitamins) and cachexia which have an adverse effect on T-cell functions and cell-mediated immunity as well as on secretory immunity and, to a smaller extent, on B-cell functions and humoral immunity.64
With regard to gastrointestinal defense functions, it is increasingly becoming known that specific food components have also a beneficial effect on the body's defense mechanisms by positively modulating them. Among these, the best immu-nomodulating food components so far studied are dietary fiber, probiotics, and prebiotics (see Section 12.5).
As discussed above (see Chapter 6), fermentable dietary fiber alters the structure and miscellaneous functions of the gut. To date, relatively few studies have been conducted on their effects on gastrointestinal defense functions. Thus, it is not possible at this time to draw conclusions on the effects of specific fibers. However, although exploration in this area is still in its infancy, animal studies have demonstrated that dietary fiber content and type have the potential to modulate defense functions, especially the immune functions of GALT. Regarding the safeguard, but nonimmune-dependent functions, it has been reported that apple pectin, wheat bran, and oat bran suppress fecal ^-glucuronidase, D-glucosidase, and nitroreductase activities in humans.65 66
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