Many types of diarrheal illnesses, with many different causes, disrupt intestinal function. The ability of probiotics to decrease the incidence or duration of certain diarrheal illnesses is perhaps the most substantiated health effect of probiotics. Probiotics can prevent or ameliorate diarrhea through their effects on the immune system. Moreover, probiotics might prevent infection because they compete with pathogenic viruses or bacteria for binding sites on epithelial cells . In the pediatric population, probiotics appear to benefit viral diarrhea, possibly by increasing the antibody secretory IgA and decreasing viral shedding, suggesting an immunological mechanism .
Studies evaluating the effect of probiotics on travelers' diarrhea are equivocal. Travelers' diarrhea (3 times or more) occurs in residents of developed countries after traveling to subtropical and tropical zones. Drinking Lactobacillus GG strain significantly decreased the incidence of diarrhea in travelers. Studies with more reliable results are still needed through appropriate selection of traveling regions .
Inflammatory bowel diseases, such as ulcerative colitis and Crohn's disease, are serious intestinal disorders that can ultimately necessitate the surgical removal of the colon. The causes of such diseases are unknown, but it has been hypothesized that an intolerance to the normal flora (bacteria) in the gut leads to inflammation and resulting pathology. The role of gut flora in the progression of these diseases has led some researchers to study the impact certain probiotic bacteria might have on maintaining the state of reduced inflammation that occurs during the diseases' remission stages. Several controlled clinical trials have shown that high levels of certain probiotic strains can extend the disease-free remission period.
In general, cancer is caused by mutation or abnormal activation of genes that control cell growth and division. Many processes or exposures can increase the occurrence of abnormal cells,.among them chemical exposures. Cancer-causing chemicals (carcinogens) can be ingested in a normal diet or generated by metabolic activity of microbes that live in the gastrointestinal system. It has been hypothesized that probiotic cultures might decrease the exposure to chemical carcinogens by (a) detoxifying ingested carcinogens; (b) altering the environment of the intestine and thereby decreasing populations or metabolic activities of bacteria that may generate carcinogenic compounds; (c) producing metabolic products (e.g., butyrate) which improve a cell's ability to die when it should die (a process known as apoptosis or programmed cell death); (d) producing compounds that inhibit the growth of tumor cells; or (e) stimulating the immune system to better defend against cancer cell proliferation .
Research suggests that the consumption of probiotic cultures may decrease cancer risk. Researchers testing the effect of the consumption of fermented milks, probiotic bacteria, and components or extracts of bacteria have found :
*A reduction in the incidence of chemically induced tumors in rats.
*A reduction of the activity of fecal enzymes, postulated to play a role in colon cancer in human and animal subjects.
*Degradation of nitrosamines.
*A weakening of mutagenic activity of substances tested in the laboratory.
*Prevention of damage to DNA in certain colonic cells.
*In vitro binding of mutagens by cell wall components of probiotic bacteria.
*Enhancement of immune system functioning.
