Uninvited Guests: The Impact of Small Intestinal Bacterial Overgrowth on Nutritional Status

27 INTRODUCTION TO SMALL INTESTINAL BACTERIAL OVERGROWTH Small intestinal bacterial overgrowth (SIBO) is an often-neglected mechanism for impaired nutrition. This review will examine the pathogenesis of SIBO and the multifaceted impact of this condition on the nutritional status of the human host. N o r m a l l y, only small numbers of bacteria are found in the small intestine when compared with that found in the colon (1). Since the small intestine is the site of digestion and absorption of food, bacterial flora are excluded from the small intestine to prevent unwanted competition with the host and abnormal entry of bacteria into the host. In addition, gas production from bacterial fermentation of food is minimized. SIBO is defined when colonic bacteria invade proximally into the ileum and jejunum and >105 o rganisms/mL of intestinal juice are present (2). The bacterial species in the small intestine in SIBO closely resemble the 300–400 species normally found in the colon. Although anatomic abnormalities and severe intestinal dysmotility are often considered to be requisite for the development of SIBO, recent discoveries linking SIBO to patients with irritable bowel syndrome suggest otherwise (3). The dramatic changes in the bacterial flora of the small bowel that occur in SIBO have a multitude of e ffects on nutritional status. The bacterial flora may compete with the host for critical nutrients, alter host metabolism, directly damage the absorptive mucosa of the host, and produce gastrointestinal symptoms that reduce or alter food intake by the host. Uninvited Guests: The Impact of Small Intestinal Bacterial Overgrowth on Nutritional Status NUTRITION ISSUES IN GASTROENTEROLOGY, SERIES #7