There is a complex interaction between food and microflora in a feedback-like system. Different types of diets can lead to changes in fecal flora, and its resultant metabolic activity can be altered. When individuals consuming a vegetarian diet were compared to those on a typical Western diet, the latter had microflora that showed greater hydrolyz-ing ability leading to a more effective metabolism of bile acids and subsequently reduced cholesterol. Similar studies in mice have shown differences with high-fat diets versus low-fat diets.
Disaccharides are broken down in the proximal small intestine by enzymes in the brush border and microvilli of the intestinal epithelium. Glucose, sucrose, lactose, and maltose are the predominant disaccharides hydrolyzed and these rarely reach the colon. When the brush border is unable to produces the enzymes needed for metabolism, the disacchar-ides are not absorbed in the small bowel and ultimately reach the colon where they interact with the abundant colonic bacteria. Subsequent fermentation causes an osmotic imbalance pulling water into the lumen and causing diarrhea. Significant and rapid production of short-chain fatty acids causes changes in the fecal pH and can irritate the colonic mucosa.
Complex carbohydrates, such as dietary fibers, are predominantly fermented in the colon by colonic bacteria, mostly anaerobic flora. Short-chain fatty acids, acetate, propionate, and butyrate are the predominant by-products and in lesser amounts carbon dioxide, hydrogen, methane, and water. Slow and regular production of short-chain fatty acids provides an energy source that helps the regeneration of colonic mucosa. Fatty acids are also used for hepatic very low density lipoprotein synthesis, which has been reported to have influence in cardiovascular disease.
Dietary protein is only partially digested in the small intestine producing amino acid (NH2) and carboxyl groups (COOH). These amines enter the colon and are hydrolyzed by enzymes of colonic bacteria. Amines and short-chain fatty acids will enter the systemic system through absorption by the colonic mucosa and portal vein into the bloodstream where they will be appropriately utilized by tissues. These substances then return to the liver through the portal circulation and are excreted as urea in the urine.
Primary bile acids originating from the liver are excreted into the small intestine and conjugate with amino acids, particularly taurine and glycine. Conjugated compounds in general are not well absorbed and cannot re-enter the hepatic circulation without further breakdown. Bacterial action hydrolyzes the conjugated amines, releasing free bile acids 7a- and 7/3-dehydroxylation of the bile acid nucleus, and hydroxyl groups C-3, C-6, and C-7. Bacteroides, Bifidobacterium, Fusobacterium, Clostridium, Lac-tobacillus, and Streptococcus are the main bacteria that assist in this hydrolysis. These free bile acids can recirculate through the enterohepatic circulation. Bile acids assist in digestion of fats in the intestine. Colonic microflora also transform excess cholesterol found in the large intestine to coprosta-nol thus reducing available cholesterol and increasing cholesterol to be excreted in stool.
Bacterial microflora of both the small and large bowel synthesizes a number of essential vitamins. Most importantly vitamin K production by the liver is dependent on the metabolic activity of bacteria in the ileum. Prothrombin, a blood-clotting factor, is synthesized in the liver. Glycoprotein arising from the prothrombin complex cannot be synthesized unless the liver contains menaquione. Bacteria in the intestine synthesize menaquione at the terminal ileum where it can be absorbed and reach the liver to promote clotting factors.
Vitamin B12 is completely synthesized from microflora in animals. Meats and dairy products from these animals is a primary source of B12 for humans, but it is also synthesized in the large bowel. However, the small bowel is the site of optimal absorption of B12 so synthesized B12 is not well absorbed. Additionally, biotin and other B complex vitamins (folic acid and thiamine) are synthesized by GI microflora.
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