Intestine digestion/absorption

Tso, P., Karlstad, M.D., Bistrian, B.R., and DeMicheUe, S J. (1995) Intestinal Digestion, Absorption, and Transport of Structured Triglycerides or Cholesterol in Rats, Am. J. Physiol. 268, G568-G577. 

JE Staggers, O Hernell, RJ Stafford, MC Carey. Physical-chemical behavior of dietary and biliary lipids during intestinal digestion and absorption. 1. Phase... 

Figure 8.13 The central role of transdeamination in metabolism of amino adds and further metabolism of the oxoacids in the liver. The box contains the reactions for conversion of the amino acids to their respective oxoacids. Processes are as follows (1) digestion of protein in the intestine and absorption of resultant amino acids, (2) degradation of endogenous protein to amino acids (primarily but not exclusively muscle protein), (3) protein synthesis, (4) conversion of amino acid to other nitrogen-containing compounds (see Table 8.4), (5) oxidation to CO2, (6) conversion to glucose via gluconeogenesis, (7) conversion to fat.

Responses to activation of the parasympathetic system. Parasympathetic nerves regulate processes connected with energy assimilation (food intake, digestion, absorption) and storage. These processes operate when the body is at rest, allowing a decreased tidal volume (increased bronchomotor tone) and decreased cardiac activity. Secretion of saliva and intestinal fluids promotes the digestion of foodstuffs transport of intestinal contents is speeded up because of enhanced peristaltic activity and lowered tone of sphincteric muscles. To empty the urinary bladder (micturition), wall tension is increased by detrusor activation with a concurrent relaxation of sphincter tonus. 

J. E. Staggers, O. Hernell, R. J. Stafford, and M. C. Carey, Physical-chemical behaviour of dietary and biliary lipids during intestinal digestion and absorption. 1. Phase behaviour and aggregation states of model lipid systems patterned after aqueous duodenal contents of healthy adult human beings, Biochemistry 29 2028-2040 (1990). 

To exert their physiological effects in vivo, bioactive peptides must be released during intestinal digestion and then reach their target sites at the luminal side of the intestinal tract or, after absorption, in the peripheral organs. Figure 1 presents a scheme of the intestinal assimilation of protein and the routes of bioactive peptide liberation. 

Q6 Vitamin Bi2 is absorbed from the terminal ileum. For successful absorption of this vitamin, intrinsic factor from the stomach is required. Since the stomach is not affected by celiac disease, production of intrinsic factor is not reduced. The terminal ileum is usually little affected by celiac disease, perhaps because the toxic components of gluten have been digested or inactivated in some way before the intestinal contents reach this part of the intestine. The absorption and blood concentration of vitamin Bi2 in celiac patients is usually within normal limits. 

Harrison EH and Hussain MM (2001) Mechanisms involved in the intestinal digestion and absorption of dietary vitamin A.131,1405-8. 

Intestinal Digestion and Accompanying Reactions The extent of intestinal digestion and the nature and the relative amounts of the digestion products taken up by the intestinal cells depend on the activity and specificity of the lipases, the rate of isomerization, and the rate of absorption of the products formed by lipase action. 

End Products of TAG, DAG, and MAG Digestion Intestinal digestion and absorption are interrelated phenomena that go on simultaneously. The chemical form in which fatty acids present in food fats are taken up depends on rates of digestion and isomerization relative to rates of uptake. The chemical form in which a fatty acid is present in the intestinal lumen or is taken up in the intestinal cells is of utmost importance because it influences its further metabolism. 

The epithelium is supported underneath by lamina propria and a layer of smooth muscle called muscularis mucosa (3-10 cells thick). These three layers, i.e., the epithelium, lamina propria, and muscularis mucosa, together constitute the intestinal mucosa.On the apical surface, the epithelium along with lamina propria projects to form villi. The cell membranes of epithelial cells that comprise the villi contain uniform microvilli, which give the cells a fuzzy border, collectively called a brush border. These structures, although greatly increase the absorptive surface area of the small intestine, provide an additional enzymatic barrier since the intestinal digestive enzymes are contained in the brush border. In addition, on the top of the epithelial layer lies another layer, the UWL, as previously described. The metabolic and biochemical components of the epithelial barrier will be discussed. 

Blockage of the bile duct caused by problems such as cholesterol-containing gallstones or duodenal or pancreatic tumors can lead to an inadequate concentration of bile salts in the intestine. Digestion and absorption of dietary lipids is diminished. Certain diseases that affect the pancreas can lead to a decrease in bicarbonate and digestive enzymes in the intestinal lumen. (Bicarbonate is required to raise the intestinal pH so that bile salts and digestive enzymes can function.) If dietary fats are not adequately digested, steatorrhea may result. Malabsorption of fats can lead to caloric deficiencies and lack of fat-soluble vitamins and essential fatty acids. 

Figure 48-3 The location of small intestinal absorption. (Modified from Morris JA, Se//vonozK S/ie/don GE Nutritional management of patients with malabsorption syndrome. Clin Gastroenterol l983 l2 463-74, based on Borgstrom 8, et al. Studies of intestinal digestion and absorption in the human, j Clin Invest I957 I52I 36.)...

Borgstrom, B., Dahlqvist, A., Lundh, G. and Sjovall, J. (1957) Studies of intestinal digestion and absorption in the human, J. Clin. Invest. 36, 1521-1536. 

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