Basic Nutrition – Carbohydrate Digestion, Absorption & Metabolism

Outline – Dietary Carbohydrates 1. 1.Digestion 2. 2.Monosaccharide Transport 3. 3.Resistant Carbohydrates 4. 4.Metabolism 5. 5.Recommendations - DRI

Pancreatic duct penetrates duodenal wall Endocrine functions Insulin and glucagon Exocrine functions Majority of pancreatic secretions Pancreatic juice secreted into small intestine –Carbohydrases  pancreatic alpha-amylase –Lipases –Nucleases –Proteolytic enzymes The pancreas

Monosaccharides are absorbed by specific transporters at the enterocyte brush border and basolateral membranes Brush border membrane Basolateral membrane Fructose Glucose Galactose Glucose Galactose Na + K+K+ Fructose SGLT-1 GLUT5 Na + K+K+ ATPase GLUT2 Disaccharidases Fructose Glucose Galactose Fructose Glucose Galactose H2OH2OH2OH2O

Short-Chain Fatty Acids Products of fiber fermentation Acetate, propionate, butyrate comprise 83% of SCFAs Found in a fairly constant molar ratio of 60:25:15 Enhance structure and function in adult rodent small intestine ; mucosal proliferation in colon, ileum and jejunum (Sakata, 1987; Kripke et al., 1989; Koruda et al., 1990) transporter activity (Tappenden et al., 1997, 1998; Tappenden and McBurney, 1998)

SCFA and Functional Adaptations Brush border membrane Basolateral membrane Fructose Glucose Galactose Glucose Galactose Na + K+K+ Fructose SGLT-1 GLUT5 Na + K+K+ ATPase GLUT2 Sucrase- Isomaltase glucose uptake Fructose Glucose Galactose Fructose Glucose Galactose H2OH2OH2OH2O

What are Dietary Reference Intakes (DRI)? Daily nutrient consumption recommendations Collective term including: Estimated Average Requirement (EAR) Recommended Dietary Allowance (RDA) Adequate Intake (AI) Tolerable Upper Intake level (UL)

EAR - Meets the requirement of half the healthy individuals in life stage group RDA - Meets the requirement of 97-98% of the healthy individuals in life stage group (RDA = EAR + 2 × SD requirement ) AI - Experimentally determined approximations for healthy individuals in life stage group (used when no RDA set) UL - Highest average daily nutrient intake level that is likely to pose no risk of adverse health effects

Determining Carbohydrate DRI Primary energy source, along with fat Preferred substrate for certain metabolic pathways Body can synthesize de novo EAR/ AI’s derived from minimum metabolic requirements and decreased disease risk

Optimal Carbohydrate Level Amount necessary for optimal health unknown Populations consuming minimal CHO for long periods show no decrease in longevity In absence of CHO, adequate protein and dietary fat needed to provide amino acids and glycerol respectively, for gluconeogenesis Brain, red and white blood cells, medulla of kidney have absolute glucose requirement

Setting the Carbohydrate DRIs In fasted adults, glucose production is about 2.8 to 3.6 g/kg/d This equals 210 to 270 g/d in a 70-kg man Minimum amount required determined by brain usage Brain glucose requirement reported to be 110 to 140 g/d

Carbohydrate DRI Values for Adults Function AgeMales RDA/ AI g/d Females* RDA/AI g/d AMDR (% total energy) Primary energy source for brain, source of calories to maintain body weight 19-30y y y >70y *Recommendations change for pregnant or lactating women