What are the accessory organs?
Salivary gland's, liver, gallbladder, pancreas
What are the organs of the gastrointestinal tract?
Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine
Salivary gland's
Release a mixture of water mucus and enzymes
Liver
Produces bile, and important secretion needed for lipid digestion
Gallbladder
Stores and releases bile, needed for a lipid digestion
Pancreas
Releases pancreatic juice that neutralizes chyme and contains enzymes needed for carbohydrates, proteins, and lipid digestion
Oral cavity
Mechanical breakdown, moist evening, and mixing of food with saliva
Pharynx
Propels food from the back of the oral cavity into the esophagus
Esophagus
transports food from the pharynx to the stomach
Stomach
Muscular contractions mix food with acid and enzymes, causing the chemical and physical breakdown of food into chyme
Small intestine
Major site of enzymatic digestion and nutrient absorption
Large intestine
Receives and prepares undigested food to be eliminated from the body as feces
glycosidic bond
Digestion involves enzymatic cleavage of the oxygen bridge called
Polysaccharides
Salivary Alpha-amylase -mouthPancreatic Alpha-amylase- small intestineResistant starchesLess salivary and pancreatic Amylase in infantsAmylase attacks only straight chain region
Amylose
The linear poly glucose chain occurs in
Amylopectin and glycogen
The branched chain polymer occurs in
digestion of amylose
Salivary gland releases salivary alpha-amylase, which hydrolyzes a-1,4 glycosidic bonds in amylose, forming dextrins
digestion of amylopectin
Salivary gland to release salivary alpha-amylase, which hydrolyzes a-1,4 glycosidic bonds in amylopectin, forming dextrins
Maltose + maltotriose
Amylose
Maltose + maltotriose + limit dextrins
Amylopectin, glycogen
2 exoglucosidases
Each carries two domains and have two activities-maltase-glucoamylase and sucrase-isomaltase
Disaccharides are
active in micro villi of enterocytes (brush orer cells)
Intestinal walls
fold into villi that are lined with absorptive mucosal cells containing microvilli
Absorption of glucose & galactose - Into cell:
active transport (symport)- specific receptor SGLT1
Absorption of glucose & galactose - Into blood:
diffusion, GLUT2
Absorption of fructose - Into cell:
facilitated transport - GLUT5
Absorption of fructose - Into blood:
GLUT2
Absorption of fructose
Slower than glucose and galactose
SGLT1 (encoded by SLC5A1 gene)
• Co-transports 2 sodium ions and one aldohexose • Km for glucose/galactose is 0.5mM and 5mM for sodium. High glucose flux even at low luminal glucose concentrations.
GLUT5 (encoded by SLC2A5 gene)
• Facilitated transport of fructose • Low affinity, 5-6mM Km
GLUT5 expression
is induced by presence of fructose in lumen
SGLT1 is
expressed at high levels during infancy, GLUT5 is low
Hexose transporter
expression increases during enterocyte differentiation and migration up the villi.
•mRNA of transporters (GLUT2, GLUT5, SGLT1)
shows diurnal variation with increase just before feeding
Activation of sweet taste receptors or SGLT1 transport activity in enteroendocrine cells
may lead to increased expression of hexose transporter genes
Factors affecting carbohydrates absorption
- Effect of physical state of starch (processing, retrograde) - Effect of transit time (fat, fiber, high osmolality) - intestinal or pancreatic disease; High AMY1 copy number - single mutation can produce defective SGLT1
Disaccharidase activity may be lost/reduced:
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gastric bypass surgery
• Bypasses 90% of the stomach, the entire duodenum, and proximal jejunum • Ileum may adapt through cellular proliferation
Use of Antidiabetic drugs
• Development of inhibitors of starch digestion to treat diabetes and obesity to control blood sugar • Acarbose, a pseudotetrasaccharide, amylase inhibitor, sucrase and maltase inhibitor; Unpleasant side effects due to undigested starch • Miglitol and Vogliobose inhibit disaccharidase activity; associated with symptoms of carbohydrate malabsorption • Newer molecules - salacinol and kotalanol (salacia reticulata herb) more effective as inhibitors
Regulation of Postprandial Glycemia
• Carbohydrates are absorbed à plasma glucose levels rise à insulin is secreted • Carbohydrate intake is accompanied by secretion of incretin hormones by gut EECs • Incretin hormones have effects on satiety, gastric emptying, endogenous glucose production and glucose utlilization. • Incretins affect insulin secretion even before plasma glucose level increases; 30 to 60% of increase in plasma insulin following an oral glucose load • Glucose-dependent insulinotropic polypeptide (GIP) - 42 amino acid peptide is secreted enteroendocrine K cells in upper small intestine • Glucagon-like peptide -1 (GLP-1) - 30 amino acid peptide is a proteolytic cleavage product of proglucagon and is secreted by enteroendocrine L cells of lower small intestine (ileum) and colon
Secretion of Incretins
GIP and GLP-1 are secreted in response to macronutrient ingestion. • Glucose in gut is most important for incretin release à through activation of SGLT1 in EECs • Involves the co-transport of sodium ions by SGLT1 à depolarizes the plasma membrane à opening of voltage-sensitive calcium channels à exocytosis of incretin-containing vesicles • GIP and GLP-1 bind to their G protein coupled receptors on pancreatic beta cells to initiate signaling (adenylate cyclase and cAMP) that culminates in the secretion of insulin by endocrine pancreas • Effects of incretins on insulin secretion are glucose dependent (higher in hyperglycemia)
Effects of Incretins
• GIP and GLP-1 promote insulin biosynthesis • Promote the survival and proliferation of pancreatic beta cells. • GLP-1 suppresses glucagon secretion in a glucose-dependent manner (Glucoregulatory role) • Plasma glucose homeostasis through other mechanisms: • Paracrine activation of EEC and enterocyte receptors • Neurocrine activation of receptors on vagal afferent terminals and receptors on enteric neurons to modulate intestinal function and in other tissues • GLP-1 regulates gastrointestinal motility by inhibiting gastric emptying during the postprandial phase
glycemic response
- Degree and duration to which blood glucose level is elevated after consuming a portion of food that would provide 50g of digestible carbohydrates and measured [AUC] for the next 2 hours after a meal
glycemic index
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Glycemic load
Amount of food used to determine glycemic index is not the amount typically consumed Glycemic load is the glycemic index normalized to serving standards - GI x g of CHO in 1 serving of foodand then divided by 100 - Global indicator of glucose response and insulin demand • Higher postprandial glycemia and hyperinsulinemia are universal mechanisms for chronic disease progression
dietary fiber
nondigestible CHO & lignin that are intact & intrinsic in plants
Functional fiber
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How do dietary fiber and functional fiber differ
- types of sugar residues - arrangement of residues (branching and substitution) - secondary structure (e.g. resistant starch); packing or arrangement can restrict enzyme access
definition of dietary fiber
Codex Alimentarius Commission definition (2010): "Carbohydrate polymers with 10 or more monomeric units that are not hydrolyzed by the endogenous enzymes in the small intestine of humans and belong to the following categories: 1. Edible carbohydrate polymers naturally occurring in the food as consumed; 2. Carbohydrate polymers, which have been obtained from food raw material by physical, enzymatic, or chemical means and which have been shown to have a physiological effect or benefit to health as demonstrated by generally accepted scientific evidence to competent authorities; and 3. Synthetic carbohydrate polymers which have been shown to have a physiological effect or benefit to health as demonstrated by generally accepted scientific evidence to competent authorities.
Cellulose
• Dietary & functional fiber • Long, linear polymer of b 1-4 linked glucose units; microfibrils • Plant cell walls; major component of dietary fiber • Sources: bran, legumes, nuts, peas, root vegetables, cabbage family, outer covering of seeds, apples • Hemicelluloses not related to cellulose; similarity in solubility properties
Pectins
• Dietary & functional fiber • Backbone = galacturonic acid • Cell wall & middle lamella in plants • Water-soluble, gel-forming • Sources: apples, strawberries, citrus
Lignin
• Main non-carbohydrate component of dietary fiber; Polyphenolic units • Structural components of plants - found in stems, seeds, bran layer
Gums
• Hydrocolloids • Site of injury • High water solubility; form viscous solutions • Mucilages protect seeds from drying (Psyllium is mucilage from husk of pysillium seeds)
Resistant Starch
• Starch that cannot be digested by humans • Arrangement of starch molecules • Types - RS1 - Physically inaccessible (but digestible) resistant starch that can be found in seeds, legumes, or unprocessed grains - RS2 - Naturally occurring semi-crystalline granules that are present in foods such as uncooked potatoes and green bananas - RS3 - From cooking & cooling or extruding foods (legumes, cooked or chilled potatoes and cornflakes and retrogradation) - RS4 - Specially processed starches that yield chemically modified starch • RS1 & RS2 = dietary fibers, RS3 & RS4 = functional fibers
Fructans--(Inulin, Oligofructose, & Fructooligosaccharides)
• Edible plant cell components • Fructose units in chains of varying length ( at least 1 fructosylfructose); linear or branched • Resistant to hydrolysis and absorption • Prebiotics • Sources: chiccory, asparagus, onions, garlic, artichokes, tomatoes, bananas
Intestinal Bacteria (Microflora), Pre- and Probiotics, and Disease
• Human gut - 100 trillion microbial organisms • Fermentation - breakdown of CHO & protein anaerobically • Lactate & short-chain fatty acids, gases • Probiotics - foods containing live bacterial cultures • Prebiotics - food ingredients that promote the growth of specific beneficial gut bacteria. • Synbiotics - combinations of both pre- and pro-biotics. • Benefits: - Reduction of diarrhea incidence -Improvements in gut health - Elimination of allergies -Prevention of infections
Selected Properties & Physiological Effects of Fiber
Paleolithic diet - 100g fiber (more fruits and vegetables) Current western diet - 15g fiber (more cereal fibers) • Important properties include: - Soluble fibers form more viscous solutions and are more fermentable than insoluble fibers - Water-soluble: some hemicelluloses, pectin, gums, b-glucans, mucilages, algal polysaccharides - Water-insoluble: cellulose, lignin, some hemicelluloses, chitosan, chitin
What factors influence fermentation rate?
*Chemical and Physical Structure -Methyl esters in pectin -Esterification of starch -Fermentation rate likely a function of links between sugars -Purification can increase fermentation and SCFA -Type and amount of crystallinity -Particle size*Processing -Extrusion cooking can change the fermentation pattern -Enzymatic treatment -Chemical treatment (alkali, hydrogen peroxide) *Combinations of fibers -both soluble and insoluble fibers -soluble fiber consumed with starch increase viscosity resulting in reduced rate of digestion
Physiological effects of Dietary Fiber in the Stomach and the Small Intestine
• Dietary fiber can delay the rate at which food enters the small intestine. - Chewing of bulky foods - Slowing of gastric filling and emptying (viscosity of chyme) • High fiber foods can promote satiety - Feeling of fullness produced by distention of the stomach (through neurological mechanisms and physical bulkiness of the fiber) - Release of satiety release hormones - Viscous fibers are more satiating; Whole foods
Physiological effects of Dietary Fiber in the Stomach and the Small Intestine (part 2)
• Dietary fiber slows the rate of nutrient absorption, lowers postprandial glucose levels and limits cholesterol absorption. - Slow gastric emptying; entrapment in gel-like matrix; interference with micelle formation; dilution of enzymes and digestible components; decreased enzyme access; impeding of mixing à slower movement - Slowing of macronutrient absorption
Dietary Fiber and Health
• Cardiovascular Disease - Lower blood pressure, improve serum lipids, and inflammation • Obesity Prevention • Type 2 Diabetes • Bowel Health - Mostly constipation and not other gastrointestinal disorders • Microbiota
Regulation of Macronutrient Metabolism at Tissue-level
• Tissues differ in structure and function (e.g. Muscle and adipose and GLUT4) • Tissues differ in their capacities to use different types of fuel. • Most tissues will be able to use glucose (postprandial) or fatty acids (starvation). Some tissues lack the ability to make this switch (RBC no mitochondria) • The liver plays a key role in modulating the mixture of circulating fuels that are supplied to the rest of the body. (e.g. Fed state; post-absorptive state; starved state) • Tissue-specific gene expression determines the structure and metabolic capacities of various tissues. • Overall expression of enzymes may differ; Differential expression or isoforms in different cells. (e.g. glycogen breakdown in muscle and liver differ in phosphorylase regulation and end product)
adipose tissue
• insulin dependent • complete oxidation • partial degradation to glycerol (for TG) • metabolized to acetyl CoA for fatty acid synthesis and storage
brain tissue
• insulin independent • complete oxidation
RBC in tissues
• insulin independent • partial oxidation to lactate
liver tissue
• insulin-independent • complete oxidation • glycogen storage • carbons for biosynthesis of other compounds • release glucose from glycogen • synthesize glucose • into other pathways
Skeletal muscle and heart
• insulin-dependent • complete oxidation • glycogen storage • Heart is aerobically metabolic all the time • Skeletal muscle has periods of high and low activity; limited anerobic capacity