Notes on Carbohydates

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Food Science and Human Nutrition
HUN 2201
All Professors

HUN 2201 – Lesson 4 – Carbohydrates • More than ½ the calories in the American diet • Over the last century, amounts and sources have changed; fiber decreased over first half of century but then whole grain consumption increased, though fiber intake did not o Much of the carbohydrate added back came from refined sugars [today, mainly corn syrup] • Refined vs unrefined carbohydrates o unrefined: whole grains, legumes, veggies, fruits, milk o whole-grain includes the bran, the germ, and the endosperm  Bran: contains most of the fiber and is a good source of vitamins  Germ: embryo at base of kernel; sprouting occurs  Endosperm: primarily starch but also contains most of the protein and some vitamins and minerals  Bran and germ are removed during refining; white flour is just produced from the endosperm [so they are enriched] • Why should we limit added sugars o not different than other sugars, except that they have been removed from their source and do not occur with all the other vitamins and minerals in natural foods o 16% of the American diet o difficult to tell from ingredient list if it is added or naturally occurring sugars • Types of Carbohydrates o One or more sugar units containing carbon, hydrogen, and oxygen in the same 2-1 proportion found in water. • Simple carbohydrates o Fruits, veggies, and milk o Monosaccharides  Glucose: blood sugar, most important fuel for body. Produced in plants and rarely occurs as a monossacharide in food; most often found as a dissacharide or starch.  Fructose: monossacharide sweeter than glucose, in fruits and veggies, does not cause blood sugar to rise as much so it can be used by diabetics.  Galactose: rarely present as a monossacharide but part of lactose, dissacharide of milk  All contain 12 hydrogen, 6 carbon and 6 oxygen atoms but differ in their arrangement o Disaccharides  Maltose: 2 molecules of glucose. Whenever starch is broken down  Sucrose: glucose to fructose; found in sugarcane, sugar beets, honey, and maple syrup…only sweetener that can be called sugar on the ingredient list of food labels  Lactose: milk sugar, glucose and galactose. Only sugar naturally occurring in animal foods, contributes about 30% of the energy in the whole cow’s milk and about 40% of energy in human milk o Making and breaking sugar chains: hydrolysis breaks, uses water to add OH group. Condensation/dehydration reaction releases a molecule of water by taking an OH group from one sugar and a hydrogen from another • Complex carbs o Generally not sweet o Short chains of 3-10 monossacharides called oligossacharides and long-chain polyssacharides including glycogen in animals and fiber in plants o Oligossacharides  Some formed in gut during polyssacharide breakdown, further digested into simple sugars. Others found naturally in legumes, artichokes, bananas. Many not digested by human enzymes and instead are broken down by intestinal microflora in the colon. Beneficial for GI tract o Glycogen  Storage form of carbohydrate in animals  Polyssacharide of highly branched chains in glucose molecules  Branched structure allows it to be broken down quickly when glucose is needed, stored in muscles and liver, not consumed in food  Only small amount storable o Starches  Storage form of carbs in plants  2 types of molecules • amylose: long straight chains of glucose molecules • amylopectin: branched chains of glucose molecules  accumulates in roots and tubers, and seeds as an energy source for the developing plant embryo.  As a starch-thickened mixture cools, amylose starches and form bonds between the molecules, forming a gel. o Fibers  Certain complex carbs and lignins  Not digested NOR absorbed into the human body  Soluble fibers form viscous solutions when mixed with water. Bacteria in large intestine break it down, producing gas and short-chain fatty acids, small quantities of which can be absorbed. Also, they are found in and around plant cells, include pectins, gums, and some hemicelluloses. Oats, apples, beans, seaweed.  Insoluble fibers cannot be broken down in large intestine and do not dissolve in water; primarily derived from structural part of plants, such as the cell walls, and include cellulose, some hemicelluloses, and lignins. Wheat bran, rye bran, broccoli.  Pectin is a soluble fiber added to food that forms a gel with sugar • Also, xanthan and locust-bean gum  Functional fibers are not naturally occurring  Pectins, gums, mucilagens, glucans are soluble/viscous, extracted and used as food additives, bulking  Viscous fibers bind to cholesterol and bile salts  Single mutated cell in colon epithelium…an initiation event that could be inhibited by fiber, and thus prevent cancer. • Carbohydrates in the digestive tract o Dissacharides and complex carbs must be digested to simple sugars to be absorbed. All humans lack digestive enzymes to completely break down a variety of oligossacharides, certain forms of starch, and fiber. • Carbohydrate digestion and absorption o Salivary amylase in the mouth breaks it into shorter polyssacharides  Dextrins to maltose o Majority of starch and dissacharide digestion occurs in the lumen, then final bit in surface of small intestine, where pancreatic amylases complete the job of breakdown. Salivary amylase no longer functions in the stomach due to high pH o Digestion of these broken down products occurs with the help of enzymes in the small intestine o Resulting monossacharides are absorbed and transported to the liver via the hepatic portal vein o LUMEN, MUCOSAL CELL, CAPILLARIES, PORTAL VEIN, LIVER o Fructose: facilitated diffusion o Glucose and galactose: active transport o Both require protein carrier • Lactose intolerance o Not enough lactase to digest lactose o Causes GI distress because undigested lactose molecules passes into large intestine where it increases the number of small molecules, which draws in water by osmosis. o Lactose is rapidly metabolized by bacteria, producing acids and gas. Together the increase in fluid in the intestine along with the acid and gas causes symptoms o Lactose intolerant people can either meet their calcium needs by gradually spacing out their consumption of dairy or eating tofu, fish, and veggies, or cheese and milk since some of the lactose is lost in the fermenting process • Indigestible carbohydrates o Either because human enzymes cannot break the bonds, OR o Resistant starch because the natural structure of the grain protects it, cooking/processing alters its digestability, or it has intentionally been modified o Presence affects transit time, type of intestinal microflora, amount of intestinal gas, and nutrient absorption  Transit time: • Increase volume of material in the lumen of the intestine and draw water into it, stimulating peristalsis and relieving constipation  Microflora: • As carbs are broken down by bacteria, short chain fatty acids are produced and the colonic contents become more acidic. • Acid prohibits growth of undesirable bacteria and may stimulate growth of beneficial bacteria, whose metabolic by-products may help prevent inflammation in the bowel and potentially protect against colon cancer  Intestinal gas • By – product of bacterial digestion  Nutrient absorption • Fiber binds certain minerals, preventing their absorption • BUT, too much fiber can reduce the absorption of these essential minerals • Soluble fiber also binds cholestoral and bile, reducing their absorption • In the stomach, fiber causes distention and slows emptying o Can be bad for small people who can’t eat a lot and may not fulfill nutrient requirements • Increase volume of intestinal contents and absorb water, forming viscous solutions which speed passage through gi tract by decreasing the amount of contact between nutrients and the absorptive surface of the small intestine, and volume and viscosity slows the absorption of glucose and other nutrients. o Beneficial because slowing the absorption of glucose reduces fluctuations in blood glucose levels. • Carbohydrates in the body o Functions  Glucose: brain and nervous system fuel. Glycogen storage in liver or muscle tissue  Monossacharide galactose: Important in nervous tissue. Combines with glucose to make lactose  Deoxyribose and ribose in DNA  Oligossacharides attached to proteins or lipids on the surface of cells, where they help signal information  Mucopolysaccharides give mucus structure  Does NOT apply to dietary fibers…fiber is not an energy-yielding nutrient o Delivering glucose to body cells  After absorption, glucose and other monos travel to liver via hepatic vein  Here, fructose and galactose area metabolized for energy  Glucose may be broken down by the liver but most is passed in general blood circulation • Amount is regulated by liver and hormones secreted in the pancreas  What affects glycemic response? • How quickly and how high blood glucose rises • Affected by amount and type of carb eaten and amount of fat and protein in that food or meal • How quickly food leaves stomach and enters intestine determines how long it takes to get into the blood • Refined sugars and starches generally cause greater glycemic response, ranked by index. 70 or more are high, 55 orless are low • Glycemic load accounts for both glycemic index of the food and amount of carbohydrate in a typical portion • Difficult to calculate because we typically eat a mixture of different portions of foods  Supplying glucose to body cells • Rise in blood glucose triggers pancreas to secrete insulin, which allows glucose to be taken into muscle and adipose tissue • In the liver, glucose promotes storage of glycogen • In muscle, it stimulates uptake for ATP production and synthesis of glycogen • Also stimulates protein synthesis and lipid synthesis…actions which remove glucose from the blood, decreasing its level • When no glucose has been consumed for a few hours, decrease in availability causes pancreas to secrete glucacon, which signals liver cells to break down glycogen into glucose, and synthesis of new glucose molecules by gluconeogenesis o Can also be stimulated by hormone epinephrine, aka adrenaline o If CHO intake is adequate, it spares proteins from being used for energy o How Glucose provides energy  Cellular respiration  1. Glycolysis: glucose breakdown in cytosol. Anaerobic. 6 carbon broken into 2 three-carbons, [pyruvate]. Generate 2 molecules of ATP for each molecule of glucose and release high energy electrons that are passed to shuttling molecules, which can transport them. Process halts if no further oxygen is available.  2. In mitochondria Acetyl-CoA formation: when oxygen is present. In MITOCHONDRIA, ONE CARBON IS REMOVED FROM PYRUVATE AND RELEASED AS co2. REMAINING 2-CARBON COMPOUND COMBINES WITH MOLECULE OF CoA to form acetyl-CoA. High- energy electrons are released and passed to shuttling molecules for transport  3. Citric acid cycle: acetyl CoA combines with oxaloacete, to form 6- carbon citric acid which then removes one carbon at a time to produce CO2. After 2 carbons have been removed this way, a 4 carbon oxaloacetate is reformed and the cycle can begin again. These chemical reactions produce 2 ATP molecules per glucose and remove electrons, which are passed to shuttling molecules.  4. Electron transport chain: inner membrane of mitochondria; proteins. Molecules passed down the chain and combine with oxygen to form water. As they are passed along, energy is used to pump hydrogen ions across the inner mitochondrial membrane. Energy is trapped and used to add a phosphate group to DP, forming ATP. o What happens when carbohydrate is limited?  Why protein is broken down to supply glucose • Gluconeogenesis, which occurs in liver and kidney cells, occurs when glucogenic amino acids form pyruvate or oxaloacetate, which can then be used to make glucose • Fatty acids and ketogenic amino acids cannot be used to produce glucose because the reactions that break them down produce 2-carbon molecules that form acetyl CoA  Why ketones are formed • To be metabolized via the citric acid cycle, acetyl CoA must combine with 4 carbon oxaloacetate. (oxa is derived from car
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