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Lecture 3

Lecture 3 - Carbohydrates, Sugars, Starches, and Fibres.doc

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Queen's University
Health Studies
HLTH 230
Jeffery Lalonde

Lecture 3 - Carbohydrates, sugars, starches, and fibres • glucose supplies energy to brain and muscles • we get most of our glucose by eating carbohydrates Carbohydrates are: -found in plant foods (whole grains, vegetables, legumes, fruits) -found in dairy (milk, yogurt, sour cream etc.) -> can come from concentrated sugars as well (soft drinks, candy, cakes, cookies) carbohydrate: compound composed of carbon, hydrogen, and oxygen arranged in monosaccharides and multiples of monosaccharides -most but not all carbs have a ratio of one carbon molecules to one water molecule Simple Carbs (the sugars) monosaccharides: single sugar disaccharides: pairs of monosaccharides Complex Carbs polysaccharides: chains of monosaccharides MONOSACCHARIDES • glucose, fructose, galactose -all contain 6 carbons, 12 hydrogens, and 6 oxygens • arrangements are different, which cause their difference in sweetness glucose -mildly sweet -essential energy source for body activities -blood sugar -one of 2 sugars in every disaccharide -make up most polysaccharides -commercially produced by refining potato start/corn starch -refined glucose = dextrose fructose -intensely sweet, sweetest of the monosaccharides -occurs naturally in fruit and honey -commercially high-fructose corn syrup is ofter used (soft drinks, cereals, dessert) -high-fructose corn syrup: syrup made from cornstarch that has been treated with an enzyme that converts some of glucose to sweeter fructose -has a fructose content of 25, 55, or 90% (rest is glucose) galactose -not sweet at all -occurs naturally in very small amounts -found in fermented milk products (ex. yogurt, aged cheese) DISACCHARIDES Lecture 3 - Carbohydrates, sugars, starches, and fibres • each have a glucose paired with galactose, fructose, or another glucose molecule • formed via condensation reaction (gives off water molecule) -hydroxyl group (OH) and hydrogen (H) come together to form water molecule • hydrolysis reaction breaks a disaccharide into two -water molecule splits to provide the H and OH -commonly occurs with digestion • maltose -glucose + glucose -produced during starch breakdown (carbohydrate digestion) -occurs during fermentation process for alcohol -minor constituent of a few foods (ex. barley) • sucrose -glucose + fructose -sweetest of disaccharides -occurs naturally in fruits, vegetables and grains -table sugar (sucrose refined from juices of sugarcane, beets) -brown and powdered sugar are also sucrose • lactose -glucose + galactose -often called “milk sugar” -principle carbohydrate of milk -provides about half the energy of skim milk POLYSACCHARIDES • contain many glucose units • sometimes contain other monosaccharides • 3 important polysaccharides in nutrition 1) glycogen: form of energy storage in animals (built from glucose units) -storage form of energy in animal body -stores glucose for future use (ex. glycogen storage in muscles) -hundred of glucose units in long branched chains -branched structure allows for rapid hydrolysis -allows for a surge of glucose to be available -stored in muscle (2/3) and liver (1/3) -not a significant source of carbohydrate from food -hydrolyzes quickly in animals after slaughter 2) starches: form of energy storage in plants (built from glucose units) -plant cells store glucose as starch -long, branched or unbranched chains of glucose molecules linked together Lecture 3 - Carbohydrates, sugars, starches, and fibres -grains are rich food sources of starch and high amounts are also found in tubers (yams and potatoes) and legumes (peas and beans) -amylopectin: branched starch -potato, tapioca, cornstarch -amylose: unbranched starch -wheat flour, rice 3) fibres: provide structure in stems, trunks, roots and leaves -variety of monosaccharides and other carbohydrate derivatives dietary fibre: nonstarch polysaccharides in plant foods, which are not digested by human digestive enzymes. Some dietary fibres are digested by GI tract bacteria -structural part of plants -found in legumes, vegetables, whole grains -most are polysaccharides -provide little or no energy (monosaccharides are not produced) Fibre Characteristics and Classifications fibre -nonstarch polysaccharides -ex. cellulose, hemicellulose, pectins, gums, mucilages -nonpolysaccharides Lecture 3 - Carbohydrates, sugars, starches, and fibres -ex. lignins (“woody” part of vegetables), cutins, tannins (ex. limes) soluble fibres -dissolve in water -sometimes called viscous fibres because they form a gel -digested by bacteria in the colon (fermentable) -found in oats, barley, legumes and citrus fruit -associated with protecting against diabetes and heart disease because they can help lower blood glucose and cholesterol -ex. gums, pectins, psyllium, some hemicelluloses insoluble fibres -do not dissolve in water -do not form gels (non-viscous) -less readily fermented -found mainly in whole grains (bran) and vegetables -promote bowel movements, alleviate constipation and help in prevention of diverticular disease (pockets in intestine) -ex. cellulose, lignins, many hemicelluloses resistant starches -escape absorption and digestion in small intestine -may be resistant due to the food’s physical properties -most common in whole/partially milled grains, legumes and just-ripened bananas -like soluble fibres, resistant starches may help support a healthy colon phytic acid -not a dietary fibre but often found in fibre rich foods -may contribute to limiting mineral absorption (unclear whether it is fibre, phytic acid or both that bind with minerals preventing absorption) • total fibre at recommended levels may help to ameliorate constipation and diverticular disease, provide fuel for colonic cells, attenuate blood glucose and lipid concentrations, and provide a source of nutrient rich, low energy density foods that could contribute to satiety -very high amounts of fibre! Fibre and Health • whole grains, vegetables, legumes, and fruits supply starch, fibre, and sugars • also provide vitamins, minerals, and are low in fat Fibre and GI Health • enhance health of large intestine walls -healthier walls help prevent absorption of unwanted components Lecture 3 - Carbohydrates, sugars, starches, and fibres • fibres increase stool weight, easing passage • large soft stool reduce pressure in lower bowel helping to prevent hemorrhoids • fibre helps prevent appendicitis because compaction of intestinal contents could block appendix and permit bacteria to invade • fibre stimulates GI tract to maintain its strength and resist bulging out pouches -> diverticula diverticular disease diverticulosis: intestinal wall develops bulges in weakened areas, most commonly colon diverticula: refers to bulging pockets diverticulitis: bulging pockets are infected and inflamed • drinking plenty of water and eating high fibre foods can help prevent constipation heart disease • diets rich in whole grains, vegetables, legumes and fruits may protect against heart attack and stroke by lowering blood pressure, improving blood lipids and reducing inflammation • high fibre diets are generally lower in animal fat and cholesterol (less saturated fat) high fibre diets lower cholesterol • -soluble fibre binds with bile acids causing its excretion -cholesterol is used to make bile acids diabetes • fibres delay digestion, therefor slow glucose absorption (less peaks in sugar) -helps to reduce fluctuations in blood glucose cancer • inverse relationship between dietary fibre intake and colon cancer • fibre helps prevent colon cancer by diluting, binding, and rapidly moving potentially cancer casing agents from colon • some fibres stimulate bacterial fermentation which produces short chain fatty acids that lower pH • lower pH can inhibit cancer growth in colon weight management • fibrous foods tend to be low in fat and refined carbs -> therefor are less energy dense • fibre absorbs digestive juice creating bulk in the stomach and therefore delaying hunger -keeps us full • weightloss products on the market sometimes contain fibre which will create bulk and fullness potentially harmful effects • excess fibre may prevent someone from meeting their caloric needs (malnourished, elderly, young vegans) • switching to a high fibre diet can cause cramping, diarrhea, and even obstruct the GI tract -must increase fibre gradually over weeks -drink plenty of fluids -select fibre rich foods from a variety of sources (f/v, whole grains, legumes) Lecture 3 - Carbohydrates, sugars, starches, and fibres What has more fibre? • 1 apple 3 1/4 inch diameter / 1 cup Romaine lettuce / 1 All-bran bar (180g) • 1 cup whole wheat cooked spaghetti / 2 slices 100% whole wheat bread / 1/2 cup lentils • whole grain lucky charms / multigrain cheerios / instant quaker oatmeal / special k red berries Carbohydrate Family Summary! Monosaccharides • glucose • fructose • galactose Disaccharides • maltose (glucose + galactose) • sucrose (glucose + fructose) • lactose (glucose + galactose) Polysaccharides • glycogen • starches (amylose and amylopectin) • fibres (soluble and insoluble) Ultimate goal of Digestion and Absorption: Glucose Carbohydrate digestion Mouth • chewing foods stimulates saliva production • amylase begins to hydrolyze starch to shorter polysaccharides and maltose Stomach • bolus mixes with stomach acid which inactivates salivary amylase • stomach acid breaks down starch to a small extent but there is no enzymatic digestion of starch in stomach • fibres stay in stomach, causing delay in gastric emptying and contributing to satiety (feeling of satisfaction that occurs after a meal, inhibits eating till next meal) Small Intestine • where majority of carb digestion happens • pancreatic amylase enters small intestine and breaks polysaccharides into shorter glucose chains and disaccharides • maltase, sucrase, and lactase from outer membranes of intestinal cells dismantle disaccharides -maltase: breaks down maltose into 2 glucose molecules -sucrase: breaks down sucrose into glucose and fructose -lactase: breaks down lactose into glucose and galactose Lecture 3 - Carbohydrates, sugars, starches, and fibres Large Intestine • 1-4 hours post meal, all sugars and most starches have been digested -only fibre remains in digestive tract • fibres in large intestine attract water, softening stool • bacteria in GI tract ferment some fibres -generates water, gas, and short chain fatty acids -metabolism of short chain fatty acids occur in liver -potentially 1.5-2.5 kcal/g of fibre (b/c of breaking down of fibre) **Carbohydrate Digestion in GI Tract Summary (Fig. 4-8) Carbohydrate Absorption Glucose & Galactose -cross lining of small intestine through active transport Fructose -cross lining of small intestine by facilitated diffusion (slows entry - smaller rise in blood glucose) • blood from intestines circulate through liver where fructose and galactose are metabolized in liver and glucose is sent out to body for energy -fructose and galactose can be converted to glucose in liver -glucose can be used immediately for energy intestinal cells normally produce enough lactase to ensure lactose is digested and absorbed • • lactase activity is generally highest immediately following birth lactose intolerance: inability to digest lactose -symptoms occur when more lactose than available lactase can handle -excess lactose that isn’t broken down by lactase (enzyme) -causes: lactase activity declines with age may occur because of damage to
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