Lecture 3 Ch 5 Carbohydrates 1 Overview Carbohydrates

Lecture 3 Ch. 5 Carbohydrates 1

Overview Carbohydrates • Sources of Carbohydrates – Simple Sugars – Complex Carbohydrates – Dietary Fiber • • Digestion and Absorption Functions Blood glucose regulation Dietary sweeteners 2

How do we get Carbohydrates ? • Figure p. 104 • Suns energy + water + CO 2 -----> O 2 + carbohydrate • Process of Photosynthesis • simple sugar produced is metabolized to more complex forms for storage • Carbon, Hydrogen and Oxygen • sugars, starches and dietary fiber 3

Mono and Di Saccharides • Mono - one ; Di - two • glucose - major form, all converted to glucose for use in our bodies • (glucose-glucose) is maltose • (glucose-fructose) is sucrose • fructose - fruit sugar • galactose - combined with glucose is lactose, milk products 4

Complex Carbohydrates • Starches and dietary fiber (polysaccharides) • highly recommended forms of food intake • starches - digestible polysacch. • amylose - straight chains of glucose (veg. , bread, pasta, rice, beans…) • storage in plants • Glycogen - branched glucose • more chains, increases breakdown • animal storage in liver (blood glucose) and muscle (movement) 5

Dietary Fiber • Insight p. 111 -115 • Indigestible polysaccharides. • Insoluble fibers • cellulose, hemicellulose, lignin • structural part of plants • do not dissolve in water • Soluble Fibers • pectin, gum, mucilage • inside and around plant cells • dissolve or swell in water • fiber rich foods are a mixture of both kinds of fiber 6

Dietary Fiber • Retains water, increasing bulk of stool, stimulates peristalsis and facilitates elimination • too little fiber, small hard stool, difficulties with elimination • hemorrhoids and diverticula • insoluble fibers increase stool size the most • require high fluid intake • may play role in preventing colon cancer 7

Dietary Fiber • Emphasize foods rather than fiber supplements • vitamin C, carotenoids, may be implicated in observed benefits • soluble fiber may reduce cholesterol as it attracts bile (produced from cholesterol) and leads to its elimination in stool • recommend 20 -35 g per day, see table 3 -4 p. 114 • MODERATION 8

Carbohydrate Digestion • Begins with cooking • salivary amylase (enzyme) – mixes with food, begins break down of starch • stops in stomach due to acid • pancreatic amylase (enzyme) – released into small intestine to breakdown to monosaccharides • Sucrase, Maltase, Lactase (enzyme) – enzymes in wall of S I produce mono from disaccharides. 9

Carbohydrate Digestion • Undigested disaccharide's used by bacteria in LI, produces acid and gas (abdominal discomfort) • Lactose intolerance • people lack sufficient lactase, usually avoid dairy • can reintroduce lactose into diet slowly, small amounts are OK. 10

Carbohydrate Absorption • Single sugars, actively absorbed by cells in wall of SI (energy) • travel via portal vein to liver, to be converted into glucose • used mainly for energy 4 kcal/g or stored as glycogen • red blood cells and brain, glucose is only source of fuel • recommend minimum 55 % of caloric intake 11

Functions of Carbohydrates • Energy source • Protein sparing effect – too little carbohydrate, leads to breakdown of body protein for energy • Ketosis – Low carbohydrate prevents proper metabolism of fats – production of ketones, altering acid/base balance and body functions • sweetener 12

Blood Glucose • Regulated very tightly • liver controls entry into body via portal vein • hyperglycemia - too high • released in urine, diabetes • hypoglycemia - too low • irritable, headache, hungry • Mainly controlled by hormones from the pancreas and adrenal glands 13

Blood Glucose • Pancreatic hormones (fig. 4 -8) • Insulin - released with onset of eating and increase in blood glucose – stimulates liver to synthesize glycogen and cells to take up glucose from the blood • Glucagon - released when blood glucose drops – stimulates breakdown of glycogen to glucose 14

Blood Glucose • • Adrenal Gland Epinephrine (aka adrenaline) released in response to stress increases blood glucose, promoting quick mental and physical action • Balance of all these hormones is necessary to maintain blood glucose • Multifaceted Control System 15

Recommendations • No RDA, minimum 50 - 100 g 55 % of total calories • reduce fat, increase fiber • sugar intake below 10 % • average ~ 16 % • only a problem if nutrient rich food is ignored in favour of sweet foods • MODERATION 16

Sweeteners • Nutritive – Sucrose – high fructose corn syrup • Alternatives – sugar alcohol • sorbitol, mannitol, xylitol, 3 kcal /g – Saccharin - (1879) • link with cancer, no ban – Aspartame - (1981) • amino acids and methanol, 200 times sweeter than sucrose • damaged by cooking • PKU (phenylketonuria) 17

Sweeteners • Alternatives – Acesulfame-K - (1988) • no energy value • can be cooked • Sucralose -(1998) – can be cooked – excreted, not used for energy • WHY? • Diabetics, calories, dental 18