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2/16/16
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Chapter 4 Carbohydrates: Simple Sugars and
Complex Chains BIOL 103, Spring 2016
What Are Carbohydrates?
• Sugars, starches, and fibers • Major food sources: Plants – Produced during photosynthesis
• Two main carbohydrate types – Simple (Sugars) – Complex (Starches and fibers)
Simple Sugars
• Monosaccharides – Glucose – Fructose – Galactose
Monosaccharides
• Glucose/Dextrose – Most abundant simple sugar in nature – Gives food a mildly sweet flavor – Usually joined to another sugar in foods to form disaccharides, starch, dietary fiber – Provides energy to body cells • Blood sugar levels • Only fuel source used by brain
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Monosaccharides
• Fructose/Levulose – “fruit sugar” – Tastes the sweetest of all the sugars – Occurs naturally in fruits and vegetables – Found in fruits, vegetables, honey, and corn syrup
High Fructose Corn Syrup
• Why do we use it? – Before 1970s à U.S. cane sugar à too expensive
– 1980s, food companies switched to cheaper corn • Pepsi/Coke switched in 1984
High Fructose Corn Syrup
• How is it made? 1. Convert glucose à fructose, using isomerase 2. Add corn syrup, then a specific ra]o of
glucose • Example: HFCS 55 (so` drinks)
Why is High Fructose Corn Syrup associated with weight gain?
1. Corn is cheap à cheaper soda à supersize à drink more à more calories
2. Fructose does NOT release insulin and lep:n à does not feel full à drink more à more calories
• However, note that overconsump]on of ANY forms of sugar will contribute to weight gain.
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Monosaccharides
• Galactose – Rarely occurs as a monosaccharide in food – Usually bonds to glucose to form lactose • Primary sugar in milk and dairy products
Simple Sugars
• Disaccharides: Consist of two monosaccharides linked together – Sucrose: glu + fru – Lactose: glu + gal – Maltose: glu + glu
• Linked by glycosidic bonds
Disaccharides
• Sucrose: glucose + fructose – “Table sugar” – Made from sugar cane and sugar beets – Listed as sugar on food labels
• Lactose: glucose + galactose – “Milk sugar” – Found in milk and milk products
Lactose Intolerance
• Who has it? – Anyone who does not have enough lactase, which normally converts (lactose à glucose + galactose)
• Why do you get it? – Lactase nonpersistence (most common) – Injury/infec]on to intes]ne usually during infancy – Developmental lactase deficiency in premature infants
– Congenital lactase deficiency: rare gene]c disorder from birth
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Disaccharides
• Maltose: glucose + glucose – “Malt sugar” – Seldom occurs naturally in foods – Product of starch breakdown – Found in germina]ng cereal grains
Complex Carbohydrates • Chains of three or more sugar molecules – Oligosaccharides • 3-‐10 sugar molecules • Examples sources: dried beans, peas, and len]ls
– Polysaccharides • Long chains of monosaccharides • Structural differences affect how they behave in water and with hea]ng • Diges]ble (e.g. starch) or nondiges]ble (e.g. fiber)
Complex Carbohydrates
• Starch – Storage form of energy in plants – Found in grains, legumes, and tubers (potatoes and yams) – Long chains of glucose units • Amylose—straight chains • Amylopec:n—branched chains – Digested more rapidly than amylose
– Resistant starch: A starch that is not digested
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Complex Carbohydrates
• Glycogen – Storage form of carbohydrates in animals – Highly branched chains of glucose units – Blood glucose – Most stored in our skeletal muscle and liver – Carbohydrate “loading”
Complex Carbohydrates
• Fiber – Nondiges]ble carbohydrates and lignins – Dietary fiber: soluble + insoluble fiber • Fruits, vegetables, legumes, and whole grains
– Func:onal fiber: Isolated and added to foods – Total fiber: Sum of dietary fiber and func]onal fiber
Types of Dietary Fiber
Soluble • Dissolves in water à gel-‐
like à slows diges]on, lowers blood cholesterol and glucose levels
• Fermentable • Example: Fruits, vegetables,
legumes, oats, oat bran, legumes (dried beans and peas)
Insoluble • Does not dissolve in water
à creates a bulk à larger, so`er stool
• Fermentable (Resistant Starch) vs. Non-‐fermentable
• Example: Bran por]on of whole grains, skins of fruits/vegetables, cauliflower, potato skins
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Complex Carbohydrates (Dietary Fiber)
• Gums and mucilages – Gums: Gel-‐forming fibers in plant cell walls and seeds
– Mucilages: gela]nous soluble fiber found in seaweed
• Pec:ns – “Sugar acids”; keeps fruits in shape – Dietary fiber found in all plants, especially fruits and berries
– Pec]ns + acid + sugar = jam
Complex Carbohydrates (Dietary Fiber)
• Beta-‐glucans – Polysaccharides of branched glucose units linked with beta glycosidic bonds
– Found in barley and oats – Help decrease blood cholesterol levels
• Lignin: – Non actually carbohydrates; composed of mul]-‐ring alcohol units that compose the only non-‐carbohydrate component of dietary fiber
– Found in cell walls of woody plants and the seeds of fruits
Complex Carbohydrates (Dietary Fiber)
• Cellulose: long, straight chains of glucose molecules with beta glycosidic bonds – Structural func]on in plants: forms the woody fibers in trees and strong plant cell walls
• Hemicelluloses: variety of monosaccharides with many branching side chains – Usually mixed with cellulose in plants – Outer bran layer in cereal grains, legumes, vegetables, nuts
Dietary Fiber
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Carbohydrate Diges]on
• Diges]on breaks down carbohydrates to single sugars – Mouth • Salivary amylase
– Stomach • HCl’s acidity stops the ac]on of salivary amylase à stops carbohydrate diges]on
Carbohydrate Diges]on
• Diges]on – Small intes]ne • Pancrea:c amylase con]nues starch diges]on • Brush border enzymes digest disaccharides • Other diges]ve enzymes: – Maltase, sucrase, and lactase
Carbohydrate Diges]on
• Diges]on – Bonds that link glucose molecules • Alpha bonds – Broken down by human enzymes (e.g. starch)
• Beta bonds – Bonds remain unbroken by human enzymes (e.g. cellulose, resistant starch)
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Carbohydrate Absorp]on
• Absorp]on in the small intes]ne – Glucose, galactose, fructose molecules travel to the liver through the portal vein – Liver converts the absorbed monosaccharaides to glucose: • Galactose à glucose • Fructose à glucose
Carbohydrates and Glucose in the Body
• Excess glucose is stored as… – Liver glycogen (~1/3) • Maintains normal blood glucose levels
– Muscle glycogen (~2/3) • Fuels muscle ac]vity
Carbohydrates and Glucose in the Body
• Glucose is our primary fuel – To drive chemical processes – To maintain normal blood glucose levels
• Other sources of energy: fat, protein • What is the order of energy usage by body?
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Consequence of Low Blood Sugar
• Sparing body protein – Adequate carbohydrates prevent body from making glucose from body proteins
– Preven]ng ketosis • Ketone bodies à ketosis à dehydra]on • Body needs a minimum of 50 to 100 grams of carbohydrate/day to prevent ketosis
Carbohydrates and Glucose in the Body
• Regula]ng blood glucose – Hyperglycemia vs. Hypoglycemia
– Controlled by hormones: • Insulin • Glucagon • Epinephrine (Adrenaline) – “Fight-‐or-‐flight” hormone
Regula]ng Blood Glucose
• Glycemic Index measures effect of food on blood glucose levels – Different foods vary in their effect on blood glucose levels – Foods with High Glycemic Index cause faster and higher rise in blood glucose
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Glycemic Index High Blood Glucose: Diabetes Mellitus
• What is diabetes?
• Normally: • Eat food with glucose à blood and cells • If too much blood glucose à pancreas releases insulin à blood glucose decreases
– If you have diabetes: • Pancreas: liqle or no insulin OR cells do not respond appropriately to insulin à hyperglycemia
High Blood Glucose: Diabetes
• Forms of diabetes mellitus: – Diabetes Type I: – Diabetes Type II: – Gesta:onal diabetes:
The Role of Carbohydrates in Our Diet
• Cons: excess simple sugar à weight gain, poor nutrient intake, tooth decay
• Pros: high intake of foods rich in dietary fiber offers many health benefits, including reduced risk of: – Obesity – Type 2 diabetes – Cardiovascular disease – Gastrointes]nal disorders
• Increase fiber intake gradually while drinking plenty of fluids
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Carbohydrates and Health
• Fiber and obesity – Possible role in weight control
• Fiber and type 2 diabetes – Beqer control of blood glucose
• Fiber and cardiovascular disease – Can lower blood cholesterol levels
• Fiber and gastrointes:nal disorders – Healthier gastrointes]nal func]oning
Carbohydrates and Health
• Nega]ve effects of excess fiber: – If add supplemental fiber too quickly, may have side effects: abdominal bloa]ng, gas – Can bind small amounts of minerals à prevent some mineral absorp]on • Examples: Zinc, Calcium, Iron
• Fiber does not have UL: because ea]ng a lot of fiber (found naturally in foods) doesn’t really affect overall health of healthy people.
Carbohydrates and Health
• Modera]ng sugar intake – Use less added sugar – Limit so` drinks, sugary cereals, candy, ice cream, and sweet desserts – Choose fresh fruits or those canned in water
The Role of Carbohydrates in Our Diet
• Choosing Carbohydrates Wisely – Increase fruit, vegetables, whole grains, low-‐fat milk
• Strategies – Eat peel fruit/vegetables – Eat legumes – Choose brown rice – Choose high-‐fiber cereal – Drink plenty of water!
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Carbohydrates and Health
• Nutri:ve sweeteners: – Monosaccharides, disaccharides, and sugar alcohols – Natural vs. refined
• Non-‐nutri:ve sweeteners: – Saccharin – Aspartame – Acesulfame K – Sucralose
Nutri]ve Sweeteners
• Nutri:ve Sweeteners: substances that sweeten food and can absorbed and yield energy in the body.
• Types: 1. Natural: mono + di-‐saccharides • e.g. honey, maple syrup
2. Refined: mono + di-‐saccharides extracted from plant food • e.g. white sugar, fructose
3. Sugar Alcohol/Polyols: may be sugar/sucrose free, but not always calorie free • e.g. xylitol
Non-‐Nutri]ve Sweeteners
• Non-‐nutri:ve sweeteners: “ar]ficial sweeteners”; yields almost no energy
• Types: 1. Saccharin (1970s; bladder cancer) 2. Aspartame (phenylalanine + aspar]c acid;
PKU) 3. Acesulfame K (stable under heat) 4. Sucralose (used in baked goods, beverages,
gela]n, etc.)