Chapter 4 Carbohydrates What Are Carbohydrates Sugars starches

Chapter 4 Carbohydrates

What Are Carbohydrates? • Sugars, starches, and fibers • Major food sources: Plants – Produced during photosynthesis • Two main carbohydrate types – Simple (sugars) – Complex (starches and fibers) © Vinicius Tupinamba/Shutter. Stock, Inc. © Ayd/Shutter. Stock, Inc. © Mircea BEZERGHEANU/Shutter. Stock, Inc. © Krunoslav Cestar/Shutter. Stock, Inc. Figure 4. 1 Cassava, rice, wheat, and corn.

Simple Sugars • Monosaccharides – Glucose – Fructose – Galactose Glucose Fructose Galactose

Simple Sugars • Disaccharides: Consist of two monosaccharides linked together – Sucrose – Lactose – Maltose Figure 4. 3 The monosaccharides: glucose, galactose, and fructose.

Monosaccharides • Glucose – Most abundant simple sugar in nature – Also called dextrose – Gives food a mildly sweet flavor – Usually joined to another sugar in foods – Provides energy to body cells • Body closely regulates blood sugar levels

Monosaccharides • Fructose – Also called levulose or fruit sugar – Tastes the sweetest of all the sugars – Occurs naturally in fruits and vegetables – Found in fruits, honey, and corn syrup

Monosaccharides • Galactose – Rarely occurs as a monosaccharide in food – Usually bonds to glucose to form lactose • Primary sugar in milk and dairy products

Monosaccharides • Other monosaccharides and derivative sweeteners – Pentoses: Single sugar molecules with five carbons • Present in food in small quantities • Essential components of nucleic acids • Synthesized in the body, not needed in food

Monosaccharides • Other monosaccharides and derivative sweeteners – Sugar alcohols: Derivatives of monosaccharides • Absorbed more slowly than sugars • Body processes them differently • Some fruits contain small amounts

Disaccharides • Joining and cleaving sugar molecules – Joined or cleaved by removal/addition of water molecule – Condensation reaction • Forms disaccharides • Joins two monosaccharides while removing H from one sugar molecule and an OH from the other to form water

Disaccharides • Joining and cleaving sugar molecules – Hydrolysis reaction • Separates disaccharides into monosaccharides • Addition of water molecule splits bond between two sugar molecules

Figure 4. 6 Condensation reactions form disaccharides. Figure 4. 7 Hydrolysis reactions split disaccharides into monosaccharides.

Figure 4. 5 The disaccharides: sucrose, lactose, and maltose.

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

Disaccharides • Maltose: glucose + glucose – “Malt sugar” – Seldom occurs naturally in foods – Product of starch breakdown – Found in germinating cereal grains

Complex Carbohydrates • Chains of two or more sugar molecules – Oligosaccharides • Three to ten sugar molecules • Examples sources: dried beans, peas, and lentils – Polysaccharides • Long chains of monosaccharides • Structural differences affect how they behave in water and with heating • Digestible or nondigestible

Complex Carbohydrates • Starch – Plants store energy as starch – Found in grains, legumes, and tubers (potatoes and yams) – Long chains of glucose units • Amylose—straight chains • Amylopectin—branched chains – Resistant starch: A starch that is not digested

Figure 4. 9 Starch and glycogen.

Complex Carbohydrates • Glycogen – Living animals store carbohydrate in the form of glycogen – Provides body glucose when blood glucose levels get low – Highly branched chains of glucose units – Most stored in our skeletal muscle and liver – Carbohydrate “loading”

Complex Carbohydrates • Fiber – Nondigestible carbohydrates and lignins – Dietary fiber: Found in plants • Fruits, vegetables, legumes, and whole grains – Functional fiber: Isolated and added to foods – Total fiber: Sum of dietary fiber and functional fiber

Data from Shils ME, Olson JA, Shike M, Ross AC, eds. Modern Nutrition in Health and Disease. 10 th ed. Philadelphia: Lippincott Williams & Wilkins, 2006. Table 4. 1 Foods Rich in Dietary Fiber

Complex Carbohydrates • Types of fiber – Cellulose • Indigestible by humans and a component of dietary fiber – Hemicelluloses • Variety of monosaccharides with many branching side chains – Pectins • Dietary fiber found in all plants, especially fruits

© J. D. Litvay/Visuals Unlimited Figure 4. 10 The structure of cellulose.

Complex Carbohydrates • Types of fiber – Gums and cilages • Gel-forming fibers that help hold plant cells together – Lignins • Indigestible substances that make up woody parts of vegetables and the seeds of fruits

Complex Carbohydrates • Types of fiber – Beta-glucans • Polysaccharides of branched glucose units • Found in barley and oats • Help decrease blood cholesterol levels – Chitin and chitosan • Primarily consumed in supplement form • Marketed as weight-loss supplements • May impair absorption of fat-soluble vitamins and some minerals

Carbohydrate Digestion and Absorption • Digestion: Breaks down carbohydrates to single sugars – Mouth • Salivary amylase begins digestion of carbohydrates. – Stomach • Acidity of stomach juices halt action of salivary amylase and stops carbohydrate digestion.

Carbohydrate Digestion and Absorption • Digestion – Small intestine • Pancreatic amylase continues starch digestion. • Brush border enzymes digest disaccharides. • Other digestive enzymes – Maltase, sucrase, and lactase split maltose, sucrose, and lactose, respectively.

Carbohydrate Digestion and Absorption • Digestion – Bonds that link glucose molecules • Alpha bonds – Broken down by human enzymes » Starch • Beta bonds – Bonds remain unbroken by human enzymes » Cellulose

Carbohydrate Digestion and Absorption • Digestion • Enzymes – Highly specific » Examples include lactase and Beano • Some carbohydrates remain intact, such as fiber and resistant starch

Figure 4. 11 Carbohydrate digestion. Most carbohydrate digestion takes place in the small intestine.

Carbohydrate Digestion and Absorption • Absorption: The small intestine swings into action – End products of carbohydrate digestion • Glucose • Galactose glucose • Fructose glucose – Liver • Stores and releases glucose as needed to maintain blood glucose levels

Figure 4. 13 Travels with carbohydrate.

Carbohydrates in the Body • Glucose is our primary fuel – Needed to burn fat efficiently – When we eat food, our bodies • Immediately use some glucose to maintain normal blood glucose levels • Store excess glucose as glycogen in liver and muscle tissue

Carbohydrates in the Body • Using glucose for energy – Liver glycogen • Maintains normal blood glucose levels – Muscle glycogen • Fuels muscle activity

Carbohydrates in the Body • Sparing body protein – Adequate carbohydrates prevent body from making glucose from body proteins. • Preventing ketosis – Ketone bodies ketosis dehydration – Body needs a minimum of 50 to 100 grams of carbohydrate/day to prevent ketosis.

Carbohydrates in the Body • Regulating blood glucose – To maintain an adequate supply for cells – Controlled by hormones • Insulin – Key “unlocking” body cells and allowing glucose to enter and fuel them • Glucagon – Breaks down glycogen to glucose • Epinephrine – “Fight-or-flight” hormone

Carbohydrates and Glucose in the Body • Regulating blood glucose – Glycemic index • Foods vary in their effect on blood glucose levels. • Index measures the effect of food on blood glucose levels.

Figure 4. 14 Regulating blood glucose levels.

Carbohydrates in the Diet • Recommended carbohydrate intake – RDA = 130 grams per day – Dietary Guidelines • Reduce the intake of added sugars • AMDR for added sugars = < 25% of daily energy intake • Choose whole grains, vegetables, fruits, and cooked dried beans and peas • Consume > ½ of all grains as whole grains

Carbohydrates in the Diet • Current consumption: How much are we eating? – Average American adult = 49– 50 percent of kcal – 13 percent of population has added sugar intake > 25 percent of kcal • High soft drink consumption • Milk consumption and vitamin and mineral quality of diet is declining

Carbohydrates in the 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! Figure 4. 16 Anatomy of a kernel of grain.

Carbohydrates in the Diet • High intake of foods rich in dietary fiber offers many health benefits, including reduced risk of – Obesity – Type 2 diabetes. – Cardiovascular disease – Gastrointestinal disorders • Increase fiber intake gradually while drinking plenty of fluids.

Carbohydrates in the Diet • Reducing added sugar intake – Use less of all nutritive sugars. – Limit consumption of soft drinks, high-sugar cereals, candy, ice cream, and sweet desserts. – Use fresh or frozen fruits for dessert and to sweeten waffles, pancakes, etc. • Sugar substitutes can help lower sugar intake, but these foods might not provide less energy.

Carbohydrates in the Diet • Nutritive sweeteners – Natural sweeteners – Refined sweeteners – Sugar alcohols • Nonnutritive (artificial) sweeteners – Examples • Saccharin • Aspartame • Acesulfame K

Carbohydrates and Health • Sugar and dental caries – High sugar intake contributes to dental caries. – Sugar-feeding bacteria produce acids that eat away tooth enamel. Figure 4. 17 Dental health.

Carbohydrates and Health • Fiber and obesity – Fibers low in fat but are filling – Possible role in weight control • Fiber and type 2 diabetes – Better control of blood glucose • Fiber and cardiovascular disease – Can lower blood cholesterol levels

Carbohydrates and Health • Fiber and gastrointestinal disorders – Healthier gastrointestinal functioning • Negative effects of excess fiber – Gradual intake and increased water consumption – Can bind small amounts of minerals