The Carbohydrates: Sugars, Starches, and FiberChapter #4

Chapter Introduction Whether you are cramming for an exam or

daydreaming about your next vacation, your brain needs carbohydrate to power its activities. Your muscles need carbohydrate to fuel their work, too, whether you are racing up the stairs to class or moving on the dance floor to your favorite music. Where can you get carbohydrate? And are some foods healthier choices than others? As you will learn from this chapter, whole grains, vegetables, legumes, and fruits naturally deliver ample carbohydrates and fiber with valuable vitamins and minerals and little or no fat. Milk products typically lack fiber, but they also provide carbohydrate along with an assortment of vitamins and minerals.

The Chemist’s View of CarbohydratesSection #4.1

Carbohydrates Humans are seldom aware that within their

brains, billions of glucose molecules are splitting to provide the energy that permits them to think and to control the activities they need and want to do.

People don’t eat glucose and glycogen directly, they get it from the foods rich in carbohydrates. Carbohydrates:

Compounds composed of carbon, oxygen, and hydrogen arranged as monosaccharides or multiples of monosaccharides.

Simple vs. Complex Simple Carbohydrates (sugars):

Monosaccharides and disaccharides. Monosaccharides:

Single sugars. Disaccharides:

Sugars composed of a pair of monosaccharides.

Complex Carbohydrates (starches and fibers): Polysaccharides composed of straight or branched

chains of monosaccharides. Polysaccharides:

Large molecules composed of chains of monosaccharides.

The Simple CarbohydratesSection #4.2

The Simple Carbohydrates Monosaccharides:

Glucose Fructose Galactose

Disaccharides: Maltose Sucrose Lactose

Monosaccharides Monosaccharides:

Carbohydrates of the general formula that consists of a single ring.

The Three Monosaccharides: Glucose:

A monosaccharide sometimes known as blood sugar or dextrose.

Fructose: A monosaccharide sometimes known as fruit sugar

or levulose. Galactose:

A monosaccharide part of the disaccharide lactose.

Disaccharides Disaccharide:

Pairs of monosaccharides linked together. To make disaccharides, a chemical reaction known as condensation

needs to take place. Condensation:

A chemical reaction in which two reactants combine to yield a larger product. To break a disaccharide into monosaccharides, hydrolysis needs to

happen. The three types of disaccharides:

Maltose: A disaccharide composed of two glucose units; sometimes known as malt

sugar. Sucrose:

A disaccharide composed of glucose and fructose; commonly known as table sugar.

Lactose: A disaccharide composed of glucose and galactose; commonly known as

milk sugar.

The Complex CarbohydratesSection #4.3

Polysaccharides The complex carbohydrates contain

many glucose units and, in some cases a few other monosaccharides.

Polysaccharides: Compounds composed of many

monosaccharides linked together.

Glycogen Glycogen is not a significant food source

of carbohydrate and is not counted as one of the complex carbohydrates in foods. Glycogen:

An animal polysaccharide composed of glucose; manufactured and stored in the liver and muscles as a storage form of glucose.

Starches Plant cells store glucose as starches.

Starches: Plant polysaccharides composed of glucose.

All starchy foods come from plants.

Fibers Fibers:

The non-starch polysaccharides that are not digested by human digestive enzymes.

Fibers include: Cellulose Hemicellulose Pectins Gums and Mucilages Nonpolysaccharide Lignins, Cutins, and Tannins Resistant Starches:

Starches that escape digestion and absorption in the small intestine of healthy people.

Fiber Characteristics Soluble Fibers:

Indigestible food components that dissolve in water to become viscous and fermentable. Viscous:

A gel-like consistency. Fermentable:

The extent to which bacteria in the GI tract can break down fibers to fragments that the body can use.

Insoluble Fibers: Indigestible food components that do not dissolve in

water. Phytic acid is often found in foods that contain fiber.

Phytic Acid: A non-nutrient component of plant seeds.

Digestion and Absorption of CarbohydratesSection #4.4

Carbohydrate Digestion (Mouth) Thoroughly chewing high-fiber foods

slows eating and stimulates the flow of saliva which contains amylase. Amylase:

An enzyme that hydrolyzes amylose (starch).

Because food is in the mouth for such as short time, very little carbohydrate digestion takes place there.

Carbohydrate Digestion (Stomach) The swallowed bolus mixes with the

stomach’s acid and protein-digesting enzymes, which inactivate salivary amylase.

The stomach’s acid continues breaking down starch, but it’s juices contain no enzymes to digest carbohydrates.

Fibers linger in the stomach and delay gastric emptying thereby providing the feeling satiety. Satiety:

The feeling of fullness and satisfaction that food brings.

Carbohydrate Digestion (Small Intestine) The small intestine performs most of the work of

carbohydrate digestion. Pancreatic amylase enters via the pancreatic duct

and continues breaking down the polysaccharides to shorter glucose chains and disaccharides.

In the outer membranes of the intestinal cells, specific enzymes dismantle specific disaccharides: Maltase: breaks down maltose into two glucose

molecules. Sucrase: breaks down sucrose into one glucose and

one fructose molecule. Lactase: breaks lactose into one glucose and one

galactose molecule. All polysaccharides and disaccharides have been

broken down to monosaccharides.

Carbohydrate Digestion (Large Intestine) Within 1 to 4 hours after a meal, all the sugars

and most of the starches have been digested and only fibers remain in the digestive tract.

Fibers in the large intestine attract water, which softens stools for passage without straining.

Bacteria in the GI tract ferment some fibers, which generates water, gas , and short-chain fatty acids. The colon uses these small fat molecules for

energy.

Carbohydrate Absorption Glucose is absorbed through the lining of the mouth

and the small intestine. Glucose and galactose traverse the cells lining the

small intestine through active transport. Fructose is absorbed by facilitated diffusion, which

slows entry and produces a smaller rise in blood sugar.

Unbranched chains of starch are digested slowly and produce a smaller rise in blood glucose than branched chains, which have many more places fro enzymes to attack and release glucose rapidly.

Blood from the intestines circulates through the liver, cells take up fructose and galactose and convert them to other compounds, such as glucose.

Lactose Intolerance Lactase activity is highest immediately after birth,

declines by about 5-10% during childhood and adolescence.

Lactose Intolerance: A condition that results from inability to digest lactose.

Lactose intolerance is caused by lactase deficiency. Lactase Deficiency:

A lack of the enzyme required to digest the disaccharide lactose into its component monosaccharides.

Lactose intolerant people can tolerate fermented milk products such as yogurt and acidophilus milk. Acidophilus Milk:

A cultured milk created by adding Lactobacillus acidophilus, a bacterium that breaks down lactose to glucose and galactose.

Glucose in the BodySection #4.5

Storing Glucose as Glycogen The liver stores about 1/3 of the body’s total

glycogen and releases glucose into the bloodstream as needed. When blood glucose levels fall, the liver cells

dismantle the glycogen by hydrolysis reactions into simple molecules of glucose and release them into the bloodstream. Glucose becomes available to supply energy to the

brain and other tissues regardless of whether the person has eaten recently.

Muscle cells can also store glucose as glycogen, using it during exercise.

Using Glucose for Energy Glucose fuels the work of most of the body’s

cells. Enzymes break glucose in half, which can be

put back together to make glucose, or they can be further broken downtown into smaller fragments that can yield energy when broken down completely to carbon dioxide and water.

The liver’s glycogen stores only last a few hours, so people need to eat dietary carbohydrate frequently to meet the body’s energy needs.

Making Glucose from Protein Glucose is the preferred source of energy for the

brain cells, other nerve cells, and developing red blood cells.

Body protein can be converted into glucose to some extent, but protein has jobs of its own that no other nutrient can do. Gluconeogenesis:

The making of glucose from a noncarbohydrate source.

To avoid gluconeogenesis, protein-sparing action needs to occur. Protein-Sparing Action:

The action of carbohydrate in providing energy that allows protein to be used for other purposes.

Making Ketone Bodies from Fat Fragments An inadequate supply of carbohydrates can shift

the body’s energy metabolism’s direction negatively by providing less glucose to meet the brain’s energy needs, and forming ketone bodies. Ketone Bodies:

The product of the incomplete breakdown of fat when glucose is not available in the cells.

Excess ketone bodies can cause ketosis. Ketosis:

An undesirably high concentration of ketone bodies in the blood and urine.

Ketosis can disrupt the bodies acid-base balance. Acid-Base Balance:

The equilibrium in the body between acid and base concentrations.

Using Glucose to Make Fat The liver breaks it into smaller molecules

and puts them together into the more permanent energy-storage compound, fat.

Then the fat travels to the fatty tissues of the body for storage. Fat cells can store unlimited quantities of fat.

This conversion of carbohydrate to fat is a relatively minor way of storing carbohydrates in the body because it is energetically expensive.

Blood GlucoseSection #4.6

The Constancy of Blood Glucose Every body cells depends on glucose for its

fuel to some extent, and the cells of the brain and the rest of the nervous system depend almost entirely on glucose for their energy.

The activity of the cells never end and continually draw on the supply of glucose in the fluid surrounding them.

A steady stream of blood moves past these cells bringing more glucose from either the intestines or the liver.

Maintaining Glucose Homeostasis To function optimally, the body must

maintain blood glucose within limits that permit the cells to nourish themselves. If blood glucose levels fall below normal

levels a person could feel dizzy or weak. If blood glucose rise above normal levels

a person could feel fatigued. Left untreated, fluctuations to the

extremes can be fatal.

The Regulating Hormones Blood glucose homeostasis is regulated primarily

by two hormones. Insulin:

A hormone secreted by the special cells in the pancreas in response to increased blood glucose concentrations.

Glucagon: A hormone that is secreted by special cells in the

pancreas in response to low blood glucose concentrations.

Epinephrine: A hormone of the adrenal gland that modulates the

stress response.

Balancing in the Normal Range The maintenance of normal blood glucose

ordinarily depends on two processes: When blood glucose falls below normal, food can

replenish it or in the absence of food, glucagon can signal the liver to break down glycogen stores.

When blood glucose rises above normal, insulin can signal the cells to take in glucose for energy.

Eating balanced meals at regular intervals helps the body maintain a happy medium between the extremes.

Falling Outside the Normal Range Foods do not govern blood glucose

concentrations, the body does. When the body fails to govern blood

glucose concentrations two conditions can occur: Diabetes Hypoglycemia People with these conditions often plan their

diets to help maintain their blood glucose within a normal range.

Diabetes Diabetes:

A disorder of carbohydrate metabolism resulting from inadequate or ineffective insulin.

Type 1 Diabetes: The less common type of diabetes in which

the person produces no insulin at all. Type 2 Diabetes:

The more common type of diabetes in which the fat cells resist insulin.

Hypoglycemia Hypoglycemia:

An abnormally low blood glucose concentration. Symptoms:

Weakness Rapid Heartbeat Sweating Anxiety Hunger Trembling

Hypoglycemia typically occurs as a consequence of poorly managed diabetes.

Most people who suffer from hypoglycemia only need to adjust their diets to limit refined carbohydrates and increase their intake of fiber-rich ones.

The Glycemic Response Glycemic Response:

The extent to which a food raises the blood glucose concentration and elicits an insulin response.

Glycemic Responses: Slow absorption, a modest rise in blood glucose, and a

smooth return to normal are desirable. Fast absorption, a surge in blood glucose, and an

overreaction that plunges glucose below normal are less desirable.

Different foods have different effects on blood glucose and are outlined in the glycemic index.

Glycemic Index: A method of classifying foods according to their potential

from raising blood glucose.

Heath Effects and Recommended Intakes of SugarsSection #4.7

Added Sugars Added Sugars:

Sugars and syrups used as an ingredient in the processing and preparation of foods such as breads, cakes, beverages, jellies, and ice cream as well as sugars eaten separately or added to foods at the table.

Health Effects of Sugars In moderate amounts, sugar can add

pleasure to meals without harming health, however, excess sugars can contribute to: Nutrient Deficiencies:

By supplying energy (kcalories) without nutrient.

Dental Caries: Decay of teeth.

Dental Plaque: A gummy mass of bacteria that grows on teeth and

can lead to dental caries and gum disease.

Accusations against Sugar Sugars have been blamed for a variety of health problems,

including: Sugar causes obesity:

Sugar is not the sole cause of obesity because obesity can occur without a high-sugar diet.

Sugar causes heart disease: For most healthy people, moderate sugar intakes do not elevate

blood lipids. Sugar causes misbehavior in children and criminal behavior

in adults: No scientific evidence supports a relationship between sugar

and hyperactivity or other misbehavior. Sugar causes cravings and addictions:

People seek carbohydrates a s a way to increase their levels of serotonin. Serotonin:

A neurotransmitter important in sleep regulation, appetite control, and sensory perception among other roles.

Recommended Intakes of Sugar The Dietary Guidelines urge people to

“choose beverages and foods to moderate your intakes of sugars.”

The Food Guide Pyramid suggests that consumers use them “sparingly.”

The DRI committee did not set up an upper limit for sugar intake, but as mentioned, excessive intakes can interfere with sound nutrition and dental health.

Health Effects and Recommended Intakes of Starch and FiberSection #4.8

Health Effects of Starch and Fiber Health Benefits of Starch and Fiber:

Protects against heart disease and stroke. Reduces the risk of type 2 diabetes. Enhances the health of the small intestine. Protects against colon cancer. Promotes weight loss.

Harmful Effects of Excess Fiber: Can inhibit other energy or nutrient needs. Can cause abdominal discomfort, gas, and

diarrhea.

Recommended Intakes of Starch and Fibers Dietary recommendations suggest that

carbohydrates provide about half of the energy requirements.

Dietary Guidelines encourage people to choose a variety of whole grains, fruits, and legumes daily.

The DRI recommendation is 14 grams per 1000-kcalorie intake.

From Guidelines to Groceries The Food Guide Pyramid suggests:

3-5 Servings Vegetables 2-4 Servings Fruits 6-11 Servings Grains

Purchasing and consuming these recommended servings can provide the recommended amount of carbohydrate and fiber that you need.

Bibliography “Understanding Nutrition, 10th Ed.”

Rolfes, S.R., Whitney, E. (2005). Thomson- Wadsworth; Belmont, CA.