Carbohydrates: Sugars, Starches, and Fibers

The Chemist's View of Carbohydrates

The dietary carbohydrate family includes:

• Monosaccharides: single sugars

• Disaccharides: sugars composed of pairs of monosaccharides

• Polysaccharides: large molecules composed of chains of monosaccharides

Monosaccharides and disaccharides (the sugars) are sometimes called simple carbohydrates, and polysaccharides (starches and fibers) are sometimes called complex carbohydrates.

To understand the structure of carbohydrates, consider the atoms within them.

Each atom can form a certain number of chemical bonds with other atoms:

• Hydrogen atoms: one

• Oxygen atoms: two

• Nitrogen atoms: three

• Carbon atoms: four

Atoms and Their Bonds

• Each atom has a characteristic number of bonds it can form with other atoms.

• Example: Ethyl alcohol molecule

  • Each H has one bond

  • O has two bonds

  • Each C has four bonds

Important Sugars in Nutrition

Represented as hexagons and pentagons.

Monosaccharides:

• Glucose

• Fructose

• Galactose

Disaccharides:

• Maltose (glucose + glucose)

• Sucrose (glucose + fructose)

• Lactose (glucose + galactose)

Monosaccharides Details

• Same numbers and kinds of atoms, but differing sweetness.

Glucose

• Blood sugar

• Essential energy source

• Part of every disaccharide

Fructose

• Sweetest of the sugars

Galactose

• Only in a few foods

Monosaccharide Definitions

• Glucose: A monosaccharide; sometimes known as blood sugar in the body or dextrose in foods.

• Fructose: A monosaccharide; sometimes known as fruit sugar or levulose. Found abundantly in fruits, honey, and saps.

• Galactose: A monosaccharide; part of the disaccharide lactose.

Chemical Structure of Glucose

• Diagrams show bonds between 6 carbon (C), 12 hydrogen (H), and 6 oxygen (O) atoms.

• Chemists use shortcuts to depict chemical structures.

• On paper, the structure of glucose is drawn flat, but in nature, the five carbons and oxygen are roughly in a plane.

• Atoms attached to the ring carbons extend above and below the plane.

• Simplified versions omit some bonds and hydrogens.

The Monosaccharides

• Similarities: All three monosaccharides have 6 carbons, 12 hydrogens, and 6 oxygens.

• Differences:

  • Fructose has a five-sided ring.

  • Galactose has a slightly different position of one OH group compared to glucose.

Disaccharides

• Pairs of monosaccharides:

  • Maltose: two glucose units

  • Sucrose: glucose and fructose

  • Lactose: galactose and glucose

• Condensation: Links two monosaccharides together, releasing water.

• Hydrolysis: Breaks a disaccharide in two, adding water.

Disaccharide Definitions

• Condensation: A chemical reaction in which water is released as two molecules combine to form one larger product.

• Hydrolysis: A chemical reaction in which one molecule is split into two molecules, with hydrogen (H) added to one and a hydroxyl group (OH) to the other (from water, H_2O).

• 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, beet sugar, or cane sugar. It also occurs in many fruits and some vegetables and grains.

• Lactose: A disaccharide composed of glucose and galactose; commonly known as milk sugar.

Condensation of Two Monosaccharides to Form a Disaccharide

• An OH group from one glucose and an H atom from another glucose combine to create a molecule of H_2O.

• The two glucose molecules bond together with a single O atom to form the disaccharide maltose.

Hydrolysis of a Disaccharide

• The disaccharide maltose splits into two glucose molecules with H added to one and OH to the other (from the water molecule).

Polysaccharides

• Glycogen: An animal polysaccharide composed of glucose; a storage form of glucose manufactured and stored in the liver and muscles. It is not a significant food source of carbohydrate and is not counted as a dietary carbohydrate in foods.

• Starches: Plant polysaccharides composed of many glucose molecules.

Polysaccharides details

Glycogen

• Storage form of energy in the body.

• Glucose molecules linked together in highly branched chains.

Starches

• Storage form of energy in plants.

• Glucose molecules linked together in branched or unbranched chains.

Glycogen and Starch Compared

• Glycogen: Contains hundreds of glucose units in highly branched chains. Requires a special protein for the attachment of the first glucose.

• Starch:

  • Amylopectin: Occasionally branched chains.

  • Amylose: Unbranched chains.

Dietary Fibers

• Structural parts of plants.

• Differ from starches.

• Soluble fibers - benefits.

• Insoluble fibers - benefits.

• Functional fibers.

• Resistant starches.

• Phytic acid.

Fibers Details

• Dietary fibers are the structural parts of plants and are found in all plant-derived foods (vegetables, fruits, whole grains, and legumes).

• Most dietary fibers are polysaccharides.

• Dietary fibers differ from starches in that the bonds between their monosaccharides cannot be broken down by digestive enzymes in the body.

• Dietary fibers are often described as nonstarch polysaccharides.

• Dietary fibers pass through the body undigested, contributing little or no energy.

• Dietary fibers are often sorted into two groups according to their solubility (soluble and insoluble fibers).

Bonds of Starch and Cellulose Compared

• Human enzymes can digest starch but cannot digest cellulose because the bonds that link the glucose molecules together are different.

Fiber Definitions

• Soluble fibers: Nonstarch polysaccharides that dissolve in water to form a gel. An example is pectin from fruit, which is used to thicken jellies.

• Insoluble fibers: Nonstarch polysaccharides that do not dissolve in water. Examples include the tough, fibrous structures found in the strings of celery and the skins of corn kernels.

Carbohydrate Digestion

• Ultimate goal: Glucose for absorption and use.

• Hydrolysis via enzymes.

Mouth

• Amylase.

Stomach

• Stomach acid and protein-digesting enzymes.

• Role of fiber.

Intestinal Role in Digestion

Small intestine

• Most carbohydrate digestion occurs here.

• Pancreatic amylase.

• Specific disaccharide enzymes:

  • Maltase.

  • Sucrase.

  • Lactase.

Large intestine

• Fibers attract water.

Digestion and Absorption of Carbohydrates - STARCH

Mouth and salivary glands

• Salivary glands secrete saliva into the mouth to moisten the food.

• The salivary enzyme amylase begins digestion:
  Amylase Starch
  ewline Small polysaccharide + Maltose

Stomach

• Stomach acid inactivates salivary enzymes, halting starch digestion.

Small intestine and pancreas

• The pancreas produces an amylase that is released through the pancreatic duct into the small intestine:
  Starch
  Pancreatic amylase
  ewline Small polysaccharides, maltose

• Then disaccharidase enzymes on the surface of the small intestinal cells hydrolyze the disaccharides into monosaccharides:

  • Maltose
    Maltase
    ewline Glucose + Glucose

  • Sucrose
    Sucrase
    ewline Fructose + Glucose

  • Lactose
    Lactase
    ewline Galactose + Glucose

• Intestinal cells absorb these monosaccharides.

Digestion and Absorption of Carbohydrates - FIBER

Mouth

• The mechanical action of the mouth crushes and tears fiber in food and mixes it with saliva to moisten it for swallowing.

Stomach

• Fiber is not digested, and it delays gastric emptying.

Small intestine

• Fiber is not digested, and it delays the absorption of other nutrients.

Large intestine

• Most fiber passes intact through the digestive tract to the large intestine.

• Here, bacterial enzymes digest fiber:
  Bacterial enzymes
  ewline Some fiber
  ewline Short-chain fatty acids, gas

• Fiber holds water, regulates bowel activity, and binds substances such as bile, cholesterol, and some minerals, carrying them out of the body.

Carbohydrate Absorption

• Active transport:

  • Glucose

  • Galactose

• Facilitated diffusion:

  • Fructose

• Liver

  • Conversion of fructose and galactose.

Absorption of Carbohydrates

• Monosaccharides, the end products of carbohydrate digestion, enter the capillaries of the intestinal villi.

• Monosaccharides travel to the liver via the portal vein.

• In the liver, galactose and fructose share metabolic pathways with glucose.

• Glucose is used by most cells in the body.

Lactose Intolerance

• Lactose intolerance: A condition that results from the inability to digest the milk sugar lactose; characterized by bloating, gas, abdominal discomfort, and diarrhea. Lactose intolerance differs from milk allergy, which is caused by an immune reaction to the protein in milk.

• Lactase deficiency: A lack of the enzyme required to digest the disaccharide lactose into its component monosaccharides (glucose and galactose).

Lactose Intolerance details

• Lactase activity is highest immediately after birth and declines with age.

• Symptoms of intolerance include bloating, gas, and abdominal discomfort.

• Causes of intolerance beyond age.

• Prevalence of lactose intolerance varies; the trait has a genetic component.

Dietary Changes with Lactose Intolerance

• Manage dairy consumption rather than restrict it.

• Gl bacteria, fermented milk products, and individualized diets can help.

• Potential nutrient deficiencies: Riboflavin, vitamin D, and calcium.

Lactose in Selected Foods (Examples)

• Whole-wheat bread, 1 slice: 0.5 g

• Dinner roll, 1: 0.5 g

• Cheddar or American cheese, 1 oz: 0.5 g

• Parmesan or cream cheese, 1 oz: 0.8 g

• Doughnut (cake type), 1: 1.2 g

• Chocolate candy, 1 oz: 2.3 g

• Sherbet, 1 c: 4.0 g

• Cottage cheese (low-fat), 1 c: 7.5 g

• Ice cream, 1 c: 9.0 g

• Milk, 1 c: 12.0 g

• Yogurt (low-fat), 1 c: 15.0 g

• Yogurt is often enriched with nonfat milk solids, which increase its lactose content to a level higher than milk's.

Glucose in the Body

• Dietary carbohydrates provide glucose that can be used by the cells for energy, stored by the liver and muscles as glycogen, or converted into fat if intakes exceed needs.

• All of the body’s cells depend on glucose; those of the central nervous system are especially dependent on it.

• Without glucose, the body is forced to break down its protein tissues to make glucose (gluconeogenesis) and to alter energy metabolism to make ketone bodies from fats.

• Blood glucose regulation depends on two pancreatic hormones: insulin to move glucose from the blood into the cells when levels are high and glucagon to free glucose from glycogen stores and release it into the blood when levels are low.

Maintaining Blood Glucose Homeostasis

1. When a person eats, blood glucose rises.

2. High blood glucose stimulates the pancreas to release insulin into the bloodstream.

3. Insulin stimulates the uptake of glucose into cells and storage as glycogen in the liver and muscles. Insulin also stimulates the conversion of excess glucose into fat for storage.

4. As the body's cells use glucose, blood levels decline.

5. Low blood glucose stimulates the pancreas to release glucagon into the bloodstream.

6. Glucagon stimulates liver cells to break down glycogen and release glucose into the blood.

7. Blood glucose begins to rise.

• The stress hormone epinephrine and other hormones also bring glucose out of storage.

Health Effects and Recommended Intakes of Sugars

• Excessive intakes of sugars may increase the risk of dental caries, displace needed nutrients and fiber, and contribute to obesity when energy intake exceeds needs.

• Concentrated sweets are relatively low in nutrients, high in kcalories, and may need to be limited; sugars that occur naturally in fruits, vegetables, and milk are acceptable.

• To control weight gain, blood glucose, and dental caries, consumers may use alternative sweeteners (artificial sweeteners, herbal products, and sugar alcohols) to limit kcalories and minimize sugar intake.

• The DRI suggests that added sugars should account for no more than 25 percent of the day’s total energy intake.

• The World Health Organization (WHO) suggests limiting added sugars to less than 10 percent of energy intake.

Nonnutritive Alternative Sweeteners

(Examples including Acesulfame Potassium, Advantame, Aspartame, Luo han guo extracts, Neotame, Saccharin, Stevia, and Sucralose, their chemical compositions, body's response, relative sweetness, energy, ADI and comments.)

Health Effects and Recommended Intakes of Starch and Fibers

• Adequate intake of fiber fosters weight management, lowers blood cholesterol, and may help prevent colon cancer, diabetes, hemorrhoids, appendicitis, and diverticulosis.

• Excessive intake of fiber displaces energy- and nutrient-dense foods, causes intestinal discomfort and distention, and may interfere with mineral absorption.

• Because starches and fibers help control body weight and prevent heart disease, cancer, diabetes, and GI disorders, the Dietary Guidelines suggest plenty of whole grains, vegetables, legumes, and fruits—enough to provide 45 to 65 percent of the daily energy intake from carbohydrate.

Starch and Fibers: Recommended Intakes

• A person consuming 2000 kcalories a day should therefore have 900 to 1300 kcalories of carbohydrate, or about 225 to 325 grams.

• This amount is more than adequate to meet the RDA for carbohydrate, which is set at 130 grams per day, based on the average minimum amount of glucose used by the brain.

• The DRI recommendation for fiber is in agreement at 14 grams per 1000-kcalorie intake.

Characteristics, Sources, and Health Effects of Fibers

(Examples including Gums, Pectins, Psyllium, some Hemicellulose, Cellulose, Lignins, Resistant starch, and Hemicellulose)

• Major Food Sources

• Actions in the Body (Viscous, Soluble, More Fermentable vs. Nonviscous, Insoluble, Less Fermentable)

• Probable Health Benefits

Fiber in Selected Foods

Grains

• Whole-grain products provide about 1 to 2 g (or more) of fiber per serving:

  • 1 slice whole-wheat, pumpernickel, rye bread

  • 1 oz ready-to-eat cereal (100% bran cereals contain 10 g or more)

  • 1/2 c cooked barley, bulgur, grits, oatmeal

Vegetables

• Most vegetables contain about 2 to 3 g of fiber per serving:

  • 1 c raw bean sprouts

  • 1/2 c cooked broccoli, brussels sprouts, cabbage, carrots, cauliflower, collards, corn, eggplant, green beans, green peas, kale, mushrooms, okra, parsnips, potatoes, pumpkin, spinach, sweet potatoes, swiss chard, winter squash

  • 1/2 c chopped raw carrots, peppers

Fruit

• Fresh, frozen, and dried fruits have about 2 g of fiber per serving:

  • 1 medium apple, banana, kiwi, nectarine, orange, pear

  • 1/2 c applesauce, blackberries, blueberries, raspberries, strawberries

• Fruit juices contain very little fiber.

Legumes

• Many legumes provide about 6 to 8 g of fiber per serving:

  • 1/2 c cooked baked beans, black beans, black-eyed peas, kidney beans, navy beans, pinto beans

• Some legumes provide about 5 g of fiber per serving:

  • 1/2 c cooked garbanzo beans, great northern beans, lentils, lima beans, split peas