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Ch. 5: Carbohydrates

Terms in this set (89)

What is the structure of carbohydrates?
includes sugar, starch and fiber

composed of carbon, hydrogen and oxygen
What are the main source of carbohydrates?
plants - during photosynthesis, plants produce glucose by using C and O from CO2 in the air, H from water and energy from the sun

plants either store the glucose or transform it to starch, fiber, fat or protein
What is the general formula for carbohydrates?
(CH2O)n - n represents the number of times the formula is repeated

EX: chemical formula for glucose is C6H12O6 or (CH2O)6
Monosaccharide
class of single sugars that are not broken down further in digestion

have the general formula of (CH2O)6

EX: glucose, fructose, galactose, sugar alcohols
Disaccharide
class of sugars formed by the chemical bonding of 2 monosaccharides

have the general formula of (CH2O)12

EX: maltose, sucrose, lactose
Oligosaccharide
complex carbohydrates that contain 3-10 single sugar units
Polysaccharide
class of complex carbohydrates containing many glucose units, from 10-1000 or more

EX: glycogen, starch, fiber
Hexose
carbohydrate containing 6 carbons; monosaccharides
Glucose
most abundant monosaccharide; also called dextrose

we eat very little of it as a monosaccharide - much of the glucose in our diets is linked together with additional sugars to form disaccharides or polysaccharides; also known as blood sugar
Fructose
monosaccharide found in fruits, vegetables, honey (which is 50% fructose / 50% glucose), and high fructose corn syrup
High fructose corn syrup
b/c it's sweeter and less expensive, it is used to sweeten many food products

presence of fructose in these products makes it a common sugar in our diets - in most north American diets, fructose accounts for about 9-11% of total energy intake
Galactose
the third major monosaccharide of nutritional importance

almost structurally identical to glucose; most of the galactose in our diets is found in combos with glucose; when they combine it forms a disaccharide called lactose which is found in milk and other dairy products
What are sugar alcohols?
derivatives of monosaccharides; used primarily as sweeteners in sugarless gum and dietetic food

EX: sorbitol, mannitol, xylitol
What are 2 additional monosaccharides found in nature that are classified as "pentoses"?
ribose & deoxyribose - meaning they contain 5 carbons

these sugars do not need to be supplied by the diet, they are v important in the body because they are part of the cell's genetic material (ribose - RNA, deoxyribose - DNA)
Condensation reaction
chemical reaction in which 2 molecules bond to form a larger molecule by releasing water -- linking of 2 monosaccharides to form a disaccharide

1 molecule of water is formed (and released) by taking a hydroxyl group (OH) from 1 sugar and a hydrogen (H) from the other sugar

one carbon on each monosaccharide participating in the condensation reaction chemically bonds with a single oxygen; 2 forms of this C-O-C bond exist in nature: alpha bonds and beta bonds
How do beta and alpha bonds differ?
beta bonds cannot be easily broken down by digestive enzymes for absorption in the small intestine

thus, foods that contain saccharide molecules linked together by beta bonds (EX: in milk & dietary fiber) are often difficult or impossible for individuals to digest b/c they lack the enzymes necessary for breaking beta bonds apart
Maltose
disaccharide that contains 2 glucose molecules joined by an alpha bond

found in seeds and alcoholic beverages

most of the maltose we ultimately digest in the small intestine is produced when we break down longer-chain polysaccharides
Sucrose
common table sugar; composed of glucose and fructose linked by an alpha bond

large amounts are found naturally in plants, such as sugarcane, sugar beets and maple tree sap - the sucrose from these sources can be purified to varying degrees

brown, white and powdered sugars are common forms of sucrose sold in grocery stores
Lactose
primary sugar in milk and milk products; consists of glucose joined to galactose by a beta bond

many people are unable to digest large amounts of lactose b/c they don't produce enough of the enzyme lactase, which is needed to break the beta bond -- this can cause intestinal gas, bloating, cramping and discomfort as the unabsorbed lactose is metabolized into acids and gases by bacteria in the large intestine
Simple sugars
monosaccharides and disaccharides
Can beta bonds in oligosaccharides be broken down?
not by our digestive enzymes

thus, when we eat foods with raffinose and stachyose, these oligosaccharides pass undigested into the large intestine, where bacteria metabolize them, producing gas and other byproducts

EX: found in onions, cabbage, broccoli, whole wheat, legumes such as kidney beans and soybeans
Raffinose
indigestible oligosaccharide made of 3 monosaccharides (galactose, glucose, fructose)
Stachyose
indigestible oligosaccharide made of 4 monosaccharides (galactose, galactose, glucose, fructose)
Beano
enzyme preparation that works in the digestive tract to break down many of the indigestible oligosaccharides
Starch
complex carbohydrates made of multiple units of glucose attached together in a form that the body can digest

the major digestible polysaccharide in our diets; is the storage form of glucose in plants
Fiber
complex carbohydrate in foods and plant origin that is made of multiple units of glucose attached together in a form that cannot be broken down by digestive processes in the stomach or small intestine
What are the 2 types of plant starch
amylose, amylopectin

both are a source of energy for plants and for animals that eat plants; found in potatoes, beans, breads, pasta, rice, and other starch products, typically in a ratio of 1:4

both contain many glucose units linked by alpha bonds
Amylose
linear, unbranched chain of glucose molecules that contains only 1 type of alpha bond (called a 1-4 bond)

amylose-rich molecules can be bonded to each other to produce modified food starch, a thickener used in baby foods, salad dressings, and instant puddings

makes up about 20% of digestible starch in plants
Amylopectin
highly branched chain structure that links glucose molecules using 2 alpha bonds (1-4 bonds link straight chains of glucose and 1-6 bonds link glucose at the branching points)

the branches allow it to retain water to form a very stable starch gel; thus, food manufacturers commonly use starches rich in amylopectin to thicken sauces and gravies; also used in many frozen foods b/c it remains stable over a wide temperature range
Alpha 1-4 bonds
broken by amylase enzymes produced in the mouth and pancreas
Alpha 1-6 bonds
broken by intestinal enzyme called alpha dextrinase
Why does amylopectin cause blood glucose levels to increase more quickly than amylose?
because the more numerous the branches in a starch, the more sites (ends) are available for enzyme action
Glycogen
the storage form of carbohydrate in humans and other animals, also contains many glucose units linked together with alpha bonds

structure is similar to amylopectin, but even more highly branched, which allows it to be broken down quickly by enzymes
What are the major storage sites for glycogen
liver and muscle cells

amount stored in these cells is influenced by the amount of carbohydrate in the diet
Glycogen storage is ___
extremely important, while limited

EX: 90 grams of glycogen stored in the liver can be converted into blood glucose to supply the body with energy, and the 300 grams of glycogen stored in muscles supply glucose for muscle use, esp during high intensity and endurance exercises
Total fiber
dietary and functional fiber in food
Dietary fiber
fiber naturally in food
Functional fiber
fiber added to food to provide health benefits
T/F: Nutrition Facts labels include functional fiber
false
What are fibers composed of?
the non-starch polysaccharides cellulose, hemiculluloses, pectins, gums and muilages joined by beta bonds

note: lignins are the only noncarbohydrate components of dietary fibers
Can fibers be broken down by digestive enzymes?
No, b/c they are joined by bonds, so the undigested fibers pass through the small intestine into the large intestine, where bacteria metabolize some and form short chain fatty acids and gas

these short chain fatty acids provide fuel for cells in the large intestine and enhance intestinal health
What are most readily digested by the intestinal bacteria?
pectins, gums, mucilages

yielding about 1.5 to 2.5 kcal/g
What are the most resistant to being broken down by bacteria?
celluose, hemicellulose and lignins
Insoluble fibers
Do not dissolve in water.
Form structural part of the plant cell.
Cellulose, hemicellulose, lignins
Seeds, whole grains
What forms the structural part of the plant cell wall in vegetables and whole grains?
cellulose, hemicellulose and lignins
Soluble fibers
Dissolve in water.
Found inside and around plant cells.
Pectins, gums, mucilages, and some hemicelluloses
Oat bran, fruits, legumes, and psyllium
What health benefits do in/soluble fibers provide?
when consumed in adequate quantities, they can lower blood cholesterol levels and blood glucose levels, thereby reducing risks of cardiovascular disease and diabetes

can decrease intestinal transit time, reducing risks of constipation, diverticular disease and colon cancer
What kinds of foods are carbohydrates found in?
table sugar, jam, jelly, fruit, fruit juices, soft drinks, baked potatoes, rice, pasta, cereals, breads
What are the best sources of starch in a diet?
plant based foods: legumes, tubers and grains (wheat, rye, corn, oats, barley, rice)

they provide ample carbohydrate and micronutrients

NOTE: fiber can be found in many of the same foods as starch, so a diet rich in grains, legumes and tubers can also provide sig amounts of dietary fiber; highly process grains are low in fiber
Where can soluble fibers be found in foods?
skins and flesh of many fruits and berries; as thickeners and stabilizers in jams, yogurts, sauces and fillings; and in products that contain psyllium and seaweed
Nutritive sweeteners
can be metabolized to yield energy

EX: the monosaccharides (glucose, fructose, galactose) and disaccharides (sucrose, lactose, maltose)
Non-nutritive (alternative) sweeteners
provide no food energy and do not promote dental caries; provide very low calorie or non-caloric sugar substitutes for people with diabetes and those trying to lose body weight; safety is determined by the FDA

include: saccharin, cyclamate, aspartame, sucralose, acesulfame-K, tagatose, stevia and monk fruit
The sweetness of ___ makes it the benchmark against which all other sweeteners are measured
sucrose (table sugar)
How is high fructose corn syrup made?
treating cornstarch with acid and enzymes to break down much of the starch into glucose; then enzymes convert some of the glucose into fructose; the final syrup is about 55% fructose, although it can range from 40-90%

used in soft drinks, candies, jam, jelly and desserts (ex: packaged cookies)
Are sugar alcohols easily metabolized?
no, not by bacteria in the mouth and thus do not promote dental caries as readily as do sugars such as sucrose
Do sugar alcohols contribute energy?
Yes, but they are absorbed and metabolized to glucose more slowly than sugars

in large quantities sugar alcohols can cause diarrhea
Cyclamate
an alternative sweetener that has been banned in US due to concerns about cancer risk

still used in Canada and other countries
Acceptable Daily Intake (ADI)
guideline on the amount of alternative sweetener considered safe for daily use or lifetime; baed on animal studies; set at 100x less than the level where no harmful effects are seen

given as mg/kg of body weight per day
Saccharin
oldest alternative sweetener; is approx 300x sweeter than sucrose

was once thought to pose a risk of bladder cancer, but is no longer

used as a tabletop sweetener and in a variety of foods in beverages; not useful in cooking b/c heating causes it to develop a bitter taste
Aspartame
used throughout the world to sweeten beverages, gelatin desserts, chewing gum, cookies and toppings / fillings of bakery goods; breaks down when heated and loses its sweetness when foods are cooked / heated

brand names: NutraSweet & Equal

is 160-200x sweeter than sucrose; only a small amount is needed to sweeten a food or beverage, so does not contribute calories to food
Neotame
approved by FDA for use as a general purpose sweetener in wide variety of food products such as baked goods, soft drinks, chewing gum, confections and frostings, frozen desserts, gelatins and puddings, jams and jellies, processed fruits and juices, toppings and syrups

heat stable and can be used in cooking and as tabletop sweetener

is 7000-13,000x sweeter than sucrose; thus small amounts do not contribute calories
Acesulfame-K
sold in the US as Sunette; 200x sweeter than sucrose

contributes no energy to the diet bc not digested by the body

can be used in baking bc doesn't lose its sweetness when heated

approved for use in chewing gum, powdered drink mixes, gelatins, puddings, candy, throat lozenges, yogurt and non dairy creamers
Sucralose
sold as Splenda; is 600x sweeter; is the only artificial sweetener made from sucrose; made by substituting 3 Cl for 3 hydroxyl groups (OH) on sucrose - the sub prevents it from being digested and absorved
Tagatose
sold as Naturlose; is an isomer of fructose

almost as sweet as sucrose and can be used in cooking and baking

poorly absorbed by the body; has a prebiotic effect bc it is fermented by bacteria in the large intestine

approved for use in cereals, diet soft drinks, health bars, frozen yogurt, fat free ice cream
Stevia
alternative sweetener derived from a plant from the Amazon; 250x sweeter but provides no energy

FDA only recently approved it use in beverages; can be purchased as a dietary supplement in natural and health food stores

in the US it is combined with sugar alcohol called crythritol and marketed as PureVia and Truvia; also blended with cane sugar and sold as Sun Crystals
Luo han guo
monk fruit is an intensely sweet, small green fruit from Asia

when concentrated, the fruit's juices is 150-300x sweeter; is heat stable and can be used in cooking / baking

currently is used mostly as tabletop sweetener; ADI has not been set, but FDA has classified as "GRAS" generally recognized to be safe
According to RDA, how much digestible carbohydrate do adults need?
130g/day
What role do digestible carbohydrates have in the body?
digestible carbohydrates are broken down to glucose; as glucose they provide a primary source of energy, spare protein from use as an energy source and prevent ketosis
What is the main function of glucose?
act as source of energy for body cells

EX: RBC and cells of CNS derive almost all of their energy from glucose

also fuels muscle cells and other body cells, although many rely on fatty acids to meet energy needs
Gluconeogenesis
when your body does not consume enough carbohydrate to yield glucose, and it is forced to break down amino acids in your muscle tissue and other organs to make glucose

when dietary carbo intake is adequate to maintain blood glucose levels, protein is "spared" from us as energy
Ketosis
Carbohydrates are needed to complete breakdown of fats.

If consumption is low:
Release of insulin decreases.
Fatty acids from adipose tissue is released to provide energy for the body.
Fatty acids are not completely broken down.
Ketone bodies are formed (ketosis).

brain and CNS cells adapt to use ketones for energy when carbo intake is inadequate - is an important adaptive mechanism for survival during starvation
Ketosis occurs in:
untreated diabetes, low carb / high fat weight reduction diets (Atkins, South Beach diet)
Effects of ketosis
suppress one's appetite, resulting in a lower calorie intake

can cause increased loss of water from the body, but over time can lead to serious consequences, such as dehydration, loss of lean body mass and electrolyte imbalances. if severe, ketosis can even cause coma and death
Carbohydrates supply about ___ of the energy intakes of adults in North America
50%

added sugars account for over 14.5%, more than 10% total energy max recommended

due to large popularity of sugar sweetened beverages
What role do indigestible carbohydrates play in maintaining integrity of the GI tract and overall health?
fiber helps prevent constipation and diverticular disease

enhances the management of body weight (high fiber foods fill us up without yielding much energy; absorb water and expand in the GI tract, contributing to satiety)

enhances the management of blood glucose levels (soluble fibers slow glucose absorption from the small intestine and decrease insulin release from the pancreas - better blood glucose regulation)

enhances the management of blood cholesterol levels (high intake of soluble fiber inhibits the absorption of cholesterol and the reabsorption of bile acids from the small intestine, reducing the risk of cardiovascular disease and gallstones)
How does fiber promote bowel health?
fiber adds bulk to the feces, making bowel movements easier; when adequate fiber and fluid are consumed, the stool is large and soft because many types of plant fibers absorb water

prevents constipation, hemorrhoids, diverticula and reduces risk of colon cancer (may be due to nutrients found in high fiber foods)
Diverticula
pouches that have extended through the exterior wall of the intestine

occurs when excessive exertion during defecation leads to the development of hemorrhoids
Diverticular disease
asymptomatic (without noticeable symptoms) in 80% of the affected people

asymptomatic form is diverticulosis and symptomatic form is diverticulitis*
How is diverticulitis remedied?
intake of fiber should be reduced to limit further bacterial activity and inflammation

once inflammation subsides, a high fiber and high fluid diet, along with physical activity is advised to restore GI tract motility
What is the goal of carbohydrate digestion?
to break down starch and sugars into monosaccharide units that are small enough to be absorbed
What is the start of carb digestion?
food preparation, because cooking softens the tough, fibrous tissues of vegetables, fruits and grains

when starches are heated, the starch granules swell as they soak up water, making them easier to digest
Where does enzymatic digestion of some carbos start?
in the mouth; saliva contains an enzyme called salivary amylase, which mixes with starch containing amylose when the food is chewed

amylase breaks down the starch into smaller polysaccharides (called dextrins) and disaccharides

b/c food is in mouth for such short time, this phase is minor in overall digestive process
What happens when the food reaches the stomach?
the salivary enzyme is inactivated by the acidity of the stomach; thus digestion of carbohydrate stops until it passes to the small intestine
What happens in the small intestine?
the polysaccharides in the food that were first acted on in the mouth are digested further by pancreatic amylase and dextrinase

disaccharides are digested to their monosaccharide units by enzymes in the absorptive cells of the small intestine

maltose --> digested by maltase to form glucose + glucose
sucrose --> digested by sucrase to form glucose + fructose
lactose --> digested by lactase to form glucose + galactose
What cannot be broken down in the small intestine?
monosaccharides that occur in food do not require further digestion

the indigestible carbos (dietary fiber and a small portion of starch in whole grain and fruits, called resistant starch) cannot and are passed into the large intestine
What happens to indigestible carbohydrates in the large intestine?
they are fermented by bacteria into acids and gases or are excreted in fecal waste
Monosaccharides are absorbed by which process?
active absorption, with the exception of fructose

following digestion, glucose and galactose are pumped into absorptive sells along with Na; ATP used in the process pumps Na back out of the absorptive cell
How is fructose transported and absorbed?
it is taken up by the absorptive cells via facilitated diffusion (carrier is used, but no energy); this is slower absorption than glucose or galactose

once glucose, galactose and fructose enter the intestinal cells, glucose and galactose remain in that form, whereas some fructose is converted to glucose; these monosaccharides are then transported via portal vein to the liver

in the liver, fructose and galactose are converted to glucose
How is glucose transported
through the bloodstream for use by the cells of the body

if blood glucose levels are adequate to meet the energy needs, the liver stores additional glucose as glycogen; although the liver's capacity to store glycogen is limited, it provides an important reserve of energy to maintain blood glucose levels and cellular function.

when carbs are consumed in very high amounts, the glycogen storage capacity of the liver is exceeded; the liver then converts the excess glucose to fat for storage in adipose tissue
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