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Carbohydrates
Carbohydrates contain only 4 kilocalories per gram, compared with 9
kilocalories per gram for fat. Thus, a diet rich in carbohydrates provides fewer
calories and a greater volume of food than a fat diet.
Plants use carbon dioxide from the air, water from the soil, and energy from the
sun to produce carbohydrates and oxygen through a process called
photosynthesis. Carbohydrates are organic compounds that contain carbon (C),
hydrogen (H), and oxygen (O) in the ratio of 1 carbon atom and 1 oxygen atom
for every 2 hydrogen atoms (CH2O). The sugar glucose, for example, contains 6
carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms, giving this vital
carbohydrate the chemical formula (C6H1206). Two or more sugar molecules can
be assembled to form increasingly complex carbohydrates. The two main types
of carbohydrates in food are simple carbohydrates (sugars) and complex
carbohydrates (starches and dietary fiber).
Simple carbohydrates Sugars composed of a single sugar molecule (a
monosaccharide) or two joined sugar molecules (a disaccharide).
Monosaccharide: Any sugar that is not broken down during digestion and has
the general formula (CH2O)n, where n= 3 to 7. For the common
monosaccharides, nutritionally important molecules, glucose, galactose, and
fructose, n= 6.
Disaccharide: Carbohydrates composed of two monosaccharide units linked by
a glycosidic bond. They include sucrose (common table sugar), lactose (milk
sugar), and maltose.
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Simple Sugars:
Monosaccharides and Disaccharides
Monosaccharides: The Single Sugars
The most common monosaccharides:
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Glucose
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Galactose
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Fructose
All three monosaccharides have 6 carbons, and all have the chemical formula
C6H1206 but each has a different arrangement of these atoms. The carbon and
oxygen atoms of glucose and galactose can form a six-sided ring. The structures
of glucose and galactose look almost identical except for the reversal of the OH
and H groups on one of the carbon atoms. The carbons and oxygen of fructose
form a five-sided ring.
Glucose
The monosaccharide glucose is the most abundant simple carbohydrate unit in
nature. In the body, glucose supplies energy to cells. Glucose is virtually the
only fuel used by the brain, except during prolonged starvation when the
glucose supply is low.
Fructose
Fructose is the fruit sugar, fructose tastes the sweetest of all the sugars—it is
what commonly sweetens colas and other soft drinks. It occurs naturally in
fruits and vegetables.
Food manufacturers use high-fructose corn syrup as an additive to sweeten
many foods, including soft drinks, desserts, candies, jellies, and jams.
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Galactose
Galactose rarely occurs as a monosaccharide in food. It usually is chemically
bonded to glucose to form lactose, the primary sugar in milk.
Other Monosaccharides and Derivative Sweeteners
Pentoses are single sugar molecules that contain five carbons. Although they are
present in foods in only small quantities, they are essential components of
nucleic acids, the genetic material of life.
Sugar ribose is part of ribonucleic acid, or RNA. Another five-carbon sugar,
deoxyribose, is a part of deoxyribonucleic acid, or DNA. Pentoses are
synthesized in the body, and therefore are not needed in the diet.
Sugar alcohols are derivatives of monosaccharides. Like other sugars, they taste
sweet and supply energy to the body. However, sugar alcohols are absorbed
more slowly than sugars and the body processes them differently. Some fruits
naturally contain minute amounts of sugar alcohols. Sugar alcohols such as
sorbitol, manitol, lactitol, and xylitol also are used as nutritive sweeteners in
foods. For example, sorbitol, which is derived from glucose, sweetens, for
example, sugarless gum.
Monosaccharides:
1) Triose contains 3 carbons (e.g. glyceraldehyde).
2) Pentose contains 5 carbons.
3) Hexose contains 6 carbons (e.g. glucose, fructose, galactose).
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Disaccharides: The Double Sugars
Disaccharides consist of two monosaccharides chemically joined by a process
called condensation.
Condensation reactions form disaccharides and hydrolysis reactions form
monosaccharides:
Sugar molecules are joined or separated (cleaved) by the removal or addition of
a molecule of water. A condensation reaction can chemically join two
monosaccharides while removing an H from one sugar molecule and an OH
from the other to form water (H2O). A hydrolysis reaction can separate
disaccharides into monosaccharides. During hydrolysis, the addition of a
molecule of water splits the bond between the two sugar molecules, providing
the H and OH groups necessary for the sugars to exist as monosaccharides.
HHydrolysis takes place during the digestion of carbohydrates.
Condensation: In chemistry, a reaction in which a covalent bond is formed
between two molecules by removal of a water molecule.
Hydrolysis: A chemical reaction in which a compound is split into two
products by the addition of water.
The following disaccharides are important in human nutrition:
Sucrose
Sucrose, most familiar to us as table sugar, is composed of one molecule of
glucose and one molecule of fructose. Manufacturers use a refining process to
extract sucrose from the juices of sugar cane or sugar beets.
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Lactose
Lactose, or milk sugar, is composed of one molecule of glucose and one
molecule of galactose. Human milk has a higher concentration of lactose than
cow’s milk.
Maltose
Maltose is composed of two glucose molecules. Maltose seldom occurs
naturally in foods, but forms whenever long molecules of starch break down.
Human digestive enzymes in the mouth and small intestine break starch down
into maltose.
Conclusions: Carbohydrates are composed of carbon, hydrogen, and
oxygen and can be categorized as simple or complex. Simple carbohydrates
include monosaccharides and disaccharides. The monosaccharides glucose,
fructose, and galactose are single sugar molecules. The disaccharides
sucrose, lactose, and maltose are double sugar molecules. A condensation
reaction joins two monosaccharides to form a disaccharide.
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Complex Carbohydrates
Complex carbohydrates are chains of more than two sugar molecules. Short
carbohydrate chains may have as few as three monosaccharide molecules, but
long chains, the polysaccharides, can contain hundreds or even
thousands.
Oligosaccharides
Oligosaccharides are short carbohydrate chains of 3 to 10 sugar molecules.
Dried beans, peas, and lentils contain the two most common oligosaccharides—
raffinose and stachyose: Raffinose is formed from three monosaccharide
molecules—one galactose, one glucose, and one fructose. Stachyose is formed
from four monosaccharide molecules—two galactose, one glucose, and one
fructose. The body cannot break down raffinose or stachyose, but they are
readily metabolized by intestinal bacteria. Human milk also contains different
types of oligosaccharides.
Polysaccharides
Polysaccharides (poly meaning “many”) are long carbohydrate chains of
monosaccharides. Some polysaccharides form straight chains, while others
branch off in all directions. Such structural differences affect how the polysaccharide behaves in water and with heating. The way monosaccharides are
linked may make them digestible (e.g., starch) or indigestible (e.g. dietary
fiber).
Complex carbohydrate: A chain of more than two monosaccharides. May be
an oligosaccharide or a polysaccharide.
Oligosaccharide: A short carbohydrate chain composed of 3 to 10 sugar
molecules.
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Polysaccharide: Long carbohydrate chains composed of more than 10 sugar
molecules. Polysaccharides can be straight or branched.
Starch: The major storage form of carbohydrate in plants; starch is composed
of long chains of glucose molecules in a straight (amylose) or branching
(amylopectin) arrangement.
Amylose: A straight-chain polysaccharide composed of glucose units.
Amylopectin: A branched-chain polysaccharide composed of glucose units.
Resistant starch: A starch that is not digested.
Glycogen: A very large, highly branched polysaccharide composed of multiple
glucose units. Sometimes called animal starch, glycogen is the primary storage
form of glucose in animals.
Dietary fiber: The indigestible parts of plants.
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Starch
Starch takes two main forms in plants: amylose and amylopectin. Amylose is
made up of long, unbranched chains of glucose molecules ( 1-4 bonds), while
amylopectin is made up of branched chains of glucose molecules ( 1-4 and 
1-4 glycosidic bonds). Amylose and amylopectin typically occur in a ratio of
about 1:4 in plants.
Although the body easily digests most starches, a small portion of the starch
in plants may remain enclosed in cell structures and escape digestion in the
small intestine. Starch that is not digested is called resistant starch. Some
legumes contain large amounts of resistant starch.
Glycogen
Glycogen, also called animal starch, is the storage form of carbohydrate in
living animals. Clycogen play an important role in our bodies as a readily
mobilizable store of glucose.
Clycogen is composed of long, highly branched chains of glucose molecules. Its
structure is similar to amylopectin, but glycogen is much more highly branched.
Glycogen in our cells can be broken down rapidly into single glucose molecules
as needed. Since enzymes can only attack the ends of glycogen chains, the
highly branched structure of glycogen multiplies the number of sites available
for enzyme activity.
Skeletal muscle and the liver are the two major sites of glycogen storage. In
muscle cells, glycogen provides a reservoir of glucose for strenuous muscular
activity. Liver cells also use glycogen to regulate blood glucose levels.
Dietary Fiber
Dietary fiber provides structure to plant cell walls, and is also found inside plant
cells. All types of plant foods contain fiber including fruits, vegetables,
legumes, and whole grains. Dietary fibers resemble starches, but are impervious
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to human digestive enzymes, so they are not digested in the GI tract. Dietary
fibers, often called nonstarch polysaccharides, include cellulose, hemicellulose,
pectins, gums, and mucilages.
Cellulose: Cellulose gives plant cell walls their strength and rigidity. It forms
the woody fibers that support tall trees. Cellulose is made up of long traightchain polysaccharide composed of hundreds of glucose units linked by beta
bonds. It is indigestible by humans and a component of insoluble dietary fiber.
Hemicelluloses: The hemicelluloses are a diverse group of polysaccharides that
vary from plant to plant. They are mixed with cellulose in plant cell walls.
Hemicelluloses are composed of a variety of monosaccharides with many
branching side chains.
Pectins: A type of soluble fiber found in fruits. Pectins are gel-forming
polysaccharides found in all plants, but especially in fruits. The pectin in fruits
acts like cement that gives body to fruits and helps them keep their shape.
Pectin forms a gel that the food industry uses to add firmness to, for example,
jellies, jams, sauces.
Gums and Mucilages:
Like pectin, gums and mucilages are thick, gel-forming fibers that help hold
plant cells together. The food industry uses plant gums such as gum Arabic to
thicken, stabilize, or add texture to foods such as puddings, pie fillings, candies,
and sauces.
Gum: A dietary fiber, which contains galactose and other monosaccharides,
found between plant cell walls.
Mucilage: A gelatinous soluble fiber containing galactose, mannose, and other
monosaccharides.
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Classification of Dietary Fiber:
Soluble and Insoluble
Dietary fibers can be classified according to their ability to dissolve in water.
Pectins, gums, mucilages, and some hemicelluloses dissolve in water and so are
classified as soluble fibers. Cellulose, some hemicelluloses, and lignin don’t
dissolve in water, and are thus insoluble fibers.
Only plant foods contain dietary fiber. Foods rich in dietary fiber include
whole-grain foods such as brown rice, rolled oats, and whole-wheat breads and
cereals; legumes such as kidney beans, chickpeas, peas, and lentils; fruits; and
vegetables. Oat bran, legumes, soybean fiber, and some fruits and vegetables
are rich in soluble fiber; whereas wheat bran and most whole grains and cereals
are rich in insoluble fiber. The soluble fiber has been shown to help lower blood
cholesterol levels.
Conclusions: Complex carbohydrates include starch, glycogen, and dietary
fiber. Starch is composed of straight or branched chains of glucose
molecules and is the storage form of energy in plants. Glycogen is
composed of highly branched chains of glucose molecules and is the storage
form of energy in animals. Dietary fibers include many different substances
that cannot be digested by enzymes in the human intestinal tract and are
found in plant foods such as whole grains, legumes, vegetables, and fruits.
Lignin: An insoluble fiber composed of multi-ring alcohol units.
soluble fiber: Dietary fiber components that dissolve in or absorb water,
including pectins, gums, mucilages, and some hemicelluloses.
Insoluble fiber: Dietary fiber components that do not dissolve in water,
including cellulose, lignin, and some hemicelluloses.
pancreatic amylase: Starch-digesting enzyme secreted by the pancreas.
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alpha bond: A chemical bond linking two monosaccharides (glycosidic bond)
that can be broken by human intestinal enzymes, releasing the individual monosaccharides.
beta bond: A chemical bond linking two monosaccharides (glycosidic bond)
that cannot be broken by human intestinal enzymes.
The main functions of glucose
Using Glucose for Energy
Glucose is the primary fuel for most cells in the body and the preferred fuel for
the brain, red blood cells, nervous system, fetus, and placenta. Even when fat is
burned for energy, a small amount of glucose is needed to metabolize fat
completely. To obtain energy from glucose, glucose from the blood must be
taken up by cells. Once glucose enters cells, a series of metabolic reactions
break it down into carbon dioxide and water, releasing energy in a form the
body can use.
Sparing Body Protein
In the absence of carbohydrate, both proteins and fats can be used for energy.
Although most cells can break down fat for energy, brain cells and developing
red blood cells require a constant supply of glucose. If glycogen stores are
depleted and glucose is not provided in the diet, the body must make its own
glucose from protein to maintain blood levels and supply glucose to the brain.
Dietary carbohydrate spares body proteins from being broken down and used to
make glucose.
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Preventing Ketosis
Even when fat provides the fuel for cells, cells require a small amount of
carbohydrate to completely break down fat to release energy. When no carbohydrate is available, the liver cannot break down fat completely for energy,
and instead produces small compounds called ketone bodies. Most cells can use
ketone bodies for energy. When ketone bodies are produced faster than they are
used ketone levels build up in the blood, causing a condition known as ketosis.
Ketosis interferes with acid/base balance, causing the blood to become too
acidic. Dehydration is a common consequence of ketosis because the body loses
water, excreting excess ketones in the urine.
Storing Glucose as Glycogen
Glucose that is not needed as a fuel source is converted into the long branched
chains of glycogen.
Liver glycogen stores are used to maintain normal blood glucose levels and
account for about one-third of body glycogen stores. Muscle glycogen stores are
used to fuel muscle activity and account for about two-thirds of body glycogen
stores.
Conclusions: Glucose circulates in the blood to provide immediate energy
to cells. The body needs adequate carbohydrate intake to prevent the
breakdown of body proteins for energy. The body needs some
carbohydrate to completely break down fat and prevent the buildup of
ketone bodies in the blood. The body stores excess glucose in the liver and
muscle as glycogen.
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Regulating Blood Glucose Levels
Two hormones produced by the pancreas tightly control blood glucose levels.
When blood glucose levels rise after a meal, special cells called beta cells in the
pancreas release the hormone insulin into the blood. Insulin acts like a key
“unlocking” the cells of the body and allowing glucose to enter and fuel them.
Insulin works on receptors on the surface of cells, increasing their affinity for
glucose and increasing glucose uptake by cells. Insulin also stimulates liver and
muscle cells to store glucose as glycogen.
When an individual has not eaten in a while, and blood glucose levels begin to
fall, alpha cells in the pancreas release another hormone called glucagon.
Glucagon stimulates the breakdown of glycogen stores to release glucose into
the bloodstream. Glucagon also stimulates gluconeogenesis, or the synthesis of
glucose from non-CHO sources.
Another hormone, epinephrine (also called adrenaline), exerts effects similar to
glucagon to ensure all body cells have adequate energy for emergencies.
High Blood Sugar: Diabetes Mellitus
Diabetes mellitus is a disease in which the body either does not produce enough
insulin or does not properly use insulin, and thus, blood glucose levels are
elevated.
Consequences of Diabetes
Abnormally high blood glucose levels increase risk of:
1) High blood pressure
2) Heart disease
3) Kidney disease.
4) Damages to the eyes.
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Forms of Diabetes
There are two main forms of diabetes:
• Type 1, previously known as insulin-dependent diabetes mellitus (IDDM)
or juvenile-onset diabetes.
• Type 2, previously known as non-insulin-dependent diabetes mellitus
(NIDDM) or adult-onset diabetes.
- Type 1 diabetes occurs when the body’s immune system attacks beta cells in
the pancreas, causing them to lose the ability to make insulin.
- Type 2 diabetes occurs when target cells (e.g., fat and muscle cells) lose the
ability to respond normally to insulin.
Low Blood Sugar: Hypoglycemia
Excess insulin results in low blood sugar, or hypoglycemia. Too much glucose
enters cells, lowering blood glucose levels. When blood glucose levels drop too
low, nervousness, irritability, hunger, headache, shakiness, rapid heartbeat and
weakness can develop. A further drop in blood glucose levels can cause coma
and death.
A person with diabetes can develop hypoglycemia in response to an overdose
of insulin or stressful exercise. In non-diabetic people, two types of
hypoglycemia occur. Reactive hypoglycemia occurs about one hour after eating
carbohydrate-rich food. The body overreacts and produces too much insulin in
response to food. Individuals can prevent reactive hypoglycemia by eating
frequent smaller meals to smooth out blood glucose responses to food. Fasting
hypoglycemia occurs because the body produces too much insulin even when
no food is eaten. Pancreatic tumors can cause fasting hypoglycemia.
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Conclusions: In healthy individuals, two hormones produced by the
pancreas closely regulate blood glucose levels. Insulin allows glucose to
enter cells and stimulates storage of glucose as glycogen, lowering blood
glucose levels. Glucagon stimulates the release of glucose from glycogen
and the formation of glucose from protein. Some individuals lack the
ability to regulate blood glucose levels properly, resulting in diabetes
(characterized by hyperglycemia) or hypoglycemia (low blood sugar).
Individuals with type 1 diabetes cannot make insulin; individuals with type
2 diabetes are resistant to the action of insulin or make inadequate
amounts.
Increasing Complex Carbohydrate Intake
Dietary Recommendations emphasize increase consumption of grains,
vegetables, and fruits. While naturally low in fat, these foods are rich in
complex carbohydrates, both starches and fibers. Legumes are rich in both
protein and complex carbohydrate.
Whole kernels of grains consist of four parts: the germ, the endosperm, the
bran, and the husk. The germ, the innermost part at the base of the kernel. It is
rich in protein, oils, vitamins, and minerals. The endosperm is the largest middle
portion of the grain kernel, and it is high in starch. The bran is composed of
layers of protective coating around the grain kernel and is rich in dietary fiber.
The husk is an inedible covering.
When grains are refined, making white flour from wheat, for example, or
making white rice from brown rice, the process removes the outer husk and bran
layers and sometimes the inner germ of the grain kernel. Since the bran and
germ portions of the grain contain much of the dietary fiber, vitamins, and
minerals, the nutrient content of whole grains is far superior to that of refined
grains.
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Lactose intolerance:
t results from a deficiency of lactase, the enzyme that splits the
disaccharide lactose into the two monosaccharides glucose and
galactose.
Lactase
Lactose
Glucose and galactose.
Lactose that is not hydrolyses into galactose and glucose remains in
the gut and acts osmotically to draw water into the intestine. Colonic
bacteria ferment the indigestible lactose, generating short chain fatty
acids, CO2 and hydrogen gas.
Consumption of quantities greater that 12 g may result in bloating,
flatulence, cramps and diarrhea.
In rare cases, a person is simply born with a deficiency of lactase.
More often, lactose deficiency develops as a consequence of another
disease, any condition that changes the intestinal villi, such as
prolonged diarrhea or use of some medicines, can lead to lactose
intolerance.
In most world’s population, normal lactose activity gradually declines
with age.
- Lactose free diet.
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Galactosemia:
Is an inborn error of metabolism in which the body cannot use galactose. In
galactosemia at least one of the enzymes required to convert galactose to
glucose is missing or defective. When infants with galactosemia are given milk,
they vomit and have diarrhea. The unmetabolized products accumulate and
follow an alternative pathway to form an abnormal product that causes growth
failure, liver enlargement and other neurological abnormalities that lead to coma
and death.
Lactose
Galactose + glucose
Galactokinase
Galactose-1-phosphate
Galactose-1-phosphate uridyl transferase
Glucose
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