arbohydrates are one of three macronutrients that
provide the body with energy ( protein and fats being the other
two). The chemical compounds in carbohydrates are found in both simple and
complex forms, and in order for the body to use carbohydrates for energy, food
must undergo digestion, absorption , and glycolysis . It is recommended that 55 to 60
percent of caloric intake come from carbohydrates.
Chemical Structure
Carbohydrates are a
main source of energy for the body and are made of carbon, hydrogen, and oxygen . Chlorophyll in plants absorbs light
energy from the sun. This energy is used in the process of photosynthesis,
which allows green plants to take in carbon dioxide and release oxygen and
allows for the production of carbohydrates. This process converts the sun's
light energy into a form of chemical energy useful to humans. Plants transform
carbon dioxide (CO 2 ) from the air, water (H 2 O) from the ground, and energy
from the sun into oxygen (O 2 ) and carbohydrates (C 6 H 12 O 6 ) (6 CO 2 + 6 H 2 O + energy = C 6 H 12 O 6 + 6 O 2 ). Most carbohydrates have a
ratio of 1:2:1 of carbon, hydrogen, and oxygen, respectively.
Humans and other
animals obtain carbohydrates by eating foods that contain them. In order to use
the energy contained in the carbohydrates, humans must metabolize , or break down, the structure of the
molecule in a process that is opposite that of photosynthesis. It starts with
the carbohydrate and oxygen and produces carbon dioxide, water, and energy. The
body utilizes the energy and water and rids itself of the carbon dioxide.
Simple Carbohydrates
Simple
carbohydrates, or simple sugars, are composed of monosaccharide or disaccharide units. Common monosaccharides
(carbohydrates composed of single sugar units) include glucose , fructose, and galactose. Glucose is
the most common type of sugar and the primary form of sugar that is stored in
the body for energy. It sometimes is referred to as blood sugar or dextrose and
is of particular importance to individuals who have diabetes or hypoglycemia . Fructose, the primary
sugar found in fruits, also is found in honey and high-fructose corn syrup (in
soft drinks) and is a major source of sugar in the diet of Americans. Galactose is
less likely than glucose or fructose to be found in nature. Instead, it often
combines with glucose to form the disaccharide lactose, often referred to as
milk sugar. Both fructose and galactose are metabolized to glucose for use by
the body.
Oligosaccharides are carbohydrates made of two to ten
monosaccharides. Those composed of two sugars are specifically referred to as
disaccharides, or double sugars. They contain two monosaccharides bound by
either an alpha bond or a beta bond. Alpha bonds are digestible by the human
body, whereas beta bonds are more difficult for the body to break down.
There are three
particularly important disaccharides: sucrose , maltose, and lactose. Sucrose is
formed when glucose and fructose are held together by an alpha bond. It is found
in sugar cane or sugar beets and is refined to make granulated table sugar.
Varying the degree of purification alters the
SUGAR COMPARISON
| Sugar |
Carbohydrate |
Monosaccharide
or disaccharide |
Additional
information |
| Beet sugar (cane
sugar) |
Sucrose |
Disaccharide
(fructose and glucose) |
Similar to white
and powdered sugar, but varied degree of purification |
| Brown sugar |
Sucrose |
Disaccharide
(fructose and glucose) |
Similar to white
and powdered sugar, but varied degree of purification |
| Corn syrup |
Glucose |
Monosaccharide |
|
| Fruit sugar |
Fructose |
Monosaccharide |
Very sweet |
| High-fructose
corn syrup |
Fructose |
Monosaccharide |
Very sweet and
inexpensive
Added to soft drinks and canned or frozen fruits |
| Honey |
Fructose and
glucose |
Monosaccharides |
|
| Malt sugar |
Maltose |
Disaccharide
(glucose and glucose) |
Formed by the
hydrolysis of starch, but sweeter than starch |
| Maple syrup |
Sucrose |
Disaccharide
(fructose and glucose) |
|
| Milk sugar |
Lactose |
Disaccharide
(glucose and galactose) |
Made in mammary
glands of most lactating animals |
| Powdered sugar |
Sucrose |
Disaccharide
(fructose and glucose) |
Similar to white
and brown sugar, but varied degree of purification |
| White sugar |
Sucrose |
Disaccharide
(fructose and glucose) |
Similar to brown
and powdered sugar, but varied degree of purification |
| SOURCE: Mahan and
Escott-Stump, 2000; Northwestern University; Sizer and Whitney, 1997; and
Wardlaw and Kessel, 2002. |
final product, but
white, brown, and powdered sugars all are forms of sucrose. Maltose, or malt
sugar, is composed of two glucose units linked by an alpha bond. It is produced
from the chemical decomposition of starch, which occurs during the germination
of seeds and the production of alcohol. Lactose is a combination of glucose and
galactose. Because it contains a beta bond, it is hard for some individuals to
digest in large quantities. Effective digestion requires sufficient amounts of
the enzyme lactase.
Complex Carbohydrates
Complex
carbohydrates, or polysaccharides, are composed of simple sugar units in
long chains called polymers. Three polysaccharides are of particular importance
in human nutrition : starch, glycogen , and dietary fiber .
Starch and glycogen
are digestible forms of complex carbohydrates made of strands of glucose units
linked by alpha bonds. Starch, often contained in seeds, is the form in which
plants store energy, and there are two types: amylose and amylopectin. Starch represents
the main type of digestible complex carbohydrate. Humans use an enzyme to break
down the bonds linking glucose units, thereby releasing the sugar to be
absorbed into the bloodstream. At that point, the body can distribute glucose
to areas that need energy, or it can store the glucose in the form of glycogen.
Glycogen is the
polysaccharide used to store energy in animals, including humans. Like starch,
glycogen is made up of chains of glucose linked by alpha bonds; but glycogen
chains are more highly branched than starch. It is this highly branched
structure that allows the bonds to be more quickly broken down by enzymes in
the body. The primary storage sites for glycogen in the human body are the
liver and the muscles.
Another type of
complex carbohydrate is dietary fiber. In general, dietary fiber is considered
to be polysaccharides that have not been digested at the point of entry into
the large intestine. Fiber contains sugars linked by bonds that cannot be
broken down by human enzymes, and are therefore
Pastas and
whole-grain breads contain complex carbohydrates, which are long strands of
glucose molecules. Nutritionists recommend that 55–60 percent of calories come
from carbohydrates, and especially complex carbohydrates.
[Photograph by
James Noble. Corbis. Reproduced by permission.]
labeled as
indigestible. Because of this, most fibers do not provide energy for the body.
Fiber is derived from plant sources and contains polysaccharides such as cellulose , hemicellulose, pectin, gums,
mucilages, and lignins.
The indigestible
fibers cellulose, hemicellulose, and lignin make up the structural part of
plants and are classified as insoluble fiber because they usually do not
dissolve in water. Cellulose is a nonstarch carbohydrate polymer made of a
straight chain of glucose molecules linked by beta bonds and can be found
in whole-wheat flour, bran, and vegetables. Hemicellulose is a nonstarch
carbohydrate polymer made of glucose, galactose, xylose, and other
monosaccharides; it can be found in bran and whole grains. Lignin, a
noncarbohydrate polymer containing alcohols and acids, is a woody fiber found
in wheat bran and the seeds of fruits and vegetables.
In contrast,
pectins, mucilages, and gums are classified as soluble fibers because they
dissolve or swell in water. They are not broken down by human enzymes, but
instead can be metabolized (or fermented) by bacteria present in the large intestine. Pectin
is a fiber made of galacturonic acid and other monosaccharides. Because it
absorbs water and forms a gel, it is often used in jams and jellies. Sources of
pectin include citrus fruits, apples, strawberries, and carrots. Mucilages and
gums are similar in structure. Mucilages are dietary fibers that contain
galactose, manose, and other monosaccharides; and gums are dietary fibers that
contain galactose, glucuronic acid, and other monosaccharides. Sources of gums
include oats, legumes , guar, and barley.
Digestion and Absorption
Carbohydrates must
be digested and absorbed in order to transform them into energy that can be
used by the body. Food preparation often aids in the digestion process. When
starches are heated, they swell and become easier for the body to break down.
In the mouth, the enzyme amylase, which is contained in saliva, mixes with food
products and breaks some starches into smaller units. However, once the
carbohydrates reach the acidic environment of the stomach, the amylase is
inactivated. After the carbohydrates have passed through the stomach and into
the small intestine, key digestive enzymes are secreted from the pancreas and
the small intestine where most digestion and absorption occurs. Pancreatic
amylase breaks starch into disaccharides and small polysaccharides, and enzymes
from the cells of the small-intestinal wall break any remaining disaccharides
into their monosaccharide components. Dietary fiber is not digested by the
small intestine; instead, it passes to the colon unchanged.
Sugars such as
galactose, glucose, and fructose that are found naturally in foods or are
produced by the breakdown of polysaccharides enter into absorptive intestinal
cells. After absorption, they are transported to the liver where galactose and
fructose are converted to glucose and released into the bloodstream. The
glucose may be sent directly to organs that need energy, it may be transformed
into glycogen (in a process called glycogenesis) for storage in the liver or
muscles, or it may be converted to and stored as fat.
Glycolysis
The molecular bonds
in food products do not yield high amounts of energy when broken down.
Therefore, the energy contained in food is released within cells and stored in
the form of adenosine triphosphate (ATP), a high-energy compound created by
cellular energy-production systems. Carbohydrates are metabolized and used to
produce ATP molecules through a process called glycolysis.
Glycolysis breaks
down glucose or glycogen into pyruvic acid through enzymatic reactions within the cytoplasm of the cells. The process results in
the formation of three molecules of ATP (two, if the starting product was
glucose). Without the presence of oxygen, pyruvic acid is changed to lactic acid , and the energy-production process
ends. However, in the presence of oxygen, larger amounts of ATP can be
produced. In that situation, pyruvic acid is transformed into a chemical
compound called acetyle coenzyme
A, a compound that begins a
complex series of reactions in the Krebs
Cycle and the electron transport
system. The end result is a net gain of up to thirty-nine molecules of ATP from
one molecule of glycogen (thirty-eight molecules of ATP if glucose was used).
Thus, through certain systems, glucose can be used very efficiently in the
production of energy for the body.
Recommended Intake
At times,
carbohydrates have been incorrectly labeled as "fattening." Evidence
actually supports the consumption of more, rather than less, starchy foods.
Carbohydrates have four calories per gram, while dietary fats
contribute nine per gram, so diets high in complex carbohydrates are likely to
provide fewer calories than diets high in fat. Recommendations are for 55 to 60
percent of total calories to come from carbohydrates (approximately 275 to 300
grams for a 2,000-calorie diet). The majority of carbohydrate calories should
come from complex rather than simple carbohydrates. Of total caloric intake,
approximately 45 to 50 percent of calories should be from complex
carbohydrates, and 10 percent or less from simple carbohydrates.
Low-Carb Diets
Low-carbohydrate
diets, such as the Atkins and South
Beach diets, are based on
the proposition that it's not fat that makes you fat. Allowing dieters to eat
steak, butter, eggs, bacon, and other high-fat foods, these diets instead
outlaw starches and refined carbohydrates on the theory that they are
metabolized so quickly that they lead to hunger and overeating. This theory,
which was first popularized in the nineteenth century, came under scathing
criticism from the medical establishment during the early 1970s when Dr. Robert
Atkins published the phenomenally popular low-carb diet bearing his name.
According to the American Medical Association (AMA), the Atkins diet was a
"bizarre regimen" that advocated "an unlimited intake of
saturated fats and cholesterol-rich foods" and therefore presented a
considerable risk of heart disease. Most doctors recommended instead a diet low
in fat and high in carbohydrates, with plenty of grains, fruits, and vegetables
and limited red meat or dairy products. This became the received wisdom during
the 1980s, at the same time that the U.S. waistline began to expand
precipitously. As dieters found that weight loss was difficult to maintain on a
low-fat diet, low-carb diets regained popularity—with as many as 30 million
people trying a low-carb diet in 2003. Several small-scale studies began to
suggest that a low-carb diet may indeed be effective and may not have the
deleterious effects its detractors have claimed; other research found that any
benefits of a low-carb diet are short-lived, and that the negative effects will
take decades to become evident. The National Institutes of Health has pledged
$2.5 million for a five-year study of the Atkins diet with 360 subjects. While
the results of this and other large-scale studies are awaited, many researchers
stress that the key issue in maintaining a healthy weight is the number of
calories consumed, not the type of calories. The National Academy of Sciences
recommends that adults obtain 45 to 65 percent of their calories from
carbohydrates, 20 to 35 percent from fat, and 10 to 35 percent from protein.
—Paula Kepos
It is important to
consume a minimum amount of carbohydrates to prevent ketosis , a condition resulting from the
breakdown of fat for energy in the absence of carbohydrates. In this situation,
products of fat breakdown, called ketone bodies, build up in the blood and
alter normal pHbalance. This can
be particularly harmful to a fetus. To avoid ketosis, daily carbohydrate intake
should include a minimum of 50 to 100 grams. In terms of dietary fiber, a
minimum intake of 20 to 35 grams per day is recommended.
Exchange System
The exchange system
is composed of lists that describe carbohydrate, fat, and protein content, as
well as caloric content, for designated portions of specific foods. This system
takes into account the presence of more than one type of nutrient in any given
food. Exchange lists are especially useful for individuals who require careful
diet planning, such as those who monitor intake of calories or certain
nutrients. It is particularly useful for diabetics, for whom carbohydrate
intake must be carefully controlled, and was originally developed for planning
diabetic diets.
Diabetes, Carbohydrate-Modified Diets, and
Carbohydrate Counting
Diabetes is a condition
that alters the way the body handles carbohydrates. In terms of diet
modifications, diabetics can control blood sugar levels by appropriately
managing the carbohydrates, proteins, and fats in their meals. The amount of
carbohydrates, not necessarily the source, is the primary issue. Blood glucose
levels after a meal can be related to the process of food preparation, the
amount of food eaten, fat intake, sugar absorption, and the combination of
foods in the meal or snack.
One method of
monitoring carbohydrate levels—carbohydrate counting—assigns a certain number
of carbohydrate grams or exchanges to specific foods. Calculations are used to
determine insulin need, resulting in better
control of blood glucose levels with a larger variety of foods. Overall,
diabetic diets can include moderate amounts of sugar, as long as they are
carefully monitored.
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This is usually called "plate service” because the food is
