Role of Carbohydrates in Human
Md. Atai Rabby
Different forms of carbohydrates in food
• The term carbohydrate covers a large number of diferent
compounds, all composed of carbon, hydrogen, and oxygen
• Classification of Carbohydrates:
• Simple Carbohydrates: Monosaccharides and Disaccharides
– Monosaccharides: Hexose (glucose, fructose, and galactose)
– Disaccharides: Sucrose, Lactose, Maltose, Mannose
• Complex Carbohydrates: Polysaccharides
Polysaccharides are further designated as digestible (starches) or indigestible
Carbohydrate as a Dietary Essential
• You need some carbohydrate daily as a source of glucose. How
much carbohydrate is enough?
• Requirements for Carbohydrate
• Intakes of less than 50 g of carbohydrate/day are associated with
adverse health effects.
• These symptoms are similar to those that develop in starvation.
• There is an unexplained loss of large amounts of both sodium and
water, which explains the rapid loss of body weight found in
people eating a carbohydrate-free diet. The loss of sodium is
followed by a loss of potassium, which usually leads to weakness.
• At the same time, protein of the body (muscles) begins to break
down, which leads to muscle wastage, further weakness, and
further weight loss.
Carbohydrate as a Dietary Essential…..
• Even more seriously, when carbohydrate is absent, the use of fat
to provide energy becomes hindered in the final series of
chemical steps normally involved in oxidizing the fat.
• This leads to the accumulation of chemical intermediates of fat
oxidation, known as ketones.
• This state, known as ketosis, is associated with mild disturbances
of cellular function and is an early indication of insufficient
carbohydrate availability in the body.
• As the ketones build up, they begin to be found as abnormal
components of blood and urine. Also, because they change the
hydrogen ion balance (or acid-base balance) of tissues, the
ketones begin to interfere with the normal functioning of various
Carbohydrate as a Dietary Essential…..
• People reaching this stage are said to be suffering from ketosis,
which usually produces symptoms of fatigue and dehydration and a
loss of stamina.
• All these undesirable effects of a carbohydrate-free diet are soon
corrected by the return of carbohydrate to the diet – a clear
indication that carbohydrate really is a dietary essential.
• Carbohydrates are also used in the synthesis of various
metabolically active complexes. Glycoproteins are important
components of cellular membranes. Glycolipids, such as
sphingolipids and gangliosides, have roles at receptor sites on cells
and in synaptic transmission.
• Mucopolysaccharides have important water-holding or binding
properties in many sites of the body; they occur in basement
membranes and in intercellular cement and form an integral part
of cartilage, tendon, skin and synovial fluid. 5
Recommended Carbohydrates Intake
• Intake of available carbohydrate (starch and sugar) that might be
ideal to support long-term health: 55 to 60 percent of total Kcal.
Further, added sugar be used in moderation (< 10% of Kcal), more
should come from complex carbohydrates and naturally occurring
sugars in foods.
• A person consuming 2000 kcal diet a day should therefore have
1100 to 1200 kcal energy from carbohydrate, or about 275 to 300
• Most of this would be starch, with its accompanying fiber, some
would be the naturally occurring sugars of fruits, vegetables, and
• Estimates of the minimum amount of carbohydrate needed by an
adult are in the range of 150-180 g carbohydrate per day.
Trends in Carbohydrates Use
• Because most diets contain at least 200 g of carbohydrate/day, a
lack of carbohydrate is unlikely to reach the level where it causes
• Also, because carbohydrate-rich foods are the cheapest source of
energy they tend to be used in amounts that ensure al least a
minimum intake, especially among those with low incomes.
• There is a big difference, however, between eating at least the
minimal amount of carbohydrate and eating the optimal amount.
• Fiber is not counted in computing energy from carbohydrate; we
do not know enough yet about its energy contributions, but they
are probably not large in comparison with those from starch and
• Fiber is an indigestible carbohydrate. It is taken in as part of many
foods, but most of it simply passes through the digestive tract
because we do not possess the enzymes needed to break it down
into a form that can be absorbed into the blood.
• However, some bacteria in the large intestine can degrade some
components of fiber, releasing products that can be absorbed into
the body and used as an energy source.
• Two categories of fiber are found in food:
– crude fiber and
– dietary fiber.
• The major component of crude fiber is the polysaccharide called
• Several other carbohydrates and related compounds called
pectins, hemilluloses, and lignins are found in plant foods and are
also resistant to digestion in the human digestive tract. These,
together with cellulose, are known collectively as dietary fiber.
• This is a general term that describes any material in food that
remains undigested in the intestine. The cellulose that is called
crude fiber is just a part of dietary fiber as whole.
• Dietary fiber can also be classified into soluble and insoluble
fiber, depending on how it reacts with water.
• Fiber Intake: With all the uncertainties, it is probably true to say
that about 20 – 30 g of dietary fiber daily is a desirable intake.
• Soluble fiber is found in fruits, some legumes, and such grains as
oats, rye, and barely. It dissolves in water to form a gel. This gel
forms within the digestive tract and serves to slow the rate at
which food passes through the small intestine. This in turn
increases the rate of absorption of nutrients from the food.
• Which is why home remedies for diarrhea often included eating
fruits rich in the soluble fiber pectin, such as apples and pears.
• Pharmaceutical manufacturers also add pectin to antidiaahea
• Anyone who has made marmalade, jam, or jelly has probably used
the soluble fiber pectin as a thickening agent.
• Insoluble fiber: composed of cellulose, hemicellulose, and lignins –
is found in vegetables and wheat bran. It tends to absorb water
and increase in bulk, greatly contributing to the volume of stools
or feces. The associated water softens the stools.
• This causes insoluble fiber to increase gastrointestinal motility, or
in other words speed the movement of material through in GI
tract, especially the colon. Bacteria will not digest insoluble fiber.
• In other words, insoluble fiber (like cellulose) is valued for its
ability to prevent or relieve constipation.
Health Benefit of Fiber
• Soluble fiber is known to have cholesterol lowering effects
because of its promotion of increased fecal excretion of bile acids,
slower cholesterol and lipid absorption, and increased production
of short-chain fatty acids by fermentation in the large intestine.
• Insoluble fiber softens the stool and increases bulk, and thus
effectively prevents constipation, which appears to play a role in
the development of colon cancer and diverticulitis.
• Theoretically, the longer potentially carcinogenic substances
contained in feces remain in the colon, the more opportunity they
have to damage the intestinal lining.
Health Benefit of Fiber….
• How do dietary fibers help to reduce the incidence of colon cancer
• The link between a lack of dietary fiber and the onset of colon cancer
has been attributed to various effects of fiber – namely:
– Changes in the populations of microorganisms in the GI tract
– The decreased binding of intestinal bile acids
– The increased time for which food residue remains in the colon
– Decreased stool weight and volume and
– Decreased frequency of defecation
• Scientists postulate that the microorganisms encouraged by a diet low
in dietary fiber enhance the formation of cancer-causing substances
(carcinogens). These microorganisms may also prevent or at least limit
the breakdown of carcinogens that are normally destroyed by the
microorganisms that thrive when there is more dietary fiber in the
Health Benefit of Fiber….
• The main alternative theory suggests that the beneficial effect of
dietary fiber results from its speeding of the passage of feces
through the large intestine.
• The logic behind this idea is that faster passage means less time for
any carcinogens present to be in contact with the intestinal wall.
Also, the bulk and water of the feces may dilute the carcinogens to
a nontoxic level.
• Some fibers bind bile and carry it out of the body; this is also
thought to reduce cancer risk.
• Diverticulosis is a condition in which the muscles of the colon
weaken, forming small pouches that can become painfully
inflamed (diverticulitis) if food residue gets trapped within them.
It often occurs in people who are chronically constipated.
Health Benefit of Fiber….
• Diverticulosis is associated with a weakening of the intestinal wall
caused by the pressure from hard stools. The weakened intestinal
wall then develops small outpouchings in which fecal material
becomes trapped. Diverticulitis is an inflammation of the intestinal
wall that develops when these outpouchings become irritated or
• Different Fibers, Different Effects
• Not all the fibers have similar effects. For example, wheat bran,
which is composed mostly of cellulose, has no cholesterol lowering
effect, whereas oat bran and the fiber od apple (pectin) do lower
• Fibers that form gels in water (pectin and guar) prolong the time of
transit of materials through the intestine, whereas insoluble fibers
(cellulose) tend to reduce the time.
Hazards of Too Much Fiber
• There is now concern that some people may be consuming too
much fiber, which can lead to a condition known as irritable bowel
• It is also possible that too much fiber may cause decreased
absorption of some mineral elements (magnesium, calcium, zinc,
and iron), either by binding to them or by simply speeding the
passage of food through the intestinal tract and reducing the
opportunity for the mineral elements to be absorbed.
• At present it is thought that 20 to 30 g of dietary fiber per day
maximizes the benefits of fiber while minimizing the possible
adverse effects of either excessive or insufficient fiber
Glycemic Effect of Foods
• The glycemic effect of a food is the effect that food has on a
person’s blood glucose and insulin response – how fast and how
high the blood glucose rises, and how quickly the body responds
by bringing it back to normal.
• The concept of glycemic index is a measure of the ability of a
carbohydrate to contribute to the concentration of blood glucose.
• High glycemic index means that the dietary carbohydrate elevates
blood glucose faster and to a higher level than a carbohydrate of
lower glycemic index.
• Diets with a low glycemic index have been shown to have various
health benefits, including reduction of blood lipids and improved
blood glucose control in diabetic subjects.
Classification of Starch according to Digestibility
• Rapidly digestible starch: digested within 20 minutes
• Slowly digestible starch: digested between 20 and 120 minutes
• Resistant starch: not digested
• Rapidly digestible starch (RDS) is readily available to digestive
enzymes and digested rapidly and completely in the small
intestine. Example: freshly cooked starchy food.
• Slowly digestible starch (SDS) digested completely in the small
intestine, but slowly. They have a specific internal structure that
delays digestion and conversion into glucose. Example: mostly raw
• Resistant starch is the name given to the components of dietary
starch that are resistant to the normal enzymatic digestion process
in the small intestine.
Starch according to Digestibility…..
• There are three type of resistant starch:
– Physically inaccessible (RS1)
– Resistant granules (RS2)
– Retrograded (RS3)
• They originated in three possible ways:
– The physical structure of the food may prevent access to digestive
enzymes if the starch is surrounded by fat.
– The nature of the cell walls around the starch granules may
– Where the starch has become retrograded by heating and cooling,
the enzymes are no longer able to break the bonds.
• It must be remembered, however, that digestibility is variable and
probably dependent on the composition of the meal.
• Resistance starches that escape digestion in the small intestine
become available for fermentation in the colon by the bacterial
• The result of this process is an increase in fecal mass owing to the
multiplication of the bacteria, production of short-chain fatty aids
(acetic, propionic and butyric acids) and a decrease in colonic pH.
• In addition, CO2, H2 and some CH4 are produced. These contribute
to a sensation of bloating and flatulence.
• It has been estimated that between 20 and 30% of the potential
energy contained in the resistant starch becomes available to the
body in the form of short-chain fatty acids absorbed from the
Which class of starch is nutritionally
the most desirable form and why?
• SDS is the most desirable form of starch, as it digested slowly thus
keep the body’s blood glucose levels relatively steady throughout
• The delayed release of glucose into the blood stream from foods
containing SDS helps to prevent the rapid rise in blood glucose
levels that can occur after a meal high in simple sugars or rapidly
• A slower rise in blood glucose level support the more efficient
movement of glucose into cell and may prevent elevated blood
insulin levels, which can contribute to the development of
• SDS has been recognized to provide control energy release, more
prolonged glucose release and helps in the management of
• Constituents of NSP
– Non-cellulosic polysaccharides: Hemicelluloses, pectins, beta-glucans,
gums and mucilages
• In the mouth, high-fiber foods generally require more chewing. This
slows down the process of eating and stimulates an increased flow
of saliva. The saliva contributes to the volume of the swallowed
• Once in the stomach, the fiber-rich food tends to absorb water and
the soluble component starts to become viscous. Both of these
changes delay stomach emptying. In the small intestine, the soluble
fiber travels slowly because of increased viscosity; this prolongs the
period of time available for the absorption of nutrients.
• The fiber may also bind some divalent ions in the small intestine,
making them unavailable for absorption at this point.
• Once in the large intestine, the soluble fiber becomes a food
source for the growth and multiplication of the bacteria flora. The
consequences of this are exactly the same as described above for
• Thus, both resistant starch and soluble NSPs contribute to
increasing bulk in the large intestine, and the production of fatty
acids and gases.
• Insoluble fiber, which has reached the colon largely unchanged,
swells by water holding, and adds further to the volume of the
colonic contents. The feces, therefore, are both bulkier and softer
because of the increased water content.
What happens if I do not have enough
• Because both too little and too much glucose are harmful, blood
levels are maintained within a narrow range through the opposing
action of the hormones insulin and glucagon.
• When blood glucose levels are high, insulin helps the cells absorb
glucose, thereby lowering the blood glucose level.
• When blood glucose levels fall beneath the acceptable limit
(hypoglycemia), the pancreas releases glucagon, which stimulates
degradation of liver glycogen to glucose.
• If the fuel crisis persists and the glycogen supply has been
depleted, the body will begin to form glucose from protein and
glycerol by gluconeogenesis.
• If the blood glucose level falls much too low, to about 60 mg/dl, a
condition called hypoglycemia (too little glucose in the blood)
• Hypoglycemia is not a disease, but a symptom by which various
diseases can be diagnosed (just as hyperglycemia is a symptom of
• In extreme cases both hypoglycemia and hyperglycemia can cause
people to fall into a coma and die.
• Reactive Hypoglycemia: A mild form of hypoglycemia, sometimes
called reactive hypoglycemia, occurs from time to time in almost
everyone. Sometimes this is simple because a person has waited
too long between meals. People who get up and get going without
eating breakfast are running on an empty fuel tank. Not
surprisingly, they report a high incidence of reactive hypoglycemia,
as a result of insufficient food. 25
• In rare instances people develop a serious condition known as
spontaneous or fasting hypoglycemia, characterized by
chronically low blood glucose levels. This condition results from an
underlying disease state, such as stimulation of excessive insulin
production by a pancreatic tumor. No matter how much people
with spontaneous hypoglycemia eat, their blood sugar remains too
• If they go for a long period without food, their blood sugar may
drop so low that their energy-starved brain loses consciousness.
• Clearly this is a life-threatening condition that requires immediate
What happens if I have high blood glucose?
• Hyperglycemia: If the blood glucose level rises too high, above
200 mg/dl, a condition called hyperglycemia (too much glucose
in the blood) results.
• In people suffering from diabetes mellitus, hyperglycemia can
• How can diabetes be controlled through diet?
• The key to treating diabetes is to keep blood glucose at a
constant and moderate level using a carefully planned diet and, if
• A diet rich in complex carbohydrates and containing non-starch
polysaccharides is recommended in the management of diabetes.
How can diabetes be controlled through diet?
• Using a menu planning guide known as the Diabetic Exchange Lists,
dietitians help clients with diabetes develop a meal plan that
specifies how much of which types of food (exchanges) can be
eaten at each meal and snack.
• A diet rich in complex carbohydrates but with small amounts of
sugar is the key to controlling blood glucose levels.
• Diabetic Exchange Lists separate individual foods into six lists: milk,
vegetable, fruit, bread, meat, and fat. The milk, starch, and meat
lists are further divided to identify high-, medium-, and low-fat
food choices. The reason for dividing foods into these lists is that
food items vary with respect to their carbohydrate, protein, fat,
and kcalorie content.
How can diabetes be controlled through diet?
• The foods in each Exchange List have roughly the same amounts
of nutrients and kcalories, which makes it easy to exchange one
food for another within a given list.
• For example, one slice of bread and ½ cup of cooked rice are
considered to be nutritionally equivalent (contains 15 g
carbohydrate, 3 g protein, trace fat and 80 kcal).
• Because each Exchange List contains a wide variety of foods,
people with diabetes have considerable leeway in developing
their own menus.
• A blood glucose level of about 90 mg/dl is referred to as the fasting
glucose level because it is the level that is maintained during
• Glucose is the main source of energy for the central nervous
system (CNS), which used about 140 g (9 Tbsp) of glucose per day.
Red blood cells need about 40 g (3 Tbsp) of glucose per day.
• Carbohydrate Effect on Brain Function
• Prolonged hypoglycemia in brain results in histologically
demonstrable and functionally irreversible damage to brain tissue.
• Recent research has indicated that the kind and amount of
carbohydrate in the diet can influence brain function and behavior.
Carbohydrate Effect on Brain Function
• When carbohydrate intake is high, comprising 70% to 80% of
total kcal, the brain produces more serotonin.
• When produced in excess, serotonin causes a carving for
carbohydrate and therefore increased consumption of
carbohydrate, which in turn stimulates the production of yet
more excess serotonin.
• This make a person sleepy and sluggish.
Some Aspects of the Role of Carbohydrate
• Protein Sparing Action of Carbohydrates
• The Need for Carbohydrates for the Oxidation of Fats
• Carbohydrates as a Source of Energy for Muscular Work
• Special Functions of Carbohydrates in Liver
• Synthesis of Ribose from Glucose
• Carbohydrate and Dental Health
Protein Sparing Action of Carbohydrates
• The body uses mainly carbohydrates as a source of energy when
they are adequately supplied in the diet, thus sparing protein for
tissue building, since meeting the energy needs of the body takes
priority over other functions like growth.
• If the diet does not supply adequate calories, the dietary protein is
oxidized as a source of energy. There is also breakdown of tissue
proteins to a greater extent.
• This function of carbohydrates serving as a source of energy and
preventing dietary protein from being oxidized and preventing
excess tissue protein breakdown in calorie deficiency in called
“protein sparing action of carbohydrates”.
• For example, the daily urinary N excretion of 135 mg in an adult rat
during fasting is reduced to 93 mg when 12 g of glucose is fed to the
rat. Glucose has spared about 33% of the body protein broken down
in the fasted rat.
The Need for Carbohydrates for the
Oxidation of Fats
• In the oxidation of fats, the acetyl CoA formed from the oxidation
of fatty acids reacts with oxaloacetic acid (formed from
carbohydrate breakdown product – pyruvic acid or formed from
the aspartic acid) to form citric acid which is oxidized through the
TCA cycle back to oxaloacetic acid through a series of reactions.
• If adequate amounts of oxaloacetic acid are not available, acetyl
CoA is formed in large amounts from the oxidation of fatty acids
and a part of it is converted into ketone bodies (acetoacetic acid
and β-hydroxy butyric acid) which accumulate in blood and tissues
and produce ketosis.
Carbohydrates as a Source of Energy for Muscular Work
• Carbohydrates are the major source of energy for muscular work.
During muscular contraction glycogen is broken down to lactic acid.
The process is known as glycolysis.
• During the recovery period lactic acid is first oxidized to pyruvic
acid and then to acetyl CoA, which in turn is oxidized through TCA
cycle to CO2 and H2O thus producing energy for muscular work.
• As a source of energy, carbohydrates are more important during
moderate or severe muscular exerction as in athletics.
Special Functions of Carbohydrates in Liver
• These include detoxifying action and a regulating influence on
protein and fat metabolism. The liver of well-fed animals is rich in
glycogen. Such a liver is more resistant to certain poisons such as
carbon-tetrachloride, alcohol or arsenic and toxins of bacteria than
the livers of under-fed animals with low glycogen reserves.
• Synthesis of Ribose from Glucose
• The pentose-ribose is present in RNA and many nucleotides. It is
formed in the body from glucose by the pentose pathway and is
incorporated in RNA and nucleotides.
Carbohydrate and Dental Caries
• Dietary carbohydrate is implicated as a major cause of dental
caries, or tooth decay.
• High intake of dietary sugars Multiplication of oral bacteria
Production of acid Low oral pH Demineralization of teeth
• Carbohydrate as Glycoproteins
• Carbohydrates are found in the form of polysaccharide chains
attached to proteins such as glycoproteins. As such they are
important constituents of tissue antigens (in the cell membrane)
and secreted proteins.
• Glucose: Glucose (also known as dextrose, blood sugar, and grape
sugar) found naturally in fruits, vegetables, and honey. These
foods, however, provide us with an average of only 18 g of glucose
per day, which is only a small fraction of the glucose used daily in
• The rest is provided by the breakdown of more complex
carbohydrates, by the conversion of other monosaccharides, or by
the metabolism of certain amino acid.
• Sorbitol: a derivative of glucose that is formed when glucose is
chemically “reduced” by the addition of one hydrogen atom.
• Sorbitol occurs naturally in such fruits as apples, pears, and
peaches and also in several vegetables.
• It is not actively absorbed from the gastrointestinal tract and is
absorbed at about one third the rate of glucose absorption.
• This means that eating food rich in sorbitol allows blood glucose
levels to remain above the fasting level for a longer time than does
eating food correspondingly rich in glucose. Thus eating sorbitol
may delay the onset of hunger.
• For this reason, sorbitol is an ingredient in some foods designed
for use in weight-reducing diets and has been used clinically as a
• Unfortunately, sorbitol also causes flatulence and diarrhea in some
people, which considerably reduces its acceptability.
• Sorbitol is also widely used as a sweetener in chewing gums
because it is less likely to promote the formation of dental cavities
than is sucrose.
• Fructose (also known as levulose or fruit sugar) occurs in many
different fruits. It also makes up one tird of the sugar in honey.
• Insulin is not needed to allow fructose to be transported into cells;
so fructose does not cause problems of high blood sugar levels in
persons with diabetes. This has led to the idea that diets high in
fructose might be a suitable way for those with diabetes to get
their supplies of carbohydrate.
• Fructose is less likely to be cariogenic (tending to cause tooth
decay) than other sugar-based sweeteners. Tooth decay,
technically known as caries, is produced when bacteria in the
mouth act on carbohydrate to produce acid.
• The level of acid production depends on what carbohydrates are
available in the mouth, with fructose resulting in a relatively low
• Galactose, the third main monosaccharides of nutritional
relevance, occurs only as a result of the breakdown during
digestion of the disaccharide lactose.
• Studies using laboratory animals have linked excessive galactose
consumption with the development of cataracts.
• The levels used, however, indicated that this was unlikely to
present any problem to humans apart from the few people with a
genetic defect of metabolism known as galactosemia, which leads
to excessive amounts of galactose in the blood.
• In galactosemia, the concentration of galactose-1-phosphate uridyl
transferase is very low. Carbohydrate intake must be controlled
when such enzyme deficiencies occur.
• If the intake of galactose is not restricted, mental retardation
• Sucrose is the most common disaccharide. The familiar white
crystals or powder of sucrose is simply known as “sugar”, which is
• Brown sugar, with its more distinctive flavor, is 97% sucrose and
is made by partially refining sucrose or by adding some molasses
to white refined sugar.
• Lactase convert lactose into glucose and galactose.
• Milk, our only source of lactose, is also our most dependable
source of calcium.
• In most mammals, intestinal lactase activity is high at birth,
declines to low levels during childhood, and remains low in
• Lactase deficiency may also arise as a secondary condition,
resulting from damage to the intestinal mucosa by some other
disease process, such as malnutrition, HIV infection and parasitic
• The low levels of lactase in the intestine (lactase insufficiency) are
associated with intolerance to milk and other milk products
containing lactose (lactose intolerance).
• People with lactose intolerance experience a lot of discomfort if they
eat large amounts of lactose-containing dairy products. 43
• They can eat fermented dairy products, such as cheese and
yogurt, prepared by fermentation because the fermentative
microorganisms convert the original lactose into lactic acid.
• Most infants, including those who become lactase insufficient in
later life, do have the appropriate levels of enzymes to thrive
solely on milk without any problems. The intolerance to lactose in
people with lactase insufficiency develops later, in early
• The availability of the enzyme lactase, purified from yeasts and
molds, has made it possible for lactose-intolerant people to drink
milk or milk products without concern.
• The enzyme, which is available in tablet or powder form, is taken
before or with lactose-containing foods or is mixed with the food
before it is consumed.
Digestion and Absorption of Carbohydrate
• Before carbohydrate can fulfill its role in the body, it must be
converted into monosaccharide units that can be absorbed by the
intestinal mucosal cells and transported to the blood.
• The required changes are brought about by the action of certain
enzymes found in the mouth and the intestine and by the acidity
of the stomach contents.
• In the mouth, salivary α-amylase, or ptyalin, which is present in
saliva, partially breaks down starch. It catalyzes the hydrolysis of
most α(1-4) linkages but not the final or “terminal” α(1-4)
linkages of starch chains, the α(1-4) linkages next to branching
points, or the α(1-6) linkages responsible for branching.
Digestion and Absorption of Carbohydrate….
• The eventual end products of α-amylase digestion (salivary and
pancreatic) are maltose, maltotriose, and α-limited dextrins
(branched polymers containing an average of eight glucose units).
• In stomach, the food particles mixed with gastric secretions
(contain no specific enzyme for the breakdown of carbohydrates),
but the HCl in stomach stops the action of salivary amylase. The
action of α-amylase is inhibited because the optimal pH value of
the enzyme is 6.7.
• Before the food mixed compltely with the gastric acid secretion, as
much as 20% to 30% of the starch may have changed to maltose.
• In the small intestine the conditions change from acidic to alkaline,
and another potent α-amylase released from the pancreas acts to
continue the digestion of starch and dextrins.
Digestion and Absorption of Carbohydrate….
• Finally enzymes of small intestine containing α-limited dextrinase,
glucoamylase, maltase, lactase, and sucrase, which are able to split
α-limited dextrins, maltotriose, maltose, lactose, and sucrose into
their respective monosaccharide units.
• The monosaccharides glucose, galactose, and fructose produced by
this process of digestion are rapidly absorbed across the intestinal
wall to the blood in the capillaries that drain into the portal vein.
The portal vein carries the monosccharides and other absorbed
nutrients to the liver.
• When absorbed, monosccharides reach the liver, and fructose and
galactose are converted into glucose, which is then carried in the
blood to the rest of the body.
Summary of Carbohydrate digestion, absorption, and circulation
• Source Enzyme Products
• Mouth Salivary α-amylase Hydrolyzes α(1-4) linkages,
• (Salivary glands) (alkaline pH) producing α-limited dextrins,
maltotriose and maltose
• Stomach: Hydrochloric acid (acid pH)
• Small Intestine Pancreatic α-amylase Hydrolyzes α(1-4) linkages,
• (Pancreas) (alkaline pH) producing α-limited dextrins,
maltotriose and maltose
• Intestinal Wall Maltase, Sucrase,Hydrolyzes maltose &
• (intestinal mucosa) Lactase maltotriose to glucose
• α-limited dextrinase Sucrose to glucose & fructose
• Absorption (alkaline pH) Lactose to galactose & glucose
• Goes to blood Starch to glucose
• Hepatic circulation to the liver Glucose Systemic circulation
Utilization of Absorbed Carbohydrates
in the Body
• The utilization of carbohydrates in the body can be discussed
under two heads:
– Formation and breakdown of glycogen
– Oxidation of glucose
How does Processing Affect Carbohydrates?
• Processing tends to reduce the nutrient density of carbohydrate-
• Sugar is extracted from sugarcane and sugar beets through a
process that purifies the sucrose molecule. This is accomplished
by removing all of the starch, fiber, minerals, and vitamins. Not
surprisingly, the resulting refined sugar is considered ‘empty
• Like sugar and starch, whole grains are refined to make white
flour. This process, which removes the fiber-rich bran and the
vitamin-rich germ, leaves mainly the powdery, white starch of
• Law in USA requires the manufacturers of any flour or flour
product to add back three of the B-complex vitamins (thiamin,
riboflavin and niacin) plus the minerals iron at the same levels in
which they were present before refining. Not
How does Processing Affect Carbohydrates?
• Replaced, however, are fiber and many other vitamins and
minerals. The result is a major source of kcalories that is much
lower in nutrient density than the whole grain fro9m which it was
• If animals store glucose in their tissues as glycogen, why are not
meats a good source of dietary carbohydrate?
• First of all, the amount of glycogen that animals, or for that
matter humans, can store is relatively small. The entire body
contains only a couple of hours’ worth of energy in the form of
• Second, any glycogen that was present in an animal’s tissues is
depleted when the animal is fasted or frightened before
Changes in Carbohydrate during Cooking
• The effects of heating on carbohydrates are predominantly
beneficial in that it break down the cell walls, solubilizes starch
and makes it more easily digested.
• The heat during cooking cause the starch within the cells to
swell. This bursts the cell walls, and the starch and other
nutrients within become accessible to digestive enzymes.
• 1. Human Nutrition by Helen A. Guthrie and Mary F. Picciano,
McGraw Hill Publisher, 1995.
• 2. Understanding Nutrition by Eleanor N. Whitney and Eva M. N.
Hamilton, West Publishing Company, Latest edition.