CARBOHYDRATES-1

Lect-2

Sara AL-Mosharruf

OBJECTIVESIntroductionCarbohydrates classificationCarbohydrate sources

lect-2

Recommended intakes of carbohydratesFunction of carbohydrateDigestion , absorption and lect-3

metabolism of carbohydrateGlucose in the body

INTRODUCTION

Carbohydrates play a major role in our life and make us healthy.

 Every cells of human body needs energy to do its work.

 People don't eat glucose directly, they eat carbohydrates.

Then their bodies convert the carbohydrates into glucose for energy and to glycogen for reserve energy.

Dietary carbohydrate provided the body's great source of fuel for energy.

The carbohydrates are compounds made of carbon (C), oxygen (O) and hydrogen (H).

These atoms can form specified number of chemical bonds, carbon form four bonds, oxygen forms two and hydrogen forms one.

C

Carbohydrate

Simple

Monosaccharide& disaccharideEx. Sugars

Complex

PolysaccharideEx. Starch &fibers

CARBOHYDRATES classification 

Carbohydrate

Monosaccharaide

Single sugars

Glucose

Fructose

Galactose

Disaccharide

Double sugars

Sucrose lactoseMaltose

PolysaccharideMultiple sugars

Starch

MONOSACCHARIDE 

Most of the monosaccharides important in nutrition are hexoses.

Monosaccharides are the simplest form of carbohydrate that can not broke down to smaller units.

The three monosaccharides are important in nutrition, all have the same number and kinds of atoms(C6H12O6), but in different arrangement.

The three kinds of monosaccharaides have a different sweetness.

Glucose on the tongue gives a mildly sweet, Galactose hardly tastes sweet at all, but Fructose is intensely sweet.

GLUCOSE The basic single sugar in body metabolism is glucose . Supply the primary fuel for cells . It is not usually found as such in the diet. The body supply comes mainly from the digestion of starch. It is also know as blood sugar or dextrose. Glucose is one of the two sugars in every disaccharide and the unit from which the polysaccharides are made almost exclusively.

FRUCTOSE

Fructose is the sweetest of the sugars.

Has the same chemical formula of glucose

but with different structure.

Fructose occurs naturally in fruits and honey.

The amount of fructose in fruits depends on

the degree of ripeness.

 As the fruit ripens, some of the stored starch turns to sugars.

It is also called fruit sugar.

GALACTOSE 

Galactose occurs naturally as a single sugar in only few foods.

Galactose has the same number and kinds of atoms as glucose and fructose in yet another arrangement.

Galactose also is not usually found as such in the diet. But rather as a part of disaccharide ”lactose=glucose+galactose”

Milk is the primary food source of galactose. Comes mainly from digestion of milk sugars (lactose).

DISACCHARIDES

• Disaccharides: pairs of monosaccharides linked together.

• Glucose occurs in all three, and the second member of the pair is either fructose, galactose, or another glucose.

• The CHO and all other energy nutrients are put together by a reaction called condensation(links two monosaccharaides together and gives out water).and are taken apart by reaction called hydrolysis (is to break a disaccharide into two, and a molecule of water splits to provide the H & OH needed to complete the resulting monosaccharide). The hydrolysis often occurs during digestion.

SUCROSE

Sucrose=glucose + fructose

Occurs naturally in fruits ,vegetables and grains.

To make a table sugar, sucrose is refined from the juices of sugarance and sugar beets, then granulated.

Is common table sugar, and from sugar cane, sugar beets. Depending to the extent to which it is refined ,the product

becomes the familiar brown, white, and powdered sugars available in the grocery stores.

Sucrose is readily hydrolyzed by acids and by enzyme sucrase in the small intestine into glucose and fructose.

LACTOSE

Lactose= glucose+ galactoseThe principle CHO of milk .

Known as milk sugar.The sugars derived primarily from plants, except for lactose and its components galactose, which come from milk and milk products.Lactose is the only common sugar not found in plants.

It is less soluble and less sweet than sucrose.

Remains in the intestine longer than other sugar encourage the growth of certain useful bacteria.It is readily hydrolyses by enzyme lactase into glucose and galactose.

MALTOSE 

Maltose= glucose + glucoseThe two single sugar units that composed maltose are both glucose.It is not usually found as such in the diet. Maltose is

produce whenever starch is break down-it is derived in the body from the intermediate digestive breakdown of starch.occurs during the fermentation process that yields alcohol. only a minor constituent of few food, most notably barely.

POLYSACCHARIDES

• Composed of many single sugar units linked together. • The important polysaccharides in nutrition include starch, glycogen and dietary fiber.

STARCH  Starch: is long branches and unbranched chains of

hundred or thousands of glucose molecules linked together.

Is insoluble polysaccharide. Is the most dietary carbohydrates worldwide found

in grains(rice and wheat), legumes(peas and beans) and tubers (potatoes and yams).

When the plant is eaten, the body hydrolyzes the starch to glucose.

Moist and heat causes grains to swell, cell may disrupt and the starch becomes soluble. Cooking renders the starch to become soluble and more accessible to digestive enzymes.

Starch grains contains two polysaccharide derived from glucose(amylose and amylopectin).

Because starch are more complex than simple sugars, they breakdown more slowly and supply energy over longer period of time.

Dextrin's are degradation product of starch in which glucose chains have been broken down to smaller units by partial hydrolysis.

The human body stores glucose as glycogen

But plant cells store glucose as starch.

GLYCOGEN

Is found only to a limited extent in meats and not at all in plants.

The human body stores much of its glucose as glycogen and it is found in the liver and muscles.

In storage many glucose molecules linked together in highly

branched chains. This arrangement permits rapid hydrolysis.

When the hormonal message arrives at the storage sites, enzymes respond by attacking glycogen simultaneously, making a surge of glucose available.

These small stores of glycogen help sustain normal blood sugar during fasting and provide immediate fuel for muscle action.

DIETARY FIBER  Dietary fibers are the structural parts of plants and thus

are found in all plants derived foods-vegetables, fruits, whole grains ,and legumes.

Most dietary fibers are polysaccharides.

Fibers differ from starches in that the bonds between their monosaccharaides cannot be broken down by human digestive enzymes. (Non starch polysaccharide).

Fibers pass through the body and they contribute no monosaccharide, and therefore little or no energy.

The bacteria of the gastrointestinal tract can break some fibers down, this is important to digestion and health.

Dietary fibers into two groups

Soluble fiber:

• Indigestible food components that dissolve in water to form a gel(viscous).

• Are easily digested by bacteria in the colon(fermentable).

• Commonly found in oats, barely, legumes, and citrus fruit.

• They are mostly associated with protecting against heart disease and diabetes by lowering blood cholesterol and glucose level,repectively.

• An example is pectin from fruit, which is used to thicken jellies.

Insoluble fiber:

• Indigestible food components that do not dissolve in water.

• Don't form gels (nonviscous),and are less readily fermented.

• Found mainly in whole grains(bran),vegetables.

• It promotes bowel movements and alleviate constipation.

Types of fibers:

1- Cellulose: cellulose is the primary constituent of

plant cell wall . Occurs in all vegetables, fruits and legumes.

2- Non-cellulose polysaccharides: such as pectin,

gums and mucilage. 

3- Lignin: woody parts of vegetables such as

carrots and the small seeds of fruits (strawberries).

Total Fibers can be the sum of Dietary fibers and Functional fibers .

Dietary fibers (all the previous).

Functional fibers : fibers usually occurs naturally in plants when these fibers have been extracted from plants or manufactured and then added to foods or used in supplements.

Resistant starches: A few starches are classified as dietary

fibers. known as resistant starches, these starches escape digestion and absorption in the small intestine.

Starch may resist digestion for several reasons, including the individual efficiency in digesting starches and the food physical properties.

Resistant starch is common in whole legumes, raw potatoes, and unripe banana.

Phytic acid It is not classified as dietary fiber, is often

found accompanying them in the same foods.

The researchers have been unable to determine whether it is dietary fiber, the phytic acid or both, that binds with minerals, preventing their absorption.

This binding presents a risk of mineral deficiencies, but the risk is minimal when total fiber intake is reasonable and mineral intake is adequate.

Clinical importance of fibers:

1-Constipation2-Colon cancer3-Obesity4-Hypoglycemia5-Hyperglycemia6-Diverticulosis

High fiber diet:

• Is the dietary fibers exceed 40 gm/day.

• Food sources: fruits, vegetables, bread & cereals.

• Disadvantage of increasing fiber in the diet are:1-Abdominal fullness2-Increasing flatulence3-Nausea and vomiting.4-Gases5-May interfere with mineral absorption.6-More seriously can obstruct the GI tract.

CARBOHYDRATE SOURCES

Chemical class name

Class members

Sources

• Polysaccharides(Multiple sugar or complex carbohydrates)

• Starch Grains and grains products Cereal, bread, pasta, rice, corn, legumes, potatoes and other vegetables.

•Glycogen •Animal tissues, liver and muscle meats.

•Dietary fiber Whole grains, fruits, Vegetables, seeds, nuts.

Table (1): Carbohydrate sources

Chemical class name

Class members

Sources

Disaccharides ( Double sugars, simple carbohydrates)

•Sucrose •Table sugar, sugar cane, sugar beets, molasses.

• Lactose •Milk.

• maltose • Barley

• Monosaccharide (single sugars, simple carbohydrate)

•Glucose Sweetener in food products Starch digestion, final Corn syrup.

•Fructose Fruits, honey.

• Galctose Lactose (milk).

  Dietary recommendations suggest that, CHO

provide more than half (55 to 60 %) of energy requirement. 

5-9 servings daily of combinations of fruits and vegetables.

6-11 servings daily of combination of breads, cereals and legumes.

 The FDA (The Food and Drug Administration) set a daily value on food for fibers at 25 gram fiber daily.

The American Dietetic Association suggests that 20 - 25 grams of dietary fibers daily. A high fiber diet is more than 40 gm/day.

Recommended intakes of carbohydrates 

 

CARBOHYDRATES-2

Lect-3

Sara AL-Mosharruf

OBJECTIVESIntroductionCarbohydrates classificationCarbohydrate sources

lect-2

Recommended intakes of carbohydratesFunction of carbohydrateDigestion , absorption and lect-3

metabolism of carbohydrateGlucose in the body

FUNCTIONS OF CARBOHYDRATES

1- Basic fuel supply  The main function of carbohydrate is to provide

the primary fuel.

 Carbohydrates burn in the body at a rate of 4 kcal/gm.

  Carbohydrates furnish readily available energy

that needed not only for physical activities but also for all the work of the body cells.

2- Reserve fuel supply

The human body reserves carbohydrate as glycogen (in liver an muscles).  

Maintain a normal blood glucose level and to prevent a breakdown of fat and protein in tissue.

 People must eat carbohydrate foods regularly and

fairly frequent intervals to meet energy demand.

3- Special tissue functions Liver:• glycogen reserves in the liver and muscle

provide constant exchange with the body's overall energy balance system.

• Reserves glycogen especially in the liver protect cell from depressed metabolic function and resulting injury.

Protein: in case of insufficient carbohydrate, the human body use the protein as a fuel supply which prevent protein to be used for its major role in tissue growth and maintenance.

Fat: • Insufficient carbohydrate for energy, rapid

breakdown of fat would produce excess materials called ketones, which result from incomplete fat oxidation in the cell.

•  These ketones are acids, and can become serious.

4- Central nervous system

• The brain has no stored supply of glucose, therefore, it is dependent on a minute-to-minute supply of glucose from the blood.

• Sustained and profound shock from blood sugar

may cause brain damage.

5- Fibers

A. Cellulose• Cellulose it remains undigested in the

gastrointestinal tract. •  And provide important bulk to the diet. •  This bulk helps 1 move the food mass along.2 stimulate normal muscle action in the intestine.3and forms the feces for elimination of waste products.

B. Noncellulose polysaccharides

Absorb water and swell to large bulk.

Slow the emptying of the food from the stomach .

Bind bile acids including cholesterol in the intestine.

 Prevent spastic colon pressure by providing bulk for normal muscle action.

 Provide fermentation material for colon bacteria to work on.

TABLE (2): SHOWN THAT, SUMMARY OF DIETARY FIBER CLASSES  Source function

• Cellulose Main cell wall constituent of plants.

• Holds water. • Reduces elevated colonic intraluminal pressure.• Binds zinc.

•Noncellulose polysaccharides1- Gums, mucilage and Seeds.2- Algal polysaccharides 3- Pectin substances  

• Secretions of plants • Algae. • Intercellular cement plant material

• Slow gastric emptying.

• Provides fermentable material for colonic bacteria with production of gas and volatile fatty acids.• Binds bile acids and cholesterol

Source function4-Hemicellulose • Cell wall

plant Material.

• Holds water and increase stool bulk.• reduces elevatedcolonic pressure.• binds bile acids

• Lignine •Woody part of plants

• Anioxidant. • Binds bile acids, cholesterol and metals.

DIGESTION , ABSORPTION AND METABOLISM OF CARBOHYDRATE

The goal is to break sugar and starches into small molecules-chiefly glucose-that the body can absorb and use.

A large starch molecule require extensive breakdown.

The disaccharide needs only to be broken once. And the monosaccharide not at all. The initial splitting begins in the mouth, and

the final splitting and absorption occur in small intestine, and the conversion to common energy (glucose) takes place in the liver.

CARBOHYDRATE DIGESTION In the mouth: Mastication: the process by which food is crushed and ground by teeth .The chewing of high-fiber food slows eating and stimulate the flow of saliva.Starch: salivary glands secretes saliva into the mouth to moisten the food.The salivary enzyme amylase begins digestion.

Starch Amylase small polyshcarrides, maltose(disacharride)

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

In the stomach:Peristalsis: wave like muscular contraction.The swallowed bolus(a portion of food swallowed at one time) mixes with stomach acids and protein digesting enzymes, which inactivate salivary amylase.Starch: stomach acid inactivates salivary enzymes, halting starch digestion.To small extent ,the stomach acid continue breaking the starch, but it juices contain no enzymes to digest CHO.Fibers: is not digested in stomach and delays gastric emptying thereby provide feeling of fullness and satiety.

In the small intestine :In it most of the work of CHO digestion . Starch: the pancreas produces an amylase that is released through pancreatic duct into the small intestine.The major CHO-digesting enzyme is the pancreatic amylase which will continue breaking polysaccharides.

Starch pancreatic amylase small polysaccharides, maltosesthe pancreatic amylase in the duodenum breaks down all small polysaccharides into disaccharides.

The final step takes place in the outer membrane of the intestinal cells. The disaccharide digestion begins at this point. There specific enzymes secreted from the intestinal glands breaks down specific disaccharides.

The disaccharide enzymes on the surface of the small intestinal cells hydrolyze the disaccharides into monosaccharaides Maltose maltase Glucose +GlucoseSucrose sucrase Fructose+GlucoseLactose lactase Galactose+Glucose==>intestinal cells absorb these monosaccharaides.

Fibers: is not digested ,and delays the absorption of other nutrients.

Large intestine:

• Within 1-4 hours after a meal, all of the sugars and most of the starches have been digested.

• Only fibers remain in the digestive tract.• Fibers in large I. attracts water, which softens the stool for passage

without straining.• Also, bacteria in the GI tract ferment some fibers.• Most fiber passes intact through digestive tract to the large intestine.

Here, bacterial enzyme digest fiber.

Some fibers Bacterial Enzymes short-chain fatty acids, gas & water

• Colon uses these small fat molecules for energy. Metabolism of short chain fatty acids occurs in the cells of liver. fiber therefore can contribute some energy. Depending on the extent to which they are broken down by bacteria an d the fatty acids are absorbed.

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

CARBOHYDRATE ABSORPTION

Peristaltic movement moves the monosaccharaides into the jejunum where digestion is completed and absorption begins.

Absorption is increases as a result of the intestinal villi(small mucus projections lining the small intestine). Each villi contain blood capillary into which the monosaccharaides passes via diffusion or active transport.

Glucose is unique that it can be absorbed to some extent through the lining of mouth, but for most part nutrient absorption takes place in the small intestine.

Glucose and galactose traverse the cell lining the small I. by active transport.

Fructose is absorbed by facilitated diffusion, which slows its entry and produces a smaller rise in blood glucose.

Likewise, unbranced chains of starch are digested slowly and produce a smaller rise in blood glucose than branched chains, which have many more places for enzymes to attack and release glucose rapidly.

o As the blood from the intestine circulates through the liver, cells there take up fructose and galactose and convert them to other compounds ,most often to glucose .

o Thus all disaccharides provide at least one glucose molecule directly ,and they can provide another one indirectly –through the conversion of fructose and galactose to glucose.

LACTOSE INTOLERANCE

A condition that results from inability to digest the milk sugar lactose, characterized by bloating, gas, abdominal discomfort, and diarrhea.

Lactose intolerance differs than milk allergy, which is caused by an immune reaction to the protein in milk.

It is a common condition that occurs when there is insuffient lactase to digest the disaccharide lactose found in milk and milk products.

Because treatment requires limiting milk intake, other sources of riboflavin, vitamin D , and calcium must ne included in diet.

GALACTOSEMIA

Galactosemia (is a rare genetic metabolic disorder that affects an individual's ability to metabolize the sugar galactose properly. Although the sugar lactose can metabolize to galactose, galactosemia is not related to and should not be confused with lactose intolerance .

Lactose in food (such as dairy products) is broken down by the enzyme lactase into glucose and galactose.

In individuals with galactosemia, the enzymes needed for further metabolism of galactose are severely diminished or missing entirely, leading to toxic levels of galactose 1-phosphate in various tissues as in the case of classic galactosemia,

resulting in hepatomegaly (an enlarged liver), cirrhosis, renal failure, cataracts, brain damage, and ovarian failure.

CARBOHYDRATE METABOLISM Glucose plays a central role in CHO metabolism. Metabolism: is the sum of various chemical processes

in a living organism by which energy is made available for the functioning of the whole organism.

Also includes building and breaking tissues. Products of metabolism is called metabolite. The cell is the functional unit of life in the humans. Energy metabolism occurs in all cells to sustain life

process. With the help of enzymes, glucose is chemically

broken down to produce energy. Excess glucose maybe converted to fat and held in

reserve in adipose tissue.

Storing glucose as glycogen• The liver stores about 1/3 of the body’s total

glycogen and release glucose into the blood stream as needed.

• After a meal ,blood glucose rises, and liver cells link the excess glucose molecules by condensation reaction into long, branching chains of glycogen.

• When blood Glucose falls liver cells breaks glycogen by hydrolysis reactions into single molecules of glucose and release them into blood stream.

• Thus glucose available to supply energy for brain and other tissues regard less if person eat recently.

• Muscle cells can also store glucose as glycogen (the two other thirds),but they hoard most of their supply, using it just for themselves during exercise. The brain maintain a small amount of glycogen, which is thought to provide an emergency energy reserve during times of severe glucose deprivation.

• The body can store glycogen for only enough for short period of time(less than a day for rest and few hours for most during exercise).

• For its long term energy reserves, for use of weeks or days of deprivation, the body uses its abundant, water free fuel, fat.

Using glucose for energy Glucose fuels the work of most of the body’s

cells. The liver glycogen stores last only 4 hrs, not

for days. To keep glucose meeting energy needs,

person has to eat CHO frequently.

Making glucose from protein

Glucose is the preferred energy source for brain cells, other nerve cells, and developing RBC.

Body protein can be concerted to glucose to some extent. But proteins has jobs of its own that no other can do it.

Body fat cannot be converted to glucose.

Thus if a person does not replenish depleted glycogen stores by eating CHO ,body proteins are broken down to make glucose to fuel these special cells.

The conversion of protein to glucose is called gluconeogensis (making of new glucose).

Only adequate dietary CHO can prevent this use of protein for energy ,and this role of CHO is known as its protein sparing action.

Making ketone bodies from fat fragments

An inadequate supply of CHO can shift the body's energy metabolism in a precarious direction.

With less CHO providing glucose to meet the brain energy needs, fat takes an alternative metabolic pathway; instead if entering the main energy pathway, fat fragments combine with each other, forming ketone bodies.

Ketone bodies provide an alternate fuel source during starvation, but when their production exceeds their use, they accumulate in blood ,causing ketosis. A condition that disturbs body normal acid-base balance.

Using glucose to make fat

After meeting the energy needs and filling its glycogen stores to capacity, the body must find a way to handle any extra glucose.

At first, energy metabolism shifts to use more glucose instead of fat.

If that is not enough to restore glucose balance, the liver breaks glucose into smaller molecules and put them together into the more permanent energy storage compound-fat.

Thus when CHO is abundant, fat is either conserved or created.

The fat then travels to the fatty tissues of the body for storage .

Unlike the liver cells, which can store only enough glycogen to meet less than a day’s energy needs, fat cells can store seemingly unlimited quantities of fat.

Metabolism of CHO

Catabolism

-Glycogenlysis: the catabolism of

glycogen-Oxidation of glucose and

synthesis of ATP.

Anabolism Glycogenesis:

synthesis of glycogen in liver

and muscle .

Every body cell depends on glucose for its energy to some extent, but the cells of the brain and the rest of the nervous system depends almost exclusively on glucose for energy.

The activities of these cells never stop, and they have limited ability to store glucose.

Day and night, they continually draw on the supply of glucose in the fluid supplying them.

To maintain the supply, a steady stream of blood moves past these cells bringing more glucose from either the intestine (food) or the liver(via glycogen breakdown or gluconeogensis).

GLUCOSE IN THE BODY

Maintaining glucose homeostasis:

To function optimally, the body must maintain blood glucose within limits that permits the cells to nourish themselves.

If blood glucose falls below normal, a person may become dizzy and weak

If it rises above normal, a person become fatigue.

Left untreated fluctuation to the extremes-either high or low-can be fatal.

The regulating hormones:

Blood glucose homeostasis is regulated primarily by two hormones:

Insulin: which move glucose from blood into cells andGlucagon: which brings glucose out of storage when necessary.

After a meal, as blood glucose rises, special cells of pancreas respond by creating insulin into blood.

In general the amount of insulin secreted corresponds with the rise of glucose.

As the circulating insulin contacts the receptors on the body's other cells, the receptors respond by ushering glucose from the blood into the cells.

Most of the cells takes only the glucose they can use for energy right away, but the liver and muscle cells can assemble the small glucose units into glycogen for storage. The liver cells can also convert glucose to fat for export to other cells.

Thus elevated blood glucose returns to normal as excess glucose is stored as glycogen and fat.

When blood glucose falls(as occurs between meals),other special cells of pancreas respond by secreting glucagon into blood.

Glucagon raises blood glucose by signaling the liver to break down its glycogen stores and release glucose into blood for use by all other body cells.

It elicits release of glucose from liver glycogen. Another hormone that signals liver cells to

release glucose is epinephrine. Epinephrine :Is a hormone of the adrenal gland

that modulate the stress response, formally called adrenaline.

Balance within normal range: (blood glucose-fasting):

Normal: 70-120 mg/dlPrediabetes: 100- 125 mg/dlDiabetes: more or equal 126 mg/dl

Falling outside the normal range:In some people, blood glucose regulation fails, which results in either: diabetes or hypoglycemia.

Diabetes: is a chronic disorder of CHO metabolism, usually resulting from insuffient or ineffective insulin.

Type 1 DM: the less common type of DM in which pancreas fails to produce insulin. The exact cause is unclear(genetic or virus..)Type 2 DM: the more common type of DM in which the cells fail to respond to insulin. Usually occurs as a result of obesity.

Hypoglycemia: an abnormally low blood glucose concentration.

• Symptoms: weakness, rapid heartbeat ,sweating, anxiety, hunger and trembling.

• Usually it is a consequence of poorly managed DM.

• It is caused by too much insulin, strenuous physical activity, inadequate food intake, or illness that causes blood glucose to plummet.

The glycemic response: the extent to which a food raises the blood glucose concentration and elicit insulin response.

• Refers to how quickly glucose is absorbed after a person eats, how high blood glucose rises, and how quickly it returns to normal.

• Low glycemic response is desirable and high glycemic response is less desirable.

Glycemic index: a method classifying food according to their potential for raising blood glucose.

What are source of blood glucose?Either from:1CHO sources1-dietary CHO(high %)2-glycogen3-lactic acid and pyruvic acid2Non-CHO sources1-protein2-fats