Transcript Disaccharides

3. Carbohydrate (CHO)
Carbohydrates are compounds made up of types of sugars. Carbohydrates
are classified according to their degree of polymerization and may be
divided initially into three principal groups, namely sugars,
oligosaccharides and polysaccharides. They constitute a large part of foods
such as rice, noodles, bread, and other grain-based products.
The major dietary carbohydrates :
Class
Sugars (1-2)
igosaccharides
(3-9)
Polysaccharides
(>9)
Sub-Group
Monosaccharide
Disaccharides
polyose
Malto-oligosaccharides
Other oligosaccharides
Components
Fructose, galactose and glucose,
lactose, sucrose and maltose .
mannitol and sorbitol
maltodextrins
fructo-oligosaccharides, raffinose
and stachyose,
Starch
Amylose, amylopectin, modified
starches
Non-starch polysaccharides Cellulose, hemicelluloses,
pectin's, hydrocolloids
Polysaccharides are often referred to as complex
carbohydrates because they are typically long multiple
branched chains of sugar units. The difference is that
complex carbohydrates take longer time to digest and
absorb than simple sugar, since their sugar units must
be separated from the chain before absorption. The
spike in blood glucose levels after ingestion of simple
sugars is thought to be related to some of the heart and
vascular diseases which have become more frequent in
recent times. Simple sugars form a greater part of
modern diets than formerly, perhaps leading to more
cardiovascular disease.
The major sources of carbohydrate in the human diet
are: 1. Cereals . 2. Root crops . 3. Sugar crops .
4. Pulses . 5. Vegetables . 6. Fruit . 7. Milk products .
The importance of Carbohydrates to health
Carbohydrates have a wide range of physiological effects which
may be important to health, such as:
1. sources of energy
2. Effects on satiety /gastric emptying‫شبع‬
3. Control of blood glucose and insulin
metabolism
4.Control of colonic epithelial cell
function
5. Protein glycosylation
6. Cholesterol and triglyceride
metabolism
7. Bile acid
dehydroxylation
8. Fermentation
9. Hydrogen/methane
production
10. Short-chain fatty
acids production
11. Bowel
habit/laxation/motor
activity
12. Effects on large
bowel microflora
Carbohydrate as an energy source :
Dietary carbohydrates have by convention been given an energy
value of 4 kcal/g. Some carbohydrates are partly or not digested in
the small intestine and are fermented in the large bowel to short
chain fatty acids. These include :•non-digestible oligosaccharides,
•starch
•non-starch polysaccharides.
Glucose and blood sugar level :
The digestion of dietary carbohydrates starts in the mouth, where
salivary amylase initiates starch degradation. The amylase
degradation of starch is completed by the pancreatic amylase active
in the small intestine.
Dietary disaccharides, as well as degradation products of starch,
need to be broken down to monosaccharides in order to be absorbed.
This final hydrolysis is accomplished by hydrolases enzymes
attached to the intestinal membrane brush-border, referred to as
"disaccharidases".
Glucose and galactose are transported actively against a
concentration gradient into the intestinal mucosal cells by
a sodium dependent transporter . Fructose undergoes
facilitated transport by another mechanism . Fructose
taken together with other sugars is better absorbed than
fructose alone.
The absorbed carbohydrates cause an elevation of the
blood glucose concentration. Fructose and galactose have
to be converted to glucose mainly in the liver and therefore
produce less pronounced blood glucose elevation.
The extent and duration of the blood glucose rise after a
meal is dependent upon:- the rate of absorption, which in
turn depends upon factors such as gastric emptying as well
as the rate of hydrolysis and diffusion of hydrolyzed
products in the small intestine.
Fermentation :
Fermentation is the colonic phase of the digestive process and
describes the breakdown in the large intestine of carbohydrates not
digested and absorbed in the upper gut. This process involves gut
micro-flora and is unique to the colon of humans because it occurs
without the availability of oxygen. It thus results in the formation of
the gases hydrogen, methane and carbon dioxide, as well as short
chain fatty acids (SCFA) (acetate, propionate and butyrate), and
stimulates bacterial growth (biomass). The gases are either absorbed
and excreted in breath, or passed out via the rectum. The major
products of such fermentation are the Short Chain Fatty Acids
(SCFA) which are rapidly absorbed and metabolized by the body.
Micro-flora
Carbohydrate which is fermented stimulates the growth of bacteria
in the large gut. This is a generalized effect which leads to an
increase in the total number of bacteria or biomass. When bacterial
growth occurs, the micro-flora synthesize protein actively from
preformed amino acids and peptides.
Carbohydrates
A. Monosaccharide's
1. Glucose(dextrose) :
The monosaccharide glucose in the body is commonly called blood sugar. It is
the major form of sugar in the blood. Normal fasting blood sugar (FBS) is 70 to
100 milligrams per 100 milliliters of serum or plasma. Impaired fasting glucose
(IFG) is 100 to 125 milligrams per 100 milliliters of serum or plasma .
Regardless of the form of sugar consumed, the body readily converts it to
glucose
2. Fructose :
Fructose is found in fruits and honey. It is the sweetest of all the
monosaccharide's. Fructose is used extensively in soft drinks, canned foods,
and various other processed foods .The human body readily converts fructose
to glucose.
3. galactose :
The monosaccharide galactose comes mainly from the breakdown of the milk
sugar lactose. Yogurt and unaged cheese may contain free galactose. It is the
least sweet of all the monosaccharide's. The body converts galactose into
glucose after ingestion.
B. Disaccharides :
1. Maltose = (glucose + glucose)
The disaccharide maltose is produced when the body
breaks starches into simpler units.
Smaller amounts of this disaccharide are present in
malt, malt products, beer, some infant formulas, and
sprouting seeds.
2. Sucrose = (glucose + fructose) :
The most prevalent disaccharide, sucrose, is ordinary
white table sugar made commercially from sugar beets
and sugar cane. Brown, granulated, and powdered
sugars are all forms of sucrose. Sucrose is also found in
molasses, maple syrup, fruits, and vegetables
3. Lactose = (glucose + galactose) Because lactose
occurs naturally only in milk, it is commonly
referred to as milk sugar. Lactose is the least sweet
of the disaccharides
At birth, lactase activity is high in the brush-border of
the small bowel of infants, but declines after weaning
and low activity in adult life.
Lactose intolerance, the term used to describe the
clinical symptoms of abdominal discomfort, flatulence
and diarrhea, associated with the ingestion of lactose
containing foods by persons with low lactase activity.
Or in injured
intestinal mucosa following acute
infection in children and in protein-energy malnutrition.
It is also found in adults, particularly in association
with celiac disease.
C. Polysaccharides :
A. Digestible fibers
1. starch :
Starch is used as a storage polysach in plants, it founds in
the form of both amylose & the branched amylopectin.
Starch found in large quantities in cereals grains,
potatoes, plantains. Starch, the major source of
carbohydrate in the diet, is found primarily in grains,
starchy vegetables, and legumes and in foods made from
grains — cereals, breads, and pasta.,
2. Glycogen :
Glycogen represents the body’s carbohydrate stores.
glucose is stored in liver and muscle tissue as the
polysaccharide Glycogen .
B. Indigestible fibers :
(plant or dietary fibers or Non-Starch Polysaccharides Fiber ) :
Dietary fiber refers to foods, mostly from plants, that the human
body cannot break down to digest and that is eliminated in
intestinal waste. Sometimes called roughage or bulk, fiber adds
almost no fuel or energy value to the diet, but it does add volume.
Bulk fills the stomach, and most experts believe a full stomach
contributes to a feeling of satiety, so further eating ceases.
1. Insoluble dietary fibers: (like beans, brown rice, maize, peas,
breads, cereals, pasta). consist mainly cellulose & hemicelluloses.
It bind to water in the colon,& stimulate peristalsis in the colon &
reduce risk of constipation & ca colon. Examples of sources of
insoluble fibers include the woody or structural parts of plants,
such as fruit and vegetable skins, and the outer coating (bran) of
wheat kernels. Insoluble fibers have been reported to promote
regularity of bowel movements and reduce the risk of
diverticular disease and some forms of cancer. The mechanism of
these effects for insoluble fiber is due to decreased intestinal
transit time and decreased intestinal pressure
2. Soluble dietary fiber (like apples, legumes, pears,
strawberries). Sources of soluble fibers include beans,
oatmeal, barley, broccoli, and citrus fruits; oat bran is a
particularly good source of soluble fiber. Soluble fibers
dissolve in water and thicken to form gels. The
reported health benefits of soluble fibers include
reduced cholesterol blood levels, regulated blood sugar
levels, and weight loss (by helping dieters control their
appetites). Soluble fiber blunts the response of blood
glucose to ingestion. The reabsorption of bile acid is
slowed by soluble fiber so increase cholesterol loses in
feces& reduce blood cholesterol level.
Dietary fiber (properties and sources)
Types of dietary fiber
1. Insoluble fibers:
Cellulose, some
hemicelluloses, lignin .
function
It holds water, or it binds to water in the
colon and increases fecal bulk, reduces
intra-luminal pressure in the colon,
stimulate peristalsis in the colon and
reduce risk of constipation and CA of
colon . promotes growth of helpful
bacteria in the colon .
source
Wheat bran, bran cereals,
whole grains(wheat and
barley), popcorn, peanuts,
apples, pears, berries,
bananas, peaches, legumes,
peas, cabbage family, corn,
spinach, sweet potato,
sunflower seeds .
1. Soluble fibers :
Pectin, some
hemicelluloses, gums .
Binds cholesterol and bile acids(slows
reabsorption of bile acids) so increases
cholesterol losses in feces and reduce
blood cholesterol level, slows gastric
emptying, provides fermentable
materials for colon bacteria with
production of volatile fatty acids and
gas, slows absorption of glucose,
preventing rapid rise in blood glucose
levels .
Oat bran, oat cereals,
barley, pumpernickel bread,
apples, citrus fruits, pears,
broccoli, Brussels, sprouts,
carrots, sweet potato, white
potato, peas, legumes,
(gums and psyllium used as
thickeners or stabilizers in
processed foods)
Function of dietary fibers :
•Preventing and treating constipation
•control of weight
•Cancer prevention
•Diabetes control (slows glucose
absorption from intestine & decreases
release of insulin).
•Reduction of blood cholesterol (inhibits
absorption of cholesterol and bile acids
therefore it prevents CVD and obesity).
Function of glucose(dextrose) :
Glucose… oxidation… CO2 + H2O + 36 – 38 ATP
1. Source of energy: dietary CHO have energy value of 4 kcal/gm.
number of CHO are only partly or not at all digested in the small
intestine & are fermented in the large bowel to short chain fatty
acids, these include the non- digestable oligosach, resistance
starch & NSP. The processes of fermentation is metabolically
less efficient than absorption in the small intestine &these CHO
provide the body with less energy.
2. control of blood glucose & insulin metabolism: absorbed CHO
cause an elevation of the blood glucose conc . Insulin is secreted
as a response to blood glucose elevation.
3. protein glycosylation: the non-enzymatic glycation of proteins is
dependant on the concentration of glucose in blood & the half
life of the protein.
4. Preventing ketosis
5. Food palatability
Glycemic Index(GI):
the blood glucose response to a given food
compared to a standard .
Effect of high glycemic index foods on the body :
1. Increase insulin release from
pancreas
2. Insulin resistance and type II DM.
3. High blood triglycerides
4. Increase fat synthesis in the liver
5. Increase fat deposition in adipose
tissue.
Classification of fruits according to Glycemic Index (GI)
Low GI
>35
Cherries (22)
Grapefruit(25)
Dried prunes(29)
Dried apricots(30)
Moderate GI
35 - 50
Apple(38)
Peach, canned in
juice(38)
Fresh pear(38)
Plum(39)
Strawberries(40)
Orange, navel(42)
Fresh peach(42)
Grapes juice(46)
Fresh prunes(49)
High GI
<50
Banana(52)
Kiwi(58)
Cocktail fruit(55)
Mango(51)
Fresh Apricots(57)
Dried figs(61)
Raisins(56)
Cantaloupe(65)
Dates(103)