Transcript Digestion & Absorption of Dietary Macronutrients & Fibre

Digestion &
Absorption of Dietary
Macronutrients &
Fibre
Hanadi Baeissa
Carbohydrates
Hanadi Baeissa
Man
Fru
Glu
Mono sacch.
Carbohydrates
Sugar
alcohol
Oligosacch.
Pentoses
Plant
Sources
Starch
Disacch.
(sucrose, maltose
& Trehalose)
NSP
Hanadi Baeissa
Di sacch.
(lactose)
Animal Sources
polysaccharides
small amount
(not important)
Figure 2. Percentage nutrient intake in three communities with different dietary
intakes.
Hanadi Baeissa
Figure 1. The principal carbohydrates in the human diet.
Hanadi Baeissa
Figure 3. Dietary
Carbohydrates
Hanadi Baeissa
Monosaccharides




Glucose is found in small amounts in fruits, vegetables and
honey. Free glucose is not found abundantly in natural foods,
but is manufactured from starch and sold commercially in a
number of proprietary preparations .
Fructose is found in fruits, vegetables, and honey. It is
present also in invert sugar, a syrup made from sucrose and
used extensively in the food industry.
Mannose is uncommon as a monosaccharide in foods but is
present in manna
Pentoses are present as constituents of the macromolecules
in the cells of the natural food stuffs, but only in small
amounts, so they are not important as a source of energy
Hanadi Baeissa
Disaccharides

Sucrose is the sugar commonly used in the home, and
is extracted commercially from sugar beet or sugar
cane. It is present also in fruit and vegetables.

Lactose is a disaccharide of glucose and galactose
that is found naturally only in milk and milk products.

Maltose is a product of the hydrolysis of starch and
comprises two molecules of glucose. It is present in
malted (sprouted) wheat and barley, from which malt
extract is produced commercially

Trehalose is a disaccharide composed of two
molecules of glucose and is known as the mushroom
sugar
Hanadi Baeissa
Oligosaccharides

•
•
•
Raffinose, stachyose and verbascose:
short-chain sugars made of galactose, glucose and fructose
Found in plant seeds – mainly legumes
Cannot be broken down by endogenous enzymes
 Fructans: single glucose + fructose chain (3-50 residues
•
•
•
depending on source)
Short chain in cereals
Inulin in artichokes (35 residues)
Also found in onions, garlic, and asparagus.
Hanadi Baeissa
Sugar Alcohols


Found in nature and are also prepared commercially
Sorbitol is found naturally in some fruit, such as cherries,
but is also made commercially. The manufactured product is
used in 'diabetic' soft drinks, jams, chocolates, and sweets. It
is only 60% as sweet as sucrose.

Mannitol and dulcitol are alcohols derived from mannose
and galactose, and both have a variety of uses in food
manufacture. Mannitol is extracted commercially from a
seaweed that grows on the coasts of Britain.

Inositol is a cyclic alcohol with six hydroxyl radicals and is
allied to glucose. It is present in many foods, especially the
bran of cereals.
Hanadi Baeissa
Polysaccharides

Starch is the major carbohydrate of the human diet and is the
main storage polysaccharide of dietary staples.

Within the plant, starch is present in the form of granules with
characteristic shapes, specific to each species

Starch consists of two main types of polysaccharide derived
from glucose. Amylose is along, virtually unbranched chain of
glucose units with α (1 → 4) linkage. Amylopectin is a highly
branched polymer

Amylopectin predominates in most starches, but the relative
amounts of amylose and amylopectin vary among different
plant sources . The majority of starches contain between 15%
and 35% amylose.
Hanadi Baeissa




Dextrins are degradation products of starch in
which the glucose chains have been broken
down to smaller units by partial hydrolysis.
They are the main source of carbohydrate in
proprietary preparations used as oral
supplements for tube feeding.
'Liquid glucose' is the mixture of dextrins,
maltose, glucose, and water. These products are
a means of giving carbohydrates in an easily
assimilated form to patients who are seriously ill.
Dextrins, being larger molecules than sucrose or
glucose, have less osmotic effect and so are
less likely to cause osmolar diarrhea.
Hanadi Baeissa

Glycogen the animal equivalent of starch,
has a structure very like that of
amylopectin, but is more highly branched.
It is present in liver and muscle, where it is
stored as a readily energy reserve.
Hanadi Baeissa
Non Starch Polysaccharides (NSP)

Definition

The original concept of 'dietary-fibre' (Hipsley 1953) was of
material derived from the plant cell wall in foods.
By 1972, dietary fibre had been defined as the skeletal remains
of plant cells that are resistant to digestion by the enzymes of
man (Trowell 1972),
but by 1978 it was suggested by Cummings & Englyst that
dietary fibre should be measured as the non-starch
polysaccharides in plant foods (James & Theander 1981).
In 1987, Englyst et al (1987b) proposed that dietary fibre
should be defined for the purposes of food labeling as
NSP, since this gives the best index of plant cell-wall
polysaccharides and is in keeping with the original concept of
dietary fibre.
Hanadi Baeissa



Fibre content of foods
Fibre content can be separated into soluble
and insoluble fractions
 Fibre values can also be separated into
cellulose and non-cellulosic polysaccharides
(NCP)

Hanadi Baeissa
Table 1. Examples of the variability in the fibre composition of different foods
based on detailed gas-liquid chromatographic analysis
Hanadi Baeissa
Figure 1. The principal carbohydrates in the human diet.
Hanadi Baeissa
Non Starch Polysaccharides (NSP)
Cellulose




Principle component of cell walls in plants
High M W linear polymer (up to 10 000 glu) linked by
β(1 → 4) bonds
Inter- and intramolecular hydrogen bonds leads to the
formation of microfibrils and fibres ( stable crystalline
structures)
Shows low chemical reactivity
Hanadi Baeissa
Hemicelluloses





Heterogeneous, branched polymers of hexoses, pentoses
and uronic acids, found in plant cell walls.
50-2000 residues long
Xylans: polymers of xylose with side chains of arabinose
and glucuronic acid (mainly in wheat, rye and barley).
Galactomannans: mannose backbone with galactose, and
glucose side chains (legumes)
Xyloglucans: glucose backbone and xylose branches
closely associated with cellulose
Hanadi Baeissa
β-Glucans:
 Water-soluble polymers of glucose linked
β(1→3) and β(1→4).
 Unlike cellulose, the glucose chains are
branched and have a relatively low degree
of polymerization.
 Cereals such as oats and barley are
particularly good sources of these
polysaccharides
 they have been implicated in the
cholesterol-reducing properties of oat bran
Hanadi Baeissa
Pectins:
Branched polymers in fruit and vegetables .
The two main types are :
 rhamno-galacturonans, which are polymers of
rhamnose and galacturonic acid with branches
of galactose and arabinose, and
 arabinogalactans, which are galactose, chains
with many short arabinose side chains.
 Used as 1) stabilizer
2) emulsifier
3) gelling agents in jams (E440).

Hanadi Baeissa

Gums: Water-soluble viscous polysaccharides of 10,000-
30 000 residue (mainly glucose, galactose, mannose,
arabinose, rhamnose and their uronic acids)
 Extracted commercially and used in the food industry as
emulsifiers, stabilizer and thickeners .
Examples:
 . Gum Arabic is obtained as an exudate from the acacia
tree
 Guargum and locust bean gum are galactomannans. They
are the storage polysaccharides of the Indian cluster bean
and the locust or carob bean, respectively.
Hanadi Baeissa
Mucilages: Structurally complex, generally



characterized by the component D-galacturonic acid.
Found in some seeds, roots, seaweeds and algea.
used as food addatitives
Examples:
a) Alginic acid: from brown seaweeds, is a polymer
of mannuronic and guluronic acids, used as a thickener
and stabilizer in ice cream etc.
b) Garrageenans: They are sulphated galactose
polymers derived from red algae.They gel in the
presence of Ca2+ or K+ ions to give or brittle gel that
is used in a large number of foods .
Hanadi Baeissa
Synthetic carbohydrates
a) Polydextrose:
 randomly cross-linked glucose polymers of various
sorts
 made by: thermal polymerization of glucose in the
presence of citric acid and sorbitol.
 Two forms: off-white amorphous powder and a light
yellow aqueous solutions
 Have similar functional properties as sucrose but nonsweet & tasteless
Hanadi Baeissa
b) Neosugar:

A mixture of short-chain fructo-oligosaccharides (3-5
residues)

synthesized from sucrose

similar characteristics to sucrose in cooking, but is
only half as sweet
Hanadi Baeissa
Figure 4. Digestion of
carbohydrates.
Hanadi Baeissa
Figure 5. Degradation of dietary glycogen by salivary or pancreatic α-amylase.
Hanadi Baeissa
Table 2. Di-and Oligosaccharidases of the Luminal Plasma Memebrane in the
Small Intestine
Hanadi Baeissa




The monosaccharide glucose, fructose, and
galactose are then transported across the
epithelial cells and enter the portal vein.
Free concentrations in the intestine or at the
mucosal surface are likely to be high enough for
passive or facilitated absorptions at the
beginning
As concentrations fall, active transport against a
concentration gradient becomes necessary and
so requires energy
Different sugars compete for transport, and
galactose and glucose are absorbed faster than
fructose
Hanadi Baeissa
Figure 6. Absorption of
monosaccharides
Hanadi Baeissa

Sorbitol is absorbed from the gut more slowly

converted to fructose in the liver

It thus has less effect on blood glucose levels than
sucrose

But episodic intakes above 50 g per day may lead to
diarrhea in diabetic patients consuming these products
Hanadi Baeissa
Factors effecting absorption:
After a mixed meal of several foods many
factors affect the rate of absorption of
carbohydrate. The rate of passage though the
stomach and upper small intestine is obviously
important, and this depends on:
1.
2.
3.
The amount of peristalsis
The viscosity of the bolus passing
Enzymic activity
Note: Glu, dextrins & sol. Starch are absorbed at equal
rates normally ie. digestion
is not a limiting factor
Hanadi Baeissa
Glucose level after a meal

Reach maximum after about 30 min

Decrease slowly to normal after 90-180 min

Height of maximum and the rate of return to normal
vary with nature of food, and give an indication of the
rate at which starchy foods are digested in the small
intestine.
Hanadi Baeissa
The Glycaemic Index (GI)

A.
B.
C.
D.


Physiological measurement used to estimate the
relative rates of glucose absorption from various
foods. Method of measuring GI is:
50 g of CHO in test food is eaten
Blood glucose is measured every 30 min for 3
hours
The area under the curve is calculated
This is compared to area when 50 g of glucose or
white bread are ingested
GI for legumes <50,but >110 for mashed
potatoes
GI is helpful in planning diabetic diets
Hanadi Baeissa
The effects of food preparation on
carbohydrate absorption

Natural starch in whole grains and seeds forms granules that are
inaccessible to digestive enzymes

It needs crushing, chopping and milling. The rate of digestion
being depending on the final particle size

Density of the product e.g. pasta, also slows digestion, and
undigested starch entering the large intestine, and is found in
feces.

Cooking
gelatinization of starch granules
the starch chains, and easier digestion
Hanadi Baeissa
dispersion of

Foods eaten raw retain their starch within granules,
and are more difficult to digest, leading to smaller GI

On cooling gelatinized starch begins recrystallization,
(known as retrogradation). This is very rapid for
amylose, but slow for amylopectin (staling of bread)

Retrogradation retards digestion, and retrograded
starch (mainly amylase) from processed cereal and
potato products have been shown to pass through the
small intestine
Note: Many factors interfere with digestion of CHO,
hence its absorption
Hanadi Baeissa
%RDS
%SDS
%RS1
%RS2
%RS3
White flour
49
48
-
3
t
Shortbread
56
43
-
-
1
White bread
94
4
-
-
2
White
spaghetti
52
43
3
-
3
Banana
Biscuits
39
23
-
38
t
Potato biscuits
47
27
-
25
1
Haricot beans
18
42
18
9
12
Pearl barley
41
41
9
-
2
Table 3. In vitro digestibility of starch in a variety of foods. The values are
expressed as a percentage of the total starch present in the food.
Hanadi Baeissa
Factors interfering with digestion of
CHO, hence its absorption:
Method of preparation and cooking ( white
bread compared to short bread)
 Structure of ingested food ( pasta vs white
bread)
 Physical accessibility of starch ( beans &
barley)
 Type of original starch granules ( white flour
vs banana and potato flour)

Hanadi Baeissa
Other factors effecting the digestibility
and absorption of starch within the small
intestine
The extent of chewing
 The concentration of amylase available for
breakdown of the starch
 The amount of starch
 The presence of other food components that
might retard enzymic hydrolysis
 The transit time of the food along the small
intestine

Hanadi Baeissa
Undigested carbohydrate entry
into the large intestine
A number of potentially fermentable
substrates enter the caecum. The principle
ones are NSP and starch, but a substantial
amount of protein also escapes digestion in
the small intestine
 For many foods, more starch than NSP
reaches the colon. The amount of starch
escaping digestion and available for
fermentation ranges from 2% for oats to 89%
for bananas

Hanadi Baeissa
Substrate
Amount (g/day)
Carbohydrates
Resistant starch
Non-starch polysaccharides
Unabsorbed sugars, sugar alcohols
Oligosaccharides
Chitin and amino sugars
8-40
8-18
2-10
2-6
1-2
Nitrogenous substrates
Dietary protein
Pancreatic enzymes and secretions
Urea and nitrate
3-9
4-6
0.5
Other substrates
Mucin
2-3
Table 4. The principal substrates available for fermentation in the human
colon. The amounts estimated are based on subjects consuming a western
diet (from Macfarlane & Cummings 1990).
Hanadi Baeissa
Colonic fermentation
The extent of fermentation depends on the form
and solubility of the substrate
e.g. soluble pectin degraded completely, insoluble
wheat bran incomplete degradation
 Steps in fermentation:

A.
B.
C.
polymers are broken down into their constituent monomers
(glucose, galactose, arabinose, xylose and uronic acids)
sugars then are converted to pyruvate
various routes followed depending on the microbial
species present and the nature of the available substrate
Hanadi Baeissa
Figure 7. Fermentation in the human colon (adapted from Cummings 1983).
Hanadi Baeissa

•
•
The principal SCFAs produced from all substrates
are :
acetate, propionate, and butyrate
Other organic acids such as isobutyrate, valerate,
isovalerate, lactate, and succinate occur in small
amounts.

These organic acids are the major anions in the
large intestine and contribute to the relatively low
pH found there (5.6-6.6)

Ratio of SCFAs depend on the substrate being
utilized
Hanadi Baeissa
SCFA produced (mg/mg polysaccharide)
Polysaccharide
Acetate
Propionate
Butyrate
Total
Starch
0.25 (50)
0.13 (22)
0.21 (29)
0.59
Arabinogalactan
0.19 (50)
0.20 (42)
0.04 (8)
0.43
Xylan
0.42 (82)
0.10 (15)
0.02 (3)
0.54
Pectin
0.27 (84)
0.06 (14)
0.01 (2)
0.34
Table 5. Short-chain fatty acids (SCFA) produced in vitro by intestinal
bacterial grown on different polysaccharide substrates. Molar ratios are
given in parentheses (from Englyst et
al 1987a)
Hanadi
Baeissa
Fates of SCFAs

Acetate and propionate are rapidly absorbed to the portal vein and
carried to the liver

Only acetate is released to other tissues

Propionate is utilized within the liver, where it may modify
carbohydrate and lipid metabolism.

Butyrate is used by colonocytes, and it is actively metabolized to
ketone bodies (acetoacetate and β-hydroxybutyrate), carbon dioxide
and water (beneficial – antitumour action)

Other SCFAs are also cleared by the liver
Hanadi Baeissa
Available energy from fermentation

60-70% of the energy potentially available if
all CHO was hydrolyzed and absorbed from
the small intestine

the old term 'unavailable carbohydrates' for
these not digested in the small intestine could
be misleading.
Hanadi Baeissa
Physiological effects of undigested CHO
1.
2.
Faecal weights (Soluble fibre is readily fermented
bacterial population, but Insoluble fibers directly
weight, thus more important as a laxative )
Effect on intestinal transit time
- The intestinal transit time is the time taken for a meal to
pass from the mouth to the anus
- Colonic transit takes approximately ten times as long as
mouth-to-caecum transit
Hanadi Baeissa
Mouth-to-caecum transit time is increased by viscous
polysaccharides leading to:
a. prolonged gastric emptying resulting in greater
staiety
b. delay in the absorption of low-molecular-weight
nutrients, e.g. glucose
Colonic transit is reduced by insoluble fibers
3.
Effect on serum Cholesterol (mainly sol. fibers)
- Physiological studies show that the addition of certain
plant fibres to the diet is accompanied by significant
reductions in serum cholesterol concentrations
- . The reduction in cholesterol is seen mostly in the lowdensity lipoprotein fraction, and is accompanied by
decreases in the cholesterol content of the liver, aorta,
and other tissues.
Hanadi Baeissa
-
Abnormal degradation of
disaccharides

The overall process of carbohydrate digestion
and absorption is so efficient in healthy
individuals that ordinarily all digestible dietary
carbohydrate is absorbed by the time the
ingested material reaches the lower jejunum

However, because predominantly
monosaccharides are absorbed, any defect in a
specific disaccharidase activity of the intestinal
mucosa causes the passage of undigested
carbohydrate into the large intestine
Hanadi Baeissa
Figure 4. Digestion of
carbohydrates.
Hanadi Baeissa
Table 2. Di-and Oligosaccharidases of the Luminal Plasma Memebrane in the
Small Intestine
Hanadi Baeissa

As a consequence of the presence of this
osmotically active material, water is drawn from
the mucosa into the large intestine, causing
osmotic diarrhea

This is reinforced by the bacterial fermentation
of the remaining carbohydrate to two- and threecarbon compounds plus large volumes of CO2
and H2 gas, causing flatulence
Hanadi Baeissa
Abnormal lactose digestion
Hanadi Baeissa
Types of Defects
1.


Hereditary defects:
Deficiencies of the individual
disaccharidases have been reported in
infants and children with disaccharide
intolerance
Partial deficiencies ( low activity ),
appearing later in life has also been
reported
Hanadi Baeissa
2.



Generalized defects:
Alteration in disaccharide degradation can also
be caused by a variety of intestinal diseases,
malnutrition, or drugs that injure the mucosa of
the small intestine
Note: Brush border enzymes are rapidly lost
in normal individual with sever diarrhea,
causing a temporary, acquired enzyme
deficiency.
Thus, patients suffering or recovering from
such a disorder cannot drink or eat significant
amounts of dairy products or sucrose without
exacerbating the diarrhea
Hanadi Baeissa
Lactase Deficiency:

Intolerance to lactose, the sugar of milk, may be
attributable to a deficiency of lactase

The syndrome should not be confused with
intolerance to milk resulting from a sensitivity
to milk proteins, usually to the β-lactoglobulin

The signs and symptoms of lactose intolerance
are the same regardless of the cause
Hanadi Baeissa
Signs and Symptoms

These include abdominal cramps, diarrhea, and
flatulence

They are attributed to accumulation of lactose,
which is osmotically active, so that it hold water,
and to the fermentative action on the sugar of
the intestinal bacteria which produce gases and
other products that serve as intestinal irritants
Hanadi Baeissa
There are 3 types of lactase deficiency:
1. Inherited lactase deficiency:
 In this syndrome, which is relatively rare,
symptoms of intolerance develop very soon
after birth
 The feeding of a lactose-free diet results in
disappearance of the symptoms
 The occurrence of lactose in the urine is a
prominent feature of this syndrome, which
appears to be attributable to an effect of lactose
on the intestine
Hanadi Baeissa
2.



Secondary low-lactase activity:
Because digestion of lactose is limited even in
normal humans, intolerance to milk is not
uncommon as a consequence of intestinal
disease
Examples are tropical and non tropical (celiac)
spure,Crohn’s disease, kwashiorkor, colitis,
and gastroenteritis.
The disorder may be noted also after surgery
for peptic ulcer
Hanadi Baeissa
3.


Primary low-lactase activity:
This is a relatively common syndrome,
particularly among non white populations
Since intolerance to lactose was not a feature
of the early life of adults with this disorder, it is
presumed to represent a gradual decline in
activity of lactase in susceptible individual
Hanadi Baeissa
Dietary Management
Depends on individual condition
 Milk and its products should be avoided in
severe cases
 Labels should be read carefully
 Use available enzyme replacement
 Use milk substitutes and ice cream substitutes
when possible

Side Effects
A negative calcium balance may result,
requiring supplementation
 Some patients may be intolerant to milk
substitutes if it is high in available
carbohydrates, and continue to exhibit the
same symptoms

Sucrase Deficiency:



There is an inherited deficiency of the
disaccharidases sucrase and isomaltase
These 2 deficiencies coexist, because sucrase
and isomaltase occur together as a complex
enzyme
Symptoms occur in early childhood and are the
same as those described in lactase deficiency
Hanadi Baeissa
Dietary Management
Avoid sucrose containing products ( read label)
 Glucose and fructose can be used as
sweeteners
 Enzyme substitute is now available as an oral
solution, but not all patients can tolerate it
 Sucrose restricted diets can be limiting in some
micronutrients ,e.g. iron, folic acid, ascorbic
acid, niacin. Therefore, care must be taken to
avoid this

Disacchariduria:


An increase in the excretion of disaccharides
may be observed in some patients with
disaccharidase deficiencies
As much as 300 mg or more of disaccharides
may be excreted in the urine of these people
and in patients with intestinal damage e.g. sprue
Hanadi Baeissa
Diagnosis:


Identification of the specific enzyme deficiencies
can be obtained by performing oral tolerance
tests with the individual disaccharides
Measurement of hydrogen gas in the breath is a
reliable test for determining the amount of
ingested carbohydrate not absorbed by the body
but rather metabolized by the intestinal flora
Hanadi Baeissa
Mono saccharide
Malabsorption:


There is a congenital condition in which
glucose and galactose are absorbed only
slowly, owing to a defect in the carrier
mechanism
Because fructose is not absorbed via the
carrier, its absorption is normal
Hanadi Baeissa