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Digestion & Absorption of Dietary Macronutrients & Fibre

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Carbohydrates

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Carbohydrates

Di sacch.(lactose)

Plant Sources Animal Sources

polysaccharides small amount

(not important)Starch NSP

Pentoses Disacch.( sucrose, maltose

&Trehalose)

Mono sacch.

Man GluFru

Oligosacch.

Sugar alcohol

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Figure 2. Percentage nutrient intake in three communities with different dietaryintakes.

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Figure 1. The principal carbohydrates in the human diet.

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Figure 3. Dietary Carbohydrates

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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

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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)

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Table 1. Examples of the variability in the fibre composition of different foods based on detailed gas-liquid chromatographic analysis

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Figure 1. The principal carbohydrates in the human diet.

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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

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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

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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

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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).

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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.

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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 .

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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

non-sweet & tasteless

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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

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Figure 4. Digestion of carbohydrates.

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Figure 5. Degradation of dietary glycogen by salivary or pancreatic α-amylase.

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Table 2. Di-and Oligosaccharidases of the Luminal Plasma Memebrane in the Small Intestine

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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

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Figure 6. Absorption of monosaccharides

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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

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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. The amount of peristalsis

2. The viscosity of the bolus passing

3. Enzymic activity

Note: Glu, dextrins & sol. Starch are absorbed at equal rates normally ie. digestion is not a limiting factor

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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.

Glucose level after a meal

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The Glycaemic Index (GI) Physiological measurement used to estimate the

relative rates of glucose absorption from various foods. Method of measuring GI is:

A. 50 g of CHO in test food is eaten B. Blood glucose is measured every 30 min for 3

hoursC. The area under the curve is calculatedD. 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

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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 dispersion of the starch chains, and easier digestion

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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

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%RDS%SDS%RS1%RS2%RS3

White flour4948-3t

Shortbread5643--1

White bread944--2

White spaghetti

52433-3

Banana Biscuits

3923-38t

Potato biscuits

4727-251

Haricot beans

184218912

Pearl barley41419-2

Table 3. In vitro digestibility of starch in a variety of foods. The values areexpressed as a percentage of the total starch present in the food.

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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)

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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

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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

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SubstrateAmount (g/day)

CarbohydratesResistant starchNon-starch polysaccharidesUnabsorbed sugars, sugar alcoholsOligosaccharidesChitin and amino sugars

8-408-182-102-61-2

Nitrogenous substratesDietary proteinPancreatic enzymes and

secretionsUrea and nitrate

3-94-60.5

Other substratesMucin

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).

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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. polymers are broken down into their constituent monomers

)glucose, galactose, arabinose, xylose and uronic acids(

B. sugars then are converted to pyruvate

C. various routes followed depending on the microbial species present and the nature of the available substrate

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Figure 7. Fermentation in the human colon (adapted from Cummings 1983).

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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

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SCFA produced (mg/mg polysaccharide)

PolysaccharideAcetatePropionateButyrateTotal

Starch0.25

(50)0.13 (22)0.21 (29)0.59

Arabinogalactan

0.19 (50)

0.20 (42)0.04 (8)0.43

Xylan0.42

(82)0.10 (15)0.02 (3)0.54

Pectin0.27

(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)

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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

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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.

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Physiological effects of undigested CHO

1. Faecal weights )Soluble fibre is readily fermented bacterial population, but Insoluble fibers directly weight, thus more important as a laxative (

2. 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

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- 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 fibers3. 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 low-density lipoprotein fraction, and is accompanied by decreases in the cholesterol content of the liver, aorta, and other tissues.

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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

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Figure 4. Digestion of carbohydrates.

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Table 2. Di-and Oligosaccharidases of the Luminal Plasma Memebrane in the Small Intestine

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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 three-carbon compounds plus large volumes of CO2 and H2 gas, causing flatulence

Abnormal lactose digestion

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Types of Defects1. 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

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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

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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

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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

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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

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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

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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

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

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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

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

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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

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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

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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