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Man
Fru
Glu
Sugar
Mono sacch.
alcohol
Pentoses
Plant
Sources
Oligosacch.
Di sacch.
(lactose)
Disacch.
(sucrose, maltose
& Trehalose)
Animal Sources
Starch
NSP polysaccharides small amount
(not important)
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Figure 2. Percentage nutrient intake in three communities with different dietary intakes.
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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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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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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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 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
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Fructans : single glucose + fructose chain (3-50 residues depending on source)
•
Short chain in cereals
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Inulin in artichokes (35 residues)
•
Also found in onions, garlic, and asparagus.
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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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 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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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 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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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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 Heterogeneous, b ranched 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.
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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.
 U sed 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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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
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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:
1.
2.
3.
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:
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
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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.
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A.
B.
C.
D.
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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
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 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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White flour
Shortbread
White bread
White spaghetti
Banana
Biscuits
Potato biscuits
Haricot beans
Pearl barley
%RDS
49
56
94
52
39
47
18
41
%SDS
48
43
4
43
23
27
42
41
%RS
-
-
-
3
-
-
18
9
1
%RS
2
3
-
-
-
38
25
9
-
%RS
3 t
1
2
3
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.
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1
12
2

 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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 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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 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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Substrate
Carbohydrates
Resistant starch
Non-starch polysaccharides
Unabsorbed sugars, sugar alcohols
Oligosaccharides
Chitin and amino sugars
Nitrogenous substrates
Dietary protein
Pancreatic enzymes and secretions
Urea and nitrate
Other substrates
Mucin
8-40
8-18
2-10
2-6
1-2
3-9
4-6
0.5
2-3
Amount (g/day)
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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The extent of fermentation depends on the form and solubility of the substrate
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A.
B.
C.
e.g. soluble pectin degraded completely, insoluble wheat bran incomplete degradation
Steps in fermentation: 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
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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)
Polysaccharide
Starch
Arabinogalactan
Xylan
Pectin
Acetate Propionate
0.25 (50)
0.19 (50)
0.42 (82)
0.27 (84)
0.13 (22)
0.20 (42)
0.10 (15)
0.06 (14)
Butyrate
0.21 (29)
0.04 (8)
0.02 (3)
0.01 (2)
Table 5. Short-chain fatty acids (SCFA) produced in vitro by intestinal bacterial grown on different polysaccharide substrates. Molar ratios are
Total
0.59
0.43
0.54
0.34

 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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 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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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
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3.
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
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.
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 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 threecarbon compounds plus large volumes of CO2 and H2 gas, causing flatulence
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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.
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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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 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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 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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1.
There are 3 types of lactase deficiency:
Inherited lactase deficiency:
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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.
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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.
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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
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 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

 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

 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
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 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

 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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 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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 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
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