Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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been found in intestine, SGLT1. In humans the activ-

ity and expression of this transporter are maintained

by the presence of luminal nutrients, as suggested

by the brush border experiments in fed and fasted

animals. The low-affinity SGLT2 is only found in

kidney, and its kinetics can be explained by reducing

the Na

/glucose coupling from 2:1 to 1:1. SGLT1 is a

glycoprotein of 75 kDa with 14 membrane-spanning

regions, existing as a homotetramer in the membrane.

The single N-linked carbohydrate side chain is not

required for function. The relative specificity of the

transporter is the same as that previously character-

ized for the active membrane transport system,

i.e., d-glucose>a-methyl-d-glucose>d-galactose>3-

O-methyl-d-glucopyranose>>>l-glucose. It is clear

that SGLT1 accounts for all of the active glucose

transport, because mutations in this protein account

for the entire phenotype in patients with hereditary

glucose-galactose malabsorption.

0023 Glucose absorbed into the enterocyte is trans-

ported across the basolateral membrane by facilitated

diffusion mediated by GLUT2. This high-capacity

transporter has 12 membrane-spanning regions, and

can transport fructose as well as glucose. Although

glucose is metabolized by the enterocyte, the pre-

ferred energy substrates for this cell are amino acids,

preferentially glutamate, glutamine, and aspartate. In

the presence of amino acids, intestinal metabolism of

glucose is decreased. After exit from the cell, glucose

enters the portal vein, and is delivered to the liver and

peripheral tissues (mostly muscle), in which tissues

the glucose is extensively metabolized.

Fructose Absorption

0024 Fructose transport occurs by an Na

-independent,

saturable system of lower capacity than that for glu-

cose or galactose. The capacity for fructose absorp-

tion in humans is limited, although theoretical

estimates of absorption capacity are relatively high.

GLUT5 mediates all or most of fructose transport

across the apical membrane of enterocytes. Human

GLUT5 transports fructose alone, but the rat homo-

log recognizes both glucose and fructose. However,

absorption of fructose in humans can be inhibited by

the presence of glucose. Thus, it is possible that a

second apical fructose transporter exists. Unlike the

relatively wide tissue distribution in humans (Table 3),

rat GLUT5 is expressed largely in the small bowel,

kidney, and brain. Fructose is poorly metabolized in

the enterocyte, and is transported from the cell by

basolateral GLUT2, and in humans by basolateral

GLUT5 as well. Expression of GLUT5 is increased in

animals fed fructose. This adaptation accompanies

the increase in sucrase-isomaltase found after fructose

or sucrose feeding.

Short-chain Fatty Acid Absorption

0025Short-chain fatty acids are the major nutrients pro-

duced by bacterial fermentation. The usual starting

substrates are carbohydrates. In humans the fermen-

tation products are produced and absorbed in the

colon. Both small bowel and colonic mucosa readily

absorb unionized short-chain fatty acids. The trans-

porter responsible for this uptake is most likely a

member of the monocarboxylate-type transport pro-

teins, perhaps by the anion exchanger AE2 found in

apical membranes of intestinal mucosal cells. The

anion gradient across the apical membrane is butyr-

ate>bicarbonate>propionate>chloride. The capacity

of this transporter is much lower than that for

SGLT1, but is still sufficient to achieve some salvage

of malabsorbed carbohydrate.

0026Unlike hexoses in the small intestine, short-chain

fatty acids are partly metabolized in the colonic cells,

and appear to be a major nutrient source. Most of the

fatty acids are metabolized intracellularly to CO

Estimates of the contribution of short-chain fatty

acid metabolism to the basal metabolic requirement

vary from low (1–2% in the pig and 6–9% in

humans) to high (30–40% in the rabbit). The import-

ance of this pathway in humans increases in patients

with sugar malabsorption, when delivery of non-

absorbed sugar is increased to the colonic lumen.

See also: Carbohydrates: Classification and Properties;

Dietary Fiber: Properties and Sources; Fructose;

Glucose: Glucose Tolerance and the Glycemic

(Glycaemic) Index; Lactose; Starch: Structure,

Properties, and Determination; Resistant Starch;

Sucrose: Dietary Importance; Sugar: Refining of

Sugarbeet and Sugarcane

Further Reading

Alpers DH (1994) Digestion and absorption of carbohy-

drates and proteins. In: Johnson LR (ed.) Physiology of

the Gastrointestinal Tract, 3rd edn, pp. 1723–1749.

New York: Raven Press.

Klein S, Cohn SM and Alpers DH (1999) Gastrointestinal

function. In: Shils ME, Olson JA, Shike M and Ross AC

(eds) Modern Nutrition in Health and Disease, 9th edn,

pp. 605–629. Baltimore: Williams & Wilkinson.

Loo DDF, Zeuthen T, Chandy G and Wright EM (1996)

Cotransport of water by the Na

/glucose cotransporter.

Proceedings of the National Academy of Science of the

USA 93: 13367.

Levin RJ (1999) Carbohydrates. In: Shils ME, Olson JA,

Shike M and Ross AC (eds) Modern Nutrition in Health

and Disease, 9th edn, pp. 49–65. Baltimore: Williams &

Wilkinson.

Thomson AB and Wild G (1997) Adaptation of intestinal

nutrient transport in health and disease. Digestive

Diseases and Sciences 42: 453–469, 470–488.

886 CARBOHYDRATES/Digestion, Absorption, and Metabolism

Traber PG (1999) Carbohydrate assimilation. In: Yamada T

(ed.) Textbook of Gastroenterology, 3rd edn, pp. 404–

428. Philadelphia: Lippincott-Raven Press.

Wright EM and Loo DD (2000) Coupling between Na

sugar, and water transport across the intestine. Annals of

the New York Academy of Science 915: 54–66.

Requirements and Dietary

Importance

I Macdonald, University of London, London, UK

Introduction

0001 Dietary carbohydrates are the cheapest source of

energy for metabolism by the body. They are cheap

because they are produced by plants for energy me-

tabolism and storage and consequently can be har-

vested in most temperate and humid climates. To

produce a million calories for human consumption

requires 0.08 ha for sucrose, 0.16 ha for potatoes,

0.4 ha for wheat and 6.88 ha for cattle. Thus a very

important factor when considering requirement of

dietary carbohydrate lies largely in economic neces-

sity. Carbohydrates have long been the mainstay in

world dietaries, with sucrose and starches being the

major sources for human consumption. In regions

where per capita incomes are low and the balance

between food production and demand is close, con-

sumption of carbohydrates as cereals or starchy root

vegetables largely meets the need for energy intake. In

some underdeveloped countries carbohydrates supply

up to 90% of the dietary energy consumed and, of

course, consumption at this level gives rise to concern

about deficiencies of the other nutrients such as pro-

teins, minerals, and vitamins. In industrial countries

dietary carbohydrates provide about half the daily

energy intake.

0002 Although the total carbohydrate intake may be

similar between some developing and developed

countries, the nature of the carbohydrates eaten and

the proportion they contribute to the energy intake

may not be similar. Carbohydrates from staple foods

such as cereals and roots are composed mainly of

starch and may represent 85% of the carbohydrate

intake in developing countries but only 62% in afflu-

ent ones. The difference is largely made up by carbo-

hydrates from fruit, sucrose, corn syrups (mainly in

North America) and, to a lesser extent, lactose.

0003 Surveys in the 1980s have shown that the consump-

tion of sucrose is changing, tending to fall in the more

developed countries while rising in the other coun-

tries, and it has been suggested that the saturation

point occurs at about 160 g day

1

per person. In de-

veloping regions, starch consumption from cereals

and roots seems to be related to income, rising when

personal income rises. In developed regions, cereal

consumption, and therefore starch intake, has fallen,

especially in Japan and Eastern Europe, with little

change in North America and Australia. These

changes might also reflect income, for at higher

income levels food choice is not limited to purchasing

power and as income rises the proportion of energy

derived from carbohydrate tends to fall.

0004Complex carbohydrates, like other insoluble com-

ponents of the diet, have no taste, only ‘mouth feel,’

but carbohydrates with a comparatively smaller mo-

lecular weight are soluble and stimulate the taste buds

for sweetness. This property of sugars, and especially

sucrose, makes them an organoleptic requirement for

many, especially the young. It has been suggested that

this property of the simple carbohydrates is useful in

making palatable those foods which are nutritionally

desirable but may not be so pleasant to consume. (See

Cereals: Contribution to the Diet; Sucrose: Dietary

Importance.)

Metabolic Importance of Dietary

Carbohydrate

0005The major carbohydrates that are found in the body

tissues in humans are glycogen, glucose, fructose,

galactose, and lactose (in lactation).

0006Glycogen, the storage form of carbohydrate, is

found in the liver and muscle. Before being released

into the blood for transport to other regions of the

body, it has to be hydrolyzed to the monosaccharide,

glucose. (See Glycogen.)

0007Glucose is the common metabolic currency of

carbohydrate in the body and all cells are able to

metabolize glucose to carbon dioxide and water

with the consequent release of energy. However, glu-

cose is not only used as a readily available source of

energy but it can also be converted to glycogen or, if

consumed in excess, to fat for storage. Although glu-

cose can be utilized by all cells, it is only in a few

organs that glucose is essential, and these include the

brain and red cells of the blood. During pregnancy

and growth, glucose is essential for the formation of

cell constituents. (See Glucose: Function and Metab-

olism.)

0008Fructose, which largely comes from the sucrose in

the diet and also from honey, does not seem to have a

specific role in the body and the only site of produc-

tion of fructose in humans seems to be in the seminal

vesicles, where it is made from glucose.

CARBOHYDRATES/Requirements and Dietary Importance 887

0009 Galactose is, along with glucose, one of the mono-

saccharides in lactose; practically the only dietary

source of galactose is milk. Galactose is synthesized

from glucose in the lactating mammary gland. (See

Galactose.)

Metabolic Sources of Carbohydrate other

than Dietary Carbohydrate

0010 As glucose is essential to the biochemistry of the body

it is not surprising that there are sources of glucose

other than those in the diet. One of these is liver or

muscle glycogen but these stores of ‘animal starch’ are

very limited. After 24 h or so of complete starvation

the stores are empty; yet, as long as water is supplied,

a normal-weight person can survive complete starva-

tion for 50–60 days, so alternative sources of glucose

must exist.

0011 Another source of glucose in the body is from the

glycerol moiety of triglyceride. This part of the trigly-

ceride molecule forms about 10% of its molecular

weight, and when triglycerides are hydrolyzed the

released glycerol can be converted to glucose. An-

other source of glucose within the body is from the

so-called glucogenic amino acids, which can be me-

tabolized to glucose. (See Triglycerides: Structures

and Properties.)

0012 Although dietary carbohydrates can be converted

to fat in the body, fat cannot be converted to carbo-

hydrate.

Metabolic Requirements for Dietary

Carbohydrate

0013 Apart from the fact that a diet with little or no carbo-

hydrate would be most unpalatable, there is a meta-

bolic need for dietary carbohydrate. Total deprivation

of energy intake has been used in the treatment of

obesity and this has provided the opportunity to study

the effects of a zero intake of dietary carbohydrate. The

effects of diets which are high in protein and fat and

very low in carbohydrate have also been studied and

the striking consequence of a low or zero carbohydrate

intake is that the breakdown of fat in the body cannot

go to completion. The final end product of fat metab-

olism is then a two-carbon chain remnant, existing in

the blood as acetoacetic acid or b-hydroxy butyrate.

The classical breath odor in this condition, known as

ketosis, is due to acetone excretion by the lungs.

Clinically, ketosis occurs in uncontrolled diabetes mel-

litus and after 24 h or more of dietary carbohydrate

deprivation in otherwise healthy individuals. Thus

carbohydrate is needed in order that the catabolism of

fat can be completed to carbon dioxide and water. (See

Fats: Digestion, Absorption, and Transport.)

0014There are two disadvantages of the ketotic state in

an individual: (1) the judgment of such a person may

be impaired and it could therefore be unwise to

handle potentially dangerous machinery (e.g., a car)

while ketotic; (2) the ketone bodies excreted in the

breath and urine contain utilizable energy and thus

represent an energy loss to the body and diminishing

body stores. Production of ketone bodies in large

quantities, as in uncontrolled diabetes mellitus, can

lead to coma and death. Thus there is a need in all

individuals for a minimal daily intake of carbohy-

drate that can supply the amount of glucose necessary

to complete the breakdown of depot fat.

0015What is the minimum desirable intake of glucose or

its equivalent? Under normal circumstances the adult

brain needs about 140 g of glucose per day and the

red blood cells need another 40 g day

1

. If diet con-

tains no sugars or starch, about 130 g of glucose can

be provided endogenously from the catabolism of

protein and from the glycerol moiety of depot fat,

thus leaving a shortfall of approximately 50 g day

1

to be obtained from the diet. It is therefore possible to

state a minimum desirable intake of glucose or its

equivalent for adults. After several days in the ketotic

state the brain, a major consumer of glucose, adapts

and can use, to some extent, the energy present in the

ketone bodies, thus lessening the minimal daily re-

quirement for dietary carbohydrate.

Metabolic Knock-on Effects of Dietary

Carbohydrate

0016All dietary carbohydrates have to be broken down to

their constituent monosaccharides (glucose, fructose,

or galactose) before they can be absorbed from the

intestine but only glucose stimulates the release of

insulin, a hormone which not only accelerates the

cellular uptake of glucose but also facilitates the

uptake of amino acids. Insulin is, in general, an ana-

bolic hormone, so that the glucose provided by the

carbohydrate in the diet can have far-reaching effects

on the metabolism of other dietary constituents,

through its ability to bring about the release of insulin.

0017The amount of energy stored in the body as carbo-

hydrate is minute when compared with that stored as

fat or protein. The total quantity of carbohydrate in

liver, muscle, kidney, and other tissues, plus the glu-

cose that circulates in the blood, amounts to about

1800 kcal (7.56 MJ). However, it has been found that

the carbohydrate stored in the skeletal muscle can be

increased considerably by reducing the proportion of

fat in the diet and replacing it with carbohydrate. This

is of importance to those who compete in endurance

sports and has led to the expression ‘carbohydrate

loading.’

888 CARBOHYDRATES/Requirements and Dietary Importance

0018 Finally, there is what used to be called the protein-

sparing effect of dietary carbohydrate. When the

energy intake in the diet is below requirement, ad-

ministration of carbohydrate (which raises insulin

levels) reduces the breakdown of body protein,

whereas dietary fat under comparable circumstances

has a negligible effect on reducing protein break-

down. (See Protein: Interactions and Reactions In-

volved in Food Processing.)

See also: Cereals: Contribution to the Diet; Fats:

Digestion, Absorption, and Transport; Galactose;

Glycogen; Protein: Interactions and Reactions Involved

in Food Processing; Sucrose: Dietary Importance;

Triglycerides: Structures and Properties

Further Reading

British Nutrition Foundation (1987) Sugars and Syrups.

London: British Nutrition Foundation.

British Nutrition Foundation (1990) Complex Carbohy-

drates in Foods. London: Chapman and Hall.

Food and Agriculture Organization / World Health Organ-

ization (1998) Carbohydrates in Human Nutrition.

Food and Nutrition paper no. 66.

Metabolism of Sugars

I Macdonald, University of London, London, UK

Background

0001 There are only three sugars that are normally present

within the body, and these are the monosaccharides,

glucose, fructose, and galactose. Glucose is the prime

carbohydrate in the body; fructose is consumed

mainly with glucose as the disaccharide sucrose and

plays a relatively minor role; galactose is associated

with lactation and milk ingestion, and is, along with

glucose, one of the monosaccharides of the lactose

molecule. (See Fructose; Galactose; Glucose: Func-

tion and Metabolism.)

Glucose

0002 To produce energy, the glucose molecule must enter a

cell from the blood and there be converted to glucose

6-phosphate in the cytoplasm. This compound can

then be metabolized in a number of ways, depending

on the needs and versatility of the cell. It can (1) be

broken down to pyruvic acid/lactic acid, (2) go down

the pentose phosphate pathway, or (3) form glycogen

(Figure 1).

0003The most important way by which energy is re-

leased from the glucose molecule is by the splitting

of the molecule to form two molecules of pyruvic

acid (glycolysis). This end product of glycolysis

may then enter the tricarboxylic acid cycle inside

the mitochondria of the cell and be broken down

completely to carbon dioxide and water with the

release of energy.

0004When quantities of pyruvic acid and hydrogen

atoms become excessive, as in severe exercise,

these two products inhibit glycolysis and react

with each other to form lactic acid.

0005The pentose phosphate pathway accounts for as

much as 30% of the glucose metabolism in the

liver and more than this in the fat cells. This path-

way supplies reducing power for fat synthesis from

carbohydrate sources. Most enzymes involved in

carbohydrate metabolism require vitamin B

metabolites as essential co-factors.

0006When glucose is not immediately required for

energy, the extra glucose that continually enters

the cells is stored as glycogen or converted to fat.

Glucose is preferentially stored as glycogen, and

only when the cell approaches saturation with

glycogen is the additional glucose then converted

to fat. (See Glycogen.)

Fructose

0007Fructose in the bloodstream is utilized about twice as

fast as blood glucose; the liver and, to a lesser extent,

the kidney and small intestine are the main sites of

Glucose

Glycogen

Glycolysis

Lactate Pyruvate

Acetyl-CoA

Tricarboxylic

acid cycle

Glucose 6-phosphate

Pentose phosphate

pathway

+ H

fig0001Figure 1 Metabolic options for glucose metabolism. Repro-

duced from Carbohydrates: Metabolism of Sugars, Encyclopaedia

of Food Science, Food Technology and Nutrition, Macrae R,

Robinson RK and Sadler MJ (eds), 1993, Academic Press.

CARBOHYDRATES/Metabolism of Sugars 889

fructose metabolism. The utilization of fructose by

other peripheral tissues seems to be negligible. The

first step in the metabolism of fructose is the formation

of fructose 1-phosphate, which then splits to form two

3-carbon molecules, namely glyceraldehyde and dihy-

droxyacetone phosphate. As shown in Figure 2,these

trioses can form pyruvate, glycerol, and, in the case of

dihydroxyacetone phosphate, glucose.

0008 Fructose is mainly converted to glucose and lactic

acid, with perhaps up to 3% of the fructose being

converted to triglycerides; ketone bodies; glycerol and

sorbitol are minor end products.

Metabolism of Sugars in the Liver

Glucose

0009 In the liver, glucose has no difficulty in crossing the

cell wall; the main factors influencing the rate of entry

are the concentration of glucose in the blood and the

ability of the enzymes in the cell to dispose of the

glucose. Much of the glucose that is absorbed from

the intestine never reaches the peripheral circulation,

as it is taken up in the first pass through the liver.

0010 The liver is also capable of releasing glucose into

the blood, either as a result of the breakdown of

glycogen or protein, or as a result of synthesis from

glycerol. The balance of input and output of glucose

by the liver is entirely controlled by hormones, acting

via the enzymes within cells:

0011 Insulin, which is produced by the b cells of the

pancreas, accelerates glycogen formation. In other

cells in the body, insulin is solely concerned with

the transfer of glucose into the cell, but in the liver,

insulin influences the synthesis of enzymes.

0012 Glucagon is produced by the a cells of the pancreas,

and causes a rapid breakdown of liver glycogen.

0013 Adrenaline, like glucagon, stimulates the break-

down of glycogen, but, unlike glucagon, it is able

to do this in muscles as well; it does not stimulate

insulin release.

0014 Glucocorticoids are produced by the adrenal gland.

These hormones maintain and aid the formation of

glycogen. (See Hormones: Adrenal Hormones.)

Fructose

0015In man, most of the fructose absorbed from the intes-

tine is taken up by the liver, hence the low blood levels

of fructose after its ingestion. Compared with glucose,

fructose has a greater ability to form lactate, but,

unlike glucose, fructose administration leads to an

increase in blood uric acid levels and, for this reason,

has been used as a screening test for gout. Also in the

liver, fructose is converted to the lipid triglyceride to a

greater extent than is glucose; again, unlike glucose,

whether given orally or intravenously, fructose speeds

up the metabolism of ethanol by the liver.

Sugars and Muscle

0016The muscle mass, because of its size and sheer need,

plays a principal role in carbohydrate metabolism.

The uptake of glucose by muscle cells is insulin-

sensitive, but the metabolism of fructose in the cell

is probably minimal, and only the pyruvate and lac-

tate formed from fructose in the liver would be of any

support to the muscle cell.

Sugars and Depot Fat

0017It has been known since 1852 that dietary carbohydrate

can be converted to depot fat, and this would appear to

be in conflict (bearing in mind the current obesity

epidemic) with the current dietary advice from several

international authorities to reduce the fat intake and

replace it by carbohydrate. However, when energy

balance is in equilibrium, high-carbohydrate/low-fat

diets do not result in appreciable de novo lipogenesis.

0018However, a high-carbohydrate diet can alter the

blood lipid pattern in a direction that may be con-

sidered undesirable in terms of cardiovascular disease,

but this change may be offset by the weight loss that

usually occurs on switching to a high-carbohydrate

diet.

0019There is no metabolic reason why sugars should

be different in their ability to be converted to depot

fat. The FAO/WHO 1998 consultation on Dietary

Guidelines proposes no specific limit for sugar con-

sumption, since any putative relationship of sugar

consumption to obesity is offset by the inverse

relationship between sugar and fat intake.

0020Fructose can enter the adipocyte, but its rate of

transport is slow, and only at high concentrations of

fructose do significant amounts enter the adipocyte.

Sugars and the Brain

0021The brain seems to be entirely dependent on glucose

and oxygen, and a reduction in the supply of either

will soon lead to irreversible damage. The brain

Glyceraldehyde Fructose

Pyruvate Glycerol Pyruvate GlycerolGlucose

Dihydroxyacetone

phosphate

fig0002 Figure 2 Metabolism of fructose. Reproduced from Carbo

hydrates: Metabolism of Sugars, Encyclopaedia of Food Science,

Food Technology and Nutrition, Macrae R, Robinson RK and Sadler

MJ (eds), 1993, Academic Press.

890 CARBOHYDRATES/Metabolism of Sugars

removes a fixed amount of glucose per unit time,

irrespective of the blood concentration of glucose,

and its uptake is not dependent on insulin. There

has been a suggestion that the insulin release

following glucose ingestion increases the level of sero-

tonin in the cerebral tissue and that this compound

diminishes the sensation of pain and produces a feel-

ing of well-being.

Sugars and the Metabolic Rate

0022 The increase in metabolic rate after the ingestion of

various sugars is greater after sucrose or a mixture

of fructose and glucose than after glucose alone,

suggesting that the body ‘handles’ fructose more

efficiently than glucose.

Sugars and the Fetus and Neonate

0023 As lipid does not cross the placenta to any great

extent, the fat present in the infant at birth must

have been synthesized in the fetus from glucose or

amino acid. Changes in the maternal blood glucose

level are quickly reflected in the fetal blood.

0024 In contrast to glucose, fructose cannot cross the

placenta, although some animal species (not humans)

can transform glucose to fructose in the placenta.

0025 In the neonate, the sole dietary source of carbo-

hydrate is the lactose in milk, but no specific role

for lactose has been identified.

Factors Affecting the Metabolic Response

to Dietary Sugars

Sex of the Consumer

0026 The increase in blood triglycerides seen in men after a

diet high in fructose is not seen in young women but

is found in postmenopausal women. Although both

sexes increase hepatic lipogenesis after fructose inges-

tion, it is possible that premenopausal women are

able to remove the blood triglycerides more rapidly.

Type of Fat Accompanying the Carbohydrate

0027 A synergistic effect of sucrose and animal fat on blood

triglycerides has been shown, and the raised levels

found after a diet high in sucrose are considerably

reduced by polyunsaturated fat accompanying the

sucrose.

Dietary Protein

0028 The recovery of serum albumin after protein deficiency

seems to be slower with sucrose in the diet than with

starch, and the interplay of the metabolism of sugars

and protein becomes more interesting when it is

appreciated that the amino acids arginine and leucine

stimulate the release of insulin. (See Protein:Inter-

actions and Reactions Involved in Food Processing.)

‘Sensitivity’ of the Consumer

0029The extent to which sugars are converted to lipids,

especially triglycerides, seems to vary between indi-

viduals. Those persons whose level of triglycerides in

fasted blood is high, and who may be more prone to

coronary heart disease, have a greater increase in

these triglycerides after consuming carbohydrates

than persons with normal lipid levels.

Species

0030There are not only within-species differences in meta-

bolic responses to sugars but also more marked dif-

ferences between the species. For example, rats can

absorb fructose from the intestine very rapidly, and

this will affect the metabolic handling of fructose

when compared with humans.

See also: Fructose; Galactose; Glucose: Function and

Metabolism; Glycogen; Hormones: Adrenal Hormones;

Protein: Interactions and Reactions Involved in Food

Processing

Further Reading

Dickens F, Randle PJ and Whelan WJ (1968) Carbohydrate

Metabolism and its Disorders, vols. 1–3. London:

Academic Press.

FAO/ WHO (1998) Carbohydrates in Human Nutrition.

Rome: FAO/WHO.

Macdonald I and Vrana A (eds) (1986) Metabolic effects of

dietary carbohydrates. Progress in Biochemical Pharma-

cology 21.

Reiser S and Hallfrisch J (1987) Metabolic Effects of Diet-

ary Fructose. Boca Raton, FL: CRC Press.

Determination

D A T Southgate, Norwich, Norfolk, UK

Introduction

0001The carbohydrates in foods are a heterogeneous

group of substances ranging from monosaccharides,

such as glucose and fructose, through to the complex

polysaccharides found in the matrix of the plant cell

walls in foods. In many cases the carbohydrate species

may be limited and typical of the food or group of

foods, such as lactose in milk and milk products, but

CARBOHYDRATES/Determination 891

many foods contain complex mixtures which present

many problems for the analyst.

0002 As in so many aspects of food analysis, the choice

of method is very much decided by the purpose of the

analysis and the uses for which the analytical infor-

mation is required. Thus, for quality control purposes

a relatively simple empirical method may be ad-

equate, whereas for nutritional research purposes

the current trend is a requirement for relatively com-

plete and chemically specific data.

0003 The food matrix itself is frequently a deciding

factor in the choice of method, as this will determine

the extraction procedures required and the proced-

ures needed to free the extracts of interfering mater-

ials. Thus many analytical procedures have been

devised that are specific for a particular group of

foods and the carbohydrates they contain.

The Carbohydrates in Foods

0004 It is possible to classify the carbohydrates in foods

according to a number of different principles. Table 1

illustrates a classification based primarily on struc-

tural terms which is linked to the physiological be-

havior of the carbohydrates once consumed: this is

the most useful classification nutritionally.

0005Nutritionally it is convenient to consider the carbo-

hydrates in foods as falling into three major groups:

0006Sugars, including mono- and disaccharides.

0007Oligosaccharides with three to nine monosacchar-

ide units.

0008Polysaccharides with 10 or more monosaccharide

units.

0009Oligosaccharides with three to nine monosacchar-

ide units fall into two subgroups: the malto series,

which are hydrolyzed by the brush border a-glucosi-

dases, and the raffino and fructo series, which are

poorly absorbed in the small intestine but are used

as substrates by the microflora of the large intestine.

The term short-chain polysaccharides has been sug-

gested for this group because analytically it is difficult

to separate some lower polysaccharides from conven-

tional oligosaccharides.

0010Polysaccharides with 10 or more (usually very

many more) monosaccharide units also fall into

two subgroups: first, the a-glucans principally, the

starches, but also including glycogen, and second,

the nonstarch polysaccharides (NSP), which include

a wide range of structural types of polysaccharides

derived from plant cell wall materials in foods and a

range of polysaccharide food additives.

tbl0001 Table 1 The major carbohydrates in foods

Number of

monosaccharide

units

Class Major examples Physiological characteristics

1 Monosaccharides Glucose Absorbed directly in the small intestine

Fructose

2 Disaccharides Sucrose Hydrolyzed in the brush border by specific disaccharidases

Lactose

Maltose

3–9 Oligosaccharides Malto- Hydrolyzed in the brush border

Fructo- Poorly absorbed from the small intestine

Raffinose Fermented by the microflora in the large intestine

Stachyose

Verbascose

< 9 Polysaccharides

Starches Hydrolyzed by gastric and pancreatic amylases and by

brush border enzymes

Amylose

Amylopectin

Nonstarch polysaccharides Not hydrolyzed in small intestine, fermented by the

microflora in the large intestine

Cell wall components Cellulose

Noncellulosic

polysaccharides

Polysaccharide food additives Gums

Mucilages

Algal

Polysaccharides

Modified starches,

pectins, etc.

892 CARBOHYDRATES/Determination

0011 Table 1 also illustrates the task facing the analyst

asked to measure all the carbohydrates in foods, even

where polysaccharides are grouped as suggested

above. It also demonstrates the need for the analyst

to consider the food matrix under examination be-

cause this will often simplify the procedures needed.

0012 During the 19th century, food chemistry was in the

early stages of development and the inadequacy of

understanding carbohydrate chemistry led to the

adoption of the so-called proximate method of food

analysis. In this method, carbohydrates were deter-

mined by difference. Moisture, protein (from total

nitrogen content), fat (by simple lipid solvent extrac-

tion) and ash (by incineration) were measured

directly and deducted from 100% to give a value for

carbohydrate. Although this method has effectively

been superseded by the growth in knowledge and the

developments in analytical chemistry, it is still in wide

use and much food composition data are still based

on this type of analysis.

Analytical Characteristics of

Carbohydrates

0013 The carbohydrates as a class, despite the wide range

of species occurring in foods, present the analyst with

a limited range of reactions that can be used analytic-

ally. As a class they are all polyhydric alcohols which

undergo substitution reactions. These are very valu-

able in structural analysis but less useful analytically.

Physical Methods

0014 These were developed within the sugar-processing

industry, where analysts were primarily concerned

with the analysis of solutions of sucrose and the prod-

ucts of its hydrolysis, glucose and fructose. The sim-

plest methods were based on the very high solubility

of sucrose in water and involved the measurement of

specific gravity or refractive indices of the solutions

which are very nearly linearly related to sucrose con-

centration. These methods are still in use for the

analysis of syrups. The second method made use of

the optical activity of the sugars: polarimetry pro-

vides a quick and reliable index of the proportions

of sucrose and glucose and fructose in an aqueous

solution.

Chemical Methods

0015 The monosaccharides have aldehyde or keto func-

tional groups which have powerful reducing proper-

ties by virtue of their ability to form enediol

arrangements. These form the basis of a series of

reductiometric procedures which are widely used.

The Munson-Walker and Lane and Eynon procedures

have formed the basic techniques for analysis in the

sugar industry. These reductiometric techniques are

highly empirical and require close attention to analyt-

ical protocols for consistent and accurate perform-

ance. Reduction of ferricyanide has also provided

the basis for colorimetric procedures using this type

of technique.

0016Nonreducing disaccharides such as sucrose require

hydrolysis before the reducing sugar methods can be

used and polysaccharides such as starch have also

been measured as monosaccharides after hydrolysis.

Each monosaccharide has, however, slightly different

reducing strengths and the analyst concerned with

mixtures of reducing sugars has to make adjustments

for the composition of the mixture.

0017The polyhydric structure of the carbohydrate also

renders them susceptible to dehydration and the for-

mation of furan structures in strong acids, and these

react with a number of chromogenic phenolic sub-

stances to form colors which are suitable for colori-

metric measurement. These reactions tend to be

relatively unspecific and careful control of conditions

is required for satisfactory measurements.

Biochemical Methods

0018The sugars (monosaccharides and disaccharides) are

involved in a series of biochemical reactions with

highly specific enzyme systems. Most of these enzymes

are available in a highly purified state and can be

coupled with NADH or NADPH to provide a very

specific and highly sensitive method for the measure-

ment of specific sugars in complex mixtures. The clin-

ical measurement of glucose uses an enzymatic method

coupled with a colorimetric method. However, in

many situations the development of chromatographic

procedures has displaced these elegant methods.

Chromatographic Methods

0019The difficulty of precisely quantifying mixtures of

sugars provided a major stimulus to the search of

separation procedures. Paper chromatography and

later thin-layer chromatography provided the first

effective qualitative and semiquantitative analytical

methods. Conventional ion-exchange techniques

were difficult to apply, although some success was

achieved in separating the weak borate complexes.

The nonvolatile nature of sugars slowed the applica-

tion of gas liquid chromatography until trimethyl silyl

derivatives were developed which gave good separ-

ations. At present most analyses are based on the use

of alditol acetate derivatives where sugars are reduced

to alditols with borohydride and then acetylated.

0020Gas liquid chromatographic techniques are widely

used, particularly in analysis of polysaccharides in

plant cell walls and in the measurement of NSP. The

derivatization stages are time-consuming and not

CARBOHYDRATES/Determination 893

completely specific because fructose gives the same

alditol as glucose (and partially as mannose).

0021 High-performance liquid chromatography (HPLC)

removes the need for the preparation of derivatives

and provides separation of all the monosaccharides,

disaccharides, and oligosaccharides. Initially the

carbohydrate columns were rather erratic in perform-

ance but now consistent separations are usual. Detec-

tion systems initially used the refractive index but

these were not very sensitive and pulsed ampero-

metric systems have been introduced which have

overcome this limitation.

Extraction and Clean-up

0022 As can be seen from the above, a selection of methods

for the end analyses of carbohydrates are used, most

of which are designed to measure monosaccharides.

Virtually all analytical methods for carbohydrate in-

volve the preparation of an extract free from interfer-

ing substances.

0023 The extraction and clean-up procedures are usually

food-specific because this simplifies the procedure

that needs to be used. This is especially important

when the analysis is being carried out for quality

control purposes, but applies across the whole range

of purposes for which carbohydrates are measured.

0024 Thus, in the sugar-processing industry the samples

are often in aqueous solution and the interfering

substances can be removed using precipitation with

metal salts. Analogously, milks and many milk prod-

ucts only require the removal of proteins to provide a

suitable extract for sugar analysis.

0025 Foods which contain more complex mixtures of

carbohydrates or when analyses of the various carbo-

hydrate species are required need more detailed

extraction and fractionation schemes. A large number

of schemes have been, and are being, used, especially

where the individual polysaccharides are being separ-

ated and measured but this article will focus on the

principles of the most widely used approaches.

0026 The major stages are illustrated in Figure 1.

0027 Sample preparation As with all analyses, this is a

critical stage. Sugars are sensitive to heat and poly-

mers containing furanoside linkages are susceptible to

acid hydrolysis under mild conditions. This means

that the preparation of samples needs to minimize

any possible changes due to heat and acid. Very acid

samples should be neutralized before drying or

extraction; freeze-drying is a suitably mild drying

procedure. The sample also needs to be finely divided

and care must be taken to prevent fractionation of the

sample into different sizes of particles.

0028 High-fat (>10%) foods need to be extracted with a

lipid solvent to assist in later extraction procedures.

0029Extraction with Aqueous Alcohol The most

common extractant is hot 80%v/v ethanol, but iso-

propanol has also been used, as has 85%v/v methanol.

This provides a very good separation of polysacchar-

ides from the other groups, but some lower polysac-

charides such as inulin are incompletely precipitated

by ethanol of this strength and some arabinans are

also partially soluble.

0030Use of Supernatant This is usually suitable for many

analytical procedures after dilution or removal of the

ethanol by rotary evaporation. Reductiometric

methods may call for clean-up with metal salts and

colorimetric methods may require removal of

strongly colored pigments using similar procedures.

Dilution prior to chromatographic separation using

HPLC is the preferred method for analyzing mixture

of sugars and oligosaccharides. Table 2 presents the

range of major options, which depend on the required

objectives of the analysis.

0031Analysis of Total Carbohydrate and Sugars The

colorimetric reactions with phenol/sulfuric and

anthrone/sulfuric are general-purpose methods

which give reasonable estimates of total sugars pre-

sent and which, with suitable standard mixtures, can

give acceptable accuracy.

0032Many of the classical reductiometric techniques

depend for their accuracy on the existence of standard

tables giving the reduction equivalents for a wide

range of mixtures of glucose and fructose and of

invert sugar and sucrose.

0033The specific enzyme procedures are based around

reactions of the type illustrated below:

0034The amount of NADPH formed is proportional to

the glucose present and can be measured by measur-

ing in the ultraviolet range at 340 and 360 nm. Any

fructose-6-phosphate produced by the hexokinase is

converted to glucose-6-phosphate with phosphoglu-

cose isomerase, which leads to formation of more

NADPH, permitting the analysis of a mixture.

Table 3 lists the enzymes used for the different carbo-

hydrate species.

0035Oligosaccharides The ethanolic extracts contain the

oligosaccharides and some polysaccharides with

lower degrees of polymerization such as inulin.

These will measure as total sugars with the colorimet-

ric reactions in strong acid but the choice of another

Glucose + ATP Glucose-6-phosphate + ADP

Hexokinase

Glucose-6-phosphate + NADP- 6-phosphogluconate

+ NADPH

Glucose-6-phosphate dehydrogenase

894 CARBOHYDRATES/Determination

tbl0002 Table 2 Options for the analysis of the ethanolic extract

Analytical objective Type of method Choice of method Limitations

Total sugars Colorimetric Phenol/sulfuric Color yield depends on sugars

present

Total sugars Reductiometric/colorimetric Ferricyanide Color yield depends on sugars

present

Hexoses Colorimetric Anthrone Color yield depends on sugars

present

Reducing sugars Reductiometric/volumetric Lane and Eynon; Munson-Walker Reduction depends on

ratios of sugars present

Copper salts in alkaline solution

Sucrose in invert sugar

solutions

Reductiometric before and

after inversion

Copper salts in alkaline solution Reduction depends on

ratio of invert sugar to sucrose

Individual sugars in

mixture

Specific enzymatic

procedures

Specific enzymes coupled with

NADH and ultraviolet measurements

Availability of enzymes

Individual sugars in

mixture

HPLC Using either refractive index or

pulsed amperometric detectors

Capital cost of equipment

In most cases, removal of ethanol before analysis is necessary.

HPLC, high-performance liquid chromatography.

Food sample

(Freeze-dried and finely ground)

If high in fat, extract with lipid solvent and dry

Extract with l80% v/v ethanol

Supernatant

Residue

Sugars, oligosaccharides

(plus pigments, organic acids, etc.)

Polysaccharides

(plus proteins, lignin etc.)

Analyze carbohydrates

Sugars

and

oligosaccharides

(short-chain carbohydrates)

Treat with DMSO

enzymatic hydrolysis

with mixture of amylolytic enzymes

Precipitate in 80%v/v ethanol

Measure glucose

Starch

Weigh

Measure protein (total N) and ash

Deduct from residue

Total dietary fiber

Residue

Hydrolyze NSP

Measure monosaccharides

and uronic acids

Nonstarch polysaccharides

fig0001 Figure 1 Typical scheme of fractionation used in the analysis of the major carbohydrates in foods. DMSO, dimethyl sulfoxide; NSP,

nonstarch polysaccharides.

CARBOHYDRATES/Determination 895