CARBOHYDRATES: NUTRITIONAL AND HEALTH...

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CARBOHYDRATES:NUTRITIONALAND HEALTHASPECTS

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ABOUT ILSI / ILSI EUROPE

The International Life Sciences Institute (ILSI) is a nonprofit, worldwide foundation established in 1978 to advance the understanding ofscientific issues relating to nutrition, food safety, toxicology, risk assessment, and the environment. By bringing together scientists fromacademia, government, industry, and the public sector, ILSI seeks a balanced approach to solving problems of common concern for the well-being of the general public. ILSI is headquartered in Washington, DC, USA. Branches include Argentina, Brazil, Europe, India, Japan, Korea,Mexico, North Africa and Gulf Region, North America, North Andean, South Africa, South Andean, Southeast Asia Region, the focal point inChina, and the ILSI Health and Environmental Sciences Institute (HESI). ILSI is affiliated with the World Health Organization as a non-governmental organisation (NGO) and has specialised consultative status with the Food and Agriculture Organization of the United Nations.

ILSI Europe was established in 1986 to identify and evaluate scientific issues related to the above topics through symposia, workshops,expert groups, and resulting publications. The aim is to advance the understanding and resolution of scientific issues in these areas. ILSIEurope is funded primarily by its industry members.

This publication is made possible by support of the ILSI Europe Dietary Carbohydrate Task Force, which is under the umbrella of the Boardof Directors of ILSI Europe. ILSI policy mandates that the ILSI and ILSI branch Boards of Directors must be composed of at least 50% publicsector scientists; the remaining directors represent ILSI’s member companies. Listed hereunder are the ILSI Europe Board of Directors andthe ILSI Europe Dietary Carbohydrate Task Force members.

ILSI Europe Board of Directors members

Prof. N.-G. Asp, Swedish Nutrition Foundation (S)Prof. P.A. Biacs, Bay Zoltán Foundation for Applied Research (H)Prof. J.W. Bridges, University of Surrey (UK)Prof. G. Eisenbrand, University of Kaiserslautern (D)Prof. M.J. Gibney, University of Dublin (IRL)Prof. A. Grynberg, National Institute for Agricultural Research (F)Dr. M.E. Knowles, Coca-Cola (B)Dr. I. Knudsen, Danish Veterinary and Food Administration (DK)Prof. R. Kroes, IRAS – Utrecht University (NL)Dr. G. Malgarini, Ferrero Group (B)Mr. J.W. Mason, Frito Lay Europe (UK)

Dr. D.J.G. Müller, Procter & Gamble European Service GmbH (D)Dr. J. O’Brien, Danone Vitapole (F)Prof. L. Serra Majem, University of Las Palmas de

Gran Canaria (E)Drs. P.J. Sträter, Südzucker AG Mannheim/Ochsenfurt (D)Prof. V. Tutelyan, National Nutrition Institute (RUS)Prof. P. van Bladeren, Nestlé Research Center (CH)Prof. W.M.J. van Gelder, Royal Numico (NL)Drs. P.M. Verschuren, Unilever Health Institute (NL)Dr. J. Wills, Masterfoods (UK)

ILSI Europe Dietary Carbohydrate Task Force member companies

Cerestar R&D Centre (B)Coca-Cola Great Britain & Ireland (UK)Danisco Sweeteners (UK)Danone Vitapole (F)Kellogg's Company of Great Britain Ltd. (UK)Masterfoods (F)Raffinerie Tirlemontoise – Orafti (B)Südzucker AG Mannheim/Ochsenfurt (D)Unilever Health Institute (NL)

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ILSI Europe

by Juliet Gray

CARBOHYDRATES:NUTRITIONAL AND

HEALTH ASPECTS

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© 2003 International Life Sciences Institute

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ISBN 1-57881-146-5

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CONTENTS

FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

CARBOHYDRATES IN FOODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

EFFECTS OF COOKING AND TECHNOLOGY ON CARBOHYDRATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

CARBOHYDRATE INTAKES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

CARBOHYDRATE DIGESTION AND ABSORPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

METABOLISM OF ABSORBED CARBOHYDRATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

CARBOHYDRATES IN HEALTH AND DISEASE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Author: Juliet Gray (UK)Scientific Editor: Arne Astrup, Royal Veterinary & Agricultural University (DK)

Scientific Referees: Cor van Loveren, Academic Centre for Dentistry Amsterdam (NL), Ellen Blaak, Maastricht University (NL), Christine Cherbut, National Institute for Agricultural Research (F),

Gabriele Riccardi, University of Naples (I)Concise Monograph Series Editor: Ron Walker, University of Surrey (UK)

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The production of this Concise Monograph wasstimulated by the publication in 1997 of the JointFAO/WHO Expert Consultation on “Carbohydrates inHuman Nutrition”, recommending the consideration ofnew terminology based on the physiological function ofcarbohydrates. Topics addressed are the classification ofcarbohydrates, carbohydrate intake, digestion andmetabolism, as well as the health effects of carbo-hydrates: control of body weight, physical activity,intestinal physiology and tolerance, disease relation-ships, oral health, cognitive function and immunesystem effects.

Further details on sugars may still be found in the ILSIEurope Concise Monograph on “Nutritional and HealthAspects of Sugars – Evaluation of New Findings”.

Janet LambertMasterfoods

FOREWORD

Over the past twenty to thirty years, knowledge of thephysiological roles of different types of carbohydratesand their involvement in health and disease hasdeveloped considerably and has challenged many long-held beliefs about sugars, starches and dietary fibre.Carbohydrates comprise a great variety of differentstructures, which in turn determine a wide range ofdifferent physiological effects in the human body.

This booklet, “Carbohydrates: Nutritional and HealthAspects”, reviews the latest scientific knowledge andunderstanding of the nutritional and health aspects ofcarbohydrates. It updates and supersedes the ILSIEurope Concise Monograph entitled “Starches andSugars: A Comparison of their Metabolism in Man” whichwas published in 1991.

Carbohydrates: Nutritional and Health Aspects 1

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CARBOHYDRATES IN FOODS

As the term implies, carbohydrates are based on theelements carbon, hydrogen, and oxygen. However,carbohydrates in foods comprise a great variety ofstructures, which in turn determine a wide range ofphysiological effects in the human body. Carbohydratescan be classified in several ways, either according to theirchemical structures or to their physiological effects.

Structural classification ofcarbohydratesA report issued by the Food and AgricultureOrganization and the World Health Organization(FAO/WHO, 1998) suggests that carbohydrates shouldbe classified primarily by molecular size, according tothe degree of polymerisation (DP), i.e. the number ofmonosaccharide units. In this classification, carbo-hydrates are divided into sugars, oligosaccharides,polysaccharides, and hydrogenated carbohydrates(polyols) (Table 1). Each group can be divided intovarious sub-groups, based on the number andcomposition of monosaccharide units.

Sugars comprise monosaccharides and disaccharides(Figure 1). The most commonly occurring monosaccha-rides are glucose (dextrose) and fructose, and the mostcommon disaccharides are sucrose and lactose.

Glucose and fructose are present in varying amounts inhoney, maple syrup, fruits, and vegetables. They arealso present in processed foods, such as soft drinks,confectionery, and bakery products, that contain starchhydrolysis and conversion products such as corn syrupsor high-fructose corn syrups. Maltodextrins, which areused as food product ingredients, may contain smallamounts of glucose.

Sucrose (composed of glucose and fructose) is the majordisaccharide in most diets, and the term “sugar” isgenerally used to describe purified sucrose. Sucroseoccurs in fruits and vegetables, honey, and maple syrup.Purified sucrose to be used as a food ingredient isderived from sugar cane and sugar beet. Sucrose is animportant ingredient in baked goods, desserts, icecream, confectionery, and soft drinks.

Lactose (milk sugar, comprising glucose and galactose)occurs in milk and milk products. It is also found infood products that contain dairy products among theiringredients, including various baked goods, and infoods that use whey as an ingredient, such as sausages.

Hydrogenated carbohydrates (polyols), such as sorbitoland xylitol (Figure 1), mannitol, maltitol, and erythritol,are the chemically reduced forms of sugars. They retainsome sweetness, but they behave quite differently inphysiological terms. Although some occur in smallamounts in fruits, they are mainly used as bulksweeteners in food products because of theirphysicochemical properties and relative sweetness.Polyols, in particular mono- and disaccharide types, arenon-cariogenic and have a low energy content and lowglycaemic response. They are used in products such aschewing gum and confectionery.

Oligosaccharides occur widely in small quantities inplant foods and in food products. In some vegetables theamounts of undigestible oligosaccharides, such as thetrisaccharide raffinose, the tetrasaccharide stachyose,and the pentasaccharide verbascose, may exceed theamounts of other simple sugars. Fructooligosaccharides(FOS) occur in cereals, such as wheat and rye, in variousvegetables, including onions, garlic, asparagus, chicory,and jerusalem artichokes, and in bananas and honey.Recent market developments are galactooligo-saccharides and condensed oligosaccharides.



4 Concise Monograph Series

Although previously overlooked, such non-digestibleoligosaccharides are now recognised as having particularphysiological significance. Digestible maltooligo-saccharides are derived from starch hydrolysis and arepresent in glucose syrups used as food ingredients.

Polysaccharides are sub-divided into starches and thenon-starch polysaccharides. They include polydextrosesand inulin, polymers of glucose and fructoserespectively, which are used as bulking agents and assucrose replacements in food products.

Starch is the main food reserve in plants andconsequently the major carbohydrate in the human diet.It is a mixture of two large polymers: amylose, whichcomprises mainly linear chains, and amylopectin,which is a highly branched polymer with a highermolecular weight (Figure 2).

Starch is produced during photosynthesis and stored aspartially crystalline granules in specific locations, suchas tubers, grains, and kernels. The shape and size of thegranules and their physical characteristics, notably the

TABLE 1

CLASS (DP*) AND SUBGROUP

Sugars (1–2)

Monosaccharides

Disaccharides

Oligosaccharides (3–9)

Maltooligosaccharides

Other oligosaccharides

Polysaccharides (>9)

Starch

Non-starch polysaccharides

Hydrogenated carbohydrates (polyols)

Monosaccharide type

Disaccharide type

Oligosaccharide type

Polysaccharide type

FOOD APPLICATION

Glucose, galactose, fructose, tagatose

Sucrose, lactose, trehalose, maltose, isomaltulose

Maltodextrins

Raffinose, stachyose, fructooligosaccharides, galactooligosaccharides

Amylose, amylopectin, modified starches

Cellulose, hemicelluloses (e.g. galactans, arabinoxylans), pectins, inulin, hydrocolloids (e.g. guar)

Sorbitol, mannitol, xylitol, erythritol

Isomalt, lactitol, maltitol

Maltitol syrups, hydrogenated starch hydrolysates

Polydextrose

A possible structural classification of the major dietary carbohydrates

*DP=Degree of polymerizationSource: Adapted from FAO/WHO Expert Consultation (1998). Carbohydrates in human nutrition.



FIGURE 1Structures of some dietary carbohydrates

MONOSACCHARIDES

DISACCHARIDES

HYDROGENATED (MODIFIED) CARBOHYDRATES


temperature at which the starch gelatinises, aredependent on the relative amounts of amylose andamylopectin present. This ratio varies with the type ofstarch (Table 2) and is an important determinant of thefunctional and nutritional properties of starches, as is thedegree of processing.

Common food starches are derived from seeds, (such aswheat, maize, rice, and barley) and roots, (such as potatoand cassava). Chemically modified starches are used insmaller quantities as food additives for their techno-logical functions in some foods, as they influencephysical properties such as viscosity, texture,appearance, emulsification, and stability.

Non-starch polysaccharides (NSP) are composed of amixture of different polysaccharides containing thepentoses (xylose and arabinose) or the hexoses(rhamnose, mannose, glucose, and galactose), and


FIGURE 2The structures of amylose and amylopectin

Linear molecule(200–2 000 glucose units)

AMYLOSE

Branched molecule(10 000–1 000 000 glucose units)

AMYLOPECTIN

Source: Gray, J. (1991). Starches and Sugars: A Comparison of theirMetabolism in Man. ILSI Europe Concise Monograph Series.Springer-Verlag, London, UK

TABLE 2

Amylose (%) Amylopectin (%)

Standard maize 24 76

Waxy maize <1 >99

High amylose maize 70 30

Potato 20 80

Rice* 18.5 81.5

Tapioca 16.7 83.3

Wheat 25 75

Approximate amylose and amylopectin contentof different starches

* Proportions vary among rice varieties.Source: Gray, J. (1991). Starches and Sugars: A Comparison of theirMetabolism in Man. ILSI Europe Concise Monograph Series.Springer-Verlag, London, UK.


uronic acids. Some are present in plant cell walls, andothers occur in the form of gums and mucilages. NSPare organised as complex structures of the plant cellwall and thus perform a structural role in plants.

Cellulose is the major structural component of plantcell walls. It consists of an unbranched (linear) chain ofseveral thousand glucose units that is resistant todigestion by human digestive enzymes.

Hemicelluloses may be present in plant foods in water-soluble or insoluble forms. They include a range ofdifferent polysaccharides with structures that compriseboth branched and linear chains of pentose units (xyloseand arabinose) and hexose units (glucose, galactose,mannose, rhamnose, glucuronic, and galacturonicacids). They have much lower molecular weights thancellulose.

In particular, the ß-glucans have generated interest as“soluble fibre”. These glucans are a major component ofthe cell wall material in oats and barley, and in recentyears oat bran has been incorporated into some foodproducts as a source of these glucans.

Pectins are present in vegetables and fruits and are usedwidely as gelling agents in foods such as jams andjellies. They are composed mainly of chains ofgalacturonic acids and rhamnose, branched with chainsof pentose and hexose units (arabinose, galactose, etc.).Pectins are water-soluble.

Hydrocolloids, which are derived from seaweedextracts, plant exudates, and seeds, include gums andmucilages, such as guar, locust gum, agar, andcarrageenan. Chemically, they comprise a wide range ofmixed polysaccharides. They are used in small amountsas thickening, gelling, stabilising, or emulsifying agentsin some food products.

Physiological classification ofcarbohydratesVarious attempts have been made to classifycarbohydrates according to their physiological effects.For example, the FAO/WHO Expert Consultation onCarbohydrates in Human Nutrition recommendedadopting the concept of glycaemic carbohydrates,defined as “providing carbohydrate for metabolism”, i.e.carbohydrates that are digested and absorbed in thesmall intestine, leading to a rise in blood glucose, todistinguish from non-glycaemic carbohydrates thatescape hydrolysis (digestion) in the small intestine, notleading to a blood glucose response, and are looselyclassified as dietary fibre (see below). Although thisgrouping of carbohydrates is nutritionally relevant,further study and debate are needed to establish moreaccurate ways of classifying carbohydrates. Forexample, it does not take account of the different rates ofdigestion of the digestible fraction of carbohydrates,dividing them into those that are rapidly digested andthose that are slowly digested.

Dietary fibre is not an exact description of a singledietary component but comprises a heterogeneousgroup of components with diverse functional properties.Originally, the term described plant cell wall materialthat passed through the gut unchanged and providedfaecal bulk, but it is now recognised that dietary fibremoderates digestion and absorption of other nutrients inthe small intestine and provides substrate forfermentation in the colon. Currently, dietary fibre isdefined as “the edible parts of plants or analogues ofcarbohydrates that are resistant to digestion andabsorption in the small intestine, with complete orpartial fermentation in the large intestine” (AmericanAssociation of Cereal Chemists, 2001).

The principal components of dietary fibre are the non-starch polysaccharides (NSP) derived from the cell walls



of plant foods in the diet. According to the currentdefinition, resistant starch (RS), defined as “starch andstarch degradation products that are not absorbed in thesmall intestine of healthy humans”, should be includedin the definition of dietary fibre (Box 1). The proportionof total starch that is resistant to digestion in somecommon foods is listed in Table 3.

According to this definition of dietary fibre, non-digestible saccharides should also be included. Forexample, fructooligosaccharides (FOS) and galacto-oligosaccharides (GOS) pass through the small intestineunchanged but are fermented by the microflora in thelarge intestine. A fraction of hydrogenated carbo-hydrates also reaches the colon, where they arefermented.

Lignin, which is a non-carbohydrate component of theplant cell wall, may also be included as dietary fibre.

Soluble and insoluble dietary fibreThe classification of dietary fibre as soluble or insolubleevolved from the early chemistry of NSP, which showedthat different fractions could be extracted by controlling

the pH and temperature of solutions. The termsprovided a useful, simple division of the physiologicalproperties of dietary fibre into NSP that principallyaffect glucose and fat absorption from the small intestine(soluble), because they are viscous and form gels in theintestine, and those that have a greater influence onbowel function (insoluble). With time, it has becomeapparent that this simple distinction is inappropriate:much insoluble fibre is rapidly fermented, and not allsoluble fibre has an effect on glucose and fat absorption.However, the use of the term “soluble” to describe gel-forming fibres has been retained.


TABLE 3

Food Total Starch Resistant Starchg/100g dry matter g/100g total starch

White bread 77 1.2

Wholemeal bread 60 1.7

Shredded wheat 71 nil

Corn flakes 78 3.8

Porridge oats 65 3.1

Rye crisp bread 61 4.9

Potato, boiled, hot 74 6.8

Potato, boiled, cold 75 13.3

Spaghetti, freshly cooked 79 6.3

Peas, cooked 20 25

Haricot beans, cooked 45 40

Proportion of total starch that is resistant insome foods

Source: Englyst, H.N., Kingman, S.M., and Cummings, J.H. (1992).Classification and measurement of nutritionally important starchfractions. European Journal of Clinical Nutrition, 46;S33–S50

BOX 1

RS1: starch that is physically enclosed, e.g., within intact cellstructures in partly milled cereal grains and seeds.

RS2: raw starch granules, e.g., in maize rich in amylose, rawpotatoes, green bananas.

RS3: retrograded amylose in processed foods. Food starchesmay be rendered partially indigestible by physical orchemical processes and by cooling, e.g. in bread,cornflakes and cooled cooked potato.

RS4: chemically modified starch.



EFFECTS OF COOKING AND TECHNOLOGY ONCARBOHYDRATES

Processing can have substantial influences oncarbohydrates, notably on the rate and extent ofdigestion of starch in the small intestine and on thecontent and functional properties of dietary fibrecomponents.

Influence of heat on starchcharacteristicsWhen starch is heated in the presence of water, anirreversible loss of the crystalline structure of the starchgranules occurs as the process of gelatinisation takesplace. The availability of starch for enzymic digestion isincreased dramatically by this process. During cookingand other processing, starch granules are not completelydissolved, although the degree of gelatinisation thatnormally occurs is sufficient to permit a largeproportion of starch to be rapidly digested. Where thisprocess is incomplete, such as in the steaming andflaking of cereal and in the manufacture of plainbiscuits, a large proportion of slowly digestible starch ispreserved.

Gelatinised starch is unstable, and as it ages or coolsdown, a progressive process of re-association of thestarch granules occurs, known as retrogradation. Thetendency towards retrogradation depends on therelative proportions of amylose and amylopectin in thestarch: amylose re-associates more quickly. Reheatingstarchy foods also influences this process.

The digestibility of starch is therefore particularlyinfluenced by the degree of processing and byretrogradation, which may reduce the digestibility ofstarch in the small intestine. Retrograded starch alsocontributes to the proportion of resistant starch in foods,e.g. in cooked and cooled potato (Table 3).

Influence of processing on content andcomposition of dietary fibreThe milling of cereal grains to produce refined floursremoves the fibre-rich outer layers of the grain andconsiderably reduces their total fibre content. Floursderived from whole grains contain large amounts ofcellulose and hemicelluloses, whereas certain hemi-celluloses – arabinoxylans – dominate in the refinedflours of wheat, rye, and maize. Although oat andbarley lose some dietary fibre during milling, therefined grains are rich in soluble ß-glucans. Heattreatment can also influence the physical structure andtherefore the functional and physiological properties ofdietary fibre.


CARBOHYDRATE INTAKES

Estimates of carbohydrate intakeCarbohydrates provide 40 to 75% of total energy intake,thus constituting the most important energy source inhuman diets. The Western countries (Western Europe,North America, and Australia) are at the low end of therange, and the countries in Asia and Africa are at the highend. The type of carbohydrate is also variable, withstarch providing 20 to 50%, and in some cases more, oftotal energy intake and sugars accounting for 9 to 27% oftotal energy intake. In developed countries, carbohydrateintake declined over the second half of the past century,generally reflecting a decreasing intake of starchycarbohydrates, although a slight increase has occurredduring the past two decades as fat intake has fallen.

Estimates of the intake of sugars, available only fordeveloped countries, reflect similar proportions ofsugars derived from cereal products, milk products, andbeverages, but there are variations in the amounts ofsugars derived from fruit, soft drinks and confectionery.

The FAO/WHO Expert Consultation on Carbohydratesin Human Nutrition recommended an optimum carbo-hydrate level of at least 55% of total energy intake,obtained from a variety of food sources, and noted thatlevels of carbohydrate consumption at or above 75% oftotal energy should be avoided because of the exclusionof adequate amounts of protein, fat, and other essentialnutrients.

Relationship between carbohydrate and fat intakesIt is recognised that there is usually an inverse relation-ship between the proportions of fat and carbohydrate inthe diet. Thus, on a percentage basis, as fat intake falls,

carbohydrate intake rises, and vice versa. While the pro-portion of energy intake from fat and carbohydrate haschanged over time, protein intake as a proportion ofenergy intake has remained relatively constant. There isnow considerable evidence to suggest that higher fatintakes are accompanied by higher total energy intakes,compared with high-carbohydrate diets. This may berelated to the lower energy density of high-carbohydratediets.

Implications for intakes of other nutrientsAs total carbohydrate intake from natural foods increasesor decreases, the intake of nutrients associated with thecarbohydrate food sources also increases or decreases. Inparticular, foods that are naturally rich in glycaemiccarbohydrates, such as cereals, pulses, seeds, fruit andvegetables, provide a wide range of importantmicronutrients (vitamins and minerals), dietary fibre,and phytochemicals, which are now recognised to have abeneficial effect on health.

Concern is frequently expressed that dietary sugars,especially added sugars, may displace micronutrientsfrom the diet. However, survey research among Britishadults indicates that micronutrient intake is notcompromised at intake levels of added sugars providingup to 17% of food energy. The highest micronutrientintakes were found among consumers of average levelsof added sugars (10 to 16% of food energy), and thelowest among those with the lowest added sugars intake(6% of food energy). On the other hand, such cross-sectional analyses of survey data do not permit anydistinction to be made between displacement ofmicronutrients as a result of sugar intake level andgenerally poor eating habits.



CARBOHYDRATE DIGESTIONAND ABSORPTION

Digestion and absorption in the smallintestineThe different processes and components of digestion andabsorption of major digestible carbohydrates, which takeplace in the digestive system, are illustrated in Figure 3.

Starch digestion begins in the mouth, by salivaryamylase, but digestion in the small intestine, bypancreatic amylase, is quantitatively more important.The cells lining the small intestine, the enterocytes,provide a highly interactive surface between the lumenof the intestine and the blood and lymphatic system.They produce the hydrolytic enzymes essential for thebreakdown of maltooligosaccharides and disaccharides(maltose, sucrose, and lactose) into their constituentmonosaccharides. The enterocyte is also the site ofabsorption of the free monosaccharides.

With the typical Western diet, which is relatively high inrapidly digestible carbohydrates, the main part isabsorbed mainly in the upper part of the small intestine.If the diet contains a greater proportion of less easilydigested carbohydrates, more carbohydrates are shiftedto the lower regions of the small intestine and to thecolon, where they are fermented.

Factors affecting rates of digestionStarch digestion is influenced by the nature and thedegree of processing of the starch and by the presence ofdietary fibre. In some foods, e.g. legumes, the starch istrapped in fibrous, thick-walled cells, so the enzymeshave limited access to it.

Digestion of carbohydrates is influenced bygastrointestinal factors, such as the rate of gastricemptying and the speed of intestinal transit. The rate ofgastric emptying itself is influenced by many factors ina mixed meal, including fat and energy content,viscosity, the ratio of solid to liquid contents of the meal,the quantity of dissolved solids, and the particle sizes ofthe food. The dietary fibre content of the diet also has animportant influence on gastric emptying. A high fibrecontent may slow gastric emptying. The mixing of somesoluble fibres with liquid meals increases the viscosityof the gastric contents, and also influences gastricemptying. Soluble fibres also build gels that entrapcarbohydrates, making them less accessible to enzymesand reducing contact with the intestinal mucosa.

Fermentation of carbohydrates in thelarge intestineCarbohydrates that resist digestion in the smallintestine, such as dietary fibre (including resistantstarch, which represents up to one fifth of ingestedstarch), many oligosaccharides (except those derivedfrom glucose syrups), and a proportion ofhydrogenated carbohydrates (polyols), pass into thelarge intestine, where they are partially or completelyfermented by the gut microflora. Lactose may alsoescape digestion and absorption in the small intestinebecause of low activity of the enzyme lactase and passinto the colon. These so-called non-digestible carbo-hydrates, originally viewed only as bulking agents, arenow seen to have important physiological effects as aresult of their fermentation.

The bacterial flora in the large bowel produces a widerange of enzymes that hydrolyse carbohydrates andgenerate short-chain fatty acids (SCFA) (acetate,propionate, and butyrate) and gases (carbon dioxide,hydrogen, and methane). The process of fermentation is




FIGURE 3Principle digestive processes of carbohydrates

Salivaryglands

Stomach

Pancreas

Small

intestine

Large intestine

Source: Gray, J. (1991). Starches and Sugars: A Comparison of their Metabolism in Man. ILSI Europe Concise Monograph Series. Springer-Verlag, London, UK

MOUTH AND STOMACH

SMALL INTESTINE

Some starch

Lumen of small intestine Interior of enterocyte

Fructose

Glucose

Lactose

SucroseStarch

Galactose

CE

LL

ME

MB

RA

NE

amylase

transport

Active

transport

Active

diffusion

FacilitatedPancreaticamylase

salivary Maltose

LARGE INTESTINE

Bacteria act on:

Some oligosaccharides“Resistant starch” short-chain fatty acidsSome complex polysaccharides }

MaltoseMaltotriose

Limit dextrinsSucrase

-isomaltase

Gluco-amylase

Lactase


essential to large bowel function. Bacteria use theproducts of fermentation to generate the energy andextract the carbon that are required for growth. One ofthe SCFA, butyric acid, is considered to be a primarynutrient for the normal growth of the epithelial cellslining the colonic mucosa and may play a role inpreventing disease of the colon. The fermentation processalso has effects on systemic metabolism, as some SCFAare absorbed and metabolised to provide energy. Theamount of energy yielded by carbohydrate fermentationis estimated to be about 8 kJ/g (2 kcal/g), which is abouthalf the energy value of carbohydrates digested in thesmall intestine. A large proportion of the gases producedduring fermentation is liberated as flatus or appears inthe breath, and some is consumed by the colonic bacteria.

The rate and extent of carbohydrate fermentationdepends on the solubility and structure of thepolysaccharides and on the accessibility of bacteria tothese polysaccharides. For example, most solublepolysaccharides, hydrogenated carbohydrates andoligosaccharides are completely and rapidly fermented,whereas resistant starch is completely fermented butonly slowly, and cellulose and hemicelluloses arepartially resistant to fermentation. The nature of thefermentation products also depends on the specificcarbohydrates, but the factors governing thepreferential production of individual SCFA have notbeen definitively ascertained.

When the enterocytes fail to produce a givencarbohydrate-splitting enzyme, the correspondingspecific sugars are not absorbed. These sugars are thentransported to the large intestine and fermented. Thismay cause laxation and abdominal pains and is referredto as “intolerance”. Such enzyme deficiencies are rare,with the exception of low lactase activity. Reducedlactase activity after weaning or later childhood,

resulting in the reduced capacity to digest and absorblactose, is a normal phenomenon in most of the worldand most prevalent in Asia and Africa. Most individualswith low lactase activity can tolerate moderate amountsof milk, especially when distributed throughout the dayand in meals.

A high intake of any fermentable carbohydrate maycause an increase in the water content of the faeces andtherefore may result in loose stools and flatulence.However, there are considerable variations inindividual response to different carbohydrates.

Gut microfloraThe gut microflora constitutes a highly complexecosystem, with more than 200 and sometimes as manyas 500 different species of anaerobic and aerobic micro-organisms in a single individual. A considerable degreeof variation in bacterial species is seen amongindividuals, which may be influenced by factors such asage and diet, but the flora is remarkably stable withinindividuals.

Recent research suggests that non-digestible carbo-hydrates, in particular oligosaccharides, are selectivelyused by certain groups of bacteria and may modify thebacterial composition of the dominant flora byincreasing bifidobacteria and other lactic acid bacteria.Consequently, several types of oligosaccharides havebeen used as “prebiotic” food additives (e.g. in foodswith claims of specific health benefits). For example,they have been added to “probiotic” dairy-basedproducts that contain bifidobacteria (e.g. yogurt), inorder to encourage the growth of these bacteria. It hasbeen suggested that this process may be beneficial tohealth maintenance, but further research withacceptable clinical end points is required to confirm this.



METABOLISM OF ABSORBEDCARBOHYDRATES

Disposal and metabolic conversion ofcarbohydratesGlucose, free and bound in digestible polymers, is themain ingested carbohydrate, and glucose is one of thebody’s main metabolic fuels. After the consumption ofmeals containing glycaemic carbohydrates, theconstituent monosaccharides, principally glucose, areabsorbed into the blood, thus increasing the bloodglucose level. After a test dose of glucose or starch hasbeen consumed, the blood glucose level reaches a peakin 20 to 30 minutes and then slowly returns to the fastinglevel over 90 to 180 minutes as glucose is taken up bydifferent tissues under the influence of insulin.

Absorbed monosaccharides enter the bloodstream viathe portal vein, which takes them to the liver. Aproportion of glucose is taken up by the liver andconverted to glycogen. Fructose is preferentially takenup by the liver and is also converted to glycogen,although at high intakes some of it (30–40%) is rapidlymetabolised in the intestinal mucosa to lactate, whichpasses to the liver and is transformed into glucose.Because the majority of the absorbed fructose andgalactose is taken up by the liver and requiresconversion to intermediates before entering the bloodand undergoing metabolism (see below), the effect ofthese sugars on blood glucose levels is very small. Anyremaining carbohydrates are taken up by other tissues,especially muscle, where they are either oxidised orconverted to glycogen for storage.

The various monosaccharides and their correspondingalcohols can be interconverted by various metabolicpathways (Figure 4). However, under normal physiolo-

gical conditions glucose is the preferred fuel for thecentral nervous system, the red blood cells, and certainmuscle fibres.

Most of the metabolic pathways that generate energy arecommon to all dietary sugars: eventually they aremetabolised to carbon dioxide and water via the citricacid cycle (Figure 4). The same enzymes and co-enzymes(often B vitamins) are involved.

The differences in metabolism of glucose and fructoseare shown in Figure 5. Galactose enters the glycolyticpathway (Figure 4). The uptake of galactose by the liveris inhibited by alcohol, leading to a transient high bloodlevel of galactose when foods containing it areconsumed simultaneously with alcohol. Whether thishas metabolic consequences is unknown.

The hierarchy for substrate oxidation

Protein, carbohydrate, and fat all supply fuel for energyexpenditure. This fuel may come from the diet or frombody stores. Alcohol is an additional source of fuel.Substrate oxidation occurs in a hierarchical order, whichis determined by the body’s storage capability for eachmacronutrient, the energy costs of converting themacronutrient to a form that can be stored moreeffectively, and the specific fuel requirements of certaintissues (Figure 6).

Alcohol takes the highest priority for oxidation, as thereis no storage pool for it and the conversion of alcohol tofat is costly in energetic terms. Amino acids are secondin the hierarchy, as again there is no specific storagepool for them. Third in the oxidative hierarchy arecarbohydrates. The body has a limited capacity to storecarbohydrates as glycogen. A typical adult male canstore 400 to 500g of glycogen, predominantly in the liver



and in muscles. However, conversion of carbohydrateto fat is energetically costly. By contrast, the body has analmost unlimited storage capacity for fat in adiposetissue, and the storage efficiency is high (96–98%). Thusfat is theoretically lowest in the oxidative hierarchy.

Regulation of blood glucoseconcentrationThe pancreas secretes insulin in response to an increasein blood glucose concentration, but insulin secretion isalso influenced by other endocrine and neural stimuli.Dietary factors other than carbohydrates also mayinfluence secretion, e.g. the amino acid composition of


FIGURE 4The metabolism of sugars

Glycogen

Glucose-1-phosphate

Glucose-6-phosphate

Fructose-6-phosphate

Glyceraldehyde-3-phosphate

ENERGY

Acetyl CoA

Citric acidcycle (Krebs

cycle)

Oxygen

Carbon dioxideWater

ENERGY

Galactose

Glucose

Xylitol

Pentose phosphate pathway

DNARNA

Fructose

TRIGLYCERIDES

PROTEIN

Source: Gray, J. (1991). Starches and Sugars: A Comparison of their Metabolism in Man.ILSI Europe Concise Monograph Series. Springer-Verlag, London, UK.



FIGURE 5Metabolism of glucose and fructose

FIGURE 6The oxidative hierarchy for macronutrients

Source: Rolls, B.J. and Hill, J.O. (1998). Carbohydrates and Weight Management. ILSI North America, ILSI Press, Washington, DC, USA.

Alcohol

Fat

CHO

Protein

Alcohol

Fat

CHO

Protein

None

Adipose tissue

Glycogen

Body protein

INTAKE – EXPENDITURE =

MINUS

STORES AUTOREGULATION

Perfect

Excellent

Excellent

Very Poor

EQUALS

FRUCTOSE

F-1-P G-6-P

GLUCOSE

GLYCOGEN

OXIDATION

F-6-P

F-1,6-P

Acetyl-CoA

FATTY ACIDS

Glycerol-3-P

TRIGLYCERIDES

• The ubiquitous enzyme (hexokinase) that catalyses the addition of a phosphate group to position 6 of glucose does not carry outthis reaction efficiently with fructose. Glucose formation from fructose is important only if available fructose provides a majorsource of dietary carbohydrate.

• The metabolism of fructose is confined mainly to the liver, where a special enzyme is present that utilises fructose (fructokinase,which phosphorylates position 1 of fructose).

• Fructose-1-phosphate bypasses the slowest step in the pathway for glucose metabolism, which results in faster metabolism offructose than glucose in the liver.

• Fructose metabolism produces end products that may be readily made into triacylglycerols (triglycerides).• Fructose also seems to facilitate the transport of fatty acids stored in adipose tissue to the liver, where they can be used to make

more triacylglycerols, which may be exported into the plasma as very low density lipoproteins (VLDL). • Fructose can slow down the removal of VLDL from the plasma, thus helping to maintain already high blood concentrations. Source: Gurr, M. (1995). Nutritional and Health Aspects of Sugars: Evaluation of New Findings. ILSI Europe Concise Monograph Series, ILSI Press,Washington, DC, USA.

slow, rate-limiting

Abbreviations:

F = fructose

G = glucose

P = phosphate


dietary proteins. The digestion of food also causes therelease of various hormones (regulatory peptides) fromthe intestine into the blood stream. Two of these, gastricinhibitory peptide (GIP) and glucagon-like peptide I(GLP I), are involved in regulating carbohydratemetabolism through influencing insulin secretion as wellas gastric emptying.

Different carbohydrates induce different blood glucoselevels and hence induce different insulin responses. Theinsulin response to carbohydrates is closely related to therate of digestion and/or absorption of its componentsugars. However, the carbohydrate in a mixed mealgenerates a greater insulin response than a similaramount of carbohydrate consumed as glucose, probablybecause of the effects of the proteins contained in themeal on insulin secretion. Table 4 summarises thevarious factors that influence insulin release.

Blood glucose responses to food andthe glycaemic indexThe rise in the blood glucose level that occurs when foodscontaining glycaemic carbohydrates are eaten is termedthe glycaemic response. The concept of glycaemic index(GI) has been developed to classify foods on the basis oftheir potential for increasing the blood glucoseconcentration.

The GI is defined as the incremental area under the bloodglucose response curve (change in blood glucoseconcentration plotted against time) (Figure 7) afterconsumption of a 50g carbohydrate portion of a test food,expressed as a percentage of the response to the sameamount of carbohydrate from a standard (reference) foodconsumed by the same subject.

The reference food, taken to have a glycaemic index of100, can be 50g of glucose or an amount of white breadcontaining 50g of starch, although the current recom-

mendation is to use glucose as the reference food. The GIvalues obtained with white bread as the reference areabout 1.4 times those obtained when glucose is thereference.

In addition to the quantity and digestibility of starchesand the quantity and sources of sugars in a meal, manyfactors influence the glycaemic responses to foods(Table 5). These include the presence of fat and protein,the degree of processing and cooking, and the amount ofdietary fibre present. The ratio of amylose to amylopectinin the starch is important in determining GI: amylopectinis more rapidly degraded than amylose.


TABLE 4

Stimulation of insulin release

1. Increase in blood glucose concentration

2. Increase in blood levels of certain amino acids

3. Glucose in intestine (probably mediated by gastric inhibitory peptide)

4. Other gut hormones such as gastrin and secretin

5. Glucagon from alpha cells of pancreas

6. Nerve (parasympathetic) stimulation

Inhibition of insulin release

1. Adrenaline

2. Nerve (sympathetic) stimulation

3. Pancreatic secretion of the hormone somatostatin

Factors influencing the release of insulin

Source: Gray, J. (1991). Starches and Sugars: A Comparison of theirMetabolism in Man. ILSI Europe Concise Monograph Series.Springer-Verlag, London, UK.


There are important differences between the GIs ofdifferent sugars. The GI of a sugar is determinedprimarily by its content of readily available glucoseresidues. Thus maltose (a disaccharide with two glucoseunits) has a GI close to 100, whereas sucrose (glucose plusfructose) has a GI of only 87. The GI of fructose (usingbread as a standard) is only 32, which is explained by thepartial conversion of fructose into glucose at high intakesof a standard test.

The use of the GI method has also revealed interestinginformation about the influence of carbohydrate foodson blood glucose levels, overturning certain long-heldbeliefs. For example, it was assumed that the bloodglucose response to starches is less than the response tosugars because of the slower digestion of starch.However, many cooked starches are digested so rapidlythat the glycaemic response is similar to that of glucose

(Table 6). The GI correlates well with the hydrolysis rateof the starch, so that, e.g., the starch in lentils generatesonly a small glycaemic effect compared with the starchin bread.

Physiological effects of GI and itspractical implicationsClinical trials with normal, diabetic, and hyperlipidaemicindividuals indicate that the consumption of mealscontaining low-GI foods is associated with reducedblood glucose and insulin responses after meals and withimproved lipid profiles, including reductions in elevatedserum triglyceride concentrations in subjects withhypertriglyceridaemia. It has also been suggested thatlow-GI foods may have longer lasting satiating effectsthan high-GI foods, but this idea is still controversial andrequires further study. Hence the practical application of


FIGURE 7Serum Glucose Responses to Ingestion of 50g CHO

Source: Krezowski, P.A., Nuttal, F.Q., Gannon, M.C., et al. (1987). Insulin and glucose responses to various starch-containing foods intype 2 diabetic subjects. Diabetes Care. HighWire Press, Palo Alto, CA, USA.

140

120

100

80

60

40

20

0

-200 30 60 120 180 240 300

Minutes after meal

mg/

dlGlucose

Lentils

Kidney Beans

Potato

Bread

Oatmeal

Rice


the GI, used in combination with other information onfood composition, looks promising in guiding foodchoice, especially for people with diabetes.

HypoglycaemiaHypoglycaemia (a low blood glucose concentration)occurs when the uptake of glucose by tissues exceeds therate of supply from the liver and/or meals. This mayoccur with or without symptoms. Contrary to popularopinion, reactive hypoglycaemia, in which symptomsoccur 2 to 5 hours after meals, in resting conditions, isnot a common but a rare disorder. Symptoms includesweating, lightheadedness, dizziness, tremor, thirst,mental lethargy, irritability, headache, and increasedrespiration rate. Treatment involves consumption offrequent small low-carbohydrate meals.

It is frequently assumed that sugars are more likely tocause such symptoms, but there may be a differingresponse to specific sugars, depending on their effectson insulin secretion and how they are metabolised. Astudy of individuals with hypoglycaemia suggestedthat whereas glucose produced symptoms in allsubjects, fructose did not produce symptoms in any ofthem, and sucrose caused symptoms in only one-thirdof the subjects.


TABLE 5

Nature and amount of carbohydrate

Nature of the monosaccharide components

Glucose

Fructose

Galactose

Nature of the starchAmylose

Amylopectin

Starch-nutrient interaction

Resistant starch

Cooking or food processing

Degree of gelatinisation of starch

Particle size

Food form

Cellular structure

Other food components

Fat and protein

Dietary fibre

Antinutrients

Organic acids

Food factors that influence glycaemic response

Source: Adapted from FAO/WHO Expert Consultation (1998).Carbohydrates in human nutrition.



TABLE 6

GI* n**Cereal products

White bread 100 ± 0 5Wholemeal flour 99 ± 3 12Rye crispbread 93 ± 2 5Corn flakes 119 ± 5 4Porridge oats 87 ± 2 8Macaroni 64 1Spaghetti, white 59 ± 4 10Spaghetti, brown 53 ± 7 2Rice, white 81 ± 3 13Rice, brown 79 ± 6 3Rice, low amylose 126 ± 4 3Rice, high amylose 83 ± 5 3Plain biscuits 71 ± 11*** 1

Potatoes

New 81 ± 8 3White, boiled 80 ± 2 3White, mashed 100 ± 2 3French fries 107 1Baked 121 ± 16 4

Legumes

Baked beans 69 ± 12 2Beans, kidney 42 ± 6 7Beans, soya 23 ± 3 3Peas, green 68 ± 7 3Lentils 38 ± 3 6

GI* n**Fruit

Apple 52 ± 3 4Apple juice 58 ± 1 2Apricots, dried 44 ± 2 2Apricots, canned 91 1Banana 83 ± 6 5Orange 62 ± 6 4Orange juice 74 ± 4 4

Dairy products

Milk, whole 39 ± 9 2Milk, skimmed 46 1Yogurt (sugar) 48 ± 1 2Yogurt (artificial sweetener) 27 ± 7 2Ice cream 84 ± 9 6

Sugars

Honey 104 ± 21 2Fructose 32 ± 2 4Glucose 138 ± 4 8Sucrose 87 ± 2 5

Glycaemic index (GI) of some selected foods using white bread as the reference standard

*GI = Glycaemic index (white bread = 100), mean ± SEM of mean values from various studies** n = Number of studies*** Brand Miller, J., Lang, V., Nantel, G., Slama, G., eds. (2001).

Data for plain biscuits are from Glycaemic Index and health: the quality of the evidence. John Libbey Eurotext.Source: FAO/WHO Expert Consultation (1998). Carbohydrates in human nutrition.


CARBOHYDRATES INHEALTH AND DISEASE

Carbohydrates and gut health andfunctionAn adequate intake of fibre-rich carbohydrates is vital tonormal gut function. Along with the long-recognisedbulking effects of fibre, the importance of carbohydratesto the health of the colon and the maintenance andstimulation of the gut microflora as well as theimportance of the fermentation product butyric acid forthe colonic epithelium is now acknowledged.

The physical characteristics of the gastric and intestinalcontents are altered by the presence of different types offibre. A greater proportion of fibre in foods producesmore bulk, and the volume of the gut contents increasesfurther because of the water-holding capacity ofparticular fibres. Within the gastrointestinal tract, somefibres swell, trapping water and nutrients, especiallythose that are water-soluble, such as sugars. Changes inthe physical characteristics of the gut contents mayinfluence gastric emptying, dilute enzymes andabsorbable compounds in the gut, prevent the hydrolysisof starch in the small intestine, and reduce the mobility ofenzymes, substrates, and nutrients to the absorptivesurface. This slows the appearance of nutrients, such asglucose and some lipids, in the blood after a meal.

Colonic motility and transit time are also modified bydifferent dietary fibres. The increased bulk and volumeof the colonic contents distends the colon wall,stimulating the muscle to propel the contents onwardsthrough the large bowel. It has also been suggested thatcompounds trapped in the fibre in the small intestine(such as biliary salts and fatty acids), which are releasedduring fermentation, stimulate motility.

Carbohydrates and cancerMost epidemiological studies that have examined therole of carbohydrates in cancer have focused on dietaryfibre and, to a much lesser extent, on monosaccharides,disaccharides, and starches. Evidence from recentintervention studies does not support any effect of fibreintake on the early phases of colorectal carcinogenesis.The effect of fibre on the late phases of colorectalcarcinogenesis is currently unknown. However, there issome evidence that the short-chain fatty acid butyrate,which is the product of carbohydrate fermentation in thelarge intestine, may have a positive influence onintestinal health, including cellular changes involved incolorectal cancer. This is the focus of current research.There are no consistent findings linking the intake ofsimple carbohydrates to cancers at specific sites.

Carbohydrates and type 2 diabetesMore than 40 studies in the scientific literature haveexamined the role of various sugars and starches in theaetiology and management of type 2 (non–insulindependent) diabetes. About half of them suggest thatthere is a positive association between sucrose intakeand diabetes, and a similar proportion suggest that thereis no association; some studies even suggest that there isan inverse association between the two. The restrictionof carbohydrate and specifically the need to avoid“simple sugars” was the mainstay of dietaryrecommendation for people with diabetes for manyyears. This was based on the premise that sugars,including sucrose, would aggravate hyperglycaemia to agreater extent than other carbohydrates. There is nowevidence to the contrary. Research on the GI of foods hasdemonstrated that blood glucose levels following anoral sucrose or fructose load are lower than after acomparable oral glucose load. Subsequent longer-termstudies, incorporating sucrose in mixed meals, haveconfirmed that a moderate intake of sucrose (up to about



50 g per day) can be consumed by people with both type1 (insulin-dependent) and type 2 diabetes.

There is little evidence that readily digested starchesincrease the risk of type 2 diabetes. However, there issupport for the contention that eating foods that are richin slowly digested (low GI) and resistant starch or highin soluble fibre, as well as the avoidance of overweightand physical inactivity, reduces the risk of developingtype 2 diabetes.

Research in the 1970s demonstrated that high-carbohydrate diets (up to 60% total energy fromglycaemic and non-glycaemic carbohydrates) wereassociated with improved blood glucose control andlower low-density lipoprotein (LDL) cholesterol levels,compared with diets providing only 40% of total energyas glycaemic and non-glycaemic carbohydrate. Moderndiabetic dietary recommendations emphasise theimportance of high carbohydrate intake as well asmodifications to fat intake because of the associatedproblems with lipid metabolism and the risk ofcardiovascular disease (CVD) among those who havediabetes. Fibre-rich carbohydrates such as staple cereals,pasta, rice, bread, pulses, vegetables, and fruit shouldprovide the major components of meals, with anemphasis on those that have a low GI and are rich insoluble types of fibre.

Carbohydrates and the insulinresistance syndromeType 2 diabetes is often associated with insulinresistance, which is an impairment of insulin action,especially the insulin-stimulated uptake of glucose bytissues and the inhibition of glucose output by the liverand of fat breakdown (lipolysis). Insulin resistance isoften associated with aberrations in other aspects of

metabolism, especially of lipids, as well as obesity. In thepast decade the concept of the “insulin resistancesyndrome”, also termed “metabolic syndrome” or“syndrome X” has developed.

The term has been adopted to describe a constellation ofconditions that are frequently associated with insulinresistance, such as impaired glucose tolerance, hyper-insulinaemia, hypertension, and dyslipidaemia, whichtend to cluster together along with patterns ofabdominal storage of excess fat. The description of thissyndrome arises from the observation that insulin-resistant patterns of glucose and fat metabolism aremore prevalent among individuals with hypertensionand abnormal blood lipid patterns than those withnormal blood pressures and blood lipids.

The genetic predisposition to type 2 diabetes and theother disorders linked to insulin resistance has resultedin a major effort to define the molecular biology of thedisorder. However, so far, there does not appear to beone or even a small group of genetic mutations thatprovide the molecular basis for insulin resistance.Increased body fat, and particularly intra-abdominal fat,in combination with a low level of physical activity,causes insulin resistance, and in people with a limitedcapacity to secrete insulin, this results in type 2 diabetes.

Among the numerous environmental factors that mayinteract with the genetic potential to develop insulinresistance, the role of dietary carbohydrate and fibreintake has been investigated, but reliable evidence issparse.

Evidence on fibre intake and insulin action is also limitedand contradictory. Some epidemiological studies havelinked high-GI diets and low cereal fibre intakes with theincidence of type 2 diabetes, but mechanistic studies to



support the epidemiology are lacking. Soluble fibres,such as ß-glucans, are of interest because of theirinfluence on blood glucose and insulin responses, butagain information on their effects on insulin action islacking.

Carbohydrates, lipid metabolism, andcardiovascular diseaseAlterations in plasma lipid concentration followinglong-term excessive consumption of large amounts ofcarbohydrates, particularly fructose and sucrose, havebeen reported from animal studies, but the significancein humans is still the subject of scientific debate. Inhumans the usual response is an elevation in triglycerideconcentration, but depression of high-densitylipoprotein (HDL) cholesterol has also been observed.

Many different dietary and environmental factors,including obesity and excessive alcohol consumption,influence blood triglyceride concentration. Some peopleare more sensitive to these effects of dietary sugars thanothers, and in such individuals blood triglyceride levelsmay increase in response to atypically high intakes ofsucrose or fructose. There is some evidence that theseeffects occur mainly in men and are less important inwomen.

Carbohydrate-induced increases in blood triglyceridelevels appear to result from the overproduction of bothvery low density lipoprotein (VLDL) triglyceride andVLDL particle numbers, as well as the impaired clearanceof VLDL from the blood. Individual responses in VLDLmetabolism are highly variable and are more apparent inindividuals who already have elevated blood triglycerideand insulin concentrations or who are obese, especiallythose with abdominal obesity. If the carbohydratecontent of a high-carbohydrate diet is predominantly inthe form of fructose, there is a greater elevation of blood

triglyceride concentration than if polysaccharidespredominate. Experimental diets containing purifiedforms of starch or monosaccharides induce the rise intriglycerides more readily than fibre-rich diets, in whichthe carbohydrate may be more slowly digested.

Most studies of these factors have been of short durationand have measured only fasting blood levels, notpostprandial concentrations (those following foodconsumption). The few long-term studies that have beenconducted indicate that the lipid-raising effects of sugarsmay be reduced after several weeks. Low-fat, high-carbohydrate diets have also been shown to havefavourable effects on other important risk factors ofcardiovascular disease, such as blood pressure, clottingand fibrinolytic factors, and endothelial function,provided that foods high in both carbohydrate anddietary fibre are adequately represented.

There are in fact various ways in which diets rich incarbohydrates may actually help reduce, rather thanincrease, cardiovascular disease risk. These includedecreasing the energy density and increasing thesatiating power of the diet, thereby helping to preventobesity; displacing fat in the diet; reducing the risks ofhigh blood glucose and insulin levels and therebymaintaining optimal insulin sensitivity; and providingmicronutrients and phytochemicals that are believed tohave a protective effect on the cardiovascular system.

Carbohydrates and regulation of bodyweightThe prevalence of overweight and obesity is rapidlyincreasing in Western countries and among affluentpeople in developing countries. It is therefore importantto understand how carbohydrates may influence bodyweight.



The maintenance of stable body weight requires abalance between the total amount of energy consumedand the total amount of energy expended. Increases inbody weight, specifically in body fat, occur when intakeexceeds expenditure. There is no conclusive evidencethat dietary sugars have a direct effect on either totalenergy intake (see below) or on energy expenditure inhuman subjects. However, current scientific evidencesuggests strongly that the ratio of fat to carbohydrateconsumed influences the regulation of body weight andthat high-fat diets that are low in carbohydrates are morelikely to promote weight gain and obesity than diets inwhich carbohydrates, including sugars, predominate.

The major reasons for this are as follows. High-carbohydrate meals are generally more bulky than high-fat meals, and carbohydrates are more satiating than fat,joule for joule (or calorie for calorie). Storage ofcarbohydrate in the body as glycogen is limited. Whenstores are full, the body adapts to oxidise anycarbohydrate that is in excess of requirements. Incomparison, storage of dietary fat is almost unlimited,and the body adapts only very slowly to oxidise excessfat. The human body has a limited ability to convertcarbohydrate into fat, and under normal circumstances itis negligible. However, each gram of fat consumed hasmore than twice the energy content of each gram ofcarbohydrate. Thus, high-fat, energy-dense diets aremore likely to promote passive over-consumption ofenergy. Energy balance is therefore more easily regulatedat high carbohydrate intakes than at high fat intakes.

A substantial amount of research has demonstrated theimportance of the energy density of the diet (joules orcalories per gram) to energy intake and body weightregulation. Consumption of high-energy-density dietsincreases the chances that a positive energy balance, andtherefore weight gain, will occur. Research consistentlyindicates that there is a spontaneous reduction in energy

intake when the energy density of the diet is reduced.Low-energy-density diets are characterised as low in fatand high in carbohydrate- and fibre-rich foods such ascereal products, fruits, and vegetables.

Appetite, satiety, and food intakeFood intake is regulated by the complex interaction ofphysiological and psychological factors that areassociated with eating and drinking. The energy densityof food is an important determinant of the amount eaten,but other properties of food may also be important.These include palatability, macronutrient composition,and the relative proportions of solids and liquids.

Studies of the effects of individual carbohydrates onhunger, satiety, and food intake have yielded conflictingresults because of the numerous methodologicaldifficulties encountered in conducting this type ofresearch. Overall, it seems unlikely that controlling asingle dietary component, such as the type of sugar orstarch, will have any significant effect on the amount ofenergy consumed, because this is also influenced byother components of the meal, including protein anddietary fibre.

The popular belief that sugar, because of its innatesweetness and hedonic properties, may over-ride theregulatory controls on food intake and promoteexcessive consumption is not supported by robustscientific evidence. Research suggests that many peopleprefer foods high in fat and sugar because of theirpalatability. A large European multicentre trialdemonstrated that under conditions in whichoverweight subjects had ad libitum access to two low-fatdiets, high either in sugars or in starchy carbohydrates,they lost similar amounts of body weight. However, itremains to be evaluated whether sugars obtained fromsoft drinks have a lesser satiating effect than sugarsobtained from solid foods.




Carbohydrates and physical activityPhysical activity requires the metabolism of body fuelreserves to provide energy for muscle contraction. Thenature of that fuel depends on whether the body is in afasting or a postprandial state and on the intensity andduration of the activity. Under fasting conditions, themetabolism of fat provides the greater proportion ofenergy for resting metabolism and for musclecontraction at low exercise intensities. Underpostprandial conditions, the greater part of energy forresting metabolism or low-intensity exercise comes fromglucose metabolism. At greater exercise intensities, themetabolism of carbohydrate reserves, including liverand muscle glycogen and blood glucose, is also thepredominant source of energy for muscle contraction. Ifthe exercise requires more energy, because of its intensityor duration, then the proportion of energy generated byfat oxidation will gradually increase relative to thatgenerated by glucose metabolism. Under normalcircumstances, protein (in the form of amino acids) isonly minimally metabolised as an energy reserve and ismobilised only under conditions of starvation or whencarbohydrate reserves are at low levels.

In recent years the optimisation of performance bymanipulation of carbohydrate availability has receivedmuch attention. It is now well established that endurancecapacity can be improved dramatically by an increasedintake of carbohydrate, which augments muscle andliver glycogen stores. For endurance sports activities, theamounts and types of dietary carbohydrates and thetiming and frequency of consumption (during training,immediately before and during exercises, andimmediately after) can be critical in determining theoptimal availability of fuel for maximising performance.Pre-exercise consumption of carbohydrate ensuresmaximal availability of carbohydrate for substrateduring the activity, whereas post-exercise carbohydrate

consumption promotes the recovery of liver and muscleglycogen reserves. Performance in endurance activitiescan generally be improved by carbohydrateconsumption during exercise. It is generallyrecommended that foods with a moderate to high GI beconsumed after exercise.

The replacement of fluids and electrolytes lost duringexercise is also important to performance. The effects ofthe ingestion of different carbohydrates on carbo-hydrate reserves and performance, and of othercomponents such as fluid and electrolytes, at varioustimes during different types of exercise and during therecovery period, have been the subject of extensiveinvestigation. Glucose, maltose, sucrose, and malto-dextrins are all absorbed and oxidised at high rates andhave been used as components of sports drinks.However, if the carbohydrate concentration of the drinkis too high, gastric emptying is reduced and the waterand carbohydrate become less available for absorption.

Research suggests that the consumption of glucose,sucrose, or glucose polymers (4–5 g/kg body weight)3 to 4 hours before exercise, and a more dilute source ofcarbohydrate (1–2 g/kg body weight) 1 hour beforeexercise, will elevate or maintain blood glucose levels,enhance carbohydrate oxidation during exercise, andimprove endurance. During exercise, when replacingfluid loss is a priority, the greatest rate of waterreplacement can be achieved by using dilute solutionsof carbohydrate (30–70 g/L) and sodium salts.

Carbohydrates and human behaviourThe idea that food, specifically sucrose, might adverselyaffect behaviour dates back to the early 1920s. It gainedmore attention in 1947, with the description of “tensionfatigue syndrome”, attributed to sugar. A plausiblemechanism for proposed links between carbohydrate


and behaviour was developed in the early 1970s, whenincreases in the synthesis of the brain neurotransmitter 5-hydroxytryptamine (serotonin) in response to carbo-hydrate consumption were demonstrated in animalstudies. Serotonin is involved in the regulation of manyaspects of human behaviour, including mood, sleep, andappetite.

Mood and cognitionThe hypothesis that carbohydrate consumption may altermood because of effects on brain serotonin has drivenmuch research in this area. However, results in healthyexperimental subjects, mostly conducted at lunchtime,have shown either no specific effect of carbohydrate, orno difference between high- and low-carbohydratemeals. Alterations in mood, including anger, depression,and reductions in vigour, have been reported in the earlyafternoon, irrespective of the type of food eaten atlunchtime. There is stronger evidence that someindividuals use carbohydrate to counteract negativemood states in various situations, including tobacco andalcohol withdrawal, premenstrual syndrome, seasonalaffective disorder, and obesity.

There is increasing evidence that carbohydrate, butspecifically glucose, can influence central nervous systemactivity and enhance cognitive function. The retention ofmemory is an important part of cognition and has beenthe focus of research both in animals and in differentgroups of human subjects, including children and elderlypeople. Acute doses of glucose have been shown toimprove memory function within 1 hour. Some studiessuggest that the effects vary with the complexity of thetask. The effect is most evident in elderly subjects, whomay have pre-existing memory deficits. However,positive effects have also been seen in cognitive tests inchildren after consumption of a carbohydrate-containingdrink. More research is required before conclusions aboutthe practical implications of these findings can be made.

Sugar and hyperactivityDuring the 1970s, the idea that sugar intake wasassociated with hyperactive behaviour in children, andeven with criminal and antisocial behaviour, prevailedstrongly, especially in the United States. It has beeninvestigated by numerous research studies over the pastfifteen to twenty years. An analysis of the results fromthese studies does not support the hypothesis thatsucrose affects hyperactivity or adversely influencesattention span or cognitive performance in children.

Carbohydrates and oral healthDental caries is one of the most widespread conditionsaffecting the oral cavity. It is a multifactorial disorder inwhich both bacteria and fermentable carbohydrate playkey roles, generating acids that demineralise the hardtissues of the teeth.

Development of dental cariesThe various interacting factors involved in thedevelopment of caries are illustrated in Figure 8.Cariogenic plaque bacteria play a central role, especiallyspecies that ferment sugars to produce lactic acid. Thesebacteria may be relatively harmless when present in thinlayers, but when they accumulate as thick layers ofplaque on the surfaces of the teeth, they are cariogenic.Salivary flow and composition are also important. Salivaprotects teeth by its buffering action, preventingextremes of acidity and alkalinity. It also containsantibacterial substances and is a source of minerals,including calcium, phosphorus, and fluoride, whichcounteract the acid demineralisation of the enamel andpromote remineralisation of early lesions. Theincorporation of fluoride during remineralisation, suchas from fluoride added to toothpaste, generates enamelthat is more resistant to decay.



Starches and sugars are important components of foodresidues trapped in niches in the mouth. They providethe nutrients for the bacteria and the material for thedevelopment of plaque. Carbohydrates cause no directdamage to the teeth, but their presence can promotecaries by creating conditions for local fermentation andacid accumulation. Starches are readily hydrolysed andfermented, and under certain circumstances they are asable to promote caries as sugars, especially when presentin forms that adhere to the teeth or lodge in nichesbetween them.

There are various ways to minimise the risks of dentalcaries and improve oral health. A key approach isimproved oral hygiene involving the removal of foodresidues and dislodging of plaque by regular brushingwith a fluoride-containing toothpaste. Additionalsources of fluoride are water, tea supplements, andrinses. Controlling the frequency of eating and drinkingoccasions, to allow remineralisation between them, alsohas an important role. Hydrogenated carbohydrates(polyols), which are not fermented by the oral flora, areused as substitutes for sugar in sugar-free confectioneryand chewing gum. Overall there have been notabledeclines in the prevalence of caries in developedcountries in the past thirty years, which reflectsimprovements in oral hygiene and the use of fluoride,especially in dental hygiene products.

Sugars and caries prevalenceFor many years, the role of sugars, specifically sucrose,has been emphasised in the aetiology of dental caries.However, sugars do not generate dangerous amounts ofacids in the mouth when plaque is absent or is presentonly in thin layers, and the presence of fluoride protectsagainst acid attacks.

Before the widespread introduction of fluoride and otherimprovements in oral hygiene, the association betweensucrose consumption and dental caries was strong. Areview of data from 47 nations up to the 1970s suggestedthat half of the variability in caries prevalence could beexplained by sucrose availability. Over the past twodecades, the situation has changed, and recent studiesindicate that the prevalence of caries is well correlatedwith sucrose consumption only where oral hygiene ispoor and where fluoride is absent, but not elsewhere.


FIGURE 8Factors influencing the development of dental caries

Source: Navia, J.M. (1994). Carbohydrates in human nutrition: theimportance of food choice in a high-carbohydrate diet. AmericanJournal of Clinical Nutrition, HighWire Press, Palo Alto, CA, USA.© American Society for Clinical Nutrition

Host F

acto

rs (

Nut

rition, Genetics, Behaviour, R

ace, Age)

Plaquebacteria

Caries

Minerals, toothenamel, and

fluoride status

Nutrientsand foodcomponents

Saliva flowand

composition


In the Netherlands, for example, the prevalence of carieshas declined rapidly in the past 25 years (Figure 9),although sucrose consumption is still more than 90% ofwhat it was in 1965. In the United Kingdom, a studyamong young children (aged 1.5 to 4.5 years) demon-strated that the association between caries and the con-sumption of sugar confectionery was present onlyamong children whose teeth were not brushed twice aday.

It can be concluded that although there is a relationshipbetween the consumption of sugars and dental caries,the disease is complex. The diet plays a role in dentalhealth by supplying nutrients to maintain mineralisationof the teeth, but it also supplies substrates for bacterialfermentation in the form of sugars and starches. Sugarsare only one factor in the complex aetiology of caries,and it is not the amount of sugars but the frequency ofintake that is important in the development of caries.Good oral hygiene and use of fluoride, together with avaried diet, are recommended for caries prevention.


FIGURE 9DMFT of 12-year old children in the Netherlands, the sugar consumption per capita per year in 1961, 1982 and1985 and the amount of toothpaste sold in the Netherlands

Adapted from: Gurr, M. (1995). Nutritional and Health Aspects of Sugars: Evaluation of New Findings. ILSI Europe Concise Monograph Series, ILSI Press, Washington, DC,USA.

Toothpaste without fluoride

Toothpaste with fluoride

Sugar consumption (kg/capita/year)

year

42.5 kg* 39 kg*38.5 kg*

5

4

3

2

1

0

10

8

6

4

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GLOSSARY

Atherosclerosis: A disease of the arteries, in which fatty,fibrous plaques develop on the inner walls,eventually disrupting blood flow.

Carcinogen: A substance capable of causing cancer.

Degree of polymerisation (DP): The number ofmonosaccharide units in a specific carbohydrate.

Dietary fibre: The edible parts of plants or analogues ofcarbohydrates that are resistant to digestion andabsorption in the small intestine and undergocomplete or partial fermentation in the large intestine(American Association of Cereal Chemists, 2001).

Dyslipidaemia: Disordered blood lipid levels.

Endothelium: A layer of cells lining the inside of certaininternal cavities such as the blood vessels.

Epithelium: The tissue covering external surfaces, suchas the skin, and lining internal cavities, such as theintestine, consisting of a single layer of cells.

Enterocyte: The predominant cell type of the smallintestinal mucosa, responsible for enzymeproduction and nutrient transport.

Epidemiology: The study of disease in relation topopulations.

Fibrinolysis: The process by which blood clots aredissolved and removed from the circulation.

Gelatinisation: Formation of a gel.

Glycaemic index (GI): A comparison of the rise in bloodglucose concentration produced by a standard amountof a test food to that produced by the same amount ofa standard (reference) food in the same person.

Glycogen: The main form of storage carbohydrate inthe body. It consists of branched chains of α-linkedglucose units.

Glucagon: Hormone secreted by the pancreas thatcauses a release of glucose from the liver, resulting inan increase in blood glucose level.

HDL (high-density lipoprotein) cholesterol: Cholesterolcarried on specific proteins in the blood out of the wallof the arteries. High levels are associated with areduced risk of cardiovascular disease.

Hyperglycaemia: Abnormal elevations of blood glucoselevels.

Hyperlipidaemia: Abnormal elevations of fat (lipids) inthe blood, including triglycerides and cholesterol.

Hypertension: High blood pressure (excessiveelevation of blood pressure, which is associated withan increased risk of organ damage, including stroke).

Hypertriglyceridaemia: Elevation of triglycerides(triacylglycerols) in the blood.

Insulin: Hormone secreted by the pancreas that has acentral role in the control of carbohydratemetabolism by lowering blood glucose levels.

Insulin resistance: An impairment of insulin action,especially the insulin-stimulated uptake of glucoseby tissues, the inhibition of glucose output by theliver, and the inhibition of fat breakdown.

LDL (low-density lipoprotein) cholesterol: Cholesterolcarried on specific proteins from the liver to othertissues. High levels are associated with an increasedrisk of heart disease.

Microflora: A bacterial population.

Postprandial: Occurring after a meal.



Prebiotic: Non-digestible food components that benefitthe host by selectively stimulating the growth and/oractivity of one or a limited number of beneficialbacteria in the colon.

Probiotic: A live microbial food ingredient that isthought to be beneficial to health.

Phytochemicals: Plant chemicals often associated withhealth effects.

Triglycerides (triacylglycerols): The most commonform of fat molecule, consisting of a molecule ofglycerol linked to three fatty acid molecules.

Very low density lipoprotein (VLDL): Triglyceride-richlipoprotein secreted into the blood by the liver.

FURTHER READING

Asp, N-G., van Amelsvoort, J.M.M., and Hautvast,J.G.A.J. eds. (1996). Nutritional implications of resistantstarch. Nutrition Research Reviews, CABI Publishing,Oxford, United Kingdom, 9:1-31.

Brand Miller, J., Slama, G., Nantel, G., and Lang, V eds.(2001). Glycaemic index and health: the quality of theevidence. Danone Vitapole Nutrition and Health Collection.John Libbey Eurotext Ltd., Paris, France.

Food and Agriculture Organization of the UnitedNations/World Health Organization (1998). Carbo-hydrates in Human Nutrition. Report of a JointFAO/WHO Expert Consultation, FAO/WHO, Rome,Italy.

McCleary, D.B.V. and Prosky, L., eds (2001). AdvancedDietary Fibre Technology. Blackwell Science, Oxford,United Kingdom.

van Loveren, C. (1992). Caries and Fluoride. NederlandseTijdschrift voor Tandheelkunde. NTvT b.v., Amsterdam,The Netherlands, 99(6):220-224.



Other ILSI Europe Publications Available from ILSI PressConcise Monographs

• A Simple Guide to Understanding and Applying the HazardAnalysis Critical Control Point Concept

• Calcium in Nutrition• Caries Preventive Strategies• Concepts of Functional Foods• Dietary Fat – Some Aspects of Nutrition and Health and

Product Development• Dietary Fibre• Food Allergy and Other Adverse Reactions to Food• Food Biotechnology – An Introduction• Health Issues Related to Alcohol Consumption• Healthy Lifestyles – Nutrition and Physical Activity• Microwave Ovens• Nutrition and Immunity in Man• Nutritional and Health Aspects of Sugars – Evaluation of New

Findings• Nutritional Epidemiology, Possibilities and Limitations• Oxidants, Antioxidants, and Disease Prevention• Principles of Risk Assessment of Food and Drinking Water

Related to Human Health• Sweetness – The Biological, Behavioural and Social Aspects• The Acceptable Daily Intake – A Tool for Ensuring Food Safety

Concise Monographs in preparation

• Antioxidants • Food Allergy (second edition)• HACCP (third edition)• Nutrition and Genetics

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ILSI Europe83 Avenue E. Mounier, Box 6B-1200 Brussels, BelgiumPhone (+32) 2 771 00 14Fax (+32) 2 762 00 44E-mail: [email protected]

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• An Evaluation of the Budget Method for Screening FoodAdditive Intake

• Applicability of the ADI to Infants and Children• Approach to the Control of Entero-haemorrhagic Escherichia

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Environment with Reference to Food Processing• Assessing Health Risks from Environmental Exposure to

Chemicals: The example of drinking water• Detection Methods for Novel Foods Derived from Genetically

Modified Organisms• Exposure from Food Contact Materials• Food Safety Management Tools• Functional Foods – Scientific and Global Perspectives• Markers of Oxidative Damage and Antioxidant Protection:

Current status and relevance to disease• Method Development in Relation to Regulatory Requirements

for the Detection of GMOs in the Food Chain• Overview of Health Issues Related to Alcohol Consumption• Overweight and Obesity in European Children and

Adolescents: Causes and consequences – prevention andtreatment

• Packaging Materials: 1. Polyethylene Terephthalate (PET) forFood Packaging Applications

• Packaging Materials: 2. Polystyrene for Food PackagingApplications

• Packaging Materials: 3. Polypropylene as a PackagingMaterial for Foods and Beverages

• Packaging Materials: 4. Polyethylene for Food PackagingApplications

• Recycling of Plastics for Food Contact Use• Safety Assessment of Viable Genetically Modified Micro-

organisms Used in Food• Safety Considerations of DNA in Foods• Salmonella Typhimurium definitive type (DT) 104: A Multi-

resistant Salmonella• Significance of Excursions of Intake above the Acceptable

Daily Intake (ADI)• The Safety Assessment of Novel Foods• Threshold of Toxicological Concern for Chemical Substances

Present in the Diet• Validation and Verification of HACCP


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