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

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the development of a high viscosity in the gut lumen.

However, gums in natural foods may not be released

in sufficient quantities to increase luminal viscosity.

Isolated gums or foods fortified with gums, however,

can have very significant effects of nutrient absorp-

tion and postprandial plasma nutrient levels. Other

important effects relate to the fermentation of the

gums by the microflora in the large intestine. A high

intake of particular gums may help in the treatment of

constipation and diabetes, and in the prevention of

colon cancer. However, only the benefits for treating

constipation have been well established, and more

research is needed in their use for other conditions.

Physiochemical Properties and

Physiological Action

0002 In this context, the term ‘gum’ covers a wide range of

polysaccharide molecules that have varied structures,

physical properties and physiological actions. The

term, for the most part, could be interchanged with

‘soluble nonstarch polysaccharide’ or soluble dietary

fiber (See Dietary Fiber: Properties and Sources).

There are many different dietary sources of gums.

They are found naturally in plant cell walls (pectins

and hemicelluloses), as plant exudates (gum arabic)

or mucilages (ispaghula

) as well as being produced

by some bacteria (xylan) and algae (alginates). The

effects of gums on human physiology will depend on

whether the gum is ingested as part of a plant cell

wall, as part of a food complex, or as an isolated

polysaccharide.

0003 Most, but not all, gums produce viscous solutions

in their isolated form, and some (such as pectin) have

cation exchange properties. The extent of the viscos-

ity or cation exchange depends on the length of the

molecule and the constituent sugars. The physio-

logical action of such gums in the diet can therefore

vary enormously and may differ from one dietary

source to another. If the gum is ingested within the

framework of a plant cell wall, as with pectin in

carrots, the pectin may not be released in sufficient

amounts to have a significant action on small intes-

tinal function but may still be available for fermenta-

tion in the large intestine. In addition, the cation

exchange properties of an isolated pectin may be

greater than pectin contained in a food because the

potential binding sites may already be taken up by

minerals or other charged molecules found in the

food matrix.

0004 Another important consideration is the amount of

gum ingested. The amount in foods, or added as

stabilizers, emulsifiers, and fat substitutes, is small,

and most of the effects described below require a

substantial intake of isolated gum. For example, the

standard dose of ispaghula for treatment of consti-

pation is 3–11 g per day. The dose recommended for

the action of guar gum on small intestinal absorption

is 15 g per day, whereas the amount of pectin in

carrots is 1.5 g per 100 g, and there is only a trace of

gums in icecream.

0005The physical characteristics of each gum are deter-

mined by sugar composition as well as the order of

sugars and degree of modification, e.g., the degree of

methylation of pectin. Viscosity results from the en-

tangling of free carbohydrate chains. This requires a

disorganized network, and so the viscosity of gums is

determined by their molecular weight and the degree

of irregular versus regular sugar patterns, the latter

allowing more ordered cross-linkage between chains.

0006The gums have a range of physiological and

nutritional effects (Table 1) mediated mostly through

effects on transit through the stomach, small intes-

tine, and colon, by reducing mixing actions in the gut,

by their actions on the interaction between nutrients,

enzymes, and mucosal cells, and finally as a result of

their fermentation by the colonic microflora.

Effects in the Stomach and Small Intestine

0007The effects of isolated gums on gastric emptying and

digestion and absorption have been well studied. The

effects of the same gums in foods are less well estab-

lished. Viscous gums eaten as part of a mainly liquid

meal can slow gastric emptying by making the liquid

meal of a more solid consistency. Guar gum, which

has one of the highest viscosities, can delay gastric

emptying half time by 60%. However, it has also been

shown that increased viscosity can facilitate the

emptying of discrete solids. In this case, they may

The terms ‘ispaghula’ and ‘psyllium’ (used later) are interchangeable.

tbl0001Table 1 Properties of gums that affect human physiology and

health

Property Consequence Main examples

Viscosity Slower absorption Guar gum, pectin,

ispaghula

(psyllium), oat

gum

Lower blood glucose

Reduced cholesterol

Increased satiety

Cation exchange Binding of minerals Pectin

Binding bile acids

Reduced cholesterol

Fermentability SCFA production Oat gum, pectin,

guar gumLow colonic pH

Altered microflora

Butyrate

production

Colonic fuel Oat gum

Colonic health

Possibly anti cancer

Stool bulking Reduced constipation Ispaghula

(psyllium), gellanDilution of carcinogens

3008 GUMS/Dietary Importance

act as lubricants and ease passage through the small

2-mm opening of the pyloric sphincter.

0008 In the small intestine, the main impact of increased

viscosity is to resist the movement of the intestinal

contractions, reducing the mixing of the digestive

enzymes with their substrates and inhibiting the

movement of the nutrients to the absorptive epithe-

lium. This can give the impression of an increase in

the thickness of the unstirred water layer adjacent to

the intestinal mucosa, through which nutrients must

pass by diffusion. These two effects, along with slowed

gastric emptying, can delay the process of digestion

and absorption, resulting in decreased postprandial

glycemia and potentially reducing fat absorption.

0009 In addition, the viscous matrix of a gum solution

can trap endogenous molecules such as bile acids. Bile

acids are essential for the emulsification of fats to

allow digestion, and for the formation of micelles to

facilitate movement of the products of fat digestion

to the mucosa for absorption. A decrease in bile acid

concentration will therefore disrupt and delay fat di-

gestion. If cation exchange properties persist, then the

bile acids may be bound to the charged gum, and this

will have an additional impact in fat digestion. The

trapped bile acids, if not released in the large intestine

during fermentation, are excreted in feces, which will

result in reduced plasma cholesterol as replacement

bile acids are synthesized from this precursor.

0010 There are, however, some problems with this sim-

plistic explanation of the effects of viscous gums on

postprandial glucose and fat absorption. The viscos-

ity shown in in vitro models to delay digestion and

reduce mobility of nutrients may not be achieved in

the part of the small intestine where most digestion

and absorption occurs.

0011 Most digestion occurs in the duodenum and je-

junum. When foods enters the duodenum, pancreatic

secretion of large amounts of bicarbonate and

enzyme rich juice is stimulated. This relatively large

volume of fluid will dilute the small intestinal con-

tents and considerably reduce the concentration

and therefore the viscosity of the food gum. The

relationship between concentration and viscosity is

not linear but exponential, and changes in concen-

tration can have a major impact on the viscosity

maintained. Experiments in rats have failed to find

any large increases in upper small intestinal viscosity

after ingestion of large amounts of guar gum. The

gums are not digested, however, so the viscosity is

regained as the nutrients and fluid are absorbed.

Thus, the viscosity in the ileum where bile acids are

reabsorbed, as part of the enterohepatic circulation,

will be much higher. It is well established, however,

that postprandial blood glucose is reduced con-

siderably after ingestion of gums. Bread containing

lower-molecular-weight guar still has the action of

reducing postprandial glycemia, again suggesting

that viscosity may not be the critical factor.

Blood Glucose and Diabetes Mellitus

0012In the past, the use of gums such as isolated solutions

of guar gum to reduce the rise in postprandial blood

glucose was thought to be of some therapeutic use in

diabetes mellitus. Although the effects of guar gum

reducing postprandial hyperglycemia and insulinemia

are well established, this approach in treating diabet-

ics was not very successful, as the gum solutions used

had poor organoleptic properties with an undesirable

mouth feel and aftertaste. Since then, the incorpor-

ation of gums such as guar and oat gum in bread

products has been explored. It has been found that

very palatable breads with proven effects on intestinal

absorption rate can be produced, but their use in the

diabetic diet has still to be established. There has also

been much recent interest in the production and

action of cereal products containing oat b-glucans,

ispaghula (psyllium), and Konjac Mannan.

Therapeutic Enteral Diets

0013Sometimes, patients, including children, have a

problem in dealing with normal foods and are fed

a simplified synthetic fluid diet of easily assimilated

nutrients. These diets often have to be liquid to allow

administration through tubes directly into the gastro-

intestinal tract. Until recently, these elemental or

polymeric diets have contained no source of dietary

fiber. This can result in constipation and, after pro-

longed use, produce atrophy of the colonic mucosa.

Some of these liquid diets have now been formulated

with soluble gums as a source of dietary fiber to

prevent these symptoms and to maintain a healthy

microflora. The gums used include soy polysacchar-

ide, pectin, and carboxymethylcellulose.

Effect of Gums on Plasma Cholesterol

0014Heart disease is one of the major killers of the de-

veloped world, and one of the main risk factors is high

plasma low-density lipoprotein (LDL) cholesterol

coupled with low high-density lipoprotein (HDL)

cholesterol. HDL cholesterol is associated with clear-

ance of sterol from the peripheral tissues to the liver.

Many of the viscous food gums (oat, pectin, guar,

psyllium, and others) have been shown to have a

significant and long-term effect in reducing LDL chol-

esterol while having little effect on, or increasing, HDL

cholesterol. The effect of the gums is modest but may

be of significance in reducing the risk of heart disease.

GUMS/Dietary Importance 3009

0015 It is not clear by which mechanisms the gums act

(Table 2). They could reduce fat digestion either

directly or by interfering with micelle formation.

Although there is likely to be only a small increase

in total fat losses, the delaying of fat absorption can

result in a change in the type of chylomicra or lipo-

proteins produced. For those gums like pectin with

cation-exchange properties, the bile acids may be

bound to the carbohydrate molecule, which are then

excreted in the feces. Viscous fibers without cation

exchange may also trap bile acids within their matrix.

Several gums, including guar, pectin, and ispaghula

(psyllium) have been shown to increase fecal bile acid

losses, and this has been associated with reduced

cholesterol. However, if the fiber is fermented, the

bile acid would be released in the large intestine

where they could be converted to secondary bile

acids and returned to the bile acid pool. Moreover,

low-molecular-weight (and therefore low-viscosity)

guar gum in bread still reduced cholesterol and not

all studies demonstrating an effect on cholesterol

show an increase in fecal bile acid. If fecal bile acid

losses are increased, cholesterol in the liver is used to

replace them, and hence plasma cholesterol falls.

0016 Cholesterol levels could also be decreased due to

the higher fecal losses of cholesterol from increased

intestinal cell loss seen with viscous fibers, and there

has been much discussion about the potential actions

of propionic acid produced from the fermentation of

the gums. However, the inhibitory effects of propi-

onic acid on hepatic cholesterol synthesis appear to

require pharmacological levels of propionate rather

than those that are produced from the fermentation

of food gums under normal conditions. Acetate is

a precursor of fat synthesis and lactulose a highly

fermentable saccharide increased serum LDL choles-

terol when fed to healthy volunteers. This was associ-

ated with higher levels of serum acetate in some

subjects. It is evident that the actions of gums on

serum cholesterol are mediated by different mechan-

isms for individual gums and for the same gums under

different circumstances.

Satiety and Obesity

0017If gums had a major effect on satiety, they could be a

useful adjunct to the prevention and treatment of the

increasing public health problem of obesity. However,

although highly viscous gums such as guar have been

shown to increase satiety and even promote some

weight loss in short-term studies, the unpalatability

of such gums and the moderate effects they have on

weight make them of unlikely benefit in weight-loss

or weight-maintenance programs.

Gums and Mineral Bioavailability

0018Some food gums with cation-exchange properties

have been shown in vitro to bind minerals such as

calcium. Even those gums without direct binding sites

may still entrap these minerals and delay or inhibit

their absorption. In vivo human studies have shown

an increase in fecal mineral output, but the overall

mineral balance is not affected in adults. The effect in

young children is unknown and is one of the reasons

why a high dietary fiber intake has not been encour-

aged in the under fives. However, there is no evidence

of significant mineral imbalance in children on high-

fiber diets, and the effects are unlikely to be great.

Moreover, there are several studies now suggesting

increased calcium absorption in the large intestine,

perhaps encouraged by the reduced pH from the

bacterial fermentation of soluble fiber.

Effects of Gums in the Large Intestine

0019Most food gums are readily fermented in the large

intestine of man by the diverse and dense microflora.

The products and consequences of this fermentation

may have a major influence on human health, and

these effects may outweigh the impact of the food

gums in the small intestine.

Colonic Fermentation

0020The human colonic microflora is thought to contain

over 400 different species (perhaps even more as new

molecular techniques, which measure bacterial DNA

and rRNA, are discovering that the majority of bac-

terial groups have never been cultured (See Micro-

flora of the Intestine: Probiotics). The mainly

anaerobic bacteria use the carbohydrates to produce

short-chain fatty acids (SCFA), mostly acetic, propi-

onic, and butyric acids, and the gases CO

,CH

tbl0002 Table 2 Possible mechanisms for the effect of gums on serum

cholesterol

Inhibition of fat digestion

Viscosity inhibits mixing of enzymes and lipids

Viscosity/cation exchange traps/binds bile acids

Viscosity inhibits formation of micelles

Viscosity inhibits movement of micelles

Increased bile acid synthesis from cholesterol

Binding of bile acids by cation exchange, viscosity

Increased fecal bile acid losses

Increased fecal cholesterol losses

Inhibited cholesterol absorption

Increased intestinal cell losses

Decreased cholesterol synthesis

Inhibition of hepatic cholesterol synthesis by propionic acid

(probably only a pharmacological effect)

3010 GUMS/Dietary Importance

and H

S. Colonic pH is also determined by the bal-

ance between the rate of fermentation and the buffer-

ing of the SCFA by bicarbonate exchanged when the

SCFA are absorbed.

0021 The fermentability of each gum depends on its

structure and water solubility. The presence of bac-

teria with the required enzymes for each step of gum

degradation is also essential. The degradation and

fermentation of a complex carbohydrate structure

may be the result of a synergy between several bacter-

ial species, and some of the enzymes may take time

to be expressed either by induction of enzymes, in

originally predominant bacteria, or by an increase in

dominance by new groups. It has been shown in rats

that it can take many weeks before the colonic micro-

flora is truly adapted to the introduction of a new

gum into the diet.

0022 The extent and rate of fermentation of food gums

are important. The rate of fermentation may deter-

mine the site of SCFA production. Most colonic

disease occurs in the distal colon, and yet most fer-

mentation occurs in the proximal colon. Therefore,

food gums that are slowly fermented may encourage

prolonged fermentation and SCFA at more distal

sites.

0023 The pattern of SCFA produced from each gum also

differs. Pectin increases the proportion of acetic acid

produced, guar gum and ispaghula tend to promote

the production of propionic acid, and oat gum

increases production of butyric acid.

Health Impact of Increased Colonic

Fermentation

0024 The colonic fermentation of carbohydrates including

food gums can result in a substantial fall in colonic

luminal pH. There are several benefits of reduced

colonic pH, which include the inhibition of undesir-

able bacterial activities such as 7a dehydroxylase,

which forms secondary bile acids that can help to

promote cancer. Reduced colonic pH can also lead

to precipitation of fatty acids, sterols, and other

molecules, which have been implicated as procarcino-

gens. Low pH also reduces the rate of colonic cell

proliferation, thought to be a risk factor for colon

cancer. However, the SCFA produced during fermen-

tation can stimulate cell proliferation, and so under

normal conditions, the effects of pH here may be

canceled out. It has been noted, however, that popu-

lations in South Africa with a low risk of colonic

cancer also have a lower colonic and fecal pH than

those from high-risk populations.

0025 The side-effects of increased colonic fermentation

and of ingestion of large amounts of food gums

include flatulence and abdominal distention. These

are usually associated with eating large amounts of

fermentable material but may reduce over time as

the colonic flora adapts to the new carbohydrate

substrate supply and adopts a more efficient fermen-

tation process. This may take over a week.

0026The SCFA have many potential actions that are

generally beneficial for health. They promote the ab-

sorption of water and electrolytes and inhibit the

growth of pathogens, reducing the risk of diarrhea.

They all stimulate cell proliferation throughout the

gut, even though they are produced in the large intes-

tine, and this may be important for wound healing

after gut surgery or after gastrointestinal disease. In

addition to these general effects, each fatty acid has its

own idiosyncratic actions.

Actions of Individual SCFA

Acetic Acid

0027Acetic acid is always the main SCFA produced, usu-

ally making up 50–70% of the SCFA. Acetic acid is

the only SCFA to reach the systemic circulation in

significant amounts. It can provide energy for muscles

and other tissues and is noninsulinogenic. Acetate has

less effect on the colonic enterocytes in terms of pro-

liferation and motility than the other SCFA. Pectin

has been shown to increase the proportion of acetate

when fermented by fecal bacteria in vitro. Acetate

produced during pectin ingestion may also benefi-

cially affect fibrin network architecture in patients

with hypercholesterolemia, reducing the risk of

heart disease.

Propionic Acid

0028Propionic acid is usually the second most abundant

SCFA produced during fermentation. Propionic acid

is absorbed into the portal vein but is mainly removed

by the liver. It is gluconeogenic, unlike the other

SCFA, and was previously thought to inhibit choles-

terol synthesis by the liver. However, although this

effect has been well demonstrated under experimental

conditions, it is now thought that not enough propi-

onic acid is formed in the colon and absorbed into the

portal vain to have this effect. However, it is worth

noting that those gums that are associated with

cholesterol-lowering effects are often those that

promote propionate production.

Butyrate

0029Butyric acid is the preferred fuel for the colonic enter-

ocyte. It has also been associated with several poten-

tial anticancer properties. An increase in the rate of

colonic cell proliferation is believed to be a risk factor

for cancer. However, all cells in the colon are

GUMS/Dietary Importance 3011

programmed with a time to die by a controlled pro-

cess called apoptosis. It is probably the balance be-

tween the levels of proliferation and apoptosis that

determine whether a cancer will or will not develop.

Although butyrate has been shown to stimulate cell

proliferation of normal colonic cells, it has been

shown to stimulate apoptosis in cancer cells in vitro.

It may also affect other important stages in carcino-

genesis such as the promotion of cell differentiation

and inhibition of histone deacetylase, which may

promote DNA repair.

Gums as Prebiotics

0030 There has been much recent interest in the beneficial

health effects of certain strains of lactic acid bacteria

or probiotics believed to have various health benefits.

These include reducing the duration of rotavirus diar-

rhea, reducing eczema in atopic infants, and poten-

tially having protective actions against intestinal

diseases, including colitis and cancer (See Microflora

of the Intestine: Probiotics). These probiotics are usu-

ally bifidobacteria, lactobacilli or streptococci but

may include certain strains of E. coli. The survival

of these bacteria in the gut can be enhanced by the

presence of certain fermentable carbohydrates. These

are called ‘prebiotics,’ but to fulfill the definition,

carbohydrates must selectively increase the popula-

tions of the probiotic organisms. The most accepted

prebiotics are fructo-oligosaccharides and resistant

starch, but some gums may also increase bacterial

populations.

Calories from Fermentation of Gums

0031 SCFA produced from fermentation can be used by the

colonic cells and other body tissues for fuel. It is now

recognized that dietary fibers, including gums, con-

tribute energy, but the exact amount depends on the

fermentability and any increased fecal nitrogen losses.

After absorption, the SCFA have to be acylated before

entering the major metabolic pathways, and this also

has an energy cost. It is now estimated that, on

average, fermentation provides 8.4 kJ (2 kcal) per

gram of carbohydrate.

Constipation

0032 If a dietary gum is poorly fermented, it may have

sufficient residual water-holding capacity to increase

stool output. Ispaghula (psyllium), which has a water-

holding capacity of approximately 7 g g

1

, has been

used successfully to treat mild to moderate constipa-

tion for many years. Gellan, a bacterially produced

gum, has also been shown to be a potent stool bulker

but is not used therapeutically. Other gums such as

karaya and xanthan, which are slowly fermented,

may increase stool wet weight and fecal SCFA

without increasing stool solids.

See also: Colon: Cancer of the Colon; Dietary Fiber:

Properties and Sources; Energy Value; Microflora of the

Intestine: Probiotics; Pectin: Properties and

Determination; Food Use; Probiotics

Further Reading

Blake DE, Hamblett CJ, Frost PG, Judd PA and Ellis PR

(1997) Wheat bran supplemented with depolymerised

guar gum, reduces the plasma cholesterol concentration

in hypocholesterolemic human subjects. American Jour-

nal of Clinical Nutrition 65: 30–42.

Bonfield C and Kritchevsky D (1997) Dietary Fiber in

Health and Disease. New York: Plenum Press.

Brown L, Rosner B, Willet WW and Sacks FM (1999)

Cholesterol lowering effects of dietary fiber: a meta-

analysis. American Journal of Clinical Nutrition 69:

30–42.

Canovale E, Tomassi G and Cummings JH (eds) (1995)

Topics in dietary fibre research. European Journal of

Clinical Nutrition 49(Supplement 3).

Eliasson AC (ed.) (1996) Carbohydrates in Foods, pp. 265–

319. New York: Marcel Dekker.

Nutritional Role of Guar Gum

P Rayment* and P R Ellis, King’s College London,

London, UK

Introduction

0001It has been recognized for many years that guar gum,

a galactomannan-rich legume flour, has considerable

potential as a therapeutic agent in the dietary man-

agement of diabetes and hyperlipidemia. For example,

it is now well established that guar gum can reduce

the postprandial rise in blood glucose and insulin

concentrations in response to glucose drinks and

starch-rich meals in both healthy and diabetic sub-

jects. Furthermore, the addition of guar gum to a

normal diet can elicit long-term improvements in

metabolic control in people with type 1 (insulin-

dependent) and type 2 (non-insulin-dependent) dia-

betes mellitus. The ease of isolation and purification

of guar gum explains why it is frequently used as a

Now at Unilever Research Colworth, UK.

3012 GUMS/Nutritional Role of Guar Gum

‘model’ non-starch polysaccharide in physiological

and clinical studies. Also, the fact that it is well char-

acterized with respect to other sources of dietary non-

starch polysaccharides makes it an ideal candidate for

clinical use. This article highlights the nutritional and

therapeutic role of guar gum, but mainly focuses on

its blood glucose- and cholesterol-lowering actions.

Also included is a brief discussion of the physico-

chemical properties of guar gum, including its rheo-

logical behavior in model systems and its influence on

the properties of human and animal digesta in vivo,

which has an important, if not crucial, influence on

gut function and metabolism.

Extraction and Composition of Guar Gum

0002Guar gum, a type of dietary fiber, is extracted from

the seed endosperm of a leguminous plant (Cyamop-

sis tetragonoloba (L.) Taub.) indigenous to the Indian

subcontinent. Figure 1 shows the pods, seeds, and

endosperm extract of the guar plant. To recover

guar gum from the seed, the endosperm halves (splits)

c d

fig0001 Figure 1 (see color plate 81) Pictures of pods, seeds, and endosperm extract of the guar plant (Cyamopsis tetragonoloba (L.) Taub.), a

member of the Leguminosae family. (a) Green pods, scale bar ¼ 1.3 cm. (b) Dried pods, scale bar ¼ 1.6 cm. (c) Seeds, scale bar ¼ 1 cm.

(d) Endosperm halves (splits), scale bar ¼ 1 cm. (e) Guar gum flour, scale bar ¼ 1 cm. (f) Scanning electron micrograph of guar gum

flour, scale bar ¼ 100 mm.

GUMS/Nutritional Role of Guar Gum 3013

must be separated from the hull and the cotyledon.

The splits are ground to a fine flour and may then be

purified by repeated alcohol washings. In this way

guar gum is commercially produced as a flour and is

frequently used as an additive (E412) in the food

industry for its thickening and stabilizing properties.

0003 Chemically, guar gum mainly consists of a high-

molecular-weight polysaccharide, namely galacto-

mannan, which is based on a mannan (M) backbone

with galactose (G) side groups, as shown in Figure 2.

The ratio of the two components (G:M) seems to vary

slightly depending on the origin of the seed, but the

gum is generally considered to contain approximately

one galactose unit for every two mannose units. The

linkages between the sugar groups cannot be hydro-

lyzed by human digestive enzymes in the small intes-

tine but are fermented by bacterial enzymes in the

large intestine. The products of this fermentation in-

clude short-chain fatty acids, mainly acetic, propi-

onic, and butyric acids, which have important

nutritional implications, notably in relation to co-

lonic function.

0004 The Food Chemicals Codex specifies a standard of

not less than 66% of galactomannan in food-grade

guar gum in the USA, although generally most samples

contain more than 80% (w/w). The other compon-

ents of guar gum, including fat and protein, which are

considered to be impurities, vary between different

sources. Table 1 lists the typical composition of a

commercially available guar gum flour.

Potential Adverse Effects of Guar Gum

0005In the food industry, guar gum is normally added to

foods in concentrations of < 1 g 100 g

1

to provide a

thickening, binding, or stabilizing function. Since guar

gum is derived from a normal food material, it is classi-

fied by the Food and Agricultural Organization/World

Health Organization as a substance of low toxicity. The

safety of guar gum is substantiated by the results of a

number of long-term studies in human subjects. In

general, adverse effects, observed in some but not all

subjects, appear to be associated with gastrointestinal

disturbances. These effects, which sometimes appear to

diminish with regular consumption of guar gum, in-

clude flatulence, gastrointestinal pain or discomfort,

nausea, and diarrhea. Some of these side-effects prob-

ably result from bacterial fermentation of guar gum in

the large intestine. The extent of the problems is gener-

ally related to the dose given, with more adverse gas-

trointestinal effects occurring when guar gum

consumption is excessive (*15–30 g day

1

). It has

also been suggested that guar gum may reduce the

bioavailability of a number of micronutrients (e.g.,

vitamins), although there is no evidence to indicate

that this is of nutritional significance.

0006There is more concern about the use of macro-

aggregates of guar gum that have the potential for

causing obstruction in the esophagus and gastrointest-

inal tract. This concern has arisen from reported inci-

dents of esophageal obstruction and rupture in patients

ingesting guar granules. There have also been a number

of cases of esophageal and small-bowel obstruction in

patients ingesting a particular type of guar-containing

diet pill. The USA and Australia have banned such

products. In the European Community (EC), food

additive legislation prohibits the use of guar gum and

other gums for the production of ‘dehydrated food-

stuffs that will rehydrate on ingestion.’ However, the

selling of guar products as medicine under license and

administered to patients under medical supervision is

permitted under EC legislation.

Physicochemical Properties of Guar Gum

Solution Properties

0007The ability of the galactomannan component of guar

gum to hydrate and increase the viscosity, or thick-

ness, of the intestinal contents is an important deter-

minant of its physiological effects. The majority of

polysaccharides used in nutritional studies exist in

solution as ‘random coils’ and the number and

fig0002 Figure 2 Structure of guar galactomannan showing the man-

nose backbone and galactose side chains.

tbl0001 Table 1 Chemical composition of a commercially available

guar gum flour

Component Concentration (g 100 g

1

)

Galactomannan 73.0–86.7

Protein 3.0–6.0

Crude fiber 1.0–4.0

Ash (total minerals) 0.8–2.0

Fat (petroleum ether extractables) 0.5–1.0

Moisture 8.0–14.0

Total impurities

13.3–27.0

Calculated as total nongalactomannan components.

3014 GUMS/Nutritional Role of Guar Gum

molecular size of the polymer chains are critical de-

terminants of viscosity and thus physiological activ-

ity. The high levels of viscosity associated with water-

soluble polysaccharides originate predominantly by

interpenetration of individual polymer chains to

form entangled networks. At low guar gum concen-

trations, the individual polymer coils occupy a separ-

ate domain within the solution with little or no

interpenetration. These solutions are described as

Newtonian because their viscosity is independent of

the rate of stirring or mixing (i.e., related to shear

rate). At higher concentrations, where the polymer

coils begin to overlap, the system becomes non-

Newtonian and viscosity then depends on the shear

rate. The more common type of non-Newtonian flow

in food systems is shear-thinning (or pseudoplastic)

behavior, where viscosity decreases with increased

mixing. Figure 3 shows the effect of shear rate and

concentration on the viscosity of guar gum solutions.

0008 The viscosity or flow properties of guar gum can be

determined by using various rheological techniques.

Rheology (from the Greek word rheos, meaning

stream) is a branch of physical science concerned

with the study of the flow and deformation of mater-

ials under externally imposed forces. However, to

predict the rheological behavior of guar gum in the

human intestine, several factors require consider-

ation. These factors include dependence on the

mode of administration, the size of the dose and the

physicochemical characteristics of the sample used as

well as the physical state of the galactomannan in the

gut lumen.

Hydration Kinetics of Guar Gum

0009 The realization that viscous polysaccharides can

be used for therapeutic benefit has led to the

development of various pharmaceutical preparations

containing guar gum. These include capsules or

macroaggregates of guar gum, such as granules and

minitablets, which have been designed to hydrate

slowly in the mouth to improve palatability. How-

ever, their beneficial effects have been reported to be

extremely variable, most probably due to their lack of

hydration in the stomach and small intestine. Conse-

quently, the hydration rate of guar gum in the gut

lumen is a critical factor. Figure 4 compares the

hydration rate profiles of a range of commercially

produced samples of guar gum. These samples were

hydrated in water and viscosity was measured over a

5-h period. Unsurprisingly, the marked differences

observed are mainly attributed to the physical nature

of the guar gum preparations. Thus, samples with a

smaller particle size (flours < 150 mm) hydrate much

more quickly than those with a larger particle size

(> 1 mm), e.g., Glucotard minitablets. The results of

many clinical trials have shown that the latter types

are often physiologically ineffective.

0010Some research workers have also reported that the

clinical efficacy of guar gum is improved when it is

incorporated directly into a food rather than when

taken as a pharmaceutical premeal supplement. Thus,

a key factor in optimizing the glucose-lowering action

Shear rate (s

−1

)

Viscosity (Pa

0.01

0.001

0.01

0.1

100

0.1 1 10 100 1000

0.1%

0.2%

0.4%

0.6%

0.8%

1.0%

1.5%

2.0%

2.5%

fig0003 Figure 3 Shear rate and concentration dependence of viscosity

for guar galactomannan solutions. Data taken from Rayment P

(1996) PhD thesis, University of London.

Ultimate

viscosity

Time (h)

Viscosity (Pa

12345

fig0004Figure 4 Hydration rate profiles of guar gum samples pre-

sented as viscosity (Pa s) versus time. Experimental points are

mean values from measurements on 4 replicate solutions of

native guar gum flours M150 (—&—) and RG30 (- -d- -), pharma-

ceutical granule preparations Guarina (. . .!. . .), Guarem (—h—)

and Lejguar (—s—), and Glucotard minitablets (—!—). The

‘ultimate viscosity’ is the measurement of viscosity taken after

24 hours of hydration following homogenisation of the guar solu-

tion with an ultra-turrax mixer. The final viscosity is taken only

when a consistent maximum reading is recorded on the visco-

meter (usually after homogenising for 4–6 min). Data taken from

Ellis PR & Morris ER (1991) Importance of the rate of hydration of

pharmaceutical preparations of guar gum; a new in vitro moni-

toring method. Diabetic Medicine 8, 378–381.

GUMS/Nutritional Role of Guar Gum 3015

of guar gum is to insure that the polymer is in intimate

contact with the starch, the main source of dietary

glucose.

Nutritional and Physiological Effects of

Guar Gum

0011 Many research groups have reported the efficacy of

guar gum in attenuating the postprandial rise in blood

glucose and insulin concentrations in healthy and

diabetic subjects (Figure 5). One reasonable explan-

ation of these postprandial effects is that guar gum

reduces the rate (and perhaps the extent) of digestion

and absorption of available carbohydrate in the gas-

trointestinal tract. (Here available carbohydrate is

defined as carbohydrate that is digested by a-amylase

in the upper gastrointestinal tract and absorbed into

the portal blood (mostly as glucose); this includes

starch, dextrins and simple sugars (e.g., sucrose, lac-

tose).) There are however problems with obtaining

quantitative estimates of glucose absorption in

humans; for example, access to the hepatic portal

vein is very difficult. Nevertheless, studies in pigs, a

useful animal model for studies on dietary polysac-

charides, have provided the first direct evidence that

guar gum decreases the rate of glucose absorption. In

these types of experiment, glucose concentrations

were measured simultaneously in the hepatic portal

vein and peripheral blood (via the mesenteric artery)

and the flow rate of the portal blood was measured

also. Significant reductions in glucose absorption

over 4 h were observed in the experimental animals

given guar gum doses equivalent to those consumed

by humans in clinical studies. In the same studies,

insulin secretion was significantly decreased over the

same period in response to the guar diet. This suggests

that the attenuation in the plasma insulin levels seen

in humans is attributed to a reduction in insulin se-

cretion from the pancreatic islet b-cells in response to

a lowered rate of glucose absorption. Also, a number

of human studies have shown that guar gum de-

creases the postprandial rise in the insulinotropic hor-

mones, plasma gastric-inhibitory polypeptide and

glucagon-like peptide-1, which may partly explain

the insulin-lowering action of guar gum. Another

interpretation of data showing decreases in the per-

ipheral blood insulin concentrations is that there has

been an increase in hepatic extraction of insulin, a

normal process by which insulin is removed from the

blood circulation. Studies carried out to substantiate

this have been largely contradictory, however.

0012In relation to long-term studies of insulin action in

experimental animals and humans, it has been

reported that guar gum increases (improves) insulin

sensitivity. A reduction in sensitivity to insulin (i.e.,

‘insulin resistance’) is seen as a decrease in the

response of tissues to insulin stimulation and is con-

sidered to be an important risk factor for both coron-

ary heart disease (CHD) and type 2 diabetes.

Mechanism of Action of Guar Gum

0013There is little doubt that the consumption of guar

gum, whether mixed in a drink or solid food, can

significantly modify digesta properties at all sites of

the gastrointestinal tract. One important feature is

the capacity of guar gum at relatively low concen-

trations to form a highly viscous network in solution.

As mentioned previously, the behavior of guar gum

in a food material is highly complex. In such systems

there is unlikely to be one mechanism to explain

fully the effects of guar gum in the human gut.

A number of physicochemical mechanisms may be

0123

Time (h)

Blood glucose (mmol l

−1

)Plasma insulin (mU l

−1

)

Control

Test

fig0005 Figure 5 Postprandial blood glucose and insulin concentra-

tions of patients with type 2 (noninsulin-dependent) diabetes

mellitus in response to control and test breakfast meals of

bread and marmalade (total available carbohydrate, mainly

starch and sugars, was 106 g). The test contained 16 g of guar

gum (in the bread) and 10 g of pectin (in the marmalade). *Statis-

tically significant differences between control and test meals at

individual time points. Figure redrawn from data published by

Jenkins DJA, Leeds AR, Gassull MA et al. (1976) Unabsorbable

carbohydrates and diabetes: decreased post-prandial hypergly-

caemia. Lancet 2: 172–174.

3016 GUMS/Nutritional Role of Guar Gum

involved, which are largely dependent on the type

and form of the guar gum ingested. Much of the

literature indicates that guar gum decreases the rate

of glucose absorption into the hepatic portal vein

by inhibiting the processes associated with digestion

and absorption of available carbohydrates. These

processes include gastric function, intestinal transit

and mixing, a-amylase–starch interactions, and the

movement of products of starch hydrolysis to the

gut mucosa.

0014 A number of experiments in humans and animals

have shown that guar gum reduces the rate of gastric

emptying of a meal. However, there are also studies

that have shown no effect or even sometimes an ac-

celeration in gastric emptying after the ingestion of

guar gum. These contradictory results probably arise

from differences in the type of guar gum used, the

way in which it was administered, the proportion of

solids and liquids in the meals, and the techniques

used to measure gastric emptying. In general, it is

considered that guar gum may reduce the rate of

gastric emptying under some conditions, but this

action is unlikely to be the only mechanism. Other

groups have shown guar gum has additional effects

on gastric function. In studies in dogs, guar gum was

reported to impair gastric trituration and sieving by

increasing the viscosity of the stomach contents. It

was shown that there was a significant increase in

the proportion of larger particles of food entering

the small intestine when guar gum was incorporated

into the diets. Possible explanations for these findings

include suggestions that the increased viscosity stabil-

izes the suspension of larger food particles, which are

likely therefore to escape maceration in the antrum of

the stomach, and/or that the guar gum alters the

contractile pattern of the terminal antrum, thus

impairing the ability of the stomach to retain large

particles.

0015 There is evidence from human and animal studies

that guar gum delays the transit of digesta in the small

intestine. The postprandial pattern of gut motility

also seems to be influenced by guar gum and other

types of dietary fiber. Thus, whereas wheat bran and

cellulose-supplemented diets were reported to pro-

duce prolonged bursts of intestinal contractions, guar

consumption caused continuous contractions with a

50% reduction in amplitude compared with the other

types of fiber. The effect of guar gum on the mixing

behavior of digesta at different sites of the gastro-

intestinal tract has yet to be investigated, but it is

likely to be extremely complex. It has been suggested

that an increase in the viscosity of digesta will pro-

duce laminar or ‘streamline’ flow, rather than turbu-

lent or disorderly flow, which is characteristic of less

viscous fluids and facilitates efficient mixing of

digesta in the gut. An inhibition of the digesta flow

rate will inhibit physical mixing of nutrients and

enzymes (e.g., pancreatic amylase). Also, laminar

flow behavior would almost certainly have an effect

on the rate at which nutrients are exposed to the

epithelial surface and then absorbed into the hepatic

portal vein.

0016However, an explanation of the precise mechanism

by which guar gum modifies gut function is still elu-

sive, despite the plethora of clinical and physiological

studies that have been undertaken. The problem is

that researchers understand little about the behavior

of guar gum in the human gastrointestinal tract.

Digesta is an extremely complex heterogeneous ma-

terial and the effects of guar gum on this system have

not been studied in great detail. Much of the work

that has been done has involved investigating digesta

in the stomach and small intestine, since it is assumed

that guar gum has negligible effects on viscosity in the

large intestine due to depolymerization by bacterial

fermentation. There are a number of problems asso-

ciated with measuring digesta rheology in vivo.In

human subjects, there are obvious practical difficul-

ties in gaining access to the sites of interest in the gut.

Notwithstanding the initial problems involved with

sampling intestinal contents, particularly in humans,

the rheological properties of digesta will depend on

the precise location of the sampling site. Thus, digesta

is subjected to dilution by gastric secretions and then

concentration by absorption of water, by both the

intestine and the guar gum, at it passes further along

the intestinal tract. Also the samples collected need to

be tested quickly to prevent drying or dissolution of

particulate materials. In the case of animal experi-

ments, this requires access to rheometers or other

measuring devices in the animal house.

0017Many factors complicate the interpretation of the

rheological behavior of digesta containing guar gum.

These include a lack of information on the hydration

kinetics of guar gum in vivo and the contribution of

undissolved food particulates to viscosity. Therefore,

to enhance our understanding of the physical proper-

ties of digesta it is important to study model systems

of entangled networks filled with particulates, al-

though in vivo other factors such as gastrointestinal

motility and fluid secretion/absorption also play a

crucial role. In recent years, the rheological properties

of guar gum solutions with increasing particulate

concentrations have been investigated. The particu-

lates studies included materials that were essentially

spherical and rod-like in shape (i.e., starch granules

and microcrystalline cellulose, respectively). The

effect of particulates or ‘fillers’ is primarily to increase

the viscosity above that of the pure guar gum system,

as shown in Figure 6. The initial Newtonian flow

GUMS/Nutritional Role of Guar Gum 3017