Download CARBOHYDRATE CONTENT OF FOODS INTAKES OF DIETARY

CARBOHYDRATES
Table 5.6
67
Major polysaccharide food additives
Type
Variants
Features
Properties
Usage
Cellulose
Carboxy methyl
p(1->4)-D-Glucans
Water-insoluble
Thickening agent
Pectins
High methoxy, low
methoxy, modified
Galacturonans with few
side-chains
Methoxyl varies with source
High methoxy pectins gel in
sugar solutions at low pH
Low methoxy forms gel
with Ca in dilute solution
Very widely used as stabilizer
and emulsifier, and as the
setting agent in jams
Gums
Guar
Locust bean
Tragacanth
Arabic
Polymers of glucose, galactose,
mannose, arabinose, rhamnose
and uronic acids
May be methoxylated or acetylated
Form viscous solutions
Wide use in foods as emulsifiers,
stabilizers and thickeners
Algal
Alginates
Copolymers of D-mannuronic
and L-guluronic acids
Insoluble in cold water; Na,
K, NH4 salts and propylene
glycol esters very soluble
Widespread use in many foods;
milk, puddings, dairy products,
ice cream, beer, confectionery, etc.
Carrageenans
Mixed galactose and
sulphated galactose polymers
One type gels strongly with
potassium ions to give a
brittle gel
One type gels strongly with
calcium ions to give an
elastic gel
One type is non-gelling
Very widely used in a large
number of types of foods
Exhibit synergic effects
(on physical properties)
with proteins
Xanthans
Glucose, mannose and glucuronic
acid in a linear chain with
mannose side-chains with
pyruvate as a side-chain
Non-gelling; soluble in hot
and cold water giving
viscous solutions
Used as a stabilizing agent
Often used in combination with
plant gums
Polydextrose
Highly branched, synthetic,
glucose polymer
Water-soluble
Used as a low-calorie filler
Neosugar
Short chains of fructose
Water-soluble
Used as a low-calorie fat substitute
In the plant, NSP are responsible for the rigidity of
cell walls. Plant cell-wall NSP may be isolated and fractionated into more or less purified preparations. Table 5.6
lists these and some other polysaccharides used as food
additives.
CARBOHYDRATE CONTENT OF FOODS
Table 5.7 gives values for the sugar, starch and NSP content of a selection of plant foodstuffs. A good source of
further values is the McCance & Widdowson food tables
and supplements (e.g. Holland et al 1988, 1991).
INTAKES OF DIETARY CARBOHYDRATES
Table 5.8 shows fibre intakes estimated on the basis of
household food surveys in European countries. There
is a wide range, from 11.8 g per person per day in the UK
to 24.7 g per person per day in Italy. The average intake
value for the countries using mostly enzymic-chemical
methods to measure fibre is 16.6 g per person per day,
and it is 20.3 g per person per day for those countries
using mainly enzymic-gravimetric methods (Cummings
1993).
Urban populations from Denmark, Finland and Japan
have NSP intakes similar to those in Britain, although
rural communities in these countries tend to consume
more. Present-day intakes of dietary fibre in affluent
countries are generally lower than they have been in the
past, when there was a greater dependence on lightly
processed carbohydrate foods.
Data for intakes of other carbohydrate fractions are less
plentiful but daily intakes per capita of about 90 g of
sugars and 130 g of total starch per day are given for the
UK National Food Survey, and rather consistent intakes
per capita of about 3 g of resistant starch per day have
been calculated for adults in several European countries.
Carbohydrate intakes in the developing world are
not well documented but there are some data from The
Gambia, West Africa: estimated daily intakes per person
of 375 g of starch and 25 g of NSP for adult males have
been reported, with lower value for women and children,
reflecting their lower body weights (Hudson & Englyst
1995). There are also some data from Latin America;
when adjusted for estimates of energy intakes, the NSP
intakes of rural Mexican men and women were similar
to those in Africa (Sanchez-Castillo et al 1997).
DIGESTION AND ABSORPTION OF
CARBOHYDRATES
The small intestine
The digestive process begins in the mouth, where chewing
breaks up the structure of compact food and mixes it
68
NUTRITIONAL SCIENCE
Table 5.7
The carbohydrate content (g/100 g as eaten) of foods (NSP, non-starch polysaccharide)
Food
Sugars
Starch
NSP
Cereals
All-Bran
Barley, pearled
Bran, wheat
Brown bread
Brown rice
Buckwheat
Corn flakes
Cornflour
Crispbread
Croissants
Digestive biscuit
Gingernut biscuit
Granary bread
Hot cross bun
Macaroni
Mince pie
Naan bread
Noodles, egg
Oat & wheat bran
Oat bran flakes
Pitta bread, white
Porridge
Puffed Wheat
Rice Krispies
Rye bread
Scones, plain
Scones, wholemeal
Shortbread
Shredded Wheat
Spaghetti, white
Spaghetti, w/meal
Sweetcorn, kernels
Water biscuit
Weetabix
Wheat flour, brown
Wheat flour, white
Wheat flour, w/meal
Wheatgerm
White bread
White rice
19.0
0
3.8
3.0
0.5
0.4
7.2
0
3.2
1.0
13.6
35.8
2.2
23.4
0.3
28.1
5.5
0.2
16.7
16.8
2.4
0
0.3
10.6
1.8
5.9
5.9
17.1
0.8
0.5
1.3
9.6
2.3
5.2
1.7
1.5
2.1
16.0
2.6
0
27.6
27.6
23.0
41.3
31.6
84.5
77.7
92.0
67.4
37.2
55.0
43.3
44.1
35.1
18.2
30.9
44.6
12.8
51.0
57.2
55.5
9.0
67.0
79.1
44.0
47.9
37.1
46.9
67.5
21.7
21.9
16.6
73.5
70.5
66.8
76.2
61.8
28.7
46.7
30.9
24.5
1.6
36.8
3.5
0.8
2.1
0.9
0.1
11.7
1.6
2.2
1.4
4.3
1.8
0.9
2.1
1.9
0.7
17.9
10.0
1.6
0.8
5.6
0.7
4.4
1.9
5.2
1.9
9.8
1.2
3.5
1.4
3.1
9.7
6.4
3.1
9.0
15.6
1.5
0.1
1.4
2.6
5.8
8.8
0.9
0.9
3.0
1.5
2.0
4.6
1.0
0.6
1.9
0
0.2
9.3
0.7
4.3
0
0.3
15.6
0.1
0.2
16.6
29.5
0.1
1.4
2.3
3.5
1.9
5.4
2.3
3.1
5.2
1.8
2.5
4.3
2.2
1.2
Vegetables
Asparagus
Aubergine
Baked bean
Beetroot
Broad bean
Broccoli, green
Brussels sprouts
Butter bean
Cabbage
Carrot
Chickpea
Chips, fried
Courgette
with saliva containing an amylase (ptyalin), but this
phase is relatively short, the amylase being inactivated
by the gastric acid when the food bolus is swallowed and
broken up in the stomach. Digestion occurs mainly in
the small intestine through the action of pancreatic aamylase, which hydrolyses starch to dextrins and maltose.
In the brush border of the intestinal epithelium there
are glucosidases that further reduce the dextrin, and
Food
Sugars
Starch
Cucumber
Kidney bean, red
Lentil, red
Lettuce
Marrow
Mushroom, fried
Okra
Old potato
Onion, fried
Parsnip
Plantain, fried
Potato crisps
Radish, red
Runner bean
Tomato
Turnip
Watercress
Yam
1.4
3.6
0.8
1.7
1.4
0.1
2.3
1.0
10.0
5.9
11.5
0.7
1.9
2.0
3.1
1.9
0.4
0.7
0.1
12.8
16.2
0
0.2
0.2
0.5
14.5
0.1
6.4
36.0
52.6
0
0.3
0
0.1
0
32.3
0.6
6.2
1.9
0.9
0.6
1.5
3.6
1.1
3.1
4.7
2.3
5.3
0.9
1.9
1.0
1.9
1.5
1.4
4.2
11.8
7.2
0.5
5.1
7.6
2.4
5.6
11.5
8.6
48.6
18.5
6.8
15.4
4.0
10.3
14.3
13.8
4.2
9.0
0
8.5
5.8
8.8
6.2
10.0
10.1
8.3
19.7
4.6
7.6
6.0
1.7
8.0
2.6
2.7
0
0
0
0
0
0.7
13.2
0
0
0
0
0
0
2.0
0.3
0
0.3
0
0
0
0
0
0
6.3
0
0
0
0
0
0
0
16.3
0
0.7
7.4
1.8
1.7
3.4
3.1
3.1
4.3
3.2
0.9
1.6
6.9
1.7
1.3
0.7
6.5
1.9
0.7
2.6
1.0
1.2
2.9
1.7
3.3
2.2
6.2
2.2
1.2
1.5
2.4
2.5
0.8
1.1
6.0
1.3
3.5
Fruits and nuts
Almond
Apple
Apricot
Avocado
Blackberry
Blackcurrant
Brazil nut
Cashew nut
Cherry
Damson
Fig
Gooseberry
Grapefruit
Grape
Hazelnut
Kiwi fruit
Lychee
Mango
Melon, Cantaloupe
Nectarine
Olive
Orange
Passion fruit
Paw-paw
Peanut
Pear
Pineapple
Plum
Prune, canned
Raspberry
Rhubarb, canned
Strawberry
Sunflower seed
Tangerine
Walnut
NSP
specific disaccharidases that hydrolyse maltose, sucrose
and lactose into their constituent monosaccharides
(glucose, fructose and galactose); these are transported
across the epithelial cells and enter the portal vein (see
Figure 5.1). Free concentrations in the intestine or at the
mucosal surface are likely to be high enough for passive or facilitated absorption, but as concentrations fall,
active transport against a concentration gradient becomes
CARBOHYDRATES
Table 5.8
Fibre intakes in Europe (after Cummings 1993)
Country
Amount
(g per person/day)
Year(s)
Analytical
method(s)
Croatia
Yugoslavia
Denmark
France
Germany (GDR)
19.0
20.6
19.1
15.9-17.5
17.3 (female)
20.7 (male)
24.7
18
22.4
15.1
11.8
1990
1990
1987
1972-1989
1988
1988
1980-1984
1990
1991
1991
1989
a
a
a, b
a, d
c
c
b +d
d
d
a
a
Italy
Norway
Spain
UK
Methods: a, NSP, Englyst; b, Association of Official Analytical
Chemists (AOAC), Prosky; c, other gravimetric; d, Southgate or
mixed.
necessary, which requires energy. The active transport
system is similar to that responsible for the absorption
of glucose from the renal tubules, and involves the
breakdown of ATP and the presence of Na + . Different
sugars compete for transport, and galactose and glucose
are absorbed faster than fructose. Surprisingly, glucose,
dextrins and some forms of starch are digested and
absorbed at equal rates (Wahlqvist et al 1978), indicating
that luminal digestion of these soluble glucans per se
is not a limiting factor in glucose absorption.
The rate of digestion and absorption of carbohydrate
from the small intestine plays a major role in determining
the metabolic effects of dietary carbohydrate. Factors
that reduce this rate include the presence of intact plant
cell walls, the dense physical structure of starchy foods
(see later), and the presence of proteins and fats. The
slowing of small intestinal absorption by increased meal
frequency (nibbling or grazing) results in reduced insulin
secretion and lower low-density lipoprotein (LDL)
cholesterol concentrations (Jenkins et al 1995).
lactose-intolerant individuals, SCC, and synthetic carbohydrates such as Polydextrose and Neosugar. Together
these provide about 5 g of carbohydrate per day in a UK
diet. In addition, an unknown amount of substrate is
available in the form of mucin secretions into the large
gut.
All the carbohydrates that reach the large intestine
(sugars, SCC, RS and NSP) may be fermented by the
colonic microflora, with the production of short-chain
fatty acids (SCFAs) and gases (Figure 5.2). The rate and
extent of fermentation depends on the form and solubility
of the substrate. Soluble carbohydrates such as pectin are
degraded almost completely, whereas insoluble polysaccharides, especially in lignified material such as wheat
bran, are more resistant.
Initially, polymers are enzymatically hydrolysed into
their constituent monomers (mainly glucose, galactose,
arabinose, xylose and uronic acids). The sugars are converted to pyruvate via the glycolytic pathway. Various
routes may be followed, depending on the microbial
species present and the identity of the substrate. The
principal SCFAs produced from all substrates are acetate,
propionate and butyrate; other organic acids such as
isobutyrate, valerate, isovalerate, lactate and succinate
may be produced in small amounts. The SCFAs are the
predominant anions in the large intestine and contribute
to the relatively low pH (5.6-6.6) of the colon contents.
The efficiency of SCFA production and the molar ratios
of acetate, propionate and butyrate generated from carbohydrate differ according to the identity of the substrate
(Table 5.5). For example, the fermentation of starch yields
SCFA with about 29% as butyrate. In contrast, only 2%
of the SCFA generated from the fermentation of pectin
is in the form of butyrate. Acetate and propionate are
rapidly absorbed from the colon and carried in the portal
vein to the liver. In experiments with healthy human
subjects, it was shown that acetate and propionate
enhanced the absorption of calcium from the distal colon
The large intestine
A variety of potentially fermentable organic molecular
species, including carbohydrates, fats and proteins,
escape digestion in the small intestine. Studies using ileostomy subjects (people who have had the large intestine
removed for medical reasons) have shown that dietary
NSP are virtually completely recovered in the ileostomy
effluent (Englyst & Cummings 1985) and will therefore
be available as substrates for fermentation in the large
intestine of intact subjects.
With normal epithelial turnover in adults on a Western
diet, endogenous carbohydrates from mucin and glycoproteins that line the small intestine contribute 3-5 g
of fermentable substrate per day. Other potential substrates for colonic fermentation include fructose and other
poorly absorbed monomers, sugar alcohols, lactose in
69
Carbohydrates reaching
the large intestine
Carbon dioxide
Hydrogen
Methane
Microbial metabolism
Energy
•*
I
Acetate
*" Butyrate
Propionate
Ammonia
Microbial growth
Breath
& flatus
Fig. 5.2
Faeces
Blood
Fermentation of carbohydrates in the large intestine.
70
NUTRITIONAL SCIENCE
(Trinidad et al 1996) and there is some evidence that
absorption of acetate in the rectum and colon is influenced by the presence of calcium and propionate
(Wolever et al 1995). SCFAs play an important role in the
regulation of the absorption of water and sodium from
the colon. Butyrate is the preferred fuel for human
colonocytes, particularly in the distal colon (AfonsoRodriguez et al 1995), and it is actively metabolized
to ketone bodies (acetoacetate and P-hydroxybutyrate),
carbon dioxide and water. The use of butyrate rather
than glucose or glutamine by the colonic epithelium is
thought to have beneficial effects; an antitumour action
has been demonstrated in vitro (Kim et al 1982, Krupitza
et al 1996). Most SCFAs are cleared by the liver, with
only acetate released (Pomare et al 1985) to be used as a
fuel by peripheral tissues (Skutches et al 1979). Propionate is utilized within the liver, where it may modify
carbohydrate and lipid metabolism.
The main gases produced during fermentation are
hydrogen and carbon dioxide. Some individuals have
the capacity to produce methane (CH4) from C 0 2 and H 2 ,
thus diminishing the total volume of gas accumulating
in the colon (C0 2 + 4H 2
CH 4 + 2H 2 0). In others,
sulphate-reducing bacteria incorporate hydrogen into
hydrogen sulphide, particularly after a high intake of
dietary sulphate (Christl et al 1992).
Excretion of gases per rectum occurs when production
of gas exceeds the absorptive capacity of the colon, but at
low levels the gases are largely absorbed and excreted
through the lungs. Measurement of the hydrogen expired
in breath is used as a marker for carbohydrate fermentation (Anderson et al 1981) and maybe useful in a clinical
setting to detect pancreatic insufficiency. However, this
technique cannot be used for reproducible assessment
of the quantity of carbohydrate entering the colon
(Cummings & Englyst 1991).
All dietary fats, proteins and carbohydrates escaping
digestion and absorption in the small intestine contribute
directly or indirectly to faecal bulk. However, dietary
fibre (NSP) is the only food fraction that has been shown
to have a universal effect on stool weight.
Two factors are involved in the faecal bulking effect
of dietary fibre. Soluble fibre is readily fermented and
leads to increased faecal weight through proliferation
of the bacterial population. Insoluble fibre, especially in
lignified material like wheat bran, partially survives
the fermentation process and acts directly as a bulking
and water-holding agent.
Stool weight is closely associated with transit time.
Faecal weights below 150 g/day are associated with increasing transit time, and constipation commonly occurs
if the faecal output falls below 100 g/day.
Transit time is the time taken for a meal to pass from
the mouth to the anus, which may be measured by
administering a marker by mouth and measuring the
time taken for this to appear in the faeces. The marker
may be a dye such as carmine red, or a capsule of tiny
radioopaque plastic shapes that can be detected by
X-rays. It is convenient to divide whole-gut transit time
into two phases, mouth-to-caecum transit, and colonic
transit, which takes approximately 10 times as long.
Mouth-to-caecum transit is affected by the rate of gastric
emptying and by small-intestinal transit, both of which
may be altered by the consumption of viscous polysaccharides such as guar gum and gum tragacanth, and
by the soluble (3-glucans in oat bran. The viscosity of
these polysaccharides delays gastric emptying, resulting
in greater satiety and a slow delivery of nutrients to the
small intestine (Holt et al 1979). Viscous polysaccharides
delay the absorption of nutrients such as sugars within
the small intestine, particularly in the distal regions where
the viscosity is increased by the absorption of water from
the gut contents.
Colonic transit is influenced less by the viscosity of
polysaccharides, which is rapidly reduced as fermentation proceeds. Insoluble polysaccharides and largely
intact cereal products like wheat bran decrease colonic
transit time, resulting in larger, softer stools. It is probable
that a number of different factors, including the retention of fluid in the fibre matrix, the presence of poorly
absorbed SCFA such as lactic acid, a low pH, which
inhibits salt and water absorption, an increase in bacterial
cell mass and distension due to gas production, all
contribute to the decrease in intestinal transit time.
THE IMPACT OF MODERN DIETARY
CARBOHYDRATES ON PUBLIC HEALTH
The world's most abundant staples are rich in carbohydrates, and the site and rate of digestion of these carbohydrates, both highly dependent on processing, have
significant implications for health. The human digestive
system is evolutionarily adapted to the diet that prevailed until the Agricultural Revolution about 10 000
years ago. In comparison with the modern 'Western' diet,
the pre-agriculture diet contained less fat and far more
dietary fibre (plant cell walls).
Carbohydrates in the pre-agriculture diet were derived
mainly from roots, seeds, fruits and tubers; cereal grains
were not a major component of this diet. The natural
encapsulation of starch and sugars within undamaged
plant cell walls (dietary fibre) in the raw or only lightly
processed foods typical of that diet slows the rate of
digestion, resulting in a sustained release of glucose. In
a classic study, using apples, apple pulp and apple juice,
Heaton and colleagues (Haber et al 1977) demonstrated
that the rate and extent of increases in blood glucose (the
glycaemic response) and insulin levels were correlated
with the extent of destruction of plant cells by processing.
The same effect has since been shown for a range of foods,
CARBOHYDRATES
including wheat, rice and potato (Heaton et al 1988).
Modern starchy food products are mainly cereal-based
and often finely milled, which disrupts the plant cell
walls, and the starch is often fully gelatinized during
processing. The release of sugars and starch from within
the cell walls, which may be removed during refining,
and the gelatinization of the starch lead to rapid digestion
and absorption of the starch in the small intestine, in
contrast to the fate of the starch in the pre-agriculture diet.
There are strong indications that the large amounts
of rapidly available glucose derived from starch and free
sugars in the modern diet, in combination with the
consumption of discrete meals, lead to periodic increases
in plasma glucose and insulin levels that are detrimental
to health in many contexts, including diabetes, coronary
heart disease, cancer and ageing. The evolutionary
timescale is too long to expect that there has been any
significant adaptation of the human gut to the cerealbased diets introduced during the Agricultural Revolution and certainly not to the highly processed foods in
the modern diet.
Current knowledge of the fate of dietary carbohydrates
means that the potentially undesirable properties of many
modern foods could be altered by using processing
techniques that yield foods with more intact plant cellwall structures. Such products would more closely
resemble the foods in the pre-agriculture diet with respect
to the rate of digestion and absorption of carbohydrate
in the small intestine, which would be of great benefit
to public health.
Dietary fibre
National dietary guidelines recommend consumption of
a naturally high-fibre diet. This diet is low in free sugars,
salt and fat, and is a good source of a range of naturally
occurring nutrients, including vitamins, minerals and
antioxidants.
Any definition or measurement of dietary fibre must
lead to values that guide the consumer in the choice
of the naturally high-fibre diet recommended in the
guidelines. The common characteristic of the plant foods
that comprise this largely unrefined high-fibre diet is
the presence of naturally occurring plant cell walls. This
is what prompted Trowell (1972) to offer the following
definition of dietary fibre material: '... the skeletal
remains of plant cells that are resistant to hydrolysis by
the enzymes of man'. This specific focus on the skeletal
remains of plant cells deliberately excluded starch and
other non-cell-wall material, and provided the source
definition of dietary fibre as endogenous plant cell-wall
material (Trowell 1972, Trowell et al 1985). Any analytical
procedure capable of measurements reflecting this
material would provide meaningful values in terms of
the dietary fibre hypothesis (Englyst & Hudson 1996).
71
The argument put forward by some of the food
industry (Bar 1994), that all the carbohydrates that reach
the large intestine have 'fibre-like' properties and should
be included as dietary fibre, is not supportable. In effect,
this is proposing a new hypothesis: namely, that all carbohydrates that reach the large intestine, irrespective of
origin or composition, are beneficial to health. There is
no evidence to support this (Johnson & Southgate 1993).
There is no justification for the inclusion of retrograded
starch, or indeed any carbohydrate other than the plant
cell-wall NSP in the definition and measurement of
dietary fibre. Only the plant cell-wall NSP are characteristic of the plant foods that constitute a true high-fibre
diet for which health benefits are known. It is important
to recognize that the beneficial effects associated with
unrefined plant foods cannot be restored by adding 'fibre
supplements' to fibre-depleted foods. The structural
integrity of the naturally occurring plant cell wall must
be retained.
Starch digestion and absorption
The gross physical form of food, the susceptibility of
starch granules to enzymatic hydrolysis and to retrogradation after cooking have a major effect on the rate
and extent of starch digestion and absorption in the small
intestine. Thus the way food is processed and prepared
in the factory and/or at home is of great importance.
Starch contained within discrete structures such as
whole grains and seeds is physically inaccessible to
pancreatic amylase. Crushing, chopping and milling all
increase the accessibility of the starch, i.e. the rate of
digestion is influenced by the final particle size (Crapo
& Henry 1988, Heaton et al 1988, O'Donnel et al 1989).
In foods such as pasta, starch hydrolysis is retarded by
the density of the product (Bornet et al 1990, Hermansen
et al 1986), which decreases enzyme access. Physical inaccessibility may cause the rate of starch hydrolysis to
be so slow that some starch enters the large intestine. In
extreme cases, starch contained within discrete structures
may be excreted in the faeces.
Cooking facilitates the hydrolysis of starch through
gelatinization and dispersion of the starch granules.
However, foods eaten raw retain their starch as granules,
which show varying degrees of resistance to digestion.
Raw starch from cereals is digested slowly within the
small intestine, giving a modest glycaemic response. Raw
starch from banana and potato shows a greater degree of
resistance, up to 90% passing undigested through the small
intestine (Englyst & Cummings 1986, Silvester et al 1995).
Retrogradation also retards digestion, and retrograded
starch (mainly amylose) from processed cereal and potato
products has been shown to pass through the small
intestine (Englyst & Cummings 1985).
Table 5.9 shows how the digestibility of starchy foods
72
NUTRITIONAL SCIENCE
Table 5.9 The effects of food processing on the in vitro digestibility
of wheat starch. The values are expressed as a percentage of total
starch
White flour
Shortbread
White bread
White spaghetti
RDS
SDS
49
56
94
52
48
43
4
43
RS,
RS2
RS3
3
—
—
—
—
3
—
trace
1
2
3
RDS, rapidly digestible starch; SDS, slowly digestible starch; RS,
resistant starch; RS,, physically inaccessible; RS2, resistant granules;
RS3, retrograded amylose.
varies with the type of food processing. Raw white wheat
flour, which consists mainly of ungelatinized starch
granules, is digested relatively slowly, 48% of the starch
measuring as slowly digestible starch (SDS). Baking the
flour into shortbread, which involves cooking in the
presence of very little water, results in limited disruption
of the granular structure and gives a product that is
also digested slowly. On the other hand, baking the flour
into bread, a process that requires a long cooking time in
the presence of water, leads to extensive gelatinization
of the starch granules and results in a rapidly digestible
product. A small amount of retrograded amylose (RS3)
is produced during cooling. The starch in wheat flour
made into spaghetti is digested more slowly than that
in bread, despite being cooked by moist heat. This is
because the dense structure of the pasta impedes enzymic
hydrolysis of the starch; 3% of the starch is measured as
RSj (physically inaccessible). Other examples of foods in
which some of the starch may be physically inaccessible
are haricot beans and pearl barley.
Normal food processing results in only small amounts
of RS. The UK diet provides an average of about 3 g
of RS per capita per day and there is no evidence that
these modest amounts of RS are detrimental to health.
However, certain types of processing and the addition
of, for example, high-amylose corn starch may result in
substantial amounts of RS in foods. Fermentation of RS
in the human large intestine has been shown to reduce
faecal ammonia, which is potentially beneficial to health.
The SCFAs produced by fermentation of carbohydrates,
including RS, have been implicated as a protective factor
against colon cancer. However, recent studies have
shown that RS can enhance tumour formation in rats
(Young et al 1996), and in a mouse model (Burn et al
1996). Although detrimental effects have been shown
only in animal studies, the desirability of increasing levels
of RS in foods requires further evaluation.
hydrate-containing foods, and allows foods to be ranked
on the basis of the rate of digestion and absorption of the
carbohydrates that they contain (Frost et al 1993). Because
GI values are normalized to a reference amount of carbohydrate, they do not take into account the amounts
of carbohydrate present in foods; e.g. a food with a low
content of carbohydrates may nevertheless have a high
GI value if that carbohydrate is digested and absorbed
rapidly in the human small intestine. This is potentially
confusing for a person wishing to control his or her blood
glucose levels by the choice of foods.
Rapidly available glucose (RAG) is the sum of free
glucose, glucose from sucrose and glucose from RDS.
RAG values are determined in vitro as the glucose released under strictly controlled conditions of extraction
and hydrolysis with pancreatin, amyloglucosidase and
invertase (Englyst et al 1992). The values are expressed
as grams of glucose per 100 grams of food as eaten.
The measurement of RAG provides values that reflect
the amount of glucose likely to be rapidly absorbed,
and thus to influence blood glucose and insulin levels.
These values can be used to compare foods on an equal
weight basis, and are important indicators for the
consumer, as food table RAG values can be used for
simple calculation of the total amount of rapidly available
glucose likely to be provided by single foods, by whole
meals and by whole diets.
When the values for 39 foods (Englyst et al 1996a) were
expressed on the basis of the available carbohydrate content of these foods, highly significant (P < 0.001) positive
correlations were observed between GI and both RDS
and RAG. This correlation between RAG intake and
glycaemic response has been confirmed with a small
number of carbohydrate-rich foods over a range of
RAG intakes (Englyst et al 1999). There has been some
controversy over the application of the glycaemic index
to mixed meals (Wolever & Bolognesi 1996). This has
led to recommendations from the American Diabetes
Association (1994) that focus on the total amount of carbohydrate consumed rather than the source of the carbohydrate. Part of the problem may be the complexity of
using the glycaemic index in a mixed diet, as it gives no
information on the amount of carbohydrate consumed.
This may be overcome by the comparatively simple
use of RAG intakes, which clearly indicate the amount
of glucose likely to be rapidly absorbed in the small
intestine. Meals with a high RAG value will result in
elevated levels of blood glucose, i.e. a large glycaemic
response, and subsequently raised levels of circulating
insulin.
The glycaemic response to dietary
carbohydrates
Non-enzymatic protein glycation
The glycaemic index (GI; Jenkins et al 1981) is an in vivo
measurement based on the glycaemic response to carbo-
Elevated levels of blood glucose are responsible for
increased non-enzymatic protein glycation, which is
CARBOHYDRATES
implicated in the development of diabetes complications
and the ageing process (Dyer et al 1993, Vlassara & Bucala
1996). The importance of the reaction of reducing sugars
with amino acids in living organisms has been appreciated only recently. Initial studies of this process were
centred on the biochemical abnormalities that occur in
diabetes, an illness characterized by high levels of blood
glucose (Koenig & Cerami 1975). The reaction products
that form in vivo have been termed advanced glycosylation end-products or AGEs (Bucala et al 1992). The
amount of carbohydrates linked to haemoglobin and
other proteins is increased in diabetes, and has proved to
be a useful marker for glycaemic control of diabetic
patients (Jovanovic & Peterson 1981). In a large trial
(Shamoon et al 1993), levels of the glycated haemoglobin
HbAlc (Koenig et al 1977) were 9% in normally controlled
diabetics but were only 7.5% in those whose blood
glucose was measured many times a day. This lowering
was associated with a dramatic decrease (75-90%) in
the number of patients who developed complications of
diabetes over a period of 9 years, demonstrating the
long-term detrimental effect of high levels of blood
glucose.
Yellow-brown pigments accumulate in the lens with
age and do so at an accelerated rate in diabetics. These
pigments are capable of cross-linking proteins, a hallmark
of cataract formation (Monnier & Cerami 1981). Protein
cross-linking has been observed also for collagen, and
again the levels are higher in diabetics (Dyer et al 1993).
Macrophages have been shown to possess specific AGE
receptors that could mediate the uptake and eventual
breakdown of AGEs (Vlassara & Bucala 1996). The net
accumulation of AGEs reflects the balance of the reaction
of glucose with the matrix proteins and their removal
by the macrophages.
The amino groups of the bases of DNA react with
reducing sugars. Although DNA is not exposed to the
high levels of glucose circulating in the blood, DNA is
very long-lived and might be expected to accumulate
AGEs. The DNA in older animals has been shown to
be cross-linked both to itself and to proteins (Bojanovic
et al 1970). The incidence of birth defects in children born
to diabetic mothers is increased many-fold, and animal
studies have shown an increased rate of mutations in
embryos that were exposed to hyperglycaemia in utero
(Lee et al 1995).
Increased levels of glycation on proteins with modest
half-lives reflect recent increases in glucose levels. On
more stable proteins, the initial glycation products
become irreversibly fixed and the accumulation of AGEs
represents a measure of exposure to glucose over time.
The cross-linking of AGEs with a second protein has
been proposed as the pathway of collagen cross-linking
that increases tissue stiffness, which may lead directly
or indirectly to kidney failure (Drickamer 1996).
73
Causes and consequences of elevated
insulin levels
The rise in plasma glucose that follows a carbohydratecontaining meal (the glycaemic response) is accompanied
by the production of insulin from the beta cells of the
pancreas. Insulin is required to facilitate the transport
of glucose across cell membranes. Insulin homeostasis
is restored by the production of insulin antagonists (e.g.
adrenaline and Cortisol) within 1-2 hours in normal
individuals. If the sensitivity of cells to insulin becomes
impaired, the body will respond by producing more
insulin and circulating levels may remain high (hyperinsulinaemia). Basal levels of circulating insulin tend to
increase with age, and hyperinsulinaemia is strongly
associated with adiposity (Eckel 1997, Mayer-Davis et al
1997), especially central fatness in men, and with lack
of fitness due to inactivity (Helmrich et al 1991, Manson
et al 1991). The amount of insulin produced in response
to a meal is related to a number of factors, including the
rate at which sugars are absorbed in the small intestine.
Elevated levels of insulin are associated with dyslipidaemia (Despres et al 1996), atherosclerosis (Stout 1996)
and the formation of tumours and increased tumour
growth in a number of cancers (Giovannucci 1995;
McKeown-Eyssen 1994). Rapidly absorbed sugars tend
to enter the lipogenic pathway and hence predispose to
the obesity that is so common in the modern Western
world.
It has been hypothesized that insulin promotes the
growth of colon tumours, and this has been demonstrated
in animal models (Tran et al 1996). Breast cancer is
associated with an elevated number of insulin receptors,
and there is some evidence that hyperinsulinaemia or
its determinants, such as waist-to-hip ratio, increases the
incidence of breast cancer. If the hypothesis is correct, the
determinants of serum insulin levels should influence
the risk of both colon and breast cancer. Fasting serum
levels are affected by insulin sensitivity, and postprandial
levels depend on how quickly glucose is absorbed in the
small intestine.
Most modern people are relatively sedentary, and
the combination of inactivity, ready access to energy-rich
foods and consumption of considerable amounts of food
at discrete meals often leads to obesity and tissue resistance to insulin. This insulin resistance results in the
inability to cope with the increased blood glucose caused
by rapid absorption of the carbohydrates in modern
sugary and starchy foods.
The highest levels of circulating insulin occur after
a meal, with the level depending on (1) the amount of
carbohydrates in the meal, (2) their form, and (3) the
degree of insulin sensitivity. The rate at which starch is
hydrolysed depends on a number of factors (see above).
In general, the starch and sugars in modern processed
74
NUTRITIONAL SCIENCE
foods are digested and absorbed more rapidly than those
in raw or traditionally cooked foods. Encapsulation of
nutrients by plant cell-wall material is an important
factor in moderating the glycaemic response. It must be
emphasised that this and most other beneficial effects
of a naturally high-fibre diet cannot be achieved by the
addition of 'fibre supplements' to fibre-depleted foods (as
discussed above).
The range of factors considered as risks for non-insulindependent diabetes mellitus (high body mass index,
central obesity, high energy intakes, high sugar intakes,
high saturated fat intakes, low fibre intakes) have all
been identified as risk factors for colon cancer. There
are strong indications that elevated plasma glucose and
insulin responses are the common factors. Support for
the promotion of colon cancer by insulin has come from
animal studies (e.g. see Tran et al 1996).
CONCLUSIONS
National dietary guidelines recommend an increase in
intake of dietary fibre and starch. The recommendations
for fibre intakes are based on the proven health benefits
of a naturally high-fibre diet (see above) and are made in
terms of obtaining fibre in the form of a balanced mixture
of fruits, vegetables and whole grains. The guidelines
discourage the use of 'fibre supplements' as a source of
dietary fibre, and there is some evidence to suggest that
these supplements may be detrimental to health (Jacobs
1983, Jacobs & Lupton 1986, Wasan & Goodlad 1996).
The recommendations to eat more starch are related to
the recommendations to eat less fat, with starch being a
cheap and readily obtainable alternative energy source.
However, recent advances in our knowledge of the physiological consequences of eating sugars and starch that
are rapidly digested and absorbed in the small intestine
suggest that, as for fibre, the form as well as the amount
of starch should be considered. Carbohydrates that are
digested and absorbed rapidly are potentially detrimental
to health in many areas, as discussed above.
It is pertinent to ask whether food labels provide
values that aid consumers in selecting a healthy diet.
The entry 'carbohydrate' is required in most countries,
and the value is usually obtained 'by difference' and used
in the calculation of energy content, but the value provides no nutritional information per se. Many processed
foods carry second-level entries, preceded by 'of which',
and here one might expect to find values for sugars,
starch and fibre. High values for sugars correctly serve
as a warning to the consumer. Since the starch may be in
the form of RDS, SDS or RS, or any mixture of these
in any proportions, a single value for starch is impossible
to interpret in nutritional terms. High levels of RDS result
in elevated plasma glucose and insulin levels known to
be detrimental to health (McKeown-Eyssen 1994). As
discussed in this chapter, the benefits or otherwise of RS
are unknown. The small amounts of RS in the Western
diet may prove to be beneficial, but large supplements
of RS are potentially detrimental to health. There is a bona
fide case for considering how best to indicate the likely
rate and extent of digestion and absorption of starch on
the food label; a third-level entry under starch 'of which
rapidly digestible' or perhaps a new entry of RAG under
carbohydrate would go some way to addressing the
problem. Whatever decision is taken by the legislators,
there must be public education concerning the potential
dangers of consuming large amounts of carbohydrates
that will be rapidly digested and absorbed in the small
intestine.
In countries where food label values for fibre are
measured as naturally occurring plant cell-wall NSP, the
labelling for fibre provides the consumer with accurate
information. In countries where fibre values are obtained
by non-specific gravimetric procedures, however, the
values are not confined to naturally occurring plant
cell-wall NSP. Indeed, these values may represent solely
material such as retrograded starch, formed as the result
of food processing, or substances added to foods. Such
labelling cannot guide the consumer in the choice of
naturally fibre-rich foods and may mistakenly encourage
the choice of foods with added 'fibre supplements', in
contradiction to the advice given in the guidelines. The
recent advances in our knowledge of the importance
of dietary carbohydrates represents an opportunity for
the food industry to make a significant contribution to
public health by restricting the use of the term 'fibre' to
naturally occurring plant cell walls, by preserving the
natural structure of plant foods as far as possible, and by
the development and marketing of starchy foods that are
more compatible with human physiology. This, together
with nutritionally sound food labelling for dietary carbohydrates, would be of great benefit to public health.
CARBOHYDRATES
75
KEY POINTS
• Dietary carbohydrates are classified into four major
groups: (1) free sugars; (2) short-chain
carbohydrates; (3) starch; and (4) non-starch
polysaccharides (NSP).
•
The in vitro measurement of rapidly available
glucose (RAG; RDS and free-sugar glucose)
provides information about the likely glycaemic
response to plant foods.
• The site, rate and extent of digestion of
carbohydrates, which are highly dependent on food
processing, have significant implications for health.
• The measurement of dietary fibre as NSP provides
a good marker of the unrefined plant foods for
which benefits to health are known.
• Starch is subdivided into rapidly digestible starch
(RDS), slowly digestible starch (SDS) and resistant
starch (RS).
•
Most of the carbohydrate that reaches the large
intestine is fermented by the colonic microflora, with
the production of short-chain fatty acids and gases.
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