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Table 1 The major dietary carbohydrates
Class (DP a ) Subgroup Principal components
Sugars (1–2) Monosaccharides Glucose, fructose, galactose
Disaccharides Sucrose, lactose, maltose,
Oligosaccharides
(3–9) (short-chain
carbohydrates)
Polysaccharides
(X10)
and an understanding of the physiological and health effects
of these macronutrients.
A chemical approach divides carbohydrates into three
main groups, sugars (DP1–2), oligosaccharides (short-chain
carbohydrates) (DP3–9) and polysaccharides (DPX10).
Sugars comprise (i) monosaccharides, (ii) disaccharides and
(iii) polyols (sugar alcohols). Oligosaccharides are either (a)
malto-oligosaccharides (a-glucans), principally occurring
from the hydrolysis of starch and (b) non-a-glucan such as
raffinose and stachyose (a galactosides), fructo- and galactooligosaccharides
and other oligosaccharides. Polysaccharides
may be divided into starch (a-1:4 and 1:6 glucans) and nonstarch
polysaccharides (NSPs), of which the major components
are the polysaccharides of the plant cell wall such as
cellulose, hemicellulose and pectin but also includes plant
gums, mucilages and hydrocolloids. Some carbohydrates,
like inulin, do not fit neatly into this scheme because they
exist in nature in multiple molecular forms. Inulin, GFN,
from plants may have from 2 to 200 fructose units and so
crosses the boundary between oligosaccharides and polysaccharides
(Roberfroid, 2005).
A variety of methodologies are available for the measurement
of the carbohydrate content of food and the components
are listed in Table 1 (Englyst et al., 2007).
Terminology
Polyols (sugar
alcohols)
Maltooligosaccharides
(a-glucans)
Non-a-glucan
oligosaccharides
Carbohydrate terminology and classification
JH Cummings and AM Stephen
trehalose
Sorbitol, mannitol, lactitol,
xylitol, erythritol, isomalt,
maltitol
Maltodextrins
Raffinose, stachyose, fructo and
galacto oligosaccharides,
Starch
polydextrose, inulin
Amylose, amylopectin, modified
(a-glucans) starches
Non-starch Cellulose, hemicellulose, pectin,
polysaccharides arabinoxylans, b-glucan,
(NSPs)
glucomannans, plant gums and
mucilages, hydrocolloids
a Degree of polymerization or number of monomeric (single sugar) units.
Based on Food and Agriculture Organization/World Health Organization
‘Carbohydrates in Human Nutrition’ report (1998), and Cummings et al.
(1997).
Total carbohydrate
Although the individual components of dietary carbohydrate
are readily identifiable, there is some confusion as to
what comprises total carbohydrate as reported in food tables.
European Journal of Clinical Nutrition
Two principal approaches to total carbohydrate are used,
first, that derived ‘by difference’ and second, the direct
measurement of the individual components that are then
combined to give a total. Calculating carbohydrate ‘by
difference’ has been used since the early 20th century and
is still widely used around the world (Atwater and Woods,
1986; United States Department of Agriculture, 2007). The
moisture, protein, fat, ash and alcohol content of a food are
determined, subtracted from the total weight of the food and
the remainder, or ‘difference’, is considered to be carbohydrate.
There are, however, a number of problems with this
approach in that the ‘by difference’ figure includes noncarbohydrate
components such as lignin, organic acids,
tannins, waxes and some Maillard products. In addition to
this error, it combines all the analytical errors from the other
analyses. Also, a single global figure for carbohydrates in
food is uninformative because it fails to identify the many
types of carbohydrates and thus to allow some understanding
of the potential health benefits of those foods.
Direct analysis of carbohydrate components and summation
to obtain a total carbohydrate value has been the basis
of carbohydrate analysis in the UK since 1929, when the first
values were published by McCance and Lawrence (1929).
Those countries that use McCance and Widdowson’s, The
Composition of Foods (Food Standards Agency/Institute
of Food Research, 2002) also express carbohydrate using
this approach. The total figure obtained is for what McCance
and Lawrence called ‘available carbohydrate’ and therefore
differs from carbohydrate by difference in that it does not
contain the plant cell wall polysaccharides (fibre). In
addition, it is not complicated by analytical difficulties with
other food components. Dietary intake of total carbohydrate
and its components using direct analysis enables examination
of geographic variations and changes in intake over
time of individual carbohydrate types and their relationship
with health outcomes. Total carbohydrate by direct measurement
is preferable and simplified methods to do this should
be developed.
Figures obtained for carbohydrate by difference and
carbohydrate analysed directly are not always the same,
particularly for complex mixtures, and foods containing fibre
or certain types of starch, like pasta (Stephen, 2006). This
results in apparently different carbohydrate intakes for the
same list of foods consumed, as shown in Table 2. Fifty-two
dietary records from a study conducted in Canada, where
carbohydrate by difference is used (Health Canada, 2005)
were subsequently analysed in the UK using values based on
McCance and Widdowson’s The Composition of Foods (Holland
et al., 1991b, 1992). In this study, energy intake was 12%
higher and carbohydrate intake 14% higher when measured
‘by difference’ (Stephen, 2006). Comparison of carbohydrate
intake among different countries should therefore be viewed
with caution if the method of carbohydrate determination is
not the same. Worldwide variations in carbohydrate intake
assumed to be due to differences in types of foods consumed,
are also, in part, due to methodology.