Digestion and Metabolism of Carbohydrates 81gestible oligosaccharides, and probably about 1–2 g
from intestinal mucopolysaccharides. These compo-
nents add up to only 18–20 g. Where does the other
10–12 g come from? It is believed to come from starch,
because experiments in humans show that about 5–
15% of the starch in foods enters the colon. Typical
Western diets contain about 120–150 g starch per day,
and, if 8% of this enters the colon, this will provide
the additional 10–12 g carbohydrate. The amount of
carbohydrate entering the colon, however, can be
increased several-fold, up to 100 g/day or more, by
changes in diet such as increased intake of NSP,
non-digestible or partially digestible carbohydrates
(ingredients in functional foods), total starch, resis-
tant starch, or slowly digested, low-GI foods.
Resistant starch
Resistant starch is starch that escapes digestion in the
small intestine and enters the colon. However, there
is controversy over the amounts of resistant starch in
foods because there is no universally accepted method
for measuring it (different methods yield different
results). The amount of resistant starch measured
chemically is generally less than that observed to enter
the colon (or leave the small intestine) in experiments
in human volunteers.
In the 1970s and early 1980s it fi rst became appar-
ent that appreciable amounts of starch are not digested
in the small bowel, from experiments showing that
breath hydrogen increased after eating normal starchy
foods. The only source of hydrogen gas in the human
body is as a product of the anaerobic fermentation of
carbohydrates by colonic bacteria (see below). If a
person consumed a load of an absorbable sugar such
as glucose, breath hydrogen did not go up. In contrast,
if lactulose (an unabsorbed disaccharide of fructose
and galactose) was consumed, breath hydrogen
increased rapidly, and the area under the breath
hydrogen curve over an 8–12 hour period was directly
proportional to the amount of lactulose consumed. If
subjects ate common starchy foods such as white
bread or potato, breath hydrogen levels increased to
an extent that suggested that 5–10% of the starch was
fermented in the colon. Subsequently, other ways of
measuring carbohydrate entering the colon were
developed. In one technique, subjects swallowed a
tube that was passed through the stomach and along
to the end of the small intestine so that the material
leaving the small intestine and about to enter the
colon could be sampled. Another method was to
study people who have had their colons removed sur-
gically and in whom the end of the ileum was sutured
to a stoma in the body wall. In this way, the material
leaving their small intestine could be collected quan-
titatively in a bag attached to their abdomen. With
these methods, the amount of carbohydrate leaving
the small intestine can be measured directly. These
methods confi rmed that a substantial amount of
starch enters the colon.
The main forms of resistant starch (RS) are physi-
cally enclosed starch, for example within intact cell
structures (known at RS 1 ); raw starch granules (RS 2 );
and retrograded amylose (RS 3 ). These kinds of starch
can be identifi ed chemically using methods developed
by Englyst and colleagues (Englyst et al. 1996).Dietary fi ber
Major interest in dietary fi ber began in the early 1970s
with the proposal by Burkitt and Trowell (1975) that
many Western diseases were due to a lack of fi ber in
the diet. However, the defi nition of dietary fi ber has
been, and continues to be, a source of scientifi c con-
troversy. Indeed, two consecutive reports from the
FAO (1997 and 1998) recommended that the term
“dietary fi ber” be phased out. Nevertheless, the term
appears to be here to stay because it is accepted by
consumers, the food industry, and governments.
A defi nition and method of measuring fi ber is
important for scientifi c studies and for food-labeling
purposes. The student must be aware that the defi ni-
tions and methods of measuring fi ber have changed
over time, and differ in different parts of the world.
Knowledge of what is meant by the term “fi ber” and
what is included in the measurement is essential for
proper interpretation of the scientifi c literature (but
often is not given in the methods section of papers
and reports).
Originally, Burkitt and Trowell (1975) defi ned fi ber
as the components of plant cell walls that are indigest-
ible in the human small intestine. Later, the defi nition
was expanded to include storage polysaccharides
within plant cells (e.g., the gums in some legumes).
Many different methods were developed to measure
dietary fi ber, but they measured different things. All
of the methods start with the drying and grinding of
the food and extraction of the fat using an organic
solvent. If the remaining material is treated with
strong acid, the chemical bonds in starch and many