Digestion and Metabolism of Carbohydrates 79glucose absorption from the gut following a meal con-
tinues for several hours after blood concentrations
have returned to fasting levels. In this later postpran-
dial period, insulin secretion is suffi cient to ensure
that the rate of glucose absorption is matched by the
rate of glucose removal from the circulation.^13 C-
Labeled substrates are being used increasingly to
investigate the kinetics of digestion, absorption, and
metabolic disposal of glucose and other sugars from a
range of foods. When continued over several years,
high rates of glucose absorption and the subsequent
challenge to the capacity of the pancreatic β-cells to
secrete insulin may be the primary determinants of
insulin resistance and eventual pancreatic failure that
contribute strongly to the etiology of diabetes and
cardiovascular disease. Such kinetic studies are likely
to be helpful in identifying foods with slower rates of
intestinal hydrolysis – information that can be used in
public health advice or in counseling of individuals.
Fructose
When glucose and fructose are available simultane-
ously after a meal containing sucrose, how does the
body select which fuel to use fi rst for oxidative
purposes? This question has been resolved by experi-
ments in which volunteers consumed, on two sepa-
rate occasions, high-sucrose test meals which were
identical except that one or other of the constituent
monomeric sugars was^13 C-labeled in each meal. The
volunteers blew into tubes at intervals after the meals
to provide breath samples for measurement of enrich-
ment of expired carbon dioxide with^13 C. The results
showed that, after the high sucrose meal, fructose was
oxidized much more rapidly and extensively than was
glucose (Figure 5.2).
This rapid oxidation of fructose may be explained
by the fact that, because it is phosphorylated in hepa-
tocytes, it bypasses 6-phosphofructokinase, one of the
key regulatory enzymes in glycolysis.
5.4 Nonglycemic carbohydrates
Carbohydrates that are not absorbed in the small
intestine enter the large bowel, where they are par-
tially or completely broken down by bacteria in the
colon by a process called fermentation. McCance and
Lawrence in 1929 were the fi rst to classify carbohy-
drates as “available” and “unavailable.” They realized
that not all carbohydrates provide “carbohydrates for
metabolism” to the body. They called these carbohy-
drates “unavailable.” This was a very useful concept
because it drew attention to the fact that some carbo-
hydrate is not digested and absorbed in the small
intestine, but rather reaches the large bowel where it
is fermented. However, it is now realized that it is
misleading to talk of carbohydrate as unavailable
because some indigestible carbohydrate can provide
the body with energy through fermentation in the
colon. Thus, “unavailable carbohydrates” are not
really unavailable. For this reason, it has been sug-
gested by the Food and Agriculture Organization
(FAO 1998) of the United Nations and World Health
Organization that the term “nonglycemic carbohy-
drates” is more appropriate.Nature of carbohydrates that enter
the colon
Carbohydrates that enter the colon can be classifi ed
either physiologically or chemically. Neither of these
classifi cations is entirely satisfactory because it is dif-
fi cult to measure the physiologically indigestible car-
bohydrate and this varies in different people. Further,
the chemical structure of carbohydrates does not
always predict their physiological behavior.Physiological classifi cation of
carbohydrates entering the colon
Carbohydrates enter the colon because (1) monosac-
charide transporters do not exist in the intestinal
mucosa or do not function at a high enough rate; (2)[^13 C]-fructose-labeled mealBreath [^13 C]-glucose-labeled meal13
COenrichment 2(atoms percent excess)Time after consumption of test meals (min)0 100 200 300 4000.020.040.060.08Figure 5.2 Enrichment of breath^13 CO 2 following ingestion of high-
sucrose test meals labeled with [^13 C]-fructose and [^13 C]-glucose.
(Redrawn from Daly et al., 2000, with permission of the American
Society for Nutrition.)