Digestion and Metabolism of Carbohydrates 77factors intrinsic to the ingested foods and to the con-
sumer infl uence these rates, including:
● food factors
● particle size
● macrostructure and microstructure of food,
especially whether cell walls are intact
● amylose–amylopectin ratio of starches
● lipid content of food
● presence (or otherwise) of enzyme inhibitors
● consumer factors
● degree of comminution in the mouth
● rate of gastric emptying
● small bowel transit time.
All three main sugars absorbed from the gut
(glucose, galactose, and fructose) are transported via
the portal vein to the liver (glucose concentrations in
the portal vein after a meal can rise to almost 10 mM),
but only glucose appears in signifi cant concentrations
in the peripheral circulation. Most of the galactose
and fructose is removed during fi rst pass through the
liver via specifi c receptors on hepatocytes, so that the
blood concentration of these sugars rarely exceeds
1 mM. Within the hepatocytes, galactose is converted
to galactose-1-phosphate by the enzyme galactoki-
nase and then to glucose-1-phosphate in three further
steps. Fructose is also phosphorylated in hepatocytes
(by fructokinase) to fructose-1-phosphate, which is
subsequently split by aldolase B to yield one molecule
of each of the glycolytic intermediates dihydroxyace-
tone phosphate and glyceraldehyde. Although the
liver removes some glucose, using the bidirectional
transporter GLUT2, most is transported in the periph-
eral circulation for utilization by muscle, adipose, and
other tissues.
Metabolic utilization of carbohydrate
Peripheral tissues utilize glucose and the above-men-
tioned intermediates from fructose and galactose via
glycolysis and the citric acid or Krebs cycle pathways.
Glycolysis, a sequence of reactions in which glucose
is converted to pyruvate, with concomitant produc-
tion of ATP, is the prelude to the citric acid cycle and
electron transport chain, which together release the
energy contained in glucose. Under aerobic condi-
tions pyruvate enters mitochondria, where it is com-
pletely oxidized to carbon dioxide and water. If the
supply of oxygen is limited, as in actively contracting
muscle, pyruvate is converted to lactate. Therefore,
complete oxidation of glucose to carbon dioxide and
water occurs under aerobic conditions through the
reactions of the glycolytic pathway (in the cell’s cyto-
plasm), the Krebs cycle, and oxidative phosphoryla-
tion (in the mitochondrion). The overall reaction can
be summarized stoichiometrically as:
C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O
Approximately 40% of the free energy (ΔG) released
by this transformation is captured by the production
of ATP (38 moles of ATP per mole of glucose oxi-
dized), which is used for a wide variety of purposes,
including powering muscle contraction, transporting
substances across membranes against a concentration
gradient, and synthesis of cell macromolecules. The
remainder of the free energy is released as heat.
When the demand for oxygen exceeds supply, as in
muscle during intense exercise, anaerobic glycolysis
produces lactic acid as a major end-product. The rela-
tive lack of oxygen means that oxidative phosphoryla-
tion cannot keep up with the supply of reduced
dinucleotides and, for glycolysis to proceed, NADH
must be recycled back to NAD+. This is achieved by
the reaction:
Pyruvate + NADH + H+ → Lactate + NAD+
which is catalyzed by the enzyme lactate dehydro-
genase. Anaerobic glycolysis provides some or all of
the ATP needs for some cells and tissues; for example,
erythrocytes, white blood cells, lymphocytes, the
kidney medulla, and eye tissues. The lactate released
from tissues undergoing anaerobic glycolysis is taken
up by other tissues that have a high number of mito-
chondria per cell, such as heart muscle, in which the
lactate is converted back to pyruvate and then enters
the Krebs cycle via acetyl coenzyme A.
In hepatic and muscle cells some glucose is con-
verted to glycogen in the glycogenesis pathway. Gly-
cogen is a readily mobilized storage form of glucose
residues linked with α-1,4-glycosidic bonds into a
large, branched polymer. Glycogen is a reservoir of
glucose for strenuous muscle activity and its synthesis
and degradation are important for the regulation of
blood glucose concentrations.Regulation of blood glucose concentration
The exocrine pancreas (and other tissues) is primed
to expect a rise in blood glucose concentration by
peptide hormones such as gastric inhibitory peptide