Digestion and Metabolism of Carbohydrates 75down to their constituent monosaccharide units, and
a battery of hydrolytic enzymes capable of splitting
the bonds between sugar residues is secreted within
the mouth, from the pancreas, and on the apical
membrane of enterocytes. While these carbohydrases
ensure that about 95% of the carbohydrate in most
human diets is digested and absorbed within the
small intestine, there is considerable variation in
bioavailability between different carbohydrate classes
and between different foods. Carbohydrates that are
digested to sugars and absorbed as such in the small
bowel are called “glycemic” carbohydrates.
Hydrolysis in the mouth and small bowel
The major carbohydrase secreted by the salivary
glands and by the acinar cells of the pancreas is the
endoglycosidase α-amylase, which hydrolyzes (digests)
internal α-1,4-linkages in amylose and amylopectin
molecules to yield maltose, maltotriose, and dextrins.
These oligosaccharides, together with the food disac-
charides sucrose and lactose, are hydrolyzed by spe-
cifi c oligosaccharidases expressed on the apical
membrane of the epithelial cells that populate the
small intestinal villi. Sucrase–isomaltase is a glycopro-
tein anchored via its amino-terminal domain in the
apical membrane that hydrolyzes all of the sucrose
and most of the maltose and isomaltose. The resulting
monomeric sugars are then available for transport
into the enterocytes.
Absorption and malabsorption in
the small bowel
Glucose and galactose are transported across the
apical membrane by the sodium–glucose transport
protein-1 (SGLT1), a process that is powered by Na+/
K+-ATPase on the basolateral membrane (Figure 5.1).
In contrast, fructose is absorbed by facilitated trans-
port via the membrane-spanning GLUT5 protein. A
member of the same family of transporter proteins,
GLUT2, is the facilitated transporter on the basolat-
eral membrane which shuttles all three monosaccha-
rides from the enterocyte towards the blood vessels
linking with the portal vein for delivery to the liver.
The capacity of the human intestine for transport
of glucose, galactose, and fructose is enormous – esti-
mated to be about 10 kg per day – so that this does
not limit absorption in healthy individuals. Carbohy-
drate malabsorption is usually caused by an inherited
or acquired defect in the brush border oligosacchari-
dases. More than 75% of human adults are lactose
intolerant because of a loss (possibly genetically
determined) of lactase activity after weaning (primary
lactose intolerance). In such individuals, ingestion of
more than very small amounts of lactose leads to the
passage of the sugar to the large bowel, where it is
fermented to produce short-chain fatty acids (SCFAs)
and gases as end-products. The appearance of hydro-
gen in the breath after ingestion of lactose is the basis
for diagnosis of malabsorption of this carbohydrate.
Diseases of the intestinal tract, such as protein-energy
malnutrition, intestinal infections, and celiac disease,
which reduce expression of lactase on the enterocyte
apical membrane, can result in secondary lactase
insuffi ciency. Sucrase–isomaltase activity, which rises
rapidly from the pylorus towards the jejunum and
then declines, is inducible by sucrose feeding. About
10% of Greenland Eskimos and 0.2% of North Amer-
icans have sucrase–isomaltase defi ciency. A missenseTable 5.1 Classes of food carbohydrates and their likely fates in the human gut
Class DP Example Site of digestion Absorbed molecules
Monosaccharides 1 Glucose Small bowel Glucose
1 Fructose Small bowela Fructose
2 Sucrose Small bowel Glucose + fructose
2 Lactoseb Small bowel Glucose + galactose
Oligosaccharides 3 Raffi nose Large bowel SCFA
3–9 Inulin Large bowel SCFA
Polysaccharides > 9 Starches Predominantly small bowelc Glucose
9 Nonstarch polysaccharides Large bowel SCFA
a Except where very large doses are consumed in a single meal.
b Except in lactose-intolerant subjects, in whom lactose fl ows to the large bowel.
c Some starch escapes small bowel digestion (resistant starch). In all these cases, the carbohydrate entering the large bowel becomes a substrate
for bacterial fermentation to short-chain fatty acids (SCFAs).
DP, degree of polymerization.