Digestion and Absorption of Carbohydrates - Adequate Food for All - page 73

insulin/glucagon secretion. This chronic stress has been implicated as a damaging
factor for pancreatic beta cells and cellular response mechanisms that contribute to
the development of type 2 diabetes (Livesey 2005).
IntestInAl bRush boRdeR glucosIdAses
And theIR substRAtes
L a C t o s e /L a C t a s e -P h L o r i z i n h y D r o L a s e
Lactose [β-d-galactopyranosyl(1,4′)d-glucopyranoside] is the sugar abundant in the
milk of mammalian species (milk sugar, from the Latin lac or milk) and is the first
carbohydrate consumed during the lifetime of humans. Breast-fed infants consume
approximately 50 g/day of lactose, but the nutritional relevance of this sugar is limited
to the lactation period, when it provides most of the caloric requirements (Marquis et
al. 2003; Foda et al. 2004). After weaning, other nutrients in foods can provide these
nutritional requirements. Thus, consumption of dairy products by adult humans is
unnecessary from a nutritional point of view. However, dairy products mainly from
bovine origin are in fact important foods for a large number of individuals.
Digestion of lactose is dependent on the enzyme called lactase-phlorizinase (LPH),
a protein bound to the apical cell membrane of the epithelial cells lining the small
intestine. LPH is formed by the two subunits lactase and phlorizinase. The lactase
subunit is the only known β-galactosidase of the intestinal lumen with the ability
to degrade lactose into its two conforming monosaccharides: glucose and galactose
(Buller et al. 1989; Montgomery et al. 1991). The defective expression of this enzyme
may cause impaired ability to digest lactose. In most of the human population, the
cessation of breast milk consumption after weaning is associated with a decrease
in the levels of LPH expression and its corresponding activity in the small intes-
tine, hampering the digestion of ingested lactose. This state, sometimes called adult
hypolactasia , is therefore a normal developmental process of humans, with results
evident during childhood or adolescence. However, in populations of north European
origin, the levels of lactase activity may remain unchanged during the entire life.
This state, sometimes referred to as persistent lactasia , is a distinctive feature of a
minority human population that acquired the genetic ability to continue the expres-
sion of the enzyme after weaning, probably resulting as an adaptation to the high
consumption of dairy products derived primarily from intensive cattle farming.
s t a r C h , m a L t o s a C C h a r i D e s , a n D m a L t o s e /m a L t a s e -
g L u C o a m y L a s e a n D s u C r a s e -i s o m a L t a s e
Starch and its derived products are by far the polysaccharides most consumed by the
human population. Natural starches are produced in reproductive tissues of plants as
a form of energy storage inside of granules in a semicrystalline form (French 1975;
Lee 1983). In these granules, starch is present in two main molecular structures:
amylose, which consists of long linear chains of glucose associated by α-1,4 gluco-
sidic linkages and occasional branching with α-1,6 glucosidic linkages, and amy-
lopectin, which consists of relatively short α-1,4-bound glucose chains of variable