Oorsig/Review
The significance of
Lactose in Milk
Dr Mark Chimes, BVSc*
Ask any veterinarian or dairy farmer why we test
Lactose levels in milk and what the test results
tell us, and you will get a blank stare. It is the
orphan of the milk analyses results. Although
the test results do not tell us a lot, it is the most
important constituent in milk that determines
the volume of milk that is produced.
INTRODUCTION
Lactose is the major carbohydrate fraction in
milk. It is a disaccharide made up of two sugar
molecules, namely Glucose and Galactose.
Lactose production in nature is limited to
mammals where the enzyme lactose synthase
binds a Glucose and Galactose molecule to form
Lactose. Lactose is less sweet than other sugars
(such as glucose, sucrose and fructose) and less
water soluble.
Cow’s milk comprises 4-5% Lactose with an
average of 4.7 – 4.9% and provides up to 30% of
the neonate’s energy needs. By contrast human
milk comprises 7.2% Lactose, which is among the
highest levels of lactose in the mammalian world
and provides up to 50% of the neonate’s energy
needs.
Lactose is a major, readily digestible source of
glucose which provides energy for the neonate.
An enzyme called lactase (ß-galactosidase)
splits Lactose into Glucose and Galactose in the
intestine of the neonate where it is absorbed
across the intestinal mucosa. The galactose is
then converted to another glucose by a different
enzyme and used as energy.
Lactase production in most humans starts to
wane in late childhood. This leads to maldigestion
12
of lactose in most cases and lactose intolerance in
more severe cases.
THE ROLE OF LACTOSE
Lactose is the principal carbohydrate in milk
and is split into two simpler sugars namely,
Glucose and Galactose, by the enzyme lactase
CH 2 OH
OH
CH 2 OH
O
H
OH H
H OH
H
H
O
OH
O
H
OH
OH
H
H
Lactose
H
OH
Lactase
CH 2 OH
H
C
OH
C
H
CH 2 OH
O
H HO
H H C C
C C OH H
OH OH
Glucose
C
H
O OH
OH H C
C C H
H OH
Galactose
in the intestine. The sugars are then absorbed
across the intestinal mucosa and used directly
as the primary source of energy by the infant.
Galactose plays an important role in the nervous
system and brain structure development
of the neonate. It is a substrate of several
macromolecules (cerebrosides, gangliosides and
mucoproteins), which are used in the synthesis
of nerve cell membranes. Galactocerebroside is