Lactose Tolerance Test
Medical Author:
Jay W. Marks, M.D.
Medical Editor: William C. Shiel,
Jr., MD, FACP, FACR
What is the lactose tolerance test?
The lactose tolerance test is a test for diagnosing an
intolerance of ingested lactose. Lactose intolerance is a genetic condition in which
individuals are unable to digest and absorb the lactose sugar in cow's milk.
What is lactose intolerance?
Lactose intolerance is the inability to digest the sugar
in milk, lactose. Milk is an important source of nutrition for children and even some adults. With
normal lactose tolerance, all of the lactose is broken down (digested) in the
intestine by an enzyme into two smaller sugars (galactose and glucose) which
then are absorbed into the body. As a result, normally no lactose reaches the
colon.
On the other hand, in individuals with lactose intolerance who lack the
intestinal enzyme that breaks down the lactose, the ingested lactose is neither
digested nor absorbed in the small intestine and reaches the colon where it is
used by the bacteria in the colon.
The bacteria produce chemicals that cause diarrhea, and also produce
gas. Abdominal pain also may occur.
How is the lactose tolerance test done?
The lactose tolerance test requires an individual to
drink a liquid that contains lactose. In the small intestine among lactose
tolerant individuals, the ingested lactose is split into galactose and glucose
which then are absorbed from the intestine and go into the blood. Several blood
samples are taken over a two hour period following the ingestion of the lactose
to measure the blood glucose level. If lactose tolerance is normal, the glucose
level in the blood rises due to the absorption of glucose from the intestine. If there is lactose
intolerance, the glucose level does not rise.
Next: What other tests can detect lactose intolerance? »
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