Lactose Intolerance (cont.)
Breath test
The hydrogen breath test is the most convenient and reliable test for lactase
deficiency and lactose intolerance. For the breath test, pure lactose, usually
25 grams (the equivalent of 16 oz of milk), is ingested with water after an
overnight fast. In persons who are lactose intolerant, the lactose that is not
digested and absorbed in the small intestine reaches the colon where the
bacteria split the lactose into glucose and galactose and produce hydrogen
(and/or methane) gas. Small amounts of the hydrogen and methane are absorbed
from the colon into the blood and then travel to the lungs where they are
excreted in the breath. Samples of breath are collected every 10 or 15 minutes
for 3-5 hours after ingestion of the lactose, and the samples then are analyzed
for hydrogen and/or methane. If hydrogen and/or methane is found in the breath,
it means that the small intestine of the person having the test was unable to
digest and absorb all of the lactose. He or she is lactase deficient. The amount
of hydrogen or methane excreted in the breath is roughly proportional to the
degree of lactase deficiency, that is, the larger the amount of hydrogen and/or
methane produced, the greater the deficiency. The amount of hydrogen and/or
methane in the breath, however, is not proportional to the severity of the
symptoms. In other words, a person who produces little hydrogen and/or methane
may have more severe symptoms than a person who produces a large amount hydrogen
and/or methane.
The breath test is the best test for determining lactase
deficiency and lactose intolerance, but it has several weaknesses. The first is
that it is a long, boring test. The second is that it suffers from the same
issue as the milk challenge test with respect to the quantity of lactose that
should be used. (See previous discussion.) Lastly, the breath test can be
falsely abnormal when there is spread of bacteria from the colon into the small
intestine, a condition called bacterial overgrowth of the small bowel. When
overgrowth occurs, the bacteria that have
moved up into the small intestine get to the lactose in the intestine before there has
been enough time for the lactose to be digested and absorbed normally, and these
bacteria produce hydrogen and/or methane. This may lead erroneously to a
diagnosis of lactose intolerance. Other conditions also interfere with the
breath test. Thus, diseases that markedly speed up transit of lactose through
the small intestine prevent lactose from being fully digested and absorbed,
leading to a misdiagnosis of lactose intolerance. Recent treatment with
antibiotics can suppress colonic bacteria and their production of hydrogen or
methane and lead to a misdiagnosis of lactose tolerance. Fortunately, these
latter conditions are uncommon and usually can be anticipated on the basis of a
person's history or symptoms.
Next: Blood glucose test »
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