One of the many frustrating aspects of irritable bowel
syndrome (IBS) is it can appear quite vague and “un-scientific” when it comes
to getting a diagnosis. In today’s modern world we expect diseases to be
diagnosed rapidly and efficiently, we go to the Drs, they take some blood and a
few days later we have our results. With IBS on the other hand, its lots of
questionnaires, symptom diaries and trial and error.
One area that does appear, on the face of it, to offer
a clear yes/no answer are breath tests. But how useful are breath tests when
it comes to an IBS diagnosis?
What are breath tests?
Breath tests are used to determine whether you absorb
or malabsorb a particular sugar. The tests offered are usually for lactose,
fructose, sorbitol and mannitol. The patient will be given a dose of one the
above sugars and then the amount of hydrogen and/or methane is measured in the
patient’s breath. The understanding is that any of the sugars that are not
absorbed are fermented by intestinal bacteria which produce the gases hydrogen
and methane. The gases are carried in the bloodstream to the lungs where they
are exhaled. A cut off point is established and if the amount of exhaled gas is
above that point then the patient is diagnosed as “intolerant” to that sugar.
For an individual that suspects they may have IBS
these tests appear very attractive because all of the mentioned sugars are
associated with the condition plus the tests promise a quick and clear result.
But…..
Sadly here comes the but. While lactose intolerance is
a recognised condition and the breath test to determine it well established,
the same cannot be said for fructose, sorbitol and mannitol. One issue is the
lack of standardisation of the test, different centres have different cut-off
points so your diagnosis may be different depending on the centre’s cut-off
point not the amount of hydrogen you produced in your test.
Focussing on fructose for a second, we all have a
limited capacity to absorb fructose. That means that at a large enough dose
every single one of us will malabsorb fructose. A study from way back in 1986
found that 8 out 10 healthy subjects malabsrobed a 50g dose of fructose,
whereas only 1 out of the 10 malabsorbed the 15g dose (1). Another study in
2014 found similar results, in a group of 16 healthy (non IBS) participants, a
40g dose of fructose was shown to distend the small bowel with water and cause
IBS type symptoms even though they were not IBS sufferers (2).
This means that depending on what dose they give you
could be wrongly labelled as intolerant to fructose or even given a false IBS
diagnosis. What has been shown is that some people are more sensitive than
others to a single dose of fructose but to label them intolerant is probably an
exaggeration.
In conclusion, while the idea of a quick test and
diagnosis is very attractive, especially in a condition such as IBS where patients
are usually desperate for a straight answer. The lack of standardisation of the
tests and the fact that we all could be diagnosed as fructose “intolerant” at
the right dose means that sadly breath tests are not a reliable way to get an
IBS diagnosis.
