Careful history taking and observation of food-related
reactions may indicate the source of food intolerance.
The food history should include:
- Identification of suspect food, drink, sweets / candy, or chewing gum.
- What is the time lag between eating and development of symptoms?
- What types of symptoms ensue?
- What amount of food is required to cause reaction?
- Does the reaction occur with every ingestion of the food?
- Does the reaction only occur under certain circumstances, e.g. exercise or stress?
- When did the last reaction occur?
- Do allergies exist, e.g. hay fever, asthma, eczema?
- Do other family members have food reactions, if so, which?
Breath tests
Breath tests are the most useful and non-invasive tests for determining the various
sugar (e.g. fructose, lactose, sucrose) and sugar alcohol (e.g. sorbitol, xylitol)
intolerances. They are well validated, widely used, but some discussion still exists
about the ideal test conditions. These tests should not be used in infants and reduced
doses of test compound are used in children. The tests procedures are identical
for all the intolerances (See
Patient breath test preparation form
Breath test analysis form
Symptom score form , Test site locations):
- Dietary restrictions, no smoking or excessive exercise from the day before testing
and no antibiotics or colonoscopy in last week pre-test.
- Ingestion of a specific amount of the sugar or sugar alcohol to be tested for intolerance.
- Regular breath samples taken for a standardized period (at clinic or at home).
- Diary of symptoms during the next day for calculation of the symptom index.
- Measurement of specific gases, e.g. hydrogen and methane, in the breath samples.
Intolerance is diagnosed based on the symptoms following the test and the gas concentrations
in the breath samples. The diagnosis is confirmed by a significant decrease in symptoms
whilst on a diet low in the poorly tolerated sugar or sugar alcohol. Expert dietary
advice is very helpful for the identification of food and drink containing the difficult-to-spot
offending ingredient. Examples of such ingredients are so-called hidden lactose
or fructans in vegetables or whole-meal products (See Food
tables).
Direct enzyme quantification in tissue biopsies
Direct enzyme levels can be determined in biopsies of the small intestine. However,
this test is invasive, expensive and performed by only a few laboratories. Sucrase-isomaltase
deficiency, a quite rare genetic disorder, is preferentially diagnosed using this
method.
Genetic tests
Genetic testing is available for some of the sugar intolerances.
Lactose intolerance: the severest genetic form of
intolerance (CC13910 / GG22018 i.e. homozygotes) can be very reliably diagnosed
using a simple genetic test with cells taken, for example, from the inside of the
cheek. The breath test is abnormal in all these individuals. Those with an intermediate
genetic form of intolerance (CT13910 / GA22018 i.e. heterozygotes), which are the
majority of individuals, have a wide range of intolerance severity to lactose. This
is most usefully assessed using the breath test measuring the actual effect of lactose
ingestion. Even a sizeable percentage of individuals without the genetic signature
of intolerance (TT13910 / AA22018) have lactose intolerance. In summary, breath
testing is overall the most useful form of testing. Genetic testing reliably identifies
a proportion of severely affected individuals and is also useful in screening larger
numbers of individuals or relatives of patients (See
Test site locations).
Fructose intolerance due to malabsorption (common fructose intolerance):
no specific genetic test is currently available. Testing is by breath test (See
Test site locations).
Hereditary fructose intolerance (HFI): about 70% of
genetic aberrations (polymorphisms) associated with HFI can currently be identified
and these should be determined in case of clinical suspicion, due to the lifelong
and possibly severe consequences of not excluding fructose meticulously from the
diet (See Test site locations).
Sorbitol and xylitol intolerance: no specific genetic
tests are currently available. Testing is by breath test (See
Test site locations).
Other tests: stool, blood
Various stool tests, including pH, have been used, but they should no longer be
advocated except for testing in infants. Measurement of blood glucose following
ingestion of lactose (lactose tolerance test) is less specific than breath testing
and is susceptible to more interference. Use of this test is diminishing and should
be discouraged.