Often many non-specific symptoms are put down to a food allergy or an intolerance. 

One of the best tests to investigate this is a double blinded placebo controlled food challenge (DBPCFC). 

With this test the patient is given either the food in question (disguised in a capsule, or something like yoghurt) or a placebo.  Neither the patient nor the doctor knows if the patient is given a placebo or the food. 

That way both the patient's and the doctor's preconceptions of what might happen are removed.  This is in fact the "gold standard" test of food allergy.  It should only be done in a monitored environment and in patients in whom the history of allergy and skin testing/blood tests do not suggest any significant risk.

In NZ self-reported rates of food allergy are up to 15%.  In studies of food allergy done by double blind test, these are usually positive in only 2 - 5%.  This means that a large number of people believe they have a food allergy when, in fact, they don't.

This is quite problematic for them for two reasons:

1 - They have to avoid food and all the social issues with food avoidance can be significant

2 - The 'real' cause of their symptoms does not get diagnosed because it is put down to food allergy.

In general symptoms such as headache, tiredness, bloating, trembling, fatigue, constipation are very unlikely to be a food allergy.  Tests such as "hair analysis" or "IgG blood testing" make claims without proof with regard to such symptoms.

For symptoms that are mostly abdominal such as tummy bloating, pain, wind, constipation or diarrhoea I recommend consulting a gastroenterologist and registered dietitian.

For symptoms or brain fog, fatigue or headaches I recommend asking your GP who is best to consult or an opinion from a general medicine physician.

Link to general medicine diabetes specialist Doctor North Shore Auckland