I recently underwent food allergy testing at the behest of my gastroenterologist to see if food allergies are causing or contributing to my irritable bowel syndrome.  It turns out I’m allergic to five foods.  My question is how do I determine if all five are causing my symptoms or just one or two?

That is a great question.  The guidelines I’m going to share with you were worked out in clinical research units.  In these settings patients are kept in a controlled environment (typically a hotel appended to the university hospital) and avoid the implicated (food allergy test positive) foods for two weeks.  It seems to take this long to fully “clean the system”.  Then, one food at a time is re-introduced.  In the research setting this is done double blinded and placebo controlled by putting the food to be tested inside gel-caps.  On a given day the patient may receive a placebo or an actual food.  Now, obviously this is a very tedious and protracted protocol.  But it has led to some basic rules to be used at home.

So, first of all, you need to avoid all five foods at the same time for two weeks.  If food allergy is causing your IBS, you will feel better at the end of two weeks. 

Then start adding a single food. You should have this food in “a usual portion size” and have it at least three days in a row.  If you have “tummy troubles” the very first day it could be due to the food or serendipity.  So, you need to repeat the experiment to verify. 

Now here is the hard part.  If you have symptoms you need to go back to the elimination diet until you feel well again before you try the next food.  Unfortunately, this could take up to two weeks again.  But you need a symptom reduced baseline to have clarity. 

Once you’ve tested all the foods there is another step you might need to take.  But this is necessary only if all the individual tests were negative.  This step involves testing food combos.  By way of example let’s say you tested positive to cheese, tomato and wheat.  Tested individually: no issue.  But tested together might reveal synergism.  So, pizza would be a mechanism to test all three.  Good luck, be patient.