Diagnosing food intolerance

In this day and age, it’s hard to imagine medical tests that don’t draw blood or that don’t involve expensive, room-filling equipment; however, for food intolerance and some immune-mediated reactions, the tried and true diagnostic requires only a pencil, paper, and food. Doctors, dietitians, and medical researchers rely on elimination diets and food challenges to identify food sensitivities, which can be used regardless of the underlying mechanism – this is important because right now we have only rough ideas of how these some of these reactions might work.

Diet investigations are deceptively simple – start from scratch and add only one potential trigger at a time until the culprit is found – but they also require a detailed knowledge of food chemicals. For this and for safety reasons – after all, you need to rule out if you actually have an allergy or another condition – you should always undergo a diet investigation with the help of a doctor or a registered dietitian. To help you prepare, let’s look at the three phases of the process: the elimination diet, the food challenges, and the modified diet. Before you start, your dietitian may also ask you to briefly keep a food and symptom journal to provide a baseline on the severity and frequency of your symptoms.

The information here is provided to give you an idea of the diet investigation process; it is not a set of instructions to carry out the procedure yourself. Always check with a doctor or a registered dietitian before modifying your diet. The elimination diet/food challenge process is not appropriate for pregnant women. People with asthma or laryngeal edema (swelling in the throat/upper airways) should only undergo food challenges in a clinic where resuscitation facilities are available. People with Crohn’s disease should not try an elimination diet without medical supervision.

Phase 1: Elimination diet

The elimination diet consists of only a few foods that are unlikely to cause symptoms in most people; depending on your history, it might be less restrictive. You must adhere to the diet even if you think that you know which food chemical makes you ill because you might be sensitive to more than one chemical. In most people, symptoms improve after two to four weeks on the elimination diet. You may experience a withdrawal reaction (a flare-up of symptoms) after the first or second week.

The elimination diet is necessary for several reasons. First, it indicates whether you are on the right track: if there is no change in symptoms after four weeks of strict adherence to the diet, then food intolerance is unlikely. Second, it removes the variability (the ‘noise’) in your symptoms so that changes can be detected during the food challenge phase. Finally, it lowers the amount of trigger chemical that you need to eat in order to see symptoms.

Phase 2: Food challenges

The food challenges determine which food chemicals are triggering symptoms. After at least two weeks on the elimination diet, specific foods are reintroduced into your diet one at a time – these are the ‘challenges.’ If a reaction occurs after a challenge, then you must wait until the symptoms subside, plus a few days, before moving on to the next food. Such challenges are referred to as ‘open’ challenges because you are aware of what you are eating. In research studies, participants are given food chemicals in capsules so they do not know which chemical is being tested or whether they are getting a placebo – these ‘blind’ challenges provide the most reliable diagnosis.

Wheat and milk are often tested first because reactions to these foods are just as common as pharmacological food intolerance reactions; then salicylate, benzoate, amines, MSG, artificial colors, etc. are tested on the advice of your dietitian. Ideally, challenge foods would only contain one trigger chemical, but this is not always possible. When challenge foods contain more than one trigger, the ordering of the challenges is important to systematically rule out all possibilities.

Phase 3: Modified diet

Your dietitian will design a modified diet for you based on your challenge results. You will be reevaluated in 1 or 2 months to see how well the restricted diet is working and to decide whether it is safe to liberalize what you eat. In this part of the process, you will be checking to see whether you have a high or low dose threshold for symptoms; you may also be able to gradually increase the amount of triggers that you eat, but, for your safety, you should always work on this with a dietitian or doctor. Since threshold doses can decrease after avoiding a food chemical, sensitive people could experience dangerous reactions when reintroducing foods to their diet, even in amounts that previously did not cause symptoms.

More information

Consult these books if you are interested in learning more about diet investigations:

© 2013 Anna (Laurie) Laforest. All rights reserved.
Photo © Can Stock Photo Inc.
FoodConnections.org – Food intolerance resource with a scientific twist


Brostoff J, Gamlin L. Food Allergies and Food Intolerance: The Complete Guide to Their Identification and Treatment. Inner Traditions/Bear; 2000. 486 p.

Clarke L, McQueen J, Samild A, Swain A. The dietary management of food allergy and food intolerance in children and adults. Australian Journal of Nutrition and Dietetics. 1996;53(3):89–98.