While there are always other possibilities, most bad reactions to food fall into two major categories, autoimmune reactions and chemical sensitivities. They are different.
Autoimmune Reactions – Both food allergies (like nut allergy) and food sensitivities (like celiac disease) are in the category of autoimmune reactions. The way an autoimmune reaction works is that your immune system has recognized an otherwise harmless protein in the specific food as being a pathogen (a harmful bacteria or virus). The body reacts to that pathogen as it would to a real pathogen (like pneumonia or food poisoning) which may mean reacting with diarrhea, vomiting, weakness, rash or problems breathing, among other things. In addition, the reaction may be time delayed. I know in my own case the digestive distress takes between one and 24 hours to show up.
What makes it an autoimmune disease is the word PROTEIN. If you can avoid the protein you can avoid the problem. So in my case, with celiac disease I can drink vodka made entirely from wheat and use grain based vinegars. These products are distilled. Distillation removes all proteins from a food. I cannot, however, use wheat germ oil or wheat starch because trace proteins remain. Note that malt vinegar has gluten provided specifically and is not safe. Neither beer nor wine are distilled so beer is not safe. It is the process of removing the proteins that makes the food safe.
Chemical Sensitivities – Chemical sensitivities do not rely on proteins. In many of the best known cases the body is unable to break down phenols (salicylates are a category of phenols) or other chemicals because of either problems with the liver or because the gut flora that break down the compounds are decimated or absent. Chemical sensitivities include but are not limited to a reaction to salicylates, MSG, additives, preservatives, and amines as well as others. They more commonly result in headaches, rashes, migraines, behavior problems, insomnia, and neurological problems, but other symptoms are also possible. With a chemical sensitivity what matters is dosage. If you have a tiny amount of the chemical you are not as likely to react at all, but once a certain threshold is reached, BOOM! reaction. Chemical sensitivity reactions are often faster (just like taking a medication) and tend to fade over time in a regular pattern.
Why does this matter?
Because if you have as many food sensitivities as we do you may start to feel like all you can eat is celery and water. It can be very frustrating. The truth is that most food is actually still available to you and a lot of processed foods that may not specifically state gluten-free may be fine. I eat things made with grain vinegar all the time because it is distilled and the proteins don’t survive. Medically speaking it is far more “gluten-free” than other foods that have 10ppm.
In addition, it is helpful in understanding what you are reacting to. If its an allergy or autoimmune food sensitivity you may need to think about what you ate yesterday as well as today. If its a chemical sensitivity the problems can start within the hour.
Now some (many?) people have both of these. Both fit with the hypothesis that the real issue is gut flora that is out of alignment and not doing its job. However, it is possible to just have one and not the other, and it is important to know which one you are dealing with.