As noted previously, for anyone having non-specific gut symptoms, it is really hard to know what causes them. We tend to attribute symptoms we have to the last thing we ate. This is a core feature of “conditioned taste aversion” or “bait shyness,” and it seems to be a very basic kind of association. Studies with many species, including humans, have reliably demonstrated that experimentally induced gut symptoms are blamed on the last thing eaten. This is especially true if the food is new or has a strong flavor, such as garlic.
But some food allergies — for instance, the “alpha-gal syndrome” discussed here under Journal Club — produce their reactions several hours after eating the allergenic food, leaving plenty of time to eat something else. So it can be difficult to know what is really causing the problem. Food sensitivities can be particularly difficult, as there is no obvious allergic reaction, just the gut symptoms. It would be nice if there were a biomarker that could be used as a test to provide an answer. There still isn’t, but there are companies that will sell you tests for them.
For instance, Amazon.com sells testing kits claiming to detect IgG in blood (or hair!) that target food antigens, and these (despite NOT being IgE antibodies) can tell you what foods you react to. Some examples:
- “5Strands Food Intolerance Test, Accurate Hair Analysis, 658 Items Tested, at Home Food Sensitivity Test Kit for Adults & Kids, Gut Health Test, Results in 4 Days.”
- “Everlywell Food Sensitivity Comprehensive Test — Learn How Your Body Responds to 204 Different Foods — at-Home Collection Kit — CLIA-Certified Labs — Ages 18+.”
Can circulating IgG and hair kits really tell you what your sensitivities are?
What the American Academy of Allergy, Asthma, and Immunology (a professional organization of allergists and immunologists) says:
It is important to understand that this test has never been scientifically proven to be able to accomplish what it reports to do. The scientific studies that are provided to support the use of this test are often out of date, in non-reputable journals and many have not even used the IgG test in question. The presence of IgG is likely a normal response of the immune system to exposure to food. In fact, higher levels of IgG4 to foods may simply be associated with tolerance to those foods.
Due to the lack of evidence to support its use, many organizations, including the American Academy of Allergy, Asthma & Immunology and the Canadian Society of Allergy and Clinical Immunology have recommended against using IgG testing to diagnose food allergies or food intolerances / sensitivities.
I couldn’t say it better!
Basically, having antibodies to food items does not necessarily mean your immune system reacts and causes symptoms to that food (Leviatan 2022). IgG levels correspond mostly to what you eat.