A lot of people in the United States claim negative reactions to wheat based products, which tends to be catagorized as celiac disease or gluten intollerance by sufferers, but which then often fail to be shown in the lab when experiments are undertaken with gluten.
The most likely answer is not that the people are making it up, or buying into some kind of nocebo (a negative placebo) in regard to wheat due to internet disinformation, but that there’s something being done with US wheat that isn’t going on in other places. This is often supported anecdotally by non-celiac gluten intolerant people who remarkably find their intolerance gone while in Europe.
An analysis of Roundup (glyphosate) usage relative to the incidence of celiac disease was compiled by computer scientist Stephanie Seneff (from MIT), published in Interdisciplinary Toxicology (which has a low impact factor and seems to just betting on its feet, for anyone that cares), shows an extremely high correlation of 0.9725. The study goes on to explain several potential mechanisms of action, and if you care I suggest reading it for yourself. Keep in mind that this scientist has no knowledge of biology, and the fact that there is no glyphosate tollerant wheat being grown commercially.
Glyphosate is a total-herbicide used in conjunction with crops which are genetically modified to be able to survive its use. The chemical itself is toxic and an endocrine disruptor, according to a string of laboratory studies conducted by Gasnier et al and published in the well-known journal Toxicology in 2009. Although toxin is a commonly used word, endocrine disruptor might need a little explanation: these are chemicals that “look like” signal molecules which your body uses to relay information to the cells: so they confuse your cells and even lead to impotence. As with every type of poison: dose is really the important factor.
Roundup has effectively soured public opinion against GMOs without most people understanding what it means, or what conventional or organic means, and connecting it through classical conditioning with the problems blamed on glyphosate useable. As levels of glyphosate usage increased, so did the number of people displaying the markers of celiac-disease or gluten intolerance, but since there is no glyphosate on wheat: is a connection remotely plausible?
The authors also found a correlation with renal disease deaths, also known as kidney failure, correlate almost perfectly. At the same time, we see that the comparison with the use of genetically modified crops does not show a likely causative relationship whatsoever. This is actually consistent with the kidney values seen in the Seralini studies, which were withdrawn due to their claims in regard to cancer and methodological errors (which were not actually significant due to the species of rat), the study may still hold a few grains of truth (but don’t bet your house on it).
Whether everything that study says is valid is a different question, and I encourage reading every study with scrutiny. Although it uncovered a few mechanisms of action and probably correctly points a finger at the use of endocrine disrupting chemicals on the food, its speculations and the specifics of farming practices are more spurious. It’s best to take what is supported and leave the rest behind, and that’s a rule we can apply when working with any study.