Just to expand on one of the parent's points, Celiac is pretty rare, and affects under 1% of the population. However, Fructose Malabsorption is much less rare, and affects closer to 5% in the US. In spite of the name, Fructose Malabsorption has little to do with dietary fructose, and instead has to do with a class of molecules called fructooligosaccharides (FOS).

Overconsumption of FOS causes bloating, nausea, "brain fog," lower GI upset, and a whole raft of other symptoms that sound an awful lot like what many self-diagnosed NCGS folks report. For folks with Fructose Malabsorption, the threshold consumption of FOS that causes symptoms is less than 10% the level that causes symptoms in the general population.

The confounding thing is that wheat is rich is FOS. So folks who believe they have NCGS stop eating wheat and sometimes feel better. They probably have Fructose Malabsorption, or are suffering from insulin spiking effects from refined carbs, or some other effect. But they heard gluten was bad for you, and when they ate the package of bread that said "gluten free," and stopped eating the normal wheat bread, they started feeling better.

"Middlebrow dismissals" of their condition don't really help them either. True, gluten is almost certainly not their problem. But gluten free food, as a side effect of lacking wheat, often helps them feel better! What are the chances that something similar is happening with MSG? Perhaps added MSG is often found along side other ingredients/contaminants that cause headaches and other reported "MSG symptoms"?