Some of these are founded--keeping monosodium glutamate to a bare minimum can help protect your brain--the stuff can ruin your brain (aspartame is at least as bad). But, there are ways around that--since calcium is involved with this, taking magnesium (and boron) before eating a food with monosodium glutamate can reduce this effect. And some people are sensitive to excessive gluten or to sugar.
However, some of these phobias are unfounded. Most people are not affected by reasonable amounts of gluten or sugar (high fructose corn syrup, however, should be kept to a bare minimum or eliminated). And, I remember when there was a low-fat craze going among the congregation (which does no good if you make up for it by adding extra sugar). As for sodium, most people would be better to shoot for an intake closer to 4,000 mg instead of 2,400--it's cadmium that drives your blood pressure up (better quit smoking--that is responsible for at least 30 mcg per pack of cancer sticks smoked).
What amazes me is when they claim to be on a health kick, and then they stop at McDonalds (which is reputed to have some of the sickest phude on the planet). They order a poisonburger (loaded with monosodium glutamate and who knows what other chemicals), a fries (with rancid polyunsaturated fat, which is the worst for your arteries both because of its instability and its already being rancid), and then a diet Coke (aspartame for your brain). And they do it every day because they are in field circus, and wish to extend their time as much as possible.