Matthew, regarding fixing gluten intolerance with diet, it really depends on what is causing the intolerance and how much damage was done before it was caught. If it's "simply" a matter of a severely handicapped digestive system, it theoretically can be cured but it takes a lot of time and attention and you have to know what you're doing. Part of the problem is that there are several different "diets" and theories that all supposedly get the digestive system back on track but there's no way of testing them but by individual trial and error. In the end, some people are able to fix their digestive systems and others are not.
There are also differences in severity between an allergy, an intolerance, a sensitivity and celiac disease.
I would think that most people who are simply gluten intolerant or sensitive could still receive Holy Communion. They may have symptoms afterwards but if not severe or life threatening they could offer it as part of their cross.
I wouldn't mess with celiacs or true allergies though. I think, unless someone else can put forth a good argument against it, that it would be good for them to receive the Blood rather than the Body of Our Lord at Holy Communion. There must be room for this adjustment in the Church's discipline.
If a person has only seen or experienced seasonal allergies with a runny nose and a little sneezing they may not realize how an allergy can actually KILL a person. To that body, that substance is a poison. I know someone with a nut allergy so severe that it's not just a matter of not eating them; he gets a reaction even if a package is opened in the same air space and he breathes in molecules of it. It's less severe than if he actually ingests it, but the throat tightens nonetheless. The severity of allergies can sound crazy until you're the one actually having the reaction. (I learned this the hard way! Used to sweep "allergies" under the rug with a bit of an eye roll like it's child's play. No longer!)