Gluten, in and of itself, is not the problem. Gluten is merely the main protein found in a few grains -- wheat, rye, and barley, predominantly. The problem arises from other factors: Wheat has been naturally bred over the last few centuries to contain higher and higher levels of gluten, because it gives bread dough its wonderful elasticity and the ability to rise. So, it's wheat that contains the most gluten, out of the gluten-containing grains. Spelt and kamut, being more ancient varieties, contain less.
Next is the problem of over-consumption in our culture. Basically, it's not breakfast unless there's some toast or a wheat-based cereal; and lunch is a sandwich or soup, salad, and bread; dinner often features rolls; and desserts and snacks often also include some kind of flour product -- cake, crackers, etc. We just plain eat too much wheat. And gluten, even if there is no intolerance, is irritating to the gut, making over-consumption more problematic.
Because of gluten's ability to irritate the gut, when digestion is compromised, gluten ingestion can become a bigger issue. When someone's under constant stress, secretory IgA, the immune system protein that coats the digestive system, is reduced, leaving the tissues more susceptible to irritations and infections. Keeping gluten intake minimized while under stress is always a good idea.
And then there's grain preparation. Sprouted wheat is easier to digest than flour products. The grain has been prepared in such a way that the protein is easier to digest, and other nutrient-inhibiting factors, such as phytates, have been neutralized. However, it does still contain all its gluten. Naturally fermented sourdough breads are likewise easier to digest, because the protein has been partially digested by the fermenting bacteria. (See
http://www.westonaprice.org/food-features/be-kind-to-your-grains for a nice article on this.)
If someone has Celiac disease, gluten is a no-no. If someone has another type of immune reaction or a digestive intolerance, abstention for a few weeks to months is important. The gut needs to be healed and in good shape, as Susan mentioned, and sometimes digestive enzymes for intolerances can be very helpful.
For all of us, gluten has been the golden reminder that relying too heavily on one food can cause us problems. But it's wrong to place the blame solely on gluten-containing grains. Rather let's look at frequency, dose, and duration; proper preparation; individual metabolic abilities and needs; and digestive function.
