Numerous people have an over-sensitivity (allergy) for gluten. But what are gluten exactly?
If you are over-sensitive to gluten (gluten intolerance or sometimes called gluten allergy, or celiac disease in its stronger form), or one of your dining guests is, then it only takes some extra attention to prepare a gluten-free meal. But have you wondered what gluten are and why they can affect people so much?
Gluten are intertwining long strains of protein (each consisting of many amino-acid molecules), with some water molecules mixed in (an excellent description of the chemistry of gluten in McGee). The more water, the more strains of protein will curl up and intertwine, forming an elastic mass.
You will have noticed that while mixing a dough, it will slowly start to firm up and become elastic. What really happened there was that you hydrated and agitated these protein strains and caused them to intertwine and give a firmer structure to the mixture. The more you mix a dough, the tougher its structure becomes. This is why many pastry recipes warn you not to over-agitate the dough - nobody likes tough cookies.
Gluten are not only harder to digest than other structures, some people (about 1%, so maybe one of your friends) have an auto-immune response to gluten, where the body goes into defense mode and starts battling the supposed attacker, causing infections in the lining of the intestines.
Gluten-free meals start with gluten-free ingredients, or ingredients with at least substantially lowered levels of gluten. These days, all supermarkets have a few shelves with gluten-free pastas, cookies and health bars.
For people who suffer from coeliakie (the auto-immune response to gluten) even traces of gluten can be enough to trigger a violent response. So do pay attention to product labels that read something like "This product was prepared in a location that also processes ...".
Bread, pasta, pizza et cetera are all loaded with gluten, if they have been made with flour. Gluten-free alternatives are often based on pea protein.