Ｔｈｅ　ｓｉｍｐｌｅｓｔ　ａｎｓｗｅｒ　ｉｓ： [小麦粉]　のアレルギーがあります。 [komugiko] no arerugii ga arimasu. komugiko is 'wheat' (I'm fructose malabsorbant. I have used this and many similar sentences many times in Japan)

I recently had cause to ask just this question of one of my Japanese language partners, as my daughter will be going to Japan with me and she is allergic to prawns. My friend suggested the following sentence: エビのアレルギーがありますから、関係(かんけい)のあるものを　のぞいてください. Naturally, you would substitute wheat, or whatever other foodstuff you are allergic to for エビ　(prawn) at the beginning of the sentence. So instead of asking what is in the food, you are saying please don't bring me anything that contains or has anything to do with............ のぞいて　＝from the verb 除く　to exclude. Hope that helps.

「すみません、アレルギーがありますので、この食べ物は何が入っているか知りたいですが。」 [sumimasen, arerugii ga arimasu no de, kono tabemono wa nani ga haitteiru ka shiritai desu ga.] Excuse, but since I have allergies, I would like to know what's in this food. Here is how I would ask. I think it gets the correct meaning across, but might not necessarily be the most natural expression.