As you quoted, some say that it is forbidden to make one's body Tamei, while others say that it is not. (See Tosfos on 2b.) However, even those who hold that it is forbidden rule that way only for one who is presently Tahor. Therefore, it does not apply to all of us who are Tamei with Tum'as Mes, as you mention.

As for your assumption that all food today is Tamei, since all people are, that is not necessarily the cae. Not all food is Tamei, because in order for it to become Tamei, it must first become "Huchshar" (prepared) to receive Tum'ah, which means wetting it. An apple picked directly from a tree by a person who is Tamei does *not* become Tamei until it becomes wet while detached.

D. Zupnik

DZ:ys