Alan gave me permission to quote one of his emails:
This mode of "the one we feed grows" teaching sometimes goes further and states that, when we feed one exclusively, the other grows weaker and weaker. This is, in my opinion, very dangerous doctrine because, while the nature that is "starved" may grow less prominent in our daily consciousness, the flesh nature undergoes no change whatsoever – it is utterly and irreversibly corrupt. I can tell you from sad experience that, if I assume that my flesh nature has become significantly "weakened", and I indulge it in any way (letting my thoughts run in an improper direction, for example), I very soon find that it is not only not weakened, but that it re-asserts itself with full ferocity! This is truly the fruit of not believing what the Word says, and is literally walking in death (because the "old man" IS dead!).