"God intervenes in the affairs of men by invitation."
Behold, I stand at the door; and knock: if any man hear my voice, and open the door; I will come in to him, and will eat with him, and he with me.
Revelation 3:20
From God's Little Instruction Book
It is true that in many areas of our life, God will not do anything if we don't invite Him to work on our behalf. And this verse certainly is talking about this. As a Christian I also understand that Jesus will not come into a person's life unless they invite Him.
But this quote from the book can appear to be limiting. It does not recognize the power of intercessory prayer. Even though a person may not invite God to intervene in their affairs, through prayer we can ask God to intervene in others affairs (in that sense God is still invited). We can ask God to even move in the life of those who do not even believe in Him. The mystery of an all powerful, all knowing, ever present God and His intervention into the life of people who have a free will is an amazing topic. More than we could ever write here.
I finish this commentary by saying I don't know about you, but I willfully invite God to work in every area of my life, even the areas that I would rather He did not work in, which would include my eating habits.