In trying to understand this verse my mind went to the words of Jesus,
(Matthew 15:11 KJV) Not that which goeth into the mouth defileth a man; but that which cometh out of the mouth, this defileth a man.It might help if you looked at the entire passage of Matthew 15:11-20.
The idea is that the source of who we are and what we produce is at our core, not in our environment. In our terms it would be the heart, to the Hebrews it was the belly that was the center of being. So what is going on here is an observation that ultimately we produce the kind of fruit in our lives that we really want. We may complain about how our kids turn out but think of all those times when they were small that you thought it was cute for them to be rebellious or disruptive.
So? Where is your belly? In our terms, where is your heart? You will reap from what you have decided to sow. Count on it. Maybe you want to plant a different crop?