Is eating what I do not need a sin?

Mark 9:29 & Matthew 17:21. "These kind only come out by prayer and fasting."
The disciples were brought a man's son who had a demon and they were trying to cast it out, but couldn't. This son was being tortured violently, and the demon would not let him go. Jesus came up and saw a crowd and asked about it, the father told Him how this demon terrorizes his son, The Lord rebuked the spirit, it threw him down one more time and then it left him.
When asked later by His disciples why they couldn't cast him out He said these kind only come out by prayer and fasting, one translation says much prayer and fasting!
If we have not committed our lives to prayer and fasting, it may very well be a sin. I've known guys who have gone to the doctor and been told unless they lose a ton of weight and start eating healthy, they will develop diseases, and they will die. Some of these people start losing weight immediately, and do very well. Others seem to not believe it, or care enough, and they go on with their lifestyles and eat what they want, and yes, die early.
The lust of our flesh is a powerful thing if given into.
But what about this diligence issue. Much prayer and fasting, this is a disciplined life is it not? Like a professional athlete it requires hard work, discipline in mind and body, and for the Christian living a prayer and fasted life, it means given over to the Holy Spirit, completely.
I was just thinking about senseless killing, like a drive by shooting that hits the wrong house...do we live senseless lives, specially those of us "hidden in Christ." Do we live this life all for Him, half and half? Zeal for Christ, burning passionately for Him, directed by His Spirit?
This can't be a thing of works, they tried to cast the demon out, but according to Jesus instructions, these kind only come out by prayer and fasting. So do we live, do I live, a prayer and fasted life. No I do not, as He has instructed me to.
If my life is His, hidden in Christ, and it's no longer I that live, but He that lives in me, what place do I have, what right do I have to now control my life? If I have given it up to be saved, shouldn't I give up all control as well? If I sell a property, and it's no longer mine, if I try and go live there still, isn't that trespassing? If I sell my car, but keep a set of keys and go and drive it, isn't that stealing?
We have become thieves robbing the temple of God that we are. We are not our own any more, we have been bought with a price, the price of Christ's life. And if we still assume control of our lives, we are thieves and trespassers!
We must not work in the flesh, but we must yield to the Spirit at all times. (Which we also must not do as a work of the flesh)
For some of us we just don't hear that we're going astray, because we are not attune to His Spirit. We have become dulled in our senses. "Lord, quicken our senses once again, whatever has caused us to lose sight of You, redeem that thing in our lives, and let us like sheep to a shepherd, hear your voice, so You can once again lead us as You will, in Jesus Name, amen!"
I have noticed in my own life that there are times I hear my voice better than His!? "Lord make that the opposite, that I hear You above all else, in Jesus Name!"