A Christian's mind should be focused on love and obedience to God.
A Christian should try to keep his body in harmony with his mind, however the body is a biological instrument. Because it is biological it will be subject to illness, and various malfunctions. The body will experience hormonal influences. The body can be biologically predisposed for depression, addiction, sexual improprieties, and lacking self control. In these cases the Christian should strive for perfection, even while realizing their own need for help.
The Christian's body will never be 100% in harmony with his mind. That is why Jesus tells us that the "spirit is willing, but the body is weak". In the end we are sinners, and need a savior.