Fasting is an excellent part of spiritual discipline. What Musashi speaks to here is as you fast your body begins to weaken. In context this would mean that you will lose strength, energy, coordination, and even mental processing will be diminished. As a samurai for Christ you must feed your soul, feed that relationship, that connection with your Lord. While Jesus fasted (an example would be when He went into the wilderness for 40 days), He spent time connecting with His Heavenly Father.
Now as they went on their way, Jesus  entered a village. And a woman named Martha welcomed him into her house. And she had a sister called Mary, who sat at the Lord's feet and listened to his teaching. But Martha was distracted with much serving. And she went up to him and said, “Lord, do you not care that my sister has left me to serve alone? Tell her then to help me.” But the Lord answered her, “Martha, Martha, you are anxious and troubled about many things, but one thing is necessary.  Mary has chosen the good portion, which will not be taken away from her.” (Luke 10:38-42, ESV)
But we cannot fast from doing either. Our on going sanctification process includes doing:
“Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’ (Matthew 7:21-23, ESV)