The Bible gives no command for Christians to fast.
The only obligatory fast ever was for the Israelites under the Mosaic Law for the annual Day of Atonement. (Leviticus 16:29-31; 23:27; Psalm 35:13) And even though Jesus Christ never commanded his disciples to fast, he and his followers fasted on Atonement Day because they were under the Mosaic Law.
But even this ended when the Mosaic Law ended when “the Christ was offered once for all time to bear the sins of many.” (Hebrews 9:24-28) And with the end of the Law, the command to fast on Atonement Day ended. This removed the only obligatory fast mentioned in the Bible.
Christians are neither under command to fast nor prohibited from doing so:
"One [man] judges one day as above another; another [man] judges one day as all others; let each [man] be fully convinced in his own mind. He who observes the day observes it to Jehovah. Also, he who eats, eats to Jehovah, for he gives thanks to God; and he who does not eat does not eat to Jehovah, and yet gives thanks to God." (Rom. 14:5, 6) NWT
One thing to also consider is that in order for Christians to “Love Jehovah your God with your whole heart and with your whole soul and with your whole mind", it may actually be more efficient to do so with your whole strength in which adequate sustenance is key in contributing toward this. (Mt. 22:37) NWT
For more, see:
Fasting - Does It Draw You Closer to God?
Should you fast? What does the Bible have to say on this subject?
Search For Bible Truths - ARCHIVE
Search For Bible Truths - Search Guide