2 And when he had fasted forty days and forty nights, he was afterward an hungred.
Matthew 4:2 King James Version (KJV)
Hunger leaves you doing the first week of fasting and what remains is habit hunger.
“When true hunger returns after around forty days, more or less, of fasting, starvation begins.” The Fasting Prayer by Franklin Hall pg. 143
3 And when the tempter came to him, he said, If thou be the Son of God, command that these stones be made bread.
Matthew 4:3 King James Version (KJV)
Breaking A long or complete (one when hunger returns) fast hastily can be harmful to you and even deadly. You must take your time to return to normal eating habits your stomach is like that of a baby. The fast here was already finished Jesus was being tempted to break the fast poorly which could result in sickness or death.
16 Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast. Verily I say unto you, They have their reward.
17 But thou, when thou fastest, anoint thine head, and wash thy face;
18 That thou appear not unto men to fast, but unto thy Father which is in secret: and thy Father, which seeth in secret, shall reward thee openly. -Matthew 6:16-18 King James
Jesus is making an assumption that the people are already fasting and giving directions on how to fast WHEN they are fasting not IF they choose to fast.
14 Then came to him the disciples of John, saying, Why do we and the Pharisees fast oft, but thy disciples fast not?
15 And Jesus said unto them, Can the children of the bride chamber mourn, as long as the bridegroom is with them? but the days will come, when the bridegroom shall be taken from them, and then shall they fast. -Matthew 9:14-15 King James Version
This text once again demonstrates, the same as Matthew 6:16-18, that fasting a basic fundamental teaching that is done by all.