Question: Jesus said, "When ye fast..." (Mt 6:16), not "IF ye fast..." What are some guidelines for fasting?

Question: Jesus said, "When ye fast..." (Mt 6:16), not "IF ye fast..." What are some guidelines for fasting? We know a young, married man who has more than once gone on a 40-day fast. He is now underweight yet considering another fast.

Response: Isaiah 58 has some pertinent things to say. "Is not this the fast that I have chosen? to loose the bands of wickedness, to undo the heavy burdens, and to let the oppressed go free, and that ye break every yoke?" (Is 58:6). Of what benefit (other than health) is a fast if not chosen by the Lord?  We believe it is only for the Holy Spirit and Scripture to tell whether, when, or for how long individuals are to fast.

Fasting is commonly promoted by demons who pose as the Virgin Mary. For example, the apparition of "Our Lady of Medjugorje" exhorts, "Christians have forgotten that they can stop war and even natural calamities by prayer and fasting" ("The Truth Will Set You Free," TBC, 5/92).

Fasting can easily become ritual and show. Jesus warned in Matthew:6:16Moreover 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.
See All...: "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."

What does the Scripture tell us about biblical fasting? Paul told married couples, "Defraud ye not one the other, except it be with consent for a time, that ye may give yourselves to fasting and prayer; and come together again, that Satan tempt you not for your incontinency" (1 Cor:7:5Defraud ye not one the other, except it be with consent for a time, that ye may give yourselves to fasting and prayer; and come together again, that Satan tempt you not for your incontinency.
See All...). There are, therefore, times for fasting and prayer.

When John's disciples questioned Jesus regarding this, the Lord replied, "Can the children of the bridechamber 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" (Mt 9:15).

Finally, Paul wrote in Colossians:2:20-23 [20] Wherefore if ye be dead with Christ from the rudiments of the world, why, as though living in the world, are ye subject to ordinances, [21] (Touch not; taste not; handle not; [22] Which all are to perish with the using;) after the commandments and doctrines of men? [23] Which things have indeed a shew of wisdom in will worship, and humility, and neglecting of the body; not in any honour to the satisfying of the flesh.
See All...: "Wherefore if ye be dead with Christ from the rudiments of the world, why, as though living in the world, are ye subject to ordinances, (Touch not; taste not; handle not; Which all are to perish with the using;) after the commandments and doctrines of men? Which things have indeed a shew of wisdom in will worship, and humility, and neglecting of the body; not in any honour to the satisfying of the flesh." 

As we can clearly see, there are times to fast and there are times not to fast. There are times when fasting is helpful for spiritual growth. Unfortunately, there are also times when fasting may be motivated by the desire of the flesh to draw attention to oneself. These things should be carefully and prayerfully considered before the Lord.