For those of us Orthodox who have friends and family in heterodox communions, it's easy for us to condemn their adversity towards fasting. We should let them continue as they will. At the same time, we are often the victims of attacks from leaders of these heterodox communions (especially if we left) who say that fasting is nothing but legalism, something that you have to do. Let's make it very clear: Fasting should be something we WANT to do. If you don't want to, don't do it because fasting is not the end, it is a means to an end.
The big problem, especially in mainline Protestantism, is a schism between the soul, the mind, and the body. The body has been exorcised from modern worship, while everything is focused on the soul and mind. Sometimes I wonder if Manichaeism and Gnosticism have really been eradicated from the church. In most Protestant buildings, there are no images, there is no communion, there is no incense. The presence of these is to elevate all the five senses. Essentially, the body is left out of prayer when it should be involved just as much. The Lord says that He should be worshiped with all our soul, mind and body, but why is the body ignored? The discipline of fasting is, to borrow from a recent book on the subject, to bring the body into the act of repentance. Though the Greek work for repentance, metanoia, means change of mind, our mind and body are united. The body is guided by the mind. We don't just train one and ignore the other. Both are involved.
Speaking of both mind and body, if the body is being disciplined so must the mind. The body is starving from certain foods which nourish it for most of the year and it groans. To prevent us from being guided by our stomachs, we must employ more and more prayer. Less food=more prayer. Even our Lord said as much.
Fasting, as fine a discipline as it is, should never be done with a possibility of harming oneself. And it should never be looked at as a means to win points with God. Bishop KALLISTOS writes:
At all times, it is important to bear in mind that 'you are not under hte law but under grace' (Rom. 6:14), and that the 'letter kills, but the spirit gives life' (2 Cor. 3:6). The rules of fasting, while they need to be taken seriously, are not to be interpreted with dour and pedantic legalism; 'for the Kingdom of God is not food and drink, but righteousness and peace and joy in the Holy Spirit' (Rom. 14:17).--"The Meaning of Great Lent" from The Lenten Triodion, p. 37
And it is important also to remember to be joyful as we do this. To fast and be despondent is not the way of the Christian life.