Throughout history, the children of God have been told that fasting without obedience is meaningless. Here is what Isaiah and an early church father had to say about fasting:
"Is not this the fast that I have chosen? to loose the bands of wickedness ... to deal thy bread to the hungry, and that thou bring the poor [brethren] that are cast out to thy house? when thou seest the naked, that thou cover him?" Isaiah 58:6-7
"I will teach thee what is a complete fast and acceptable to the Lord ... God desireth not such a vain fast [the accustomed fasts] but fast thou [unto God] such a fast as this; DO NO WICKEDNESS IN THY LIFE, AND SERVE THE LORD WITH A PURE HEART; OBSERVE HIS COMMANDMENTS..." Shepherd of Hermas (See notes below)
The Biblical and historical understanding of the fast that God requires has to do with righteousness; not food. God has promised that he will bless those who keep His commandments - and curse those who do not. (whether we fast or not) Deut 28
Historical notes - J B Lightfoot, The Apostolic Fathers, p 203, Baker Book House 1956
According to Lightfoot, "The work [Shepherd of Hermas] is found in general circulation in the Eastern and Western churches, soon after the middle of the second century ... It is quoted by Irenaeus in Gaul, by Tertullian in Africa, by Clement and Origen in Alexandria. All these fathers - even Tertullian, before he became a Montanist - either cite it as scripture, or assign to it a special authority as in some sense inspired and quasi-canonical."
Jamieson Fausset & Brown's Commentary on Zech 7:5 points out the difference between the futile fasts of the captivity and the true "fast unto the Lord" of Romans 14:6, we read:
"did ye ...fast unto me? --No: it was to gratify yourselves in hypocritical will-worship. If it had been "unto me" ye would have "separated yourselves: not only from food, but from your sins (Isaiah 58:3-7). They falsely made the fast an end, intrinsically meritorious in itself, not a means towards God's glory in their sanctification ... (Romans 14:6)" Jamieson Fausset & Brown Bible Commentary Vol. 11,p.855 1888