While they were worshipping the Lord and fasting, the Holy Spirit said, ‘Set apart for me Barnabas and Saul for the work to which I have called them.’ So after they had fasted and prayed, they placed their hands on them and sent them off. (Acts 13:2-3)
In Greek, the verb ‘to fast’ is νηστεύω nesteuo from ne,implying negation, and ἐσθίω esthio ‘to eat’, thus nesteuo is literally ‘to not eat’. But in the context of the Bible, this is abstinence not starvation. Thus, a Biblical definition of a fast is to refuse food in order to better focus on God – or put in the negative, and especially in a modern context, fasting without prayer is merely dieting.
God Breathed Words 