There is an ancient catechism called the Didache (ca. 100 AD) that designates Wednesday (the day of the Lord's betrayal) and Friday (the crucifixion) as fast days. These were to supercede the Old Testament tradition of Tuesdays and Thursdays. If one feels it is the will of God to follow these, then it should be fine since some ancient Christians did so.Although days of the week are not in scripture, Christ still called us to fast (Matthew 6:16,Mark 2:19, Matthew 17:21) & St. Paul clearly fasted (1 Corinthians 7:5, 2 Corinthians 6:5). Cornelius fasted before St Peter preached the Gospel to the gentiles (Acts 10:30).Of course, there is a wrong way to fast (Luke 18:12) and a right way (Matthew 6:18). In the Orthodox Church, we observe a vegan diet (as much as possible) on these days; one may surely choose abstinence too according to one's ability. No one is called to anything to harm their health. God bless.