The disciples were unable to drive out a demon from a boy, but Jesus drove it out. When the disciples asked how he did it, he said that this kind can be driven out only “by prayer and fasting” (Mark 9:29). This shows that because the disciples were not fasting they could not drive out the demon, and, that Jesus could drive it out because he was fasting. Some copyists attempted to change this verse by leaving out the words “and fasting”. This is how, for example, the Revised Standard Version reads. But this reading gives the passage an impossible meaning that Jesus’s disciples were not praying either.
Great example of how some versions of the Bible try and water down the power of God's word.
Here's a list of the top five best-selling English translations of the Bible. The ones in red, the King James and the New King James, are the only ones who keep the words "and fasting" in Mark 9:29.
New International Version.
King James Version.
New Living Translation.
New King James Version.
English Standard Version.
Compare the rest at this site.
Luke 5:33 quoted above reveals that (a) fasting means abstinence from eating and drinking
The Bible tells us of three different kinds of fasting.
There's a regular fast like the one that Jesus subjected himself to after his baptism, where he ate nothing, but obviously drank.
There's the partial, or specific type of fast that Daniel underwent, where you give up one or more specific items. He said,
I ate no pleasant bread, neither came flesh nor wine in my mouth.
And there's the total fast, like what Paul subjected to after his conversion, where nothing is eaten or drunk.
And he was three days without sight, and neither did eat nor drink. This is also the kind of fast that Queen Esther called for:
Go, gather together all the Jews that are present in Shushan, and fast ye for me, and neither eat nor drink three days, night or day: I also and my maidens will fast likewise;
It seems like three days is the max for that kind of fast.