According to this NY Times blog, "It’s long been known that glucose fuels many brain functions. Having a bite to eat appears to help boost a person’s willpower."
So, if you're learning to fast, when you're struggling, just have a bite to eat. That will help. Hmmm...
I fasted a bit a few weeks back. I wish I was more spiritual because it wasn't fun. It took a lot of focus. Maybe it's me just getting older but it was tough. David talked in Psalm 109 about being weak when fasting and I guess I experienced that.
But i do recognize that something deep happens during the midst of it. I really can't explain it. Rewards of Fasting and Too Busy Not to Pray are a couple books that I read during my fast. Both are helpful. I would recommend each of them, although the former is the harder read.
4 Benefits of Fasting...
1-increased hunger for God
4-increased focus in prayer life
Scripture References to Fasting