Ketogenic diets were created in the 1920s because doctors noticed a lot of folks got better as they starved. A diabetic would get sick enough, get bed ridden, and then suddenly get better because. There are other diseases for which ketogenic diets help too.
But here we are, somewhere in Lent, and when I first started to realize how this works physiologically, I thought it was cool that church had this thing all along. Now it is 2015 I am somewhat annoyed that church hasn't properly figured this stuff out.
I think our ancestors were doing the best they could with the knowledge they had, but now we have better knowledge, so we can fast better. Unfortunately, I don't think people want to accept that our success or failure rests on our physiological state. There is no evidence that there is an 'apart'- i.e. that the spirit somehow succeeds as we beat our flesh into submission.
What happens is we fast until ketone bodies start being produced in large enough numbers for us to be able to stop being grumpy human beings and start being able to focus. Focus is necessary for prayer. So, certainly, to many of our ancestors, this process has a 'feel' of discipline, and with our new understanding of why fasting works, it will still be discipline- i.e. you still have to stay away from the twinkies. Except, if you want to get to the right physiological state, eating fat helps. The simplest thing would be to start an Atkins like diet a few days before you need to fast proper, and then when you get to a proper fast, drop to eating small amounts of fat every so often.
But those that fast tend to be sticklers for whatever rules were laid down in their churches.