Re: fasting while underweight
You are certainly going to absorb more nutrients from herbs and other whole foods than you are from any type of synthetic vitamin. However, even with the herbs, if you are low on enzymes and you are taking your herbs in capsules, you may have a hard time breaking the capsule down so that you can get to the herb.
One of the ways you can tell whether or not you have enough enzymes is by how you feel when you eat. Do you have indigestion, heartburn, acid reflux, constipation, gas, bloating, etc.
As far as fasting goes, I think one of the easiest programs to follow is the one Paul Bragg lays out in his book, "The Miracle of Fasting". He suggests fasting on water one day a week, three days per month and then for a week twice a year - usually as the seasons change - such as in the Spring and Fall. If we were to adopt this as a habit, along with a good diet, we would have an on-going maintenance program for our body - cleansing and nourishing.