Thanks Kat and 30yrvet for your contributions. I think I do suffer from mild hypoglycemia which is why I get the shakes. However, I seem less likely to get them on a water fast than on a juice fast. It may be the "diet-induced" hypoglycemia that John refers to because of the lack of protein. The body digests the juice and absorbs the sugar so fast and then you get the rapid drop that induces the shakes.
I manage to fast if I'm having surgery but still generally feel light headed and, of course, that's less than 24 hours. Given the risks for some people, I wonder why hospitals don't ask more questions before requiring individuals to fast. I wonder whether severly hypoglycemic individuals can get surgery scheduled for first thing in the morning to minimize the time they are fasting.
