Earlier quoted context omitted.
See Roy Walford et al out of UCLA on the CRON diet. Its not a nice way to live, but it seems to be safe and healthy.
I'll have to look into it more, but from what I've seen, it seemed that people had to restrict calories so much that their immune system was underactive (which may be a large part of why it works).
Interesting, i last looked at this over 10 years ago, i assume this came to light more recently than that.