I am familiar with this. I haven't finished the book, but Breuss seemed to think that people's bodies were much more capable of handling fasting if fed an adequate supply of vitamins and minerals from the juices and teas, the cancer being much more susceptible to being wiped out completely than the individual. It's easy to see how this is a no-brainer for some if they have nothing to lose.
One thing he also mentions early on in the book is that people without cancer who just want to heal some chronic condition, lose some weight, etc. do perfectly fine with a 21-day fast. He insists that 42 days is minimum to starve cancer though.
I've considered a 7-day fast for awhile similar to the controlled water fast Westside always talks about, but 7 days of veggie juice actually sounds more sensible (and appetizing).
Do you know anybody personally who has done a 40+ day fast? By personally, I mean observe their progress first-hand?