I got the distinct impression from the OP that rather than this being properly and appropriately linked to something like a "fasting window" that it was one of those totally whacky ideas that float round under the guises of "natural cancer treatment".
I've got to say that as one of the longest survivors every in the whole of the world that I'm VERY aware that I'm here testament to first rate medical science, innovative treatment, fantastic scientific research and development, clinical trials of some serious pharmaceuticals and good luck!
Like others have mentioned I am a huge advocate for just a normal well balanced diet and exercise and activity. It irritates the heck out of me when I hear the latest "fad" about such as fasting to detox! carb free, dairy free, blah blah blah.
CAUTION: The following might offend
Over the past decades I've heard some bonkers stuff about natural cure for cancer. I used to just smile sweetly while tuning my ears to off. Nowadays though I've switched off my censor and be polite buttons and when someone tells me such as "eat loads of broccoli, it cures cancer" ... and yes I've been told that! I respond with something along the lines of "don't be so stupid. If broccoli cured cancer people wouldn't be going through really difficult treatment and making desperate efforts to stay alive. Furthermore I eat brocolli & I'm not so stupid as to think that there's a simple cure all for anything"
And at the risk of offending and being entirely inappropriate have you noticed how so many of these stupid ideas come from people who look and are really unhealthy!
Heck I have leukaemia and have had it for more than 2 decades and yet in my 60's I run a business doing a very physical job: training horses and people who undertake equestrian sports.