Remember, fasting didn't work out so well for Jobs.
This same diet landed Ashton Kutcher in the hospital when he tried to go method for Jobs.
https://www.psychologytoday.com/us/blog/naughty-nutrition/20...
It seems that the best parallel we can do is cancer == DNA "software" bugs.
Software debugging is a complex art and what works in one case definitely doesn't work in all cases.
https://www.scientificamerican.com/article/fasting-might-boo...
The lack of protein or fat in the juice means that it cannot help the rest of the body rebuild itself as the tumor is sucking up all available resources. Shipments of essential fats or amino acids are being hijacked by piratical cancer cells, and never reach their intended destinations. If there were an oversupply, some might still get through.
Not to be super contrarian, but AFAIU nutritional treatment of cancer actually is a very big surprise for traditional cancer treatment/oncologists.
E.g. a podcast I listened to with Dominic D’Agostino mentioned walking through a cancer ward and most/all patients were being fed Ensure, a sugary "protein" drink (ingredient list starts with water, maltodextrin, sugar), to keep them from losing weight.
Which is a valid traditional concern, but ends up supporting the cancer's glucose-based metabolism (disclaimer IANAE) vs. triggering what this article cites (for the subset of cancers that are susceptible to it).
I don't think that is the model ...
I think the idea is that the cancer cells are a luxury that your body supports because external queues indicate that it's living in the land of plenty. Why not support the energy needs of these extraneous cells ? There's plenty to go around ...
The fasting forces your body to act in a miserly fashion and like someone forced onto a budget, the extraneous luxuries are the first thing to go.
It's your body that starves the cancer cells.
In doing so, they make themselves more sensitive to the resource supply. The "good citizen" cells will put themselves into a low-energy resting state or commit apoptosis to return resources for the common good when the energy supply gets low, as the cancer cells continue selfish business as usual, perhaps by growing more blood vessels into their tumor.
While fasting, many of your "good citizen" cells will stop dividing. Chemotherapy drugs or radiation treatments have greater impact on rapidly-dividing cells, like those in hair follicles, so the damage will then be more focused than ever on cancer cells, who disobey any signals to slow down, and keep dividing anyway. If you could send out a signal that says, "all cells should stay in their homes and shelter-in-place for the next 72 hours" then the cancer cells will ignore it and be the only ones out partying in the streets, and your attack helicopter drugs can be flying around, shooting at anything that moves. Then the immune cell cops can come out, and finish off the stragglers if they can.
It isn't the weakness of individual cells, it's the weakness of the intercell relationships in a tumor. "Good citizen" cells will commit apoptosis on demand in order to spare cells that are more important to the whole body. Cancerous cells have generally already ignored their suicide orders, and bluffed past or warded off the macrophages, in order to continue existing. That greed can be used against the cancer. They indiscriminately consume resources, so ordering the villagers to stop drinking water until further notice, and then poisoning the well, will kill or weaken anyone that disobeys. Even just poisoning the well without the prior protective order will tend to disproportionately kill more cancer cells, just because they drink as much as they can while the other cells are taking just what they need.
But when you can issue that protective order, by means of fasting, you get higher survival rates for obedient cells. If, for some reason, the cancer cells are resistant to the treatment, they will still hedonistically party themselves to death once the food is gone, as all the normal cells have either gone into brumation or killed themselves to lessen the burden.