https://en.wikipedia.org/wiki/Metformin is a type 2 diabetes drug that, through usage, has appeared to yield cancer-suppressing benefits. Studies underway, and some notable scientists are beginning to take it prophylatically.
That's the short version. The more differentiated view is that:
- this is found to be the case "for many perhaps most" --- not all cancer/tumors
- in a glucose-starved context, "many perhaps most" cancers/tumors seem to be able to switch to glutamine or IIRC even other amino acids
- once it's "there": zero blood glucose is impossible to achieve and would be deadly, if blood glucose is mostly scavenged by the tumor(s) it stands to reason that the liver will happily waste away lean tissue over time to provide the strictly necessary blood glucose levels (there are a very few other healthy cells that also can "run" only on glucose and not metabolize fat or ketones or lactate etc, such as red blood cells, around half (or less) of the brain, and all cells that lack mitochondria)
Most cancers/tumors seem to show both: myriads of genetic mutations and severely disrupted/dysfunctional/degenerate mitochondria. The latter necessitates "anaerobic glycolysis even in the presence of oxygen", aka the Warburg effect. The debate is still ongoing which of these 2 characteristics really came first and perhaps caused the second, with mainstream largely siding with the DNA-mutations-first perspective. Anaerobic glycolysis is fermentation of glucose (or easily convertible alternatives such as glutamine) instead of oxidation of glucose (or other sources of kcal) --- abnormal for cells with functioning/healthy mitochondria except perhaps in brief anaerobic-emergency moments, and sometimes viewed as the original "ancient, pre-oxygen cellular energy generation" modus operandi.